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Collapse susceptibility map in abandoned mining areas by microgravity survey: A case study in Candado hill (Málaga, southern Spain)
Journal of Applied Geophysics 130 (2016) 101–109
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Collapse susceptibility map in abandoned mining areas by microgravity survey: A case study in Candado hill (Málaga, southern Spain) F.J. Martínez-Moreno a,⁎, J. Galindo-Zaldívar b,c, L. González-Castillo b, J.M. Azañón b,c a b c
IDL-Universidade de Lisboa, Faculdade de Ciências, Campo Grande, Ed. C8, Lisboa, Portugal Departamento de Geodinámica, Universidad de Granada, 18071 Granada, Spain Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, 18071 Granada, Spain
a r t i c l e
i n f o
Article history: Received 24 December 2015 Received in revised form 22 April 2016 Accepted 27 April 2016 Available online 28 April 2016 Keywords: Construction collapse Abandoned mine galleries Microgravity survey Residual anomaly map Collapse susceptibility areas
a b s t r a c t The presence of disused gypsum mine galleries in Candado hill (Málaga, southern Spain) has caused constructions at the western end of the hill to collapse. The mine, which closed in the 1960s, is disconnected from the surface. Therefore, the precise dimensions and position of the galleries are unknown, making it essential to undertake a thorough microgravity study to assess the collapse susceptibility of the area. On the one hand, we analyze the accuracy of the terrain correction and Bouguer anomaly calculation, varying density values. This study shows that higher density values produce more pronounced residual minima corresponding to the mine galleries. On the other hand, the approximate mine gallery positions and dimensions were derived through forward modelling, then correlated with the gravity residual anomalies produced in the hill. Altogether, these results can be presented as a collapse susceptibility map relating the areas containing mine galleries with shallow voids on the hill. © 2016 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Geological setting. . . . . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Differential GPS . . . . . . . . . . . . . . . . . . . . . 3.2. Microgravity measurements. . . . . . . . . . . . . . . . 4. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Bouguer and regional anomaly maps . . . . . . . . . . . . 4.2. Residual anomaly maps with different terrain corrections . . 4.3. Residual anomaly map and microgravity forward models . . 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Microgravity studies in void location . . . . . . . . . . . . 5.2. Analysis of different densities applied to the terrain correction 5.3. Collapse susceptibility map based on microgravity results . . 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (F.J. Martínez-Moreno).
http://dx.doi.org/10.1016/j.jappgeo.2016.04.017 0926-9851/© 2016 Elsevier B.V. All rights reserved.
Sinkholes and karst fissures formed in gypsum bedrock are common features in sedimentary rock (Martinez et al., 1998; Galve et al., 2008, 2009a; Gutiérrez et al., 2008). A significant minority of such sinkholes having an anthropogenic origin may be related to the collapse of
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Fig. 1. Collapse areas in Candado hill (Málaga, S Spain). (a) Location of Candado hill on the Iberian Peninsula; (b) Collapse areas marked with red line and illustration of the collapsed construction.
cavities. Often times associated groundwater seepage or upward artesian flow from regional karst aquifers underlies evaporitic rocks at the surface (Klimchouk et al., 1996; Calaforra and Pulido-Bosch, 1999; Gutiérrez et al., 2007). However, the main anthropogenic geohazard linked to gypsum and chalk bedrock is related to the collapse of manmade shallow cavities produced in the past century, when this mineral was widely mined in Europe (i.e. Waltham et al., 2007). Around the world, most old mines were excavated using the room and pillar method. This method, however, led to substantial geohazards long after the mine was abandoned, posing a serious public safety issue today (Gutiérrez et al., 2009). Shallow man-made gypsum cavities (depth b 50 m) can be affected by local collapses and large regional collapses (Whittaker and Reddish, 1990; Bell et al., 1992, 2000, 2005; Singh and Dhar, 1997; Deb and Choi, 2006). On the surface, the impact of collapse appears as subsidence or crown-holes. A major collapse would cover a large surface and can culminate in a small earthquake or rockburst — e.g., the local magnitude registered in one gypsum mine was close to 3.1 ML (Wang et al., 2008). A collapse can be dynamic and brutal, and may produce a domino effect due to the load transfer (Ma et al., 2012). Candado hill is a small mountain (100 m high) by the eastern outskirts of the city of Málaga (Costa del Sol, Spain). It was profusely mined during the first half of the 20th century. Gypsum mining ended
