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Subsurface structural imaging of Ceboruco Volcano area, Nayarit, Mexico using high-resolution aeromagnetic data
Journal of Volcanology and Geothermal Research 371 (2019) 162–176
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Subsurface structural imaging of Ceboruco Volcano area, Nayarit, Mexico using high-resolution aeromagnetic data Rashad Sawires a,b,⁎, Essam Aboud c,d a
Centro de Sismología y Volcanología de Occidente (SisVOc), Universidad de Guadalajara, Av. Universidad 293, Del. Ixtapa, 48280 Puerto Vallarta, Jalisco, Mexico Department of Geology, Faculty of Science, Assiut University, 71516 Assiut, Egypt National Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Cairo, Egypt d King Abdulaziz University, Geohazard Research Centre, 80206, Jeddah 21589, Saudi Arabia b c
a r t i c l e
i n f o
Article history: Received 1 August 2018 Received in revised form 14 December 2018 Accepted 4 January 2019 Available online 10 January 2019 Keywords: 3D inversion Aeromagnetic data Geothermal prospect area Ceboruco Volcano Mexico
a b s t r a c t Ceboruco volcano is one of the largest volcanoes of the Trans-Mexican Volcanic Belt (TMVB), which extends along central Mexico. Among the western TMVB, Ceboruco Volcano comes in the second place after Colima Volcano in its activity. Also, it is considered as the only one that has historically-documented eruptions. Few geophysical studies have been published with the aim of studying the internal structure of the volcano. In the current work, and throughout the support of the CeMIEGeo-P24 geothermal exploration project, we aim to delineate the possible subsurface structural trends, to determine the approximate depth to the basement surface, and to provide an illustrative 3D model for its subsurface structure. To achieve such goals, a detailed analysis of the aeromagnetic data for the volcano area was performed. 2D interpretation was carried out for the aeromagnetic data. Edge detection filters, such as Tilt Derivative, and Analytic Signal, were used to map the contact/faults within the study area. In addition, 3D inversion was used to image the subsurface structure of the volcano. Aeromagnetic data was inverted using the GMSYS-3D inversion program by applying Parker algorithm. Four subsurface layers at depths of 250, 500, 750 and 1000 m were assumed during the inversion process. Results indicate that the limits “boundary” of the subsurface magma chamber of the volcano can be traced easily from the first subsurface layer. A secondary structure line appears towards the east from the volcano boundary and it can be observed from the inversion of the second and third layers. This indicates that both, the base of the volcano and the structure line, stem from the same source. Finally, they disappear in the inversion results for the last layer, which concludes that we reach the basement above which the volcano is formed. Combination of all obtained results with the geological information, helped to understand the main structure of the Ceboruco volcano. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ceboruco volcano (Fig. 1) is one of the largest volcanoes of the TransMexican Volcanic Belt (TMVB), which extends along the centralsouthern parts of the Mexican Republic. It is considered the most active volcano in the western TMVB after Colima Volcano and also the only one that has historically-documented eruptions (Sieron and Siebe, 2008). More than 40% of the Mexican population (e.g., in Mexico City, Guadalajara, and Puebla) lives around this TMVB. Despite the benefits of the volcanic activities (e.g., fertile agricultural soil, mineral resources such as gold, nickel, copper, silver and lead, and cement industry), the risk of volcanic eruptions is an important issue to be taken into consideration.
⁎ Corresponding author at: Department of Geology, Faculty of Science, Assiut University, 71516 Assiut, Egypt. E-mail address: [email protected] (R. Sawires).
https://doi.org/10.1016/j.jvolgeores.2019.01.012 0377-0273/© 2019 Elsevier B.V. All rights reserved.
Ceboruco volcano has been investigated by the national power company (the Mexican Federal Commission of Electricity) to evaluate its potentiality for geothermal energy. Some of the obtained results for the drilled boreholes and the core samples have been published by Ferrari et al. (2000, 2003). Since that time, the volcano has been studied by a number of researchers (e.g., Browne and Gardner, 2004; Sieron and Siebe, 2008). They focused on studying the geological features and the eruption history of the volcano. However, limited seismological and geophysical studies (e.g., Sánchez et al., 2009; Rodríguez-Uribe et al., 2013; Fernandez-Cordoba et al., 2017) have been published recently with the aim of studying the seismic activity and the internal structure of the volcano. Sánchez et al. (2009) studied the seismicity at the Ceboruco Volcano during the period 2003 to 2008. They classified the earthquake events that have been located at Ceboruco region into three main categories, volcanotectonic, low-frequency, and hybrid “mixed-frequency” earthquakes. They concluded that the occurrence of volcanic-type
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Fig. 1. Location map of Ceboruco volcano showing its spatial relation to the surrounding tectonic plate boundaries and the Trans-Mexican Volcanic Belt (modified after Núñez-Cornú et al., 2002).
earthquakes in the Ceboruco volcano region is mainly related to the presence of a stress regime which leads to possible faulting and fracturing at a very low propagation rate. They mentioned that stress accommodation could be due to either tectonic origin or variations in the pressure of the subsurface magma. On the other hand, they mentioned that the occurrence of low-frequency events is due to changes in the pressure of the subsurface fluids. However, hybrid-earthquake events represent an indication for the near-surface processes that may take place close to the interior caldera where active fumaroles exist. Rodríguez-Uribe et al. (2013) analyzed, in more details, a number of low-frequency earthquakes that taken place in the region during the same period. Based on the seismogram shapes, spectral content, frequency and time-amplitude content of the studied events, they classified these earthquakes into four different types of source mechanisms (extended coda, short duration, bobbin and modulated amplitude) that are acting in the volcano region. More recently, Fernandez-Cordoba et al. (2017) performed a ground gravity survey using a Scintrex CG-5 AUTOGRAV gravity meter following the available routes and tracks around the volcano area. After the application of the necessary corrections, three main significant negative gravity anomalies at the volcano region has been identified on the Bouguer anomaly map. The first one is located towards the southwestern and western portions of the mapped area. The second anomaly is mainly related to the presence of surrounding valleys, while the third one that represents the principal anomaly (from −130 to −144 mGal) is coinciding directly with the volcano calderas. Moreover, they constructed a simple 3D gravity model for the volcano area using average density values of 2690 and 2340 kg/m3 for the surrounding medium and intruded material, respectively. Their inversion showed that
an isolated body, of about 163 km3 in volume, is appeared at the central part of the volcano region. They mentioned that this anomalous body coincides roughly with the volcano internal structure “magma chamber”. This is why they recommended further geophysical and seismological studies for the volcano area to help understanding and confirming its subsurface structural conditions. In the current work, and throughout the application of aeromagnetic data, an attempt to apply a different geophysical method in order to analyze the possible structural trends prevailing in the study region and define the dimensions of the anomalies bodies has been carried out. Aeromagnetic data are widely used in a variety of geological applications and purposes. They are of great assistance in defining the lithological contacts and in the delineation of geologic structures such as lineaments, dykes or faults. It can be used along with ordinary geologic maps for the evaluation of the Earth's natural resources. Moreover, it helps in studying and defining the structural trends and the crustal heterogeneities that are commonly associated with earthquake activity. In 1995, the Ceboruco volcano was covered by aeromagnetic surveys by the Mexican Geological Service (Servicio Geológico Mexicano, 1995). The primary goal of this survey (Ixtlán F13-D42, Nayarit) was the exploration (e.g., geothermal energy and mineral exploration). The main objectives of the present work are to delineate the possible subsurface structural trends, and image the limits and dimensions of the subsurface magma chamber using these aeromagnetic data. This research is supported by the Mexican CeMIEGeo-P24 project entitled “Passive and Magnetotelluric Seismic Exploration in the Geothermal Fields of the Ceboruco Volcano and La Caldera de la Primavera”. The abovementioned targets are achieved by 2D and 3D interpretation techniques. The 2D technique involves edge detection using the Tilt DeRivative