in 1960, when pits, exploitation rooms and extraction galleries were abandoned. Some 30 years later, a residential resort named ‘Montegolf’ was built close to Candado hill summit, and extensive urban development of the area ensued. Several years later, the construction of a highway along the eastern side of Candado hill produced a highway cutslope 70 m in height. On September 11th, 2000, an apparent shallow slope failure produced the partial collapse of the Montegolf housing complex (Fig. 1). Yet the debris flow that tilted the foundations of three houses is actually a consequence of a vast deformation affecting the entire hill. Deformations at the top of Candado hill point to a collapse inside as the trigger of the debris flow. Moreover, anthropogenic activity on Candado hill may affect the general stress field and ground deformation (Azañón et al., 2012). Subsidence, slope deformations, landslides and other related damage are very significant problems in mining areas. This type of geohazard is not easy to predict in a timely manner, especially in the case of abandoned mines. Identifying the causes of collapse, while a very delicate matter, is absolutely necessary in order to improve our understanding of such events. The detection of voids by means of geophysical methods is widely documented (Rybakov et al., 2001; Vadillo et al., 2012; Martínez-Moreno et al., 2013, 2014, 2015b). Geophysical methods, including microgravity studies, may be applied to detect cavities along
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Fig. 2. Geological setting of Candado hill. (a) Location of the study area (highlighted with red dots) in the Betic Cordillera; (b) Simplified geological map of Candado hill and surroundings. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Modified from Vera-Peláez et al., 2004.
profiles and maps (Colley, 1963; Butler, 1984; Al-Rawi and Rezkalla, 1987; Rybakov et al., 2001; Mochales et al., 2008; Gambetta et al., 2011). In addition to defining the areas where voids extend, gravity maps allow us to determine their approximate depth and dimensions. This research was undertaken to construct a collapse susceptibility map of a disused mine area without outside access and whose dimensions and location were basically unknown. Through microgravity measurements in map distribution, we define the areas in Candado hill where the mine galleries are located and pinpoint the location of each void.
reaching up to the Middle Miocene (Chalouan and Michard, 1990; Martín-Algarra et al., 2009) (Fig. 2). The hill extends 700 × 400 m, its base being located at 65 m high, while the top reaches 125 m high. From base to top, it is composed by red clays, gypsum and conglomerates with a sandy matrix from Permo-Trias that crop out around the hill (Vera-Peláez et al., 2004). The disused mine under study is located in this layer. Over the base, there are levels of limestone and dolostone from the JurassicCretaceous with bedding that is NE–SW oriented and 20° SE dipping. The series culminates in Quaternary sands and conglomerates. 3. Methods
2. Geological setting Candado hill is located in the Malaguide Complex, which pertains to the Internal Zones of the Betic Cordillera (Azema, 1961; Serrano et al., 1995). This complex is formed by Paleozoic rocks partially affected by Variscan metamorphism and other late sedimentary sequences
This research entailed microgravity prospection positioned with differential GPS. Microgravity prospection calls for a fixed positioning, as to a ± 1 m topographic error causes a variation of ±0.3 mGal (Telford and Sheriff, 1990). Such a variation clearly exceeds the maximum permitted error when considering the values of studied anomalies.
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Fig. 3. Gravity anomaly maps of Candado hill. Regional anomaly trend (black lines) over Bouguer anomaly at a reference density of 2.67 g/cm3.
3.1. Differential GPS The equipment used in this study included the differential Global Position System (GPS) Leica 1200+ system, which uses error correction data to fine-tune the position of each measurement station with an accuracy of ±0.5–20 mm. The GPS is connected with the Andalusian Positioning Network, known as RAP (Junta de Andalucía, 2015). This equipment incorporates the stakeout program, which enables one to create measurement grids with an equidistance of 10 × 10 m. New additional position measurements allowed us to improve the previous digital terrain model (DTM) and calculate a more accurate DTM for terrain correction. The system used for GPS data was UTM and the datum of reference was ETRS89–Zone 30N. 3.2. Microgravity measurements Geophysical gravity methods make it possible to determine underground density changes through the measurement of local gravity variations. These differences are caused by the presence of bodies whose density contrasts with that of the host rock —i.e. cavities with a density of ~0 g/cm3, or filled by water ~1 g/cm3, enclosed within host rock of ~2.67 g/cm3. To acquire gravity data we used a Scintrex Autograv CG5 on a tripod, with an accuracy of ± 0.001 mGal. The measurements were taken at 317 stations with a spacing of 10 × 10 m over a regular grid in opencast and non-constructed areas, while in constructed areas the measurements were taken along the streets with roughly the same spacing. All proper practices to enhance accuracy in
microgravity surveys were taken into account: cycles shorter than 3 h; before measurements, the gravimeter was leveled and calibrated on the tripod to minimize hysteresis effects; measurements in each station were repeated during 60–90 s until minimizing the obtained error under ± 0.005 mGal. After tidal and instrumental corrections, the Bouguer anomaly was determined using the standard density of 2.67 g/cm3, close to the average density of the lithology of the studied area. The terrain correction consists of removing the visible topography. It was calculated by means of the Hammer circle method (Hammer, 1939, 1982) using our local DTM combined with the IGN digital terrain model (Instituto Geográfico Nacional, 2005), with 5 m horizontal resolution. The maximum radius covered during terrain correction was from 2 m to 9902 m around each gravity site (zones B to K). Accurate analysis of results requires extracting the residual gravity anomaly from the Bouguer anomaly. For this purpose, the regional anomalies were removed through polynomial regression first order on the Bouguer anomaly tendency (Martínez-Moreno et al., 2015a) due to the elongated NW–SE morphology of the anomaly. In addition, we calculated different residual gravity maps with density variation (both Bouguer anomaly and terrain correction) from 2.5 to 2.7 g/cm3, to perform analysis of the density variation for local studies near areas with voids. Furthermore, we performed forward models along perpendicular profiles crossing the center of the hill. These anomalies were modeled by means of GRAVMAG v.1.7, from the British Geological Survey (Pedley et al., 1993), with 2.5D approximation according to the
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Fig. 4. Residual anomaly maps with different terrain corrections calculated from several densities. The topography is marked over the residual anomaly maps with white lines.
geological information. The 2.5D modeling allowed us to assign perpendicular extensions to the modeled bodies. 4. Results The obtained results are analyzed in light of the Bouguer anomaly map of the study area and shallow density variations (residual anomaly maps). Comparison of the different densities used in the terrain correction and Bouguer anomaly is achieved. Finally, we propose forward models corresponding to the position and shape of the mine.
correction with different density values (Fig. 4). The values were between 2.5 g/cm3 and 2.7 g/cm3, with steps of 0.05 g/cm3, reaching the maximum density assigned to carbonates. The resulting maps show the most pronounced residual minima values to be located at the center of the hill, just slightly displaced to the NW of the highest topography. The residual minima produced by the presence of mine galleries are larger for the high density values (2.7 g/cm3) than for the low ones (2.5 g/cm3). The minima shape is similar for all the maps, yet with varying extension. 4.3. Residual anomaly map and microgravity forward models
4.1. Bouguer and regional anomaly maps The gravity results show a Bouguer anomaly (Fig. 3) presenting a linear NW–SE trend with values ranging from 42.2 to 43.7 mGal, which increases from SW to NE. In the center of Candado hill the lineal tendency is interrupted by low gravity values. This regional anomaly is produced by density variations in depth. 4.2. Residual anomaly maps with different terrain corrections The appropriate density for a hill made up of carbonates and voids was obtained through calculation of the Bouguer anomaly and terrain
Our analysis of the residual gravity anomaly produced for the mine galleries is achieved considering a standard density of 2.67 g/cm3. The anomaly map (Fig. 5) shows maxima values from 0.3 to 0.4 mGal at the N and S edges, and minima values of −0.1 mGal enclosing a more pronounced minimum from − 0.3 to − 0.5 mGal at the center of the hill. These minima correspond to the presence of mine galleries in the area. To support our analysis of the minima, we calculated 2.5D models along two perpendicular profiles crossing the main minimum of the study area (Figs. 5 and 6). For both profiles, two main densities were considered: 2.67 g/cm3 belonging to the carbonates of the hill, and
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Fig. 5. Residual anomaly maps for a terrain correction with standard density of 2.67 g/cm3. The position of the modeled profiles is marked with white lines.