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(TDR) and the Analytic Signal (AS) methods. Furthermore, the 3D inversion technique of the aeromagnetic data to model the Ceboruco volcano internal structure has been involved. 2. Geologic setting and eruptive history The Ceboruco volcano (elevation of 2280 m above sea level) is located in the State of Nayarit, Mexico, within the Tepic Zacoalco Rift (TZR) in the western part of the Trans-Mexican Volcanic Belt (TMVB), as shown in Fig. 1. The TZR zone marks the tectonic boundary between the Jalisco Block micro-plate (to the southwest) and the North American Plate (to the northeast), and consists of a number of en echelon fault-bounded basins trending mainly in the NNW-SSE direction. A triple junction appears clearly near the city of Guadalajara (Fig. 1). At the junction meet the NW-SE oriented Tepic-Zacoalco graben, the N–S trending Colima rift and the E–W oriented Chapala graben (Fig. 1). From a geological point of view, the Ceboruco volcano is an andesitic to dacitic stratovolcano (Nelson, 1980). It is characterized by successive periods of volcanic activity suffering big tectonic deformations. SuárezPlascencia (1998) has defined four periods of eruptive activity for the Ceboruco Volcano. The earliest and first volcanic activity took place about 8 Ma ago and it was probably related to the extensional tectonics in the region. This activity produced basaltic and andesitic lava flow successions, covering the Jalisco Block rocks (see Ferrari et al., 2003). This period of volcanic activity was followed later by a period of repose. The latter period ended with a Plinian eruption that produced pyroclastic flows, together with the formation of the first caldera (Gardner and Tait, 2000). The second period of extension-related volcanic activity took place during Late Pliocene. During this period, a domal formation in the first caldera, accompanied by basaltic and andesitic lava flows, was formed. Rock outcrops dated at about 3.8 Ma can be observed in the vicinity of Buenavista and Ixtlan (Ferrari et al., 2000). This phase ended by the formation of the second caldera of the Ceboruco volcano. The third period of activity took place during the Early Pliocene in which the Jala ignimbrites and rhyolites were cut by a fault, forming the northeastern limits of the rift (Sieron and Siebe, 2008). During this
period, the formation of the second caldera and a production of andesitic lava flows occurred. As a final period of activity, on 21–23 February 1870, inhabitants of the surrounding areas witnessed extensive volcanic activity in the form of big fumaroles and ash fall. A rhyolitic-dacitic lava, ensued after the fumaroles and ash fall, was flowing down to the southwestern slopes, covering the more recent pyroclastic deposits (Sieron and Siebe, 2008). Currently, some fumaroles (Fig. 3) surrounding the main summit of the Ceboruco volcano with a temperature range below 100 °C can be observed (Sieron and Siebe, 2008). Lithologically, the rock succession (Fig. 2) in the mapped area begins, chronologically, with the Cordilleran Basement (Eocene), then the Sierra Madre Occidental volcanism (Oligocene to Late Miocene), and ends with the TMVB sequence (Pliocene to Quaternary). Ferrari et al. (2000) stated that in some isolated outcrops, near the city of Mexpan, there are some younger Sierra Madre Occidental Ignimbrites with an age of about 20 Ma. Sometimes, they are overlain by the Jala silicic Rhyolites and Ignimbrites or the Ixtlán and Buenavista Basalt-Andesitic sequence that has an age of about 4.7 to 3.8 Ma. This succession was recorded from the cutting and core samples that have been collected during the drilling of the exploratory CB-1 borehole, which is located about 3.0 km southwest of the volcano calderas (see Ferrari et al., 2003). From a structural point of view, the Ceboruco volcano is bounded by a NW–SE trending normal fault from the northeastern direction (Figs. 2, 4). Ferrari et al. (2002) and Sieron and Siebe (2008) observed a clear fault scarp running through the Jala Rhyolites and Ignimbrites sequences (Figs. 2, 4). It is also bounded by another fault, which deduced by the alignment of some lava domes, more or less bounding the northern boundary of the Tepic-Zacoalco rift. However, from the southwestern borders of the volcano, several uplifts of the Sierra El Guamuchil Rhyolites and Ignimbrites (Figs. 2, 4) have been observed without a clear indication for faulting (Sieron and Siebe, 2008). 3. Aeromagnetic data High-quality aeromagnetic data are available for the Ceboruco volcano (an area of about 60 km2) from the Mexican Geological Service
Fig. 2. Detailed geologic map showing rock units, fault trends and topographic elevations in and around the Ceboruco volcano (compiled from Sieron and Siebe, 2008; Ixtlán F13-D42, Nayarit geologic map).
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Fig. 3. Field picture of the fumaroles that surround the main summit of the Ceboruco volcano (taken on June 2018).
Fig. 4. Geologic model of the Ceboruco volcano showing the prevailed tectonic setting (after Venegas, 1995).
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Fig. 5. Color-shaded total magnetic intensity map of the study area.
(Servicio Geológico Mexicano). The aeromagnetic survey was carried out by the Mexican Geological Service with an ISLANDER BN2-A21 aircraft, using a Geometer G-822A magnetometer, Cesium Vapor optical pump, the Picodas P-101 system, video camera of Gem System GSM19 Overhauser, a radar altimeter Sperry RT-220 and a navigation system Ashtech GG24 GPS + GLONAS, 16 m. The flight was made at 300 m of constant height above the ground level, with N–S course exploration parallel lines with 1000 m separation. The whole survey was completed in January 1995. After the application of the necessary corrections, the resultant aeromagnetic data were plotted, contoured, and displayed on a 2D map (Ixtlán F13-D42, Nayarit) with a scale of 1:50,000 and a contour interval of 20 nT. The total magnetic intensity (TMI) map for the Ceboruco volcano (Fig. 5) displays different positive (red and magenta) and negative (green, light and dark blue) anomalies. The map reveals positive magnetic features in elliptical and circular shapes concentrated mainly on the middle and to the southern and the southwestern parts of the study area. However, the negative magnetic features are mostly located towards the northern and northeastern parts of the study area. An important elongated negative magnetic anomaly feature, trending in the NW-SE direction, is clearly observed at the middle part (dark blue color) of the mapped region. This feature is matched well with the inferred normal fault (Fig. 2) associated to the Tepic-Zacoalco rift zone. A steep gradient for the magnetic values along this anomaly is observable. Fig. 5 displays some structural-related trends. These trends do not occur randomly, but usually aligned along definite axes forming some structural features that can be used later to define the different magnetic provinces. According to the dominance of the magnetic anomalies, the NW–SE direction constitutes the major prevailing trend. On the other hand, there are few magnetic anomalies trending in the NE–
SW and E–W directions. These trends reflect the subsurface structure of the study area. 4. Interpretation of aeromagnetic data 4.1. Reduction to the north magnetic pole To interpret the aeromagnetic map/data, the first and most common procedure is to reduce the data to pole using what is called ReductionTo-Pole technique (RTP) in the frequency domain. RTP technique transforms the data as it would have been measured at the north pole, and not affected by the magnetic variation with latitude. Once data are reduced, they is ready for qualitative and quantitative interpretation. Since Ceboruco volcano is located at a low latitude (between 21°N and 21.25°N latitudes) in the northern hemisphere; the aeromagnetic data are subjected to a reduction to the northern magnetic pole. By using an Inclination (I) of 48.56° and a Declination (D) of 6.78° East for the mapped area, the RTP map was generated (Fig. 6). RTP is considered as the best and common way used for removing magnetic distortions. The resulting RTP map shows a direct good correlation between magnetic anomalies and their causative sources (which can be deduced from the geologic map). When looking to the RTP map (Fig. 6), in comparison with the original TMI map (Fig. 5), the northward shift in the positions of the magnetic anomalies can be observed easily. This is due to the elimination of the inclination and the declination effect of the magnetic field at the mapped site. Moreover, the locations and shapes of the anomalous bodies are centered over their respective causative bodies, which also correlates very well with the geologic map (Fig. 2). The RTP map elucidates some major magnetic belts that are scattered and distributed along the mapped
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Fig. 6. Color-shaded RTP map of the study area.
Fig. 7. Power spectrum curve of the aeromagnetic data showing the corresponding average regional and residual components and depths in the study area.
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Fig. 8. a) Color-shaded regional-magnetic component (low-pass filter) and b) Color-shaded residual-magnetic component (high-pass filter), using a cut-off frequency of 0.30 cycle/km to the RTP aeromagnetic data.
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Fig. 9. a) Color-shaded Tilt DeRivative (TDR) map of the study area and the corresponding associated trends, and b) Rose diagram showing the length percentages for the lineaments trends defined from the TDR map.
region. First, the positive magnetic anomalies have an amplitude of more than 350 nT (red and magenta in color), expressed as circular to elongated anomalies, located mainly at the middle, northeastern and southwestern parts of the mapped area, and trending to the NW–SE and N–S directions. Second, the negative magnetic anomalies show an amplitude of less than −200 nT (light and dark blue in color). Two parallel NW-SE trending negative magnetic belts are observed in the RTP map surrounding a high positive magnetic anomaly. The latter anomaly is located at the middle of the mapped area. These two negative belts trending principally on the NW-SE direction, and correlate very well with the inferred faults displayed on the geologic map (Fig. 2). Furthermore, some small semi-
elongated negative anomalies trending mainly N–S and E–W appear scattered all over the mapped area. Finally, the RTP map (Fig. 6) shows a very good correlation between some strong magnetic signatures and the exposed geologic rock units (Fig. 2) of the study area, suggesting that the magnetic anomalies correspond with the contacts or the edges of subsurface geologic structures. 4.2. Separation of the regional and residual anomalies Filtering the potential field data (gravity and magnetic) is an essential step prior to the analysis and the interpretation. The objective of
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Fig. 10. Color-shaded analytical signal (AS) contour map of the study area.