0 g/cm3 assigned to the unsaturated voids. Profile 1 (Fig. 6a), with NW– SE direction and 200 m-length, presents a principal gravity minimum of − 0.55 mGal in its central part, and local minima of − 0.3 mGal at 50, 120 and 160 m-lengths. The minima having shorter wavelengths were adjusted by modeling shallow smaller voids (5–10 m-long in the profile direction, and 10–20 m perpendicular to it). The longest wavelength
located along the profile was adjusted with a deeper and bigger void, which has dimensions of 90 m long in the profile direction and 80 m long perpendicular to the profile direction. Profile 2, having a NE–SW direction, shows a residual minimum of −0.5 mGal located between 60 and 200 m. Three additional local minima with lower wavelengths were identified at 30, 120 and 155 m in length. The main minimum
Fig. 6. Microgravity models along the study profiles showing the galleries marked in Fig. 4. (a) NW–SE profile and (b) SW–NE profile.
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Fig. 7. Collapse susceptibility map over (a) residual anomaly map and (b) orthophoto. The red line highlights the main gallery and the yellow line marks the area having shallower voids. Purple circles mark the areas where major collapse occurred. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
was adjusted with a void located 50 m-deep, 80 m-length and 90 m perpendicular; the secondary minimum was modeled with a shallow void, 20–40 m-length and having a perpendicular extension of 10–20 m. 5. Discussion 5.1. Microgravity studies in void location Microgravity enables one to detect underground voids in different contexts. This method was applied to cross-sections of maps elaborated from gravity data that highlight cavities as gravity minima, showing different values and wavelengths depending on the gallery dimensions and depth. Equidistant spacing of the measurement stations is necessary to obtain useful maps fitting the analysis of an anomaly produced by voids (Leucci and De Giorgi, 2010). It is furthermore possible to obtain cavity morphologies as well as depth estimations through forward modeling (Beres et al., 2001; Mochales et al., 2008; Martínez-Moreno et al. 2014), ideally in conjunction with other geophysical methods. Our microgravity study resulted in a residual anomaly map (Fig. 5) highlighting minima associated with the position of voids. Still, the morphology, depth and dimensions were unknown. Independent and simultaneous 2.5D forward modeling of the anomaly along the two profiles served to obtain approximate real morphologies. In both models, the perpendicular lengths of the mine are concordant, the cave points coincide, and the observed and calculated anomaly match. These models reflect contours of the residual anomaly map that fit the surface projection of the mine area. 5.2. Analysis of different densities applied to the terrain correction A detailed terrain correction in this research study was deemed essential to obtain reliable results (Debeglia and Dupont, 2002). Such calculations call for an adjusted DTM due to the fact that nearby
topography substantially influences the final anomaly maps (Martínez-Moreno et al., 2015a). In this case we used a DTM of 5 × 5 m spacing improved with acquired differential GPS measurements. In addition, the density value applied in the terrain (Hammer, 1939, 1982) and Bouguer anomaly corrections are determinant for the reliability of results. While in typical study areas a standard density value of 2.67 g/cm3 can be applied as the initial approximation, we applied different values, from 2.5 g/cm3 to 2.7 g/cm3, as the possible densities associated with a hill of the given lithologies (Fig. 4). Firstly, results show that the terrain correction is well calculated, because the features do not only correlate with the main topography of the hill. The most pronounced minima are not located at the highest topographies, but rather to the NW. The contour anomaly shapes are likewise not restricted to the topography shape. In most of the resulting maps (Fig. 4), it is seen that high densities applied in the terrain correction imply more highlighted and expanded minima for the residual gravity anomaly, pertaining to the mine gallery location. Whereas the topographic correction gives higher values for higher densities in gravity calculation, the Bouguer anomaly shows a reversed and more pronounced sense. Accordingly, the higher density values correspond to more pronounced minima obtained in the residual anomaly map. However, as the average density of unaltered limestones and dolostones is close to 2.67 g/cm3, we applied this standard density to obtain the most suitable residual anomaly map (Fig. 5).
5.3. Collapse susceptibility map based on microgravity results Susceptibility maps for construction collapse may be based on Geographic Information System (GIS) (Yilmaz, 2007), geomorphic factors (Papadopoulou-Vrynioti et al., 2013) or sinkholes (Galve et al., 2009b). It is less common for susceptibility maps to be derived from microgravity to highlight areas where collapse might occur owing to the presence of voids.