filtering is to condition the data, to deliver the resulting presentation, to make it easy for final interpretation, and to obtain anomalies in terms of their causative bodies or what is called by “geological sources” (Bird, 1997). Various filtering techniques can be used. However, one of the most traditional and common filters is the isolation of deep (long wavelength) and shallow (short wavelength) anomalies. The success of such filter depends mainly on the proper selection of the cut-off wavelength applied during the filter design. The cut-off wavelengths and the contribution of short- and long-wavelengths can be obtained from the radially-averaged power spectrum technique (Maus and Dimri, 1996). The radially-averaged power spectrum (Fig. 7) can be used to estimate the average depths of the causative sources (e.g., basement complex, volcanic intrusions, or subsurface geological structures). Several authors (e.g., Bhattacharyya, 1966; Spector and Grant, 1970) mentioned that the slope of the accompanied two linear segments can be used to estimate the average depths to the top of the deep-seated, and near-surface magnetic sources. These depths (Fig. 7) are average values and do not reflect the topographic features of the hidden basement surface. The 2D power spectrum curve (Fig. 7) shows two linear segments (based on the change of the slope of the obtained spectrum curve) related to the long- and short-wavelength components. They have frequency bands ranging from 0.05 to 0.30 cycle/km, and from 0.30 to 1.10 cycle/km, respectively. The first component (long wavelength and low frequency) is related to a deep source and defined as the
regional component, while the second one (short wavelength and high frequency) has a shallower origin, and is defined as the nearsurface component. Meanwhile, the high-frequency signal, displayed in the spectrum, was considered as noise. In the current study, the frequency band (cutoff wavelength of 0.30 cycle/km) was used through the band-pass filter technique using Oasis Montaj Geosoft 9.3.1 (Oasis Montaj Program v.9.3.1, 2018) software to isolate the regional and residual magnetic components. The band-pass filter is very critical to the cut-off frequencies that pass or reject specific frequency bands. The low-pass (regional component) and high-pass (residual component) maps have been generated and plotted in Fig. 8a,b. The regional-magnetic component (low-pass) map (Fig. 8a) displays the deep-seated high-amplitude anomalies, while the residualmagnetic component (high-pass) map (Fig. 8b) shows the shallow low-amplitude anomalies. Most of the magnetic anomalies appearing on the RTP map (Fig. 6) continue to appear in the residual-magnetic component map (Fig. 8b). The latter shows clearly the different clusters of positive and negative magnetic anomalies distributed all over the mapped area, which are of higher resolution than those displayed on the RTP map (Fig. 6). These anomalies have semi-circular, circular to elongated shapes characterized by their relatively short wavelengths and high frequencies. The major structural trends that can be deduced clearly from the residual aeromagnetic anomaly map (Fig. 8b) are in the NW–SE, NE–SW and E–W directions, related mainly to the nearsurface structures.
Fig. 11. Color-shaded relief maps for the subsurface from sea level for the inversion results; a) for the first layer (L250) at 250 m, b) for the second layer (L500) at 500 m, c) for the third layer (L750) at 750 m, and d) for the fourth layer (L1000) at 1000 m.
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4.3. 2D edge detection interpretation techniques In the current study, we focused more on the 2D and 3D structural interpretation of the Ceboruco volcano. In 2D interpretation, edge detection filters, such as Tilt DeRivative (TDR) and Analytic Signal (AS) were applied. These filters are commonly used to trace faults/contacts within the study area. To achieve a 3D interpretation, a 3D inversion method was applied to image the causative body in the subsurface of the mapped area. All the obtained results have been compared and confirmed with the geologic and tectonic features in the study area. This is to avoid the ambiguity in the final conclusions. Edge detection of the causative magnetic sources is one of the most crucial steps towards the modeling of the magnetic anomalies (Bournas and Baker, 2001; Ardestani and Motavalli, 2007). Several methods have been used by many authors (e.g., Pilkington and Keating, 2004; Cooper and Cowan, 2008; Cooper, 2009; Arisoy and Dikmen, 2013). The TitleDeRivative (TDR) and the Analytical Signal (AS) methods are considered the most common techniques among many others. In this work, the TDR and AS techniques were used for the edge detection of the prevailing subsurface structures in the study area. It was very interesting to compare between the obtained results, and show the degree of similarity when applying each individual technique. More details about the applied methodologies and their corresponding results will be shown in the following context. 4.3.1. Tilt-DeRivative (TDR) method TDR has been selected to be applied on potential field data by Miller and Singh (1994) and Verduzco et al. (2004). More recently, it was developed and used by several authors (e.g., Salem et al., 2008; Fairhead et al., 2008). Many researchers have applied this filter to their data because of its practical and fundamental simplicity (Hinze et al., 2013). This filter together with its total horizontal derivative (THDR) are usually useful in the mapping and detection of the geological contacts and edges which may be related to the mineral exploration targets or to shallow basement structures. TDR is computed by the division of the Vertical DeRivative component (VDR) by the Total Horizontal DeRivative component (THDR) of the magnetic field (Verduzco et al., 2004): TDR ¼ tan−1
VDR THDR
0
1
∂f =∂z B C ¼ tan−1 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA 2 2 ð∂f =∂xÞ þ ð∂f =∂yÞ
ð1Þ
where f is the magnetic field, “∂f/∂x, ∂f/∂y, ∂f/∂z” are the first derivatives of the magnetic field in the directions x, y and z, respectively. The advantage of the TDR technique is that its zero-contour line is located directly or very close to the contact or the edge of the causative source (e.g., lineation, fault, contact). So, it can be applied to the aeromagnetic data to delineate the possible edges in the mapped area (Miller and Singh, 1994). Moreover, positive values of the TDR are located above the magnetic source, while the negative ones are located far from it. In the current study, the TDR filter was applied to the aeromagnetic data, and the obtained results were plotted directly as a gridded image in Fig. 9a. A large number of linear magnetic anomalies can be traced easily from the TDR map, trending mainly in the NW-SE, ENE-WSW, and E-W directions. Those anomalies represent the prevailed subsurface contacts or fault trends in the study area. A rose diagram from the obtained lineaments map has been prepared and plotted in Fig. 9b. It confirms the previously-mentioned structural trends defined from Fig. 9a. 4.3.2. Analytical signal (AS) method The analytic signal (AS) or the total gradient method is another powerful method that is normally used in the evaluation and detection of the buried subsurface structures which usually appear as linear magnetic anomalies (e.g., faults, fractures or dykes) (Saada, 2016). The
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analytical signal approach has been used successfully to locate dykes from the profile data by Nabighian (1972, 1974, 1984) and Atchuta et al. (1981). Later, it was developed by Roest et al. (1992) to be used in the interpretation of aeromagnetic maps. Further improvements on the technique to interpret aeromagnetic data were done by Hsu et al. (1996, 1998) and Thurston and Smith (1997). Saada (2016) mentioned that the AS method is helpful in the determination of the different parameters of the magnetic anomalies due to the independence on the magnetization direction (inclination). The analytical signal is the square root of the summation of the squares of the different derivatives in the x, y and z directions (Roest et al., 1992): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ∂f ∂f ∂f AS ¼ þ þ ∂x ∂y ∂z where
∂f ; ∂x
∂f ∂f ; ∂y ∂z
ð2Þ
are the first-order derivatives of the total intensity
magnetic field. Both horizontal and vertical derivatives of the anomaly are Hilbert transform pairs of each other (Nabighian, 1972). In the present analysis, the analytical signal was computed and plotted (Fig. 10) using the Oasis Montaj Geosoft 9.3.1 (Oasis Montaj Program v.9.3.1, 2018) software. It clearly appears that an elongated NW-SE high anomaly is located at the middle part of the mapped region, which is surrounded by low to moderate N–S and E–W trending anomalies. Hence, it can be realized that the analytical signal method is very successful in defining the outer limits of the magma chamber beneath the Cebouruco Volcano. 4.4. Modeling of the data with 3D inversion techniques Qualitative magnetic or gravity data interpretation provides a general information about the subsurface. When more detailed information about the subsurface is needed, quantitative models of the subsurface are required. This is the concept of the geophysical inversion. The 3D inversion technique gives a 3D image about the physical properties of the subsurface as density (from gravity data) or susceptibility (from magnetic data). However, constraints are required in order to control the inversion process (e.g. boreholes or lithological information). In the current study, the aeromagnetic data of Ceboruco volcano was inverted using the GMSys-3D inversion program from Geosoft (Popowski et al., 2006) where a model is defined by a number of stacked surface grids with susceptibility distributions specified for the layer below each surface. GMSys-3D calculations were performed in the wave number domain using the Parker algorithm (Parker, 1973). The approach assumes that the magnetic response of the subsurface layer is caused by a series of vertical square-ended prisms of infinite depth extent, where its horizontal dimensions equal to the input grid cell size. Before starting the inversion, we assumed that there are four subsurface layers (L250, L500, L750, and L1000) at various depths (250, 500, 750 and 1000 m, respectively). Instead of assuming magnetic susceptibility for each layer, we calculated the apparent magnetic susceptibility for each layer using the susceptibility filter. A susceptibility filter is a compound filter that performs a reduction to the pole, downward continuation to the source depth, correction for the geometric effect and division by the total magnetic field to yield susceptibility. Apparent susceptibility mapping assumes a simple geometric model. The required information to get the apparent susceptibility are the total magnetic field (F), declination (D), inclination (I), and depth to find susceptibility. The obtained apparent magnetic susceptibility was used within the inversion process as a physical property of the layer. When working on 3D inversion, the forward modeling should be estimated initially. This forward modeling generates three maps, observed, calculated, and error (misfit) maps. From those maps, we can invert the assumed layers (L250, L500, L750, and L1000) in terms of the apparent magnetic susceptibility. We inverted each layer
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a) 250 m
b) 500 m
c) 750 m Fig. 12. A three-dimensional model for the study area at the different studied layers/levels (shallow, medium, and deep).