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In this case, the residual anomaly map was considered together with the forward models of the mine galleries to arrive at a collapse susceptibility map underlining key areas related to the mine galleries (Figs. 6 and 7). The extension and position of the galleries along the profiles determine the anomaly contour (Fig. 7a): that of − 0.15 mGal (marked with red line in Fig.7) fits the mine limits and restricts the high collapse susceptibility area. Further, shallow voids in the hill define a lower collapse susceptibility area (−0.05 mGal, yellow line in Fig.7). These contours can be highlighted over the orthophoto to verify the marked areas (Fig. 7b). The mine is seen to lie mainly E of the construction area. However, on the W and NW sides of the hill the red line is located over some buildings. The areas marked with purple circles in Fig. 7b, matching the red line and constructions, indicate major collapses and destruction on the hill. 6. Conclusions The presence of a disused mine beneath Candado hill originated construction collapse along the W side of the hill. The gypsum mine comprised several galleries, but the lack of outside access impedes determination of its morphology and position. Accurate microgravity research to the hill provided new data for assessing its collapse susceptibility. Density analysis, applied to the terrain correction and Bouguer anomaly, revealed that the optimal values of the host rocks is close to the standard 2.67 g/cm3. Two profiles were modeled crossing each other to derive the approximate gallery morphology, dimensions and depth. Altogether, our results produce a collapse susceptibility map supported by the residual gravity anomaly contours, highlighting areas where collapses could occur. Moreover, a second wide area is marked, where shallower and smaller voids might also originate local collapse. The area undergoing the principal construction collapse is underlined, matching the highly susceptible area. This study serves to confirm that gravity research is an adequate tool for detecting voids and elaborating collapse susceptibility maps that condition future construction in areas where subsoils may contain anthropic or natural voids. Acknowledgements We thank Jean Sanders for reviewing the English grammar. Our research was supported by projects CGL2010-21048, P09-RNM-5388 and RNM148. In addition, we are grateful with the positive comments and reviews made by the two anonymous reviewers who significantly helped to improve the manuscript. References Al-Rawi, F.R., Rezkalla, J.S., 1987. The application of microgravity survey for cave detection in a karstic area. Large Rock Caverns, Proceedings of the International Symposium, Helsinki, Finland. 2, pp. 25–28. Azañón, J., Puertas, E., Ureña, C., Gallego, R., Romero-Gómez, F., 2012. Collapse in a rock massif induced by lateral confinement loss: the case of Montegolf (Málaga, Spain). EGU General Assembly Conference Abstracts, p. 12527. Azema, J., 1961. Etude géologique des abords de Málaga (Espagne). Estud. Geol. 17, 131–160. Bell, F.G., Cripps, J.C., Moorlock, B.S.P., Culshaw, M.G., 1992. Subsidence and ground movements in chalk. Bull. Int. Assoc. Eng. Geol. 45, 75–82. Bell, F.G., Donnelly, L.J., Genske, D.D., Ojeda, J., 2005. Unusual cases of mining subsidence from Great Britain, Germany and Colombia. Environ. Geol. 47, 620–631. Bell, F.G., Stacey, T.R., Genske, D.D., 2000. Mining subsidence and its effect in the environment: some differing examples. Environ. Geol. 40, 135–152. Beres, M., Luetscher, M., Olivier, R., 2001. Integration of ground-penetrating radar and microgravimetric methods to map shallow caves. J. Appl. Geophys. 46, 249–262. Butler, D.K., 1984. Microgravimetric and gravity gradient techniques for detection of subsurface cavities. Geophysics 49, 1084–1096. Calaforra, J.M., Pulido-Bosch, A., 1999. Gypsum karst features as evidence of diapiric processes in the Betic Cordillera, Southern Spain. Geomorphology 29, 251–264. Chalouan, A., Michard, A., 1990. The Ghomarides nappes, Rif coastal range, Morocco: a Variscan chip in the Alpine belt. Tectonics 9, 1565–1583.