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individually where the GMSys-3D program is based on 1-layer case inversion. The inversion results for the first layer (L250) are shown in Fig. 11a. The results show clearly the depth values for the causative body below the volcano are ranging from −499 to 1605 m. In the second layer (L500) (Fig. 11b), we can still see the boundary of the causative body and there is a zone, probably a volcanic intrusion, outlined by a yellow contour (depths from −750 to 1870 m). For the third layer (L750; Fig. 11c), we can still trace the base of the volcano as well as the yellow structural contour. This indicates that the base of the volcano and the yellow margin stem come from the same source. They could be connected at depth (depths from −1500 to 1211 m). Finally, for the last layer (L1000 m; Fig. 11d), the base boundary of the volcano disappeared, as well as the yellow structural line. It indicates that we reached the basement above which the volcano was formed. Fig. 12 shows a three-dimensional display for the different considered layers, and the depth values in each case. 5. Discussion and conclusions A number of infinite interpretations are usually available for most of the geophysical problems. Among them, only one solution is the correct answer. To reach this geophysical model, all known geologic and tectonic information should be taken into consideration. In the current study, the available geological, structural and tectonic knowledge was incorporated into the aeromagnetic analysis, interpretation and modeling for the Ceboruco Volcano area. On June 2018, a specific field trip to the volcano region has been arranged to confirm the geologic and structural setting of the volcano. In addition, a step-by-step comparison for the obtained maps/results was done in relation with the geologic map (specially for the RTP map) and the available previous geological and geophysical studies (e.g., Sieron and Siebe, 2008; Fernandez-Cordoba et al., 2017). Moreover, the structural interpretation for the possible related fault trends from the aeromagnetic data has been compared with the general tectonic and structural setting (e.g., Tepic-Zacoalco rift) of the volcano region. Thus, in turn, confirm all the obtained results. By applying such information, in the current work, a detailed 2D and 3D modeling of the aeromagnetic data for the volcano area, that may represent the actual conditions of the study region, were achieved. In this work, high-resolution aeromagnetic data for the Ceboruco Volcano was analyzed to delineate the possible subsurface geologic structures, and to provide an illustrative 3D model for the volcano. To reach such goals, several steps were followed: reduction of the total intensity aeromagnetic values to the north magnetic pole, isolation of the regional and residual magnetic components, application of the analytical signal and tilt derivative methods, and finally the 3D inverse modeling. Results indicated that there is a large number of linear magnetic anomalies, observable specially from the TDR and AS maps, that may correspond to contacts, faults or dykes. The major structural trends extracted from these maps are the NW-SE, ENE-WSW and E-W trends. The first and the most dominant trend is correlating very well with the wellknown NW-SE oriented Tepic Zacoalco rift, which bounds the volcano from the northwestern side. On the other hand, the inversion of the aeromagnetic data shows clearly the limits of the causative body at the shallower subsurface layers. At deeper levels, a structure marked by a yellow contour appears towards the eastern side of the studied area which means that both of the base of the volcano and the structure arise from the same magmatic source. Finally, at the deepest inverted subsurface layer, the limits of the magma chamber and the structural line disappeared, which means that the basement above which the volcano is formed has been reached. Our results and models, that have been obtained from the analysis of aeromagnetic data, are complementing and confirming the results, obtained from the interpretation of gravity data, by Fernandez-Cordoba et al. (2017). The principal negative gravity anomaly defined by them on their Bouguer anomaly map, that is located at the middle part of
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the study region, is coinciding perfectly with the positive magnetic anomaly defined in our study. This anomaly represents the location of the subsurface magma chamber of the volcano. However, there is some differences between the other defined small anomalies delineated from gravity and aeromagnetic data. Another significant topic is that the application of the aeromagnetic data in the definition of structural trends and possible faults/contacts is more successful in our study than in the Fernandez-Cordoba et al. (2017) work that depends on gravity. This definition of possible prevailing structure was confirmed by the field check and by comparison with the geologic maps. In conclusion, these models, from one side, serve as groundwork in the search for geothermal resources, which in turn help decisionmakers concerning the production of electricity using the possible geothermal energy. This is the principal target of the CEMIE-Geo Project. From the other side, volcanoes are directly threaten the surrounding population centers and could have a direct influence on the socioeconomic development of such regions. Thus, volcanic risk assessment is an important scientific, economic, and political issues, especially in high-density populated areas such as Mexican cities. To assess the volcanic hazard, a scientist needs to identify the behavior of the volcanic system in the past to make it possible use such information to forecast the possible activity in the future. To do this, a compilation of all existing geological, geophysical, structural and volcanic eruption data should be taken into account. That is why our geophysical model could play an important role in any future assessment for the volcanic hazard assessment and mitigation for the study region. The obtained results show that the aeromagnetic data are very successful in the imaging of the possible subsurface geologic structures and estimating the approximate depths of the basement of the Ceboruco Volcano area. This work represents the first step of investigation for the CeMIEGeo-P24 geothermal exploration project, which will be succeeded later by detailed ground magnetic and magneto-telluric surveys for the anomalies of interest. Seismological studies are highly recommended for the volcano site, especially after this study. Such studies should shed more light on the relationship between the defined structural trends and the location of earthquake hypocenters. Acknowledgments This research work was supported by the Mexican CeMIEGeo-P24 project entitled “Passive and Magnetotelluric Seismic Exploration in the Geothermal Fields of the Ceboruco Volcano and La Caldera de la Primavera” throughout a postdoctoral fellowship stay for the first author at the Centro de Sismología y Volcanología de Occidente (SisVOc), Universidad de Guadalajara, Campus of Puerto Vallarta, Jalisco, Mexico. In addition, we would like to thank Editor Jurgen Neuberg and the anonymous reviewers for their thoughtful comments. Thanks also to Dr. J.A. Peláez (Universidad de Jaén, Spain) who reviewed the English language of the manuscript. References Ardestani, V.E., Motavalli, H., 2007. Constraints of analytic signal to determine the depth of gravity anomalies. J. Earth and Space Phys. 33, 77–83. Arisoy, M., Dikmen, U., 2013. Edge detection of magnetic sources using enhanced total horizontal derivative of the Tilt angle. Yerbilimleri 34, 73–82. Atchuta, R.D., Ram Babu, H.V., Sanker, P.V., 1981. Narayan Interpretation of magnetic anomalies due to dikes: the complex gradient method. Geophysics 46, 1572–1578. Bhattacharyya, B.K., 1966. Continuous spectrum of the total magnetic field anomaly due to a rectangular prismatic body. Geophysics 31, 97–121. Bird, D., 1997. Interpreting Magnetic Data: Geophysical Corner, EXPLORER, AAPG and SEG, May, 1997. Bournas, N., Baker, H.A., 2001. Interpretation of magnetic anomalies using the horizontal gradient analytic signal. Ann. Geophys. 44, 506–526. Browne, B.L., Gardner, J.E., 2004. The nature and timing of caldera collapse as indicated by accidental lithic fragments from the AD 1000 eruption of Volcan Ceboruco, Mexico. J. Volcanol. Geotherm. Res. 130, 93–105. Cooper, G.R.J., 2009. Balancing images of potential field data. Geophysics 74, 17–20. Cooper, G.R.J., Cowan, D.R., 2008. Edge enhancement of potential field data using normalized statistics. Geophysics 73, 1–4.
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Fairhead, J.D., Salem, A., Williams, S., Samson, E., 2008. Magnetic interpretation made easy: the tilt-depth-dip-Dk method. 2008 Annual International Meeting Expanded Abstracts. Soc. Explor. Geophys., pp. 779–783 Fernandez-Cordoba, J., Zamora-Camacho, A., Espindola, J.M., 2017. Gravity survey at the Ceboruco Volcano área (Nayarit, Mexico): a 3-D model of the subsurface structure. Pure Appl. Geophys. 174, 3905–3918. Ferrari, L., Conticelli, S., Vaggeli, G., Petrone, C.M., Manetti, P., 2000. Late Miocene volcanism and intra-arc tectonics during the early development of the Trans-Mexican Volcanic Belt. Tectonophysics 318, 161–185. Ferrari, L., López-Martínez, M., Rosas-Elguera, J., 2002. Ignimbrite flare-up and deformation in the southern Sierra Madre Occidental, western Mexico: implications for the late subduction history of the Farallón plate. Tectonics 21, 17–1/24. Ferrari, L., Petrone, C.M., Francalanci, L., Tagami, T., Eguchi, M., Conticelli, S., Manetti, P., Venegas-Salgado, S., 2003. Geology of the San Pedro-Ceboruco Graben, western Trans-Mexican Volcanic Belt. Rev. Mex. Cienc. Geol. 20, 165–181. Gardner, J.E., Tait, S., 2000. The caldera-forming eruption of Volcán Ceboruco, Mexico. Bull. Volcanol. 62, 20–33. Hinze, W.J., Von Frese, R.R.B., Saad, A.H., 2013. Gravity and Magnetic Exploration Principles, Practices, and Applications. Cambridge University Press, New York. Hsu, S.K., Coppens, D., Shyu, C.T., 1998. Depth to magnetic source using the generalized analytic signal. Geophysics 63, 1947–1957. Hsu, S.K., Sibunet, J.C., Shyu, C.T., 1996. High-resolution detection of geologic boundaries from potential field anomalies. An enhanced analytic signal technique. Geophysics 61, 373–386. Maus, S., Dimri, V., 1996. Depth estimation from the scaling power spectrum of potential fields. Geophys. J. Int. 124, 113–120. Miller, H.G., Singh, V., 1994. Potential field tilt—a new concept for location of potential field sources. J. Appl. Geophys. 32, 213–217. Nabighian, M.N., 1972. The analytical signal of two-dimensional magnetic bodies with polygonal cross-section, its properties and use for automated interpretation. Geophysics 37, 507–517. Nabighian, M.N., 1974. Additional comment on the analytical signal of two-dimensional magnetic bodies with polygonal cross-section. Geophysics 39, 85–92. Nabighian, M.N., 1984. Towards a three dimensional automatic interpretation of potential field data via generalized Hilbert transform, fundamental relations. Geophysics 47, 780–786. Nelson, S.A., 1980. Geology and petrology of Volcán Ceboruco, Nayarit, Mexico. Bull. Geol. Soc. Am. 91, 2290–2431. Núñez-Cornú, F.J., Marta, R.L., Nava, F.A., Reyes-Dávila, G., Suárez-Plascencia, C., 2002. Characteristics of the seismicity in the coast and north of Jalisco block, Mexico. Phys. Earth Planet. Inter. 132, 141–155.