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Contents lists available at ScienceDirect
Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
Collapse susceptibility map in abandoned mining areas by microgravity survey: A case study in Candado hill (Málaga, southern Spain) F.J. Martínez-Moreno a,⁎, J. Galindo-Zaldívar b,c, L. González-Castillo b, J.M. Azañón b,c a b c
IDL-Universidade de Lisboa, Faculdade de Ciências, Campo Grande, Ed. C8, Lisboa, Portugal Departamento de Geodinámica, Universidad de Granada, 18071 Granada, Spain Instituto Andaluz de Ciencias de la Tierra, CSIC-Universidad de Granada, 18071 Granada, Spain
a r t i c l e
i n f o
Article history: Received 24 December 2015 Received in revised form 22 April 2016 Accepted 27 April 2016 Available online 28 April 2016 Keywords: Construction collapse Abandoned mine galleries Microgravity survey Residual anomaly map Collapse susceptibility areas
a b s t r a c t The presence of disused gypsum mine galleries in Candado hill (Málaga, southern Spain) has caused constructions at the western end of the hill to collapse. The mine, which closed in the 1960s, is disconnected from the surface. Therefore, the precise dimensions and position of the galleries are unknown, making it essential to undertake a thorough microgravity study to assess the collapse susceptibility of the area. On the one hand, we analyze the accuracy of the terrain correction and Bouguer anomaly calculation, varying density values. This study shows that higher density values produce more pronounced residual minima corresponding to the mine galleries. On the other hand, the approximate mine gallery positions and dimensions were derived through forward modelling, then correlated with the gravity residual anomalies produced in the hill. Altogether, these results can be presented as a collapse susceptibility map relating the areas containing mine galleries with shallow voids on the hill. © 2016 Elsevier B.V. All rights reserved.
Contents 1. 2. 3.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Geological setting. . . . . . . . . . . . . . . . . . . . . . . . Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Differential GPS . . . . . . . . . . . . . . . . . . . . . 3.2. Microgravity measurements. . . . . . . . . . . . . . . . 4. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Bouguer and regional anomaly maps . . . . . . . . . . . . 4.2. Residual anomaly maps with different terrain corrections . . 4.3. Residual anomaly map and microgravity forward models . . 5. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. Microgravity studies in void location . . . . . . . . . . . . 5.2. Analysis of different densities applied to the terrain correction 5.3. Collapse susceptibility map based on microgravity results . . 6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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1. Introduction
⁎ Corresponding author. E-mail address: [email protected] (F.J. Martínez-Moreno).
http://dx.doi.org/10.1016/j.jappgeo.2016.04.017 0926-9851/© 2016 Elsevier B.V. All rights reserved.
Sinkholes and karst fissures formed in gypsum bedrock are common features in sedimentary rock (Martinez et al., 1998; Galve et al., 2008, 2009a; Gutiérrez et al., 2008). A significant minority of such sinkholes having an anthropogenic origin may be related to the collapse of
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Fig. 1. Collapse areas in Candado hill (Málaga, S Spain). (a) Location of Candado hill on the Iberian Peninsula; (b) Collapse areas marked with red line and illustration of the collapsed construction.
cavities. Often times associated groundwater seepage or upward artesian flow from regional karst aquifers underlies evaporitic rocks at the surface (Klimchouk et al., 1996; Calaforra and Pulido-Bosch, 1999; Gutiérrez et al., 2007). However, the main anthropogenic geohazard linked to gypsum and chalk bedrock is related to the collapse of manmade shallow cavities produced in the past century, when this mineral was widely mined in Europe (i.e. Waltham et al., 2007). Around the world, most old mines were excavated using the room and pillar method. This method, however, led to substantial geohazards long after the mine was abandoned, posing a serious public safety issue today (Gutiérrez et al., 2009). Shallow man-made gypsum cavities (depth b 50 m) can be affected by local collapses and large regional collapses (Whittaker and Reddish, 1990; Bell et al., 1992, 2000, 2005; Singh and Dhar, 1997; Deb and Choi, 2006). On the surface, the impact of collapse appears as subsidence or crown-holes. A major collapse would cover a large surface and can culminate in a small earthquake or rockburst — e.g., the local magnitude registered in one gypsum mine was close to 3.1 ML (Wang et al., 2008). A collapse can be dynamic and brutal, and may produce a domino effect due to the load transfer (Ma et al., 2012). Candado hill is a small mountain (100 m high) by the eastern outskirts of the city of Málaga (Costa del Sol, Spain). It was profusely mined during the first half of the 20th century. Gypsum mining ended
in 1960, when pits, exploitation rooms and extraction galleries were abandoned. Some 30 years later, a residential resort named ‘Montegolf’ was built close to Candado hill summit, and extensive urban development of the area ensued. Several years later, the construction of a highway along the eastern side of Candado hill produced a highway cutslope 70 m in height. On September 11th, 2000, an apparent shallow slope failure produced the partial collapse of the Montegolf housing complex (Fig. 1). Yet the debris flow that tilted the foundations of three houses is actually a consequence of a vast deformation affecting the entire hill. Deformations at the top of Candado hill point to a collapse inside as the trigger of the debris flow. Moreover, anthropogenic activity on Candado hill may affect the general stress field and ground deformation (Azañón et al., 2012). Subsidence, slope deformations, landslides and other related damage are very significant problems in mining areas. This type of geohazard is not easy to predict in a timely manner, especially in the case of abandoned mines. Identifying the causes of collapse, while a very delicate matter, is absolutely necessary in order to improve our understanding of such events. The detection of voids by means of geophysical methods is widely documented (Rybakov et al., 2001; Vadillo et al., 2012; Martínez-Moreno et al., 2013, 2014, 2015b). Geophysical methods, including microgravity studies, may be applied to detect cavities along
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Fig. 2. Geological setting of Candado hill. (a) Location of the study area (highlighted with red dots) in the Betic Cordillera; (b) Simplified geological map of Candado hill and surroundings. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) Modified from Vera-Peláez et al., 2004.