Oasis Montaj Program v.9.3.1, 2018. Geosoft Mapping and Processing System, Version 9.3.1, 2018. Parker, R.L., 1973. The rapid calculation of potential anomalies. Geophys. J. R. Astron. Soc. 31, 447–455. Pilkington, M., Keating, P., 2004. Contact mapping from gridded magnetic data: a comparison of techniques. Explor. Geophys. 35, 206–311. Popowski, T., Connard, G., French, R., 2006. GMSYS-3D: 3D Gravity and Magnetic Modeling for Oasis Montaj – User Guide. Northwest Geophysical Associates, Corvallis, Oregon, p. 32. Rodríguez-Uribe, M.C., Núñez-Cornú, F.J., Nava Pichardo, F.A., Suárez-Plascencia, C., 2013. Some insights about the activity of the Ceboruco Volcano (Nayarit, Mexico) from recent seismic low-frequency activity. Bull. Volcanol. 75, 755. Roest, W.R., Verhoef, J., Pilkington, M., 1992. Magnetic interpretation using the 3-D analytic signal. Geophysics 57, 116–125. Saada, S.A., 2016. Edge detection and depth estimation of Galala El Bahariya Plateau, Eastern Desert-Egypt, from aeromagnetic data. Geomech. Geophys. Geo-energ. Georesour. 2, 25–41. Salem, A., Williams, S., Fairhead, D., Smith, R., Ravat, D., 2008. Interpretation of magnetic data using tilt-angle derivatives. Geophysics 73, L1–L10. Sánchez, J.J., Núñez-Cornú, F.J., Suárez-Plascencia, C., Trejo-Gómez, E., 2009. Seismicity at Ceboruco Volcano, Mexico. Seismol. Res. Lett. 823–830. Servicio Geológico Mexicano, 1995. Carta Magnética de Campo Total (escala 1:50000) Ixtlán F13-D42, Nayarit. www.sgm.gob.mx. Sieron, K., Siebe, C., 2008. Revised stratigraphy and eruption rates of Ceboruco stratovolcano and surrounding monogenetic vents (Nayarit, Mexico) from historical documents and new radiocarbon dates. J. Volcanol. Geotherm. Res. 176, 241–264. Spector, A., Grant, F.S., 1970. Statistical models for interpreting aeromagnetic data. Geophysics 35, 293–302. Suárez-Plascencia, C., 1998. Análisis De las Variables del Riesgo Volcánico en el Volcán Ceboruco, Nayarit, México. Master thesis. Centro de Investigacíon Cientifca y de Educacíon Superior de Ensenada, p. 149. Thurston, J.B., Smith, R.S., 1997. Automatic conversion of magnetic data to depth, dip, and susceptibility contrast using the SPI (TM) method. Geophysics 62, 807–813. Venegas, S., 1995. Resultados de la exploración geotérmica en el Volcán Ceboruco en el Estado de Nayarit, Mexico. Geotermia 11, 71–86. Verduzco, B., Fairhead, J.D., Green, C.M., MacKenzie, C., 2004. New insights into magnetic derivatives for structural mapping. Lead. Edge 23, 116–119.
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Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
Subsurface structural imaging of Ceboruco Volcano area, Nayarit, Mexico using high-resolution aeromagnetic data Rashad Sawires a,b,⁎, Essam Aboud c,d a
Centro de Sismología y Volcanología de Occidente (SisVOc), Universidad de Guadalajara, Av. Universidad 293, Del. Ixtapa, 48280 Puerto Vallarta, Jalisco, Mexico Department of Geology, Faculty of Science, Assiut University, 71516 Assiut, Egypt National Research Institute of Astronomy and Geophysics (NRIAG), 11421 Helwan, Cairo, Egypt d King Abdulaziz University, Geohazard Research Centre, 80206, Jeddah 21589, Saudi Arabia b c
a r t i c l e
i n f o
Article history: Received 1 August 2018 Received in revised form 14 December 2018 Accepted 4 January 2019 Available online 10 January 2019 Keywords: 3D inversion Aeromagnetic data Geothermal prospect area Ceboruco Volcano Mexico
a b s t r a c t Ceboruco volcano is one of the largest volcanoes of the Trans-Mexican Volcanic Belt (TMVB), which extends along central Mexico. Among the western TMVB, Ceboruco Volcano comes in the second place after Colima Volcano in its activity. Also, it is considered as the only one that has historically-documented eruptions. Few geophysical studies have been published with the aim of studying the internal structure of the volcano. In the current work, and throughout the support of the CeMIEGeo-P24 geothermal exploration project, we aim to delineate the possible subsurface structural trends, to determine the approximate depth to the basement surface, and to provide an illustrative 3D model for its subsurface structure. To achieve such goals, a detailed analysis of the aeromagnetic data for the volcano area was performed. 2D interpretation was carried out for the aeromagnetic data. Edge detection filters, such as Tilt Derivative, and Analytic Signal, were used to map the contact/faults within the study area. In addition, 3D inversion was used to image the subsurface structure of the volcano. Aeromagnetic data was inverted using the GMSYS-3D inversion program by applying Parker algorithm. Four subsurface layers at depths of 250, 500, 750 and 1000 m were assumed during the inversion process. Results indicate that the limits “boundary” of the subsurface magma chamber of the volcano can be traced easily from the first subsurface layer. A secondary structure line appears towards the east from the volcano boundary and it can be observed from the inversion of the second and third layers. This indicates that both, the base of the volcano and the structure line, stem from the same source. Finally, they disappear in the inversion results for the last layer, which concludes that we reach the basement above which the volcano is formed. Combination of all obtained results with the geological information, helped to understand the main structure of the Ceboruco volcano. © 2019 Elsevier B.V. All rights reserved.
1. Introduction Ceboruco volcano (Fig. 1) is one of the largest volcanoes of the TransMexican Volcanic Belt (TMVB), which extends along the centralsouthern parts of the Mexican Republic. It is considered the most active volcano in the western TMVB after Colima Volcano and also the only one that has historically-documented eruptions (Sieron and Siebe, 2008). More than 40% of the Mexican population (e.g., in Mexico City, Guadalajara, and Puebla) lives around this TMVB. Despite the benefits of the volcanic activities (e.g., fertile agricultural soil, mineral resources such as gold, nickel, copper, silver and lead, and cement industry), the risk of volcanic eruptions is an important issue to be taken into consideration.
⁎ Corresponding author at: Department of Geology, Faculty of Science, Assiut University, 71516 Assiut, Egypt. E-mail address: [email protected] (R. Sawires).
https://doi.org/10.1016/j.jvolgeores.2019.01.012 0377-0273/© 2019 Elsevier B.V. All rights reserved.
Ceboruco volcano has been investigated by the national power company (the Mexican Federal Commission of Electricity) to evaluate its potentiality for geothermal energy. Some of the obtained results for the drilled boreholes and the core samples have been published by Ferrari et al. (2000, 2003). Since that time, the volcano has been studied by a number of researchers (e.g., Browne and Gardner, 2004; Sieron and Siebe, 2008). They focused on studying the geological features and the eruption history of the volcano. However, limited seismological and geophysical studies (e.g., Sánchez et al., 2009; Rodríguez-Uribe et al., 2013; Fernandez-Cordoba et al., 2017) have been published recently with the aim of studying the seismic activity and the internal structure of the volcano. Sánchez et al. (2009) studied the seismicity at the Ceboruco Volcano during the period 2003 to 2008. They classified the earthquake events that have been located at Ceboruco region into three main categories, volcanotectonic, low-frequency, and hybrid “mixed-frequency” earthquakes. They concluded that the occurrence of volcanic-type
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Fig. 1. Location map of Ceboruco volcano showing its spatial relation to the surrounding tectonic plate boundaries and the Trans-Mexican Volcanic Belt (modified after Núñez-Cornú et al., 2002).