profiles and maps (Colley, 1963; Butler, 1984; Al-Rawi and Rezkalla, 1987; Rybakov et al., 2001; Mochales et al., 2008; Gambetta et al., 2011). In addition to defining the areas where voids extend, gravity maps allow us to determine their approximate depth and dimensions. This research was undertaken to construct a collapse susceptibility map of a disused mine area without outside access and whose dimensions and location were basically unknown. Through microgravity measurements in map distribution, we define the areas in Candado hill where the mine galleries are located and pinpoint the location of each void.
reaching up to the Middle Miocene (Chalouan and Michard, 1990; Martín-Algarra et al., 2009) (Fig. 2). The hill extends 700 × 400 m, its base being located at 65 m high, while the top reaches 125 m high. From base to top, it is composed by red clays, gypsum and conglomerates with a sandy matrix from Permo-Trias that crop out around the hill (Vera-Peláez et al., 2004). The disused mine under study is located in this layer. Over the base, there are levels of limestone and dolostone from the JurassicCretaceous with bedding that is NE–SW oriented and 20° SE dipping. The series culminates in Quaternary sands and conglomerates. 3. Methods
2. Geological setting Candado hill is located in the Malaguide Complex, which pertains to the Internal Zones of the Betic Cordillera (Azema, 1961; Serrano et al., 1995). This complex is formed by Paleozoic rocks partially affected by Variscan metamorphism and other late sedimentary sequences
This research entailed microgravity prospection positioned with differential GPS. Microgravity prospection calls for a fixed positioning, as to a ± 1 m topographic error causes a variation of ±0.3 mGal (Telford and Sheriff, 1990). Such a variation clearly exceeds the maximum permitted error when considering the values of studied anomalies.
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Fig. 3. Gravity anomaly maps of Candado hill. Regional anomaly trend (black lines) over Bouguer anomaly at a reference density of 2.67 g/cm3.