earthquakes in the Ceboruco volcano region is mainly related to the presence of a stress regime which leads to possible faulting and fracturing at a very low propagation rate. They mentioned that stress accommodation could be due to either tectonic origin or variations in the pressure of the subsurface magma. On the other hand, they mentioned that the occurrence of low-frequency events is due to changes in the pressure of the subsurface fluids. However, hybrid-earthquake events represent an indication for the near-surface processes that may take place close to the interior caldera where active fumaroles exist. Rodríguez-Uribe et al. (2013) analyzed, in more details, a number of low-frequency earthquakes that taken place in the region during the same period. Based on the seismogram shapes, spectral content, frequency and time-amplitude content of the studied events, they classified these earthquakes into four different types of source mechanisms (extended coda, short duration, bobbin and modulated amplitude) that are acting in the volcano region. More recently, Fernandez-Cordoba et al. (2017) performed a ground gravity survey using a Scintrex CG-5 AUTOGRAV gravity meter following the available routes and tracks around the volcano area. After the application of the necessary corrections, three main significant negative gravity anomalies at the volcano region has been identified on the Bouguer anomaly map. The first one is located towards the southwestern and western portions of the mapped area. The second anomaly is mainly related to the presence of surrounding valleys, while the third one that represents the principal anomaly (from −130 to −144 mGal) is coinciding directly with the volcano calderas. Moreover, they constructed a simple 3D gravity model for the volcano area using average density values of 2690 and 2340 kg/m3 for the surrounding medium and intruded material, respectively. Their inversion showed that
an isolated body, of about 163 km3 in volume, is appeared at the central part of the volcano region. They mentioned that this anomalous body coincides roughly with the volcano internal structure “magma chamber”. This is why they recommended further geophysical and seismological studies for the volcano area to help understanding and confirming its subsurface structural conditions. In the current work, and throughout the application of aeromagnetic data, an attempt to apply a different geophysical method in order to analyze the possible structural trends prevailing in the study region and define the dimensions of the anomalies bodies has been carried out. Aeromagnetic data are widely used in a variety of geological applications and purposes. They are of great assistance in defining the lithological contacts and in the delineation of geologic structures such as lineaments, dykes or faults. It can be used along with ordinary geologic maps for the evaluation of the Earth's natural resources. Moreover, it helps in studying and defining the structural trends and the crustal heterogeneities that are commonly associated with earthquake activity. In 1995, the Ceboruco volcano was covered by aeromagnetic surveys by the Mexican Geological Service (Servicio Geológico Mexicano, 1995). The primary goal of this survey (Ixtlán F13-D42, Nayarit) was the exploration (e.g., geothermal energy and mineral exploration). The main objectives of the present work are to delineate the possible subsurface structural trends, and image the limits and dimensions of the subsurface magma chamber using these aeromagnetic data. This research is supported by the Mexican CeMIEGeo-P24 project entitled “Passive and Magnetotelluric Seismic Exploration in the Geothermal Fields of the Ceboruco Volcano and La Caldera de la Primavera”. The abovementioned targets are achieved by 2D and 3D interpretation techniques. The 2D technique involves edge detection using the Tilt DeRivative
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(TDR) and the Analytic Signal (AS) methods. Furthermore, the 3D inversion technique of the aeromagnetic data to model the Ceboruco volcano internal structure has been involved. 2. Geologic setting and eruptive history The Ceboruco volcano (elevation of 2280 m above sea level) is located in the State of Nayarit, Mexico, within the Tepic Zacoalco Rift (TZR) in the western part of the Trans-Mexican Volcanic Belt (TMVB), as shown in Fig. 1. The TZR zone marks the tectonic boundary between the Jalisco Block micro-plate (to the southwest) and the North American Plate (to the northeast), and consists of a number of en echelon fault-bounded basins trending mainly in the NNW-SSE direction. A triple junction appears clearly near the city of Guadalajara (Fig. 1). At the junction meet the NW-SE oriented Tepic-Zacoalco graben, the N–S trending Colima rift and the E–W oriented Chapala graben (Fig. 1). From a geological point of view, the Ceboruco volcano is an andesitic to dacitic stratovolcano (Nelson, 1980). It is characterized by successive periods of volcanic activity suffering big tectonic deformations. SuárezPlascencia (1998) has defined four periods of eruptive activity for the Ceboruco Volcano. The earliest and first volcanic activity took place about 8 Ma ago and it was probably related to the extensional tectonics in the region. This activity produced basaltic and andesitic lava flow successions, covering the Jalisco Block rocks (see Ferrari et al., 2003). This period of volcanic activity was followed later by a period of repose. The latter period ended with a Plinian eruption that produced pyroclastic flows, together with the formation of the first caldera (Gardner and Tait, 2000). The second period of extension-related volcanic activity took place during Late Pliocene. During this period, a domal formation in the first caldera, accompanied by basaltic and andesitic lava flows, was formed. Rock outcrops dated at about 3.8 Ma can be observed in the vicinity of Buenavista and Ixtlan (Ferrari et al., 2000). This phase ended by the formation of the second caldera of the Ceboruco volcano. The third period of activity took place during the Early Pliocene in which the Jala ignimbrites and rhyolites were cut by a fault, forming the northeastern limits of the rift (Sieron and Siebe, 2008). During this
period, the formation of the second caldera and a production of andesitic lava flows occurred. As a final period of activity, on 21–23 February 1870, inhabitants of the surrounding areas witnessed extensive volcanic activity in the form of big fumaroles and ash fall. A rhyolitic-dacitic lava, ensued after the fumaroles and ash fall, was flowing down to the southwestern slopes, covering the more recent pyroclastic deposits (Sieron and Siebe, 2008). Currently, some fumaroles (Fig. 3) surrounding the main summit of the Ceboruco volcano with a temperature range below 100 °C can be observed (Sieron and Siebe, 2008). Lithologically, the rock succession (Fig. 2) in the mapped area begins, chronologically, with the Cordilleran Basement (Eocene), then the Sierra Madre Occidental volcanism (Oligocene to Late Miocene), and ends with the TMVB sequence (Pliocene to Quaternary). Ferrari et al. (2000) stated that in some isolated outcrops, near the city of Mexpan, there are some younger Sierra Madre Occidental Ignimbrites with an age of about 20 Ma. Sometimes, they are overlain by the Jala silicic Rhyolites and Ignimbrites or the Ixtlán and Buenavista Basalt-Andesitic sequence that has an age of about 4.7 to 3.8 Ma. This succession was recorded from the cutting and core samples that have been collected during the drilling of the exploratory CB-1 borehole, which is located about 3.0 km southwest of the volcano calderas (see Ferrari et al., 2003). From a structural point of view, the Ceboruco volcano is bounded by a NW–SE trending normal fault from the northeastern direction (Figs. 2, 4). Ferrari et al. (2002) and Sieron and Siebe (2008) observed a clear fault scarp running through the Jala Rhyolites and Ignimbrites sequences (Figs. 2, 4). It is also bounded by another fault, which deduced by the alignment of some lava domes, more or less bounding the northern boundary of the Tepic-Zacoalco rift. However, from the southwestern borders of the volcano, several uplifts of the Sierra El Guamuchil Rhyolites and Ignimbrites (Figs. 2, 4) have been observed without a clear indication for faulting (Sieron and Siebe, 2008). 3. Aeromagnetic data High-quality aeromagnetic data are available for the Ceboruco volcano (an area of about 60 km2) from the Mexican Geological Service
Fig. 2. Detailed geologic map showing rock units, fault trends and topographic elevations in and around the Ceboruco volcano (compiled from Sieron and Siebe, 2008; Ixtlán F13-D42, Nayarit geologic map).
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Fig. 3. Field picture of the fumaroles that surround the main summit of the Ceboruco volcano (taken on June 2018).
Fig. 4. Geologic model of the Ceboruco volcano showing the prevailed tectonic setting (after Venegas, 1995).
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Fig. 5. Color-shaded total magnetic intensity map of the study area.
(Servicio Geológico Mexicano). The aeromagnetic survey was carried out by the Mexican Geological Service with an ISLANDER BN2-A21 aircraft, using a Geometer G-822A magnetometer, Cesium Vapor optical pump, the Picodas P-101 system, video camera of Gem System GSM19 Overhauser, a radar altimeter Sperry RT-220 and a navigation system Ashtech GG24 GPS + GLONAS, 16 m. The flight was made at 300 m of constant height above the ground level, with N–S course exploration parallel lines with 1000 m separation. The whole survey was completed in January 1995. After the application of the necessary corrections, the resultant aeromagnetic data were plotted, contoured, and displayed on a 2D map (Ixtlán F13-D42, Nayarit) with a scale of 1:50,000 and a contour interval of 20 nT. The total magnetic intensity (TMI) map for the Ceboruco volcano (Fig. 5) displays different positive (red and magenta) and negative (green, light and dark blue) anomalies. The map reveals positive magnetic features in elliptical and circular shapes concentrated mainly on the middle and to the southern and the southwestern parts of the study area. However, the negative magnetic features are mostly located towards the northern and northeastern parts of the study area. An important elongated negative magnetic anomaly feature, trending in the NW-SE direction, is clearly observed at the middle part (dark blue color) of the mapped region. This feature is matched well with the inferred normal fault (Fig. 2) associated to the Tepic-Zacoalco rift zone. A steep gradient for the magnetic values along this anomaly is observable. Fig. 5 displays some structural-related trends. These trends do not occur randomly, but usually aligned along definite axes forming some structural features that can be used later to define the different magnetic provinces. According to the dominance of the magnetic anomalies, the NW–SE direction constitutes the major prevailing trend. On the other hand, there are few magnetic anomalies trending in the NE–
SW and E–W directions. These trends reflect the subsurface structure of the study area. 4. Interpretation of aeromagnetic data 4.1. Reduction to the north magnetic pole To interpret the aeromagnetic map/data, the first and most common procedure is to reduce the data to pole using what is called ReductionTo-Pole technique (RTP) in the frequency domain. RTP technique transforms the data as it would have been measured at the north pole, and not affected by the magnetic variation with latitude. Once data are reduced, they is ready for qualitative and quantitative interpretation. Since Ceboruco volcano is located at a low latitude (between 21°N and 21.25°N latitudes) in the northern hemisphere; the aeromagnetic data are subjected to a reduction to the northern magnetic pole. By using an Inclination (I) of 48.56° and a Declination (D) of 6.78° East for the mapped area, the RTP map was generated (Fig. 6). RTP is considered as the best and common way used for removing magnetic distortions. The resulting RTP map shows a direct good correlation between magnetic anomalies and their causative sources (which can be deduced from the geologic map). When looking to the RTP map (Fig. 6), in comparison with the original TMI map (Fig. 5), the northward shift in the positions of the magnetic anomalies can be observed easily. This is due to the elimination of the inclination and the declination effect of the magnetic field at the mapped site. Moreover, the locations and shapes of the anomalous bodies are centered over their respective causative bodies, which also correlates very well with the geologic map (Fig. 2). The RTP map elucidates some major magnetic belts that are scattered and distributed along the mapped
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Fig. 6. Color-shaded RTP map of the study area.
Fig. 7. Power spectrum curve of the aeromagnetic data showing the corresponding average regional and residual components and depths in the study area.
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Fig. 8. a) Color-shaded regional-magnetic component (low-pass filter) and b) Color-shaded residual-magnetic component (high-pass filter), using a cut-off frequency of 0.30 cycle/km to the RTP aeromagnetic data.
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Fig. 9. a) Color-shaded Tilt DeRivative (TDR) map of the study area and the corresponding associated trends, and b) Rose diagram showing the length percentages for the lineaments trends defined from the TDR map.
region. First, the positive magnetic anomalies have an amplitude of more than 350 nT (red and magenta in color), expressed as circular to elongated anomalies, located mainly at the middle, northeastern and southwestern parts of the mapped area, and trending to the NW–SE and N–S directions. Second, the negative magnetic anomalies show an amplitude of less than −200 nT (light and dark blue in color). Two parallel NW-SE trending negative magnetic belts are observed in the RTP map surrounding a high positive magnetic anomaly. The latter anomaly is located at the middle of the mapped area. These two negative belts trending principally on the NW-SE direction, and correlate very well with the inferred faults displayed on the geologic map (Fig. 2). Furthermore, some small semi-
elongated negative anomalies trending mainly N–S and E–W appear scattered all over the mapped area. Finally, the RTP map (Fig. 6) shows a very good correlation between some strong magnetic signatures and the exposed geologic rock units (Fig. 2) of the study area, suggesting that the magnetic anomalies correspond with the contacts or the edges of subsurface geologic structures. 4.2. Separation of the regional and residual anomalies Filtering the potential field data (gravity and magnetic) is an essential step prior to the analysis and the interpretation. The objective of
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Fig. 10. Color-shaded analytical signal (AS) contour map of the study area.
filtering is to condition the data, to deliver the resulting presentation, to make it easy for final interpretation, and to obtain anomalies in terms of their causative bodies or what is called by “geological sources” (Bird, 1997). Various filtering techniques can be used. However, one of the most traditional and common filters is the isolation of deep (long wavelength) and shallow (short wavelength) anomalies. The success of such filter depends mainly on the proper selection of the cut-off wavelength applied during the filter design. The cut-off wavelengths and the contribution of short- and long-wavelengths can be obtained from the radially-averaged power spectrum technique (Maus and Dimri, 1996). The radially-averaged power spectrum (Fig. 7) can be used to estimate the average depths of the causative sources (e.g., basement complex, volcanic intrusions, or subsurface geological structures). Several authors (e.g., Bhattacharyya, 1966; Spector and Grant, 1970) mentioned that the slope of the accompanied two linear segments can be used to estimate the average depths to the top of the deep-seated, and near-surface magnetic sources. These depths (Fig. 7) are average values and do not reflect the topographic features of the hidden basement surface. The 2D power spectrum curve (Fig. 7) shows two linear segments (based on the change of the slope of the obtained spectrum curve) related to the long- and short-wavelength components. They have frequency bands ranging from 0.05 to 0.30 cycle/km, and from 0.30 to 1.10 cycle/km, respectively. The first component (long wavelength and low frequency) is related to a deep source and defined as the
regional component, while the second one (short wavelength and high frequency) has a shallower origin, and is defined as the nearsurface component. Meanwhile, the high-frequency signal, displayed in the spectrum, was considered as noise. In the current study, the frequency band (cutoff wavelength of 0.30 cycle/km) was used through the band-pass filter technique using Oasis Montaj Geosoft 9.3.1 (Oasis Montaj Program v.9.3.1, 2018) software to isolate the regional and residual magnetic components. The band-pass filter is very critical to the cut-off frequencies that pass or reject specific frequency bands. The low-pass (regional component) and high-pass (residual component) maps have been generated and plotted in Fig. 8a,b. The regional-magnetic component (low-pass) map (Fig. 8a) displays the deep-seated high-amplitude anomalies, while the residualmagnetic component (high-pass) map (Fig. 8b) shows the shallow low-amplitude anomalies. Most of the magnetic anomalies appearing on the RTP map (Fig. 6) continue to appear in the residual-magnetic component map (Fig. 8b). The latter shows clearly the different clusters of positive and negative magnetic anomalies distributed all over the mapped area, which are of higher resolution than those displayed on the RTP map (Fig. 6). These anomalies have semi-circular, circular to elongated shapes characterized by their relatively short wavelengths and high frequencies. The major structural trends that can be deduced clearly from the residual aeromagnetic anomaly map (Fig. 8b) are in the NW–SE, NE–SW and E–W directions, related mainly to the nearsurface structures.
Fig. 11. Color-shaded relief maps for the subsurface from sea level for the inversion results; a) for the first layer (L250) at 250 m, b) for the second layer (L500) at 500 m, c) for the third layer (L750) at 750 m, and d) for the fourth layer (L1000) at 1000 m.
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4.3. 2D edge detection interpretation techniques In the current study, we focused more on the 2D and 3D structural interpretation of the Ceboruco volcano. In 2D interpretation, edge detection filters, such as Tilt DeRivative (TDR) and Analytic Signal (AS) were applied. These filters are commonly used to trace faults/contacts within the study area. To achieve a 3D interpretation, a 3D inversion method was applied to image the causative body in the subsurface of the mapped area. All the obtained results have been compared and confirmed with the geologic and tectonic features in the study area. This is to avoid the ambiguity in the final conclusions. Edge detection of the causative magnetic sources is one of the most crucial steps towards the modeling of the magnetic anomalies (Bournas and Baker, 2001; Ardestani and Motavalli, 2007). Several methods have been used by many authors (e.g., Pilkington and Keating, 2004; Cooper and Cowan, 2008; Cooper, 2009; Arisoy and Dikmen, 2013). The TitleDeRivative (TDR) and the Analytical Signal (AS) methods are considered the most common techniques among many others. In this work, the TDR and AS techniques were used for the edge detection of the prevailing subsurface structures in the study area. It was very interesting to compare between the obtained results, and show the degree of similarity when applying each individual technique. More details about the applied methodologies and their corresponding results will be shown in the following context. 4.3.1. Tilt-DeRivative (TDR) method TDR has been selected to be applied on potential field data by Miller and Singh (1994) and Verduzco et al. (2004). More recently, it was developed and used by several authors (e.g., Salem et al., 2008; Fairhead et al., 2008). Many researchers have applied this filter to their data because of its practical and fundamental simplicity (Hinze et al., 2013). This filter together with its total horizontal derivative (THDR) are usually useful in the mapping and detection of the geological contacts and edges which may be related to the mineral exploration targets or to shallow basement structures. TDR is computed by the division of the Vertical DeRivative component (VDR) by the Total Horizontal DeRivative component (THDR) of the magnetic field (Verduzco et al., 2004): TDR ¼ tan−1
VDR THDR
0
1
∂f =∂z B C ¼ tan−1 @qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiA 2 2 ð∂f =∂xÞ þ ð∂f =∂yÞ
ð1Þ
where f is the magnetic field, “∂f/∂x, ∂f/∂y, ∂f/∂z” are the first derivatives of the magnetic field in the directions x, y and z, respectively. The advantage of the TDR technique is that its zero-contour line is located directly or very close to the contact or the edge of the causative source (e.g., lineation, fault, contact). So, it can be applied to the aeromagnetic data to delineate the possible edges in the mapped area (Miller and Singh, 1994). Moreover, positive values of the TDR are located above the magnetic source, while the negative ones are located far from it. In the current study, the TDR filter was applied to the aeromagnetic data, and the obtained results were plotted directly as a gridded image in Fig. 9a. A large number of linear magnetic anomalies can be traced easily from the TDR map, trending mainly in the NW-SE, ENE-WSW, and E-W directions. Those anomalies represent the prevailed subsurface contacts or fault trends in the study area. A rose diagram from the obtained lineaments map has been prepared and plotted in Fig. 9b. It confirms the previously-mentioned structural trends defined from Fig. 9a. 4.3.2. Analytical signal (AS) method The analytic signal (AS) or the total gradient method is another powerful method that is normally used in the evaluation and detection of the buried subsurface structures which usually appear as linear magnetic anomalies (e.g., faults, fractures or dykes) (Saada, 2016). The
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analytical signal approach has been used successfully to locate dykes from the profile data by Nabighian (1972, 1974, 1984) and Atchuta et al. (1981). Later, it was developed by Roest et al. (1992) to be used in the interpretation of aeromagnetic maps. Further improvements on the technique to interpret aeromagnetic data were done by Hsu et al. (1996, 1998) and Thurston and Smith (1997). Saada (2016) mentioned that the AS method is helpful in the determination of the different parameters of the magnetic anomalies due to the independence on the magnetization direction (inclination). The analytical signal is the square root of the summation of the squares of the different derivatives in the x, y and z directions (Roest et al., 1992): sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 2 ∂f ∂f ∂f AS ¼ þ þ ∂x ∂y ∂z where
∂f ; ∂x
∂f ∂f ; ∂y ∂z
ð2Þ
are the first-order derivatives of the total intensity
magnetic field. Both horizontal and vertical derivatives of the anomaly are Hilbert transform pairs of each other (Nabighian, 1972). In the present analysis, the analytical signal was computed and plotted (Fig. 10) using the Oasis Montaj Geosoft 9.3.1 (Oasis Montaj Program v.9.3.1, 2018) software. It clearly appears that an elongated NW-SE high anomaly is located at the middle part of the mapped region, which is surrounded by low to moderate N–S and E–W trending anomalies. Hence, it can be realized that the analytical signal method is very successful in defining the outer limits of the magma chamber beneath the Cebouruco Volcano. 4.4. Modeling of the data with 3D inversion techniques Qualitative magnetic or gravity data interpretation provides a general information about the subsurface. When more detailed information about the subsurface is needed, quantitative models of the subsurface are required. This is the concept of the geophysical inversion. The 3D inversion technique gives a 3D image about the physical properties of the subsurface as density (from gravity data) or susceptibility (from magnetic data). However, constraints are required in order to control the inversion process (e.g. boreholes or lithological information). In the current study, the aeromagnetic data of Ceboruco volcano was inverted using the GMSys-3D inversion program from Geosoft (Popowski et al., 2006) where a model is defined by a number of stacked surface grids with susceptibility distributions specified for the layer below each surface. GMSys-3D calculations were performed in the wave number domain using the Parker algorithm (Parker, 1973). The approach assumes that the magnetic response of the subsurface layer is caused by a series of vertical square-ended prisms of infinite depth extent, where its horizontal dimensions equal to the input grid cell size. Before starting the inversion, we assumed that there are four subsurface layers (L250, L500, L750, and L1000) at various depths (250, 500, 750 and 1000 m, respectively). Instead of assuming magnetic susceptibility for each layer, we calculated the apparent magnetic susceptibility for each layer using the susceptibility filter. A susceptibility filter is a compound filter that performs a reduction to the pole, downward continuation to the source depth, correction for the geometric effect and division by the total magnetic field to yield susceptibility. Apparent susceptibility mapping assumes a simple geometric model. The required information to get the apparent susceptibility are the total magnetic field (F), declination (D), inclination (I), and depth to find susceptibility. The obtained apparent magnetic susceptibility was used within the inversion process as a physical property of the layer. When working on 3D inversion, the forward modeling should be estimated initially. This forward modeling generates three maps, observed, calculated, and error (misfit) maps. From those maps, we can invert the assumed layers (L250, L500, L750, and L1000) in terms of the apparent magnetic susceptibility. We inverted each layer
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a) 250 m
b) 500 m
c) 750 m Fig. 12. A three-dimensional model for the study area at the different studied layers/levels (shallow, medium, and deep).
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individually where the GMSys-3D program is based on 1-layer case inversion. The inversion results for the first layer (L250) are shown in Fig. 11a. The results show clearly the depth values for the causative body below the volcano are ranging from −499 to 1605 m. In the second layer (L500) (Fig. 11b), we can still see the boundary of the causative body and there is a zone, probably a volcanic intrusion, outlined by a yellow contour (depths from −750 to 1870 m). For the third layer (L750; Fig. 11c), we can still trace the base of the volcano as well as the yellow structural contour. This indicates that the base of the volcano and the yellow margin stem come from the same source. They could be connected at depth (depths from −1500 to 1211 m). Finally, for the last layer (L1000 m; Fig. 11d), the base boundary of the volcano disappeared, as well as the yellow structural line. It indicates that we reached the basement above which the volcano was formed. Fig. 12 shows a three-dimensional display for the different considered layers, and the depth values in each case. 5. Discussion and conclusions A number of infinite interpretations are usually available for most of the geophysical problems. Among them, only one solution is the correct answer. To reach this geophysical model, all known geologic and tectonic information should be taken into consideration. In the current study, the available geological, structural and tectonic knowledge was incorporated into the aeromagnetic analysis, interpretation and modeling for the Ceboruco Volcano area. On June 2018, a specific field trip to the volcano region has been arranged to confirm the geologic and structural setting of the volcano. In addition, a step-by-step comparison for the obtained maps/results was done in relation with the geologic map (specially for the RTP map) and the available previous geological and geophysical studies (e.g., Sieron and Siebe, 2008; Fernandez-Cordoba et al., 2017). Moreover, the structural interpretation for the possible related fault trends from the aeromagnetic data has been compared with the general tectonic and structural setting (e.g., Tepic-Zacoalco rift) of the volcano region. Thus, in turn, confirm all the obtained results. By applying such information, in the current work, a detailed 2D and 3D modeling of the aeromagnetic data for the volcano area, that may represent the actual conditions of the study region, were achieved. In this work, high-resolution aeromagnetic data for the Ceboruco Volcano was analyzed to delineate the possible subsurface geologic structures, and to provide an illustrative 3D model for the volcano. To reach such goals, several steps were followed: reduction of the total intensity aeromagnetic values to the north magnetic pole, isolation of the regional and residual magnetic components, application of the analytical signal and tilt derivative methods, and finally the 3D inverse modeling. Results indicated that there is a large number of linear magnetic anomalies, observable specially from the TDR and AS maps, that may correspond to contacts, faults or dykes. The major structural trends extracted from these maps are the NW-SE, ENE-WSW and E-W trends. The first and the most dominant trend is correlating very well with the wellknown NW-SE oriented Tepic Zacoalco rift, which bounds the volcano from the northwestern side. On the other hand, the inversion of the aeromagnetic data shows clearly the limits of the causative body at the shallower subsurface layers. At deeper levels, a structure marked by a yellow contour appears towards the eastern side of the studied area which means that both of the base of the volcano and the structure arise from the same magmatic source. Finally, at the deepest inverted subsurface layer, the limits of the magma chamber and the structural line disappeared, which means that the basement above which the volcano is formed has been reached. Our results and models, that have been obtained from the analysis of aeromagnetic data, are complementing and confirming the results, obtained from the interpretation of gravity data, by Fernandez-Cordoba et al. (2017). The principal negative gravity anomaly defined by them on their Bouguer anomaly map, that is located at the middle part of
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the study region, is coinciding perfectly with the positive magnetic anomaly defined in our study. This anomaly represents the location of the subsurface magma chamber of the volcano. However, there is some differences between the other defined small anomalies delineated from gravity and aeromagnetic data. Another significant topic is that the application of the aeromagnetic data in the definition of structural trends and possible faults/contacts is more successful in our study than in the Fernandez-Cordoba et al. (2017) work that depends on gravity. This definition of possible prevailing structure was confirmed by the field check and by comparison with the geologic maps. In conclusion, these models, from one side, serve as groundwork in the search for geothermal resources, which in turn help decisionmakers concerning the production of electricity using the possible geothermal energy. This is the principal target of the CEMIE-Geo Project. From the other side, volcanoes are directly threaten the surrounding population centers and could have a direct influence on the socioeconomic development of such regions. Thus, volcanic risk assessment is an important scientific, economic, and political issues, especially in high-density populated areas such as Mexican cities. To assess the volcanic hazard, a scientist needs to identify the behavior of the volcanic system in the past to make it possible use such information to forecast the possible activity in the future. To do this, a compilation of all existing geological, geophysical, structural and volcanic eruption data should be taken into account. That is why our geophysical model could play an important role in any future assessment for the volcanic hazard assessment and mitigation for the study region. The obtained results show that the aeromagnetic data are very successful in the imaging of the possible subsurface geologic structures and estimating the approximate depths of the basement of the Ceboruco Volcano area. This work represents the first step of investigation for the CeMIEGeo-P24 geothermal exploration project, which will be succeeded later by detailed ground magnetic and magneto-telluric surveys for the anomalies of interest. Seismological studies are highly recommended for the volcano site, especially after this study. Such studies should shed more light on the relationship between the defined structural trends and the location of earthquake hypocenters. Acknowledgments This research work was supported by the Mexican CeMIEGeo-P24 project entitled “Passive and Magnetotelluric Seismic Exploration in the Geothermal Fields of the Ceboruco Volcano and La Caldera de la Primavera” throughout a postdoctoral fellowship stay for the first author at the Centro de Sismología y Volcanología de Occidente (SisVOc), Universidad de Guadalajara, Campus of Puerto Vallarta, Jalisco, Mexico. In addition, we would like to thank Editor Jurgen Neuberg and the anonymous reviewers for their thoughtful comments. Thanks also to Dr. J.A. Peláez (Universidad de Jaén, Spain) who reviewed the English language of the manuscript. References Ardestani, V.E., Motavalli, H., 2007. Constraints of analytic signal to determine the depth of gravity anomalies. J. Earth and Space Phys. 33, 77–83. Arisoy, M., Dikmen, U., 2013. Edge detection of magnetic sources using enhanced total horizontal derivative of the Tilt angle. Yerbilimleri 34, 73–82. Atchuta, R.D., Ram Babu, H.V., Sanker, P.V., 1981. Narayan Interpretation of magnetic anomalies due to dikes: the complex gradient method. Geophysics 46, 1572–1578. Bhattacharyya, B.K., 1966. Continuous spectrum of the total magnetic field anomaly due to a rectangular prismatic body. Geophysics 31, 97–121. Bird, D., 1997. Interpreting Magnetic Data: Geophysical Corner, EXPLORER, AAPG and SEG, May, 1997. Bournas, N., Baker, H.A., 2001. Interpretation of magnetic anomalies using the horizontal gradient analytic signal. Ann. Geophys. 44, 506–526. Browne, B.L., Gardner, J.E., 2004. The nature and timing of caldera collapse as indicated by accidental lithic fragments from the AD 1000 eruption of Volcan Ceboruco, Mexico. J. Volcanol. Geotherm. Res. 130, 93–105. Cooper, G.R.J., 2009. Balancing images of potential field data. Geophysics 74, 17–20. Cooper, G.R.J., Cowan, D.R., 2008. Edge enhancement of potential field data using normalized statistics. Geophysics 73, 1–4.
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