3.1. Differential GPS The equipment used in this study included the differential Global Position System (GPS) Leica 1200+ system, which uses error correction data to fine-tune the position of each measurement station with an accuracy of ±0.5–20 mm. The GPS is connected with the Andalusian Positioning Network, known as RAP (Junta de Andalucía, 2015). This equipment incorporates the stakeout program, which enables one to create measurement grids with an equidistance of 10 × 10 m. New additional position measurements allowed us to improve the previous digital terrain model (DTM) and calculate a more accurate DTM for terrain correction. The system used for GPS data was UTM and the datum of reference was ETRS89–Zone 30N. 3.2. Microgravity measurements Geophysical gravity methods make it possible to determine underground density changes through the measurement of local gravity variations. These differences are caused by the presence of bodies whose density contrasts with that of the host rock —i.e. cavities with a density of ~0 g/cm3, or filled by water ~1 g/cm3, enclosed within host rock of ~2.67 g/cm3. To acquire gravity data we used a Scintrex Autograv CG5 on a tripod, with an accuracy of ± 0.001 mGal. The measurements were taken at 317 stations with a spacing of 10 × 10 m over a regular grid in opencast and non-constructed areas, while in constructed areas the measurements were taken along the streets with roughly the same spacing. All proper practices to enhance accuracy in
microgravity surveys were taken into account: cycles shorter than 3 h; before measurements, the gravimeter was leveled and calibrated on the tripod to minimize hysteresis effects; measurements in each station were repeated during 60–90 s until minimizing the obtained error under ± 0.005 mGal. After tidal and instrumental corrections, the Bouguer anomaly was determined using the standard density of 2.67 g/cm3, close to the average density of the lithology of the studied area. The terrain correction consists of removing the visible topography. It was calculated by means of the Hammer circle method (Hammer, 1939, 1982) using our local DTM combined with the IGN digital terrain model (Instituto Geográfico Nacional, 2005), with 5 m horizontal resolution. The maximum radius covered during terrain correction was from 2 m to 9902 m around each gravity site (zones B to K). Accurate analysis of results requires extracting the residual gravity anomaly from the Bouguer anomaly. For this purpose, the regional anomalies were removed through polynomial regression first order on the Bouguer anomaly tendency (Martínez-Moreno et al., 2015a) due to the elongated NW–SE morphology of the anomaly. In addition, we calculated different residual gravity maps with density variation (both Bouguer anomaly and terrain correction) from 2.5 to 2.7 g/cm3, to perform analysis of the density variation for local studies near areas with voids. Furthermore, we performed forward models along perpendicular profiles crossing the center of the hill. These anomalies were modeled by means of GRAVMAG v.1.7, from the British Geological Survey (Pedley et al., 1993), with 2.5D approximation according to the
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Fig. 4. Residual anomaly maps with different terrain corrections calculated from several densities. The topography is marked over the residual anomaly maps with white lines.
geological information. The 2.5D modeling allowed us to assign perpendicular extensions to the modeled bodies. 4. Results The obtained results are analyzed in light of the Bouguer anomaly map of the study area and shallow density variations (residual anomaly maps). Comparison of the different densities used in the terrain correction and Bouguer anomaly is achieved. Finally, we propose forward models corresponding to the position and shape of the mine.
correction with different density values (Fig. 4). The values were between 2.5 g/cm3 and 2.7 g/cm3, with steps of 0.05 g/cm3, reaching the maximum density assigned to carbonates. The resulting maps show the most pronounced residual minima values to be located at the center of the hill, just slightly displaced to the NW of the highest topography. The residual minima produced by the presence of mine galleries are larger for the high density values (2.7 g/cm3) than for the low ones (2.5 g/cm3). The minima shape is similar for all the maps, yet with varying extension. 4.3. Residual anomaly map and microgravity forward models
4.1. Bouguer and regional anomaly maps The gravity results show a Bouguer anomaly (Fig. 3) presenting a linear NW–SE trend with values ranging from 42.2 to 43.7 mGal, which increases from SW to NE. In the center of Candado hill the lineal tendency is interrupted by low gravity values. This regional anomaly is produced by density variations in depth. 4.2. Residual anomaly maps with different terrain corrections The appropriate density for a hill made up of carbonates and voids was obtained through calculation of the Bouguer anomaly and terrain
Our analysis of the residual gravity anomaly produced for the mine galleries is achieved considering a standard density of 2.67 g/cm3. The anomaly map (Fig. 5) shows maxima values from 0.3 to 0.4 mGal at the N and S edges, and minima values of −0.1 mGal enclosing a more pronounced minimum from − 0.3 to − 0.5 mGal at the center of the hill. These minima correspond to the presence of mine galleries in the area. To support our analysis of the minima, we calculated 2.5D models along two perpendicular profiles crossing the main minimum of the study area (Figs. 5 and 6). For both profiles, two main densities were considered: 2.67 g/cm3 belonging to the carbonates of the hill, and
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Fig. 5. Residual anomaly maps for a terrain correction with standard density of 2.67 g/cm3. The position of the modeled profiles is marked with white lines.
0 g/cm3 assigned to the unsaturated voids. Profile 1 (Fig. 6a), with NW– SE direction and 200 m-length, presents a principal gravity minimum of − 0.55 mGal in its central part, and local minima of − 0.3 mGal at 50, 120 and 160 m-lengths. The minima having shorter wavelengths were adjusted by modeling shallow smaller voids (5–10 m-long in the profile direction, and 10–20 m perpendicular to it). The longest wavelength
located along the profile was adjusted with a deeper and bigger void, which has dimensions of 90 m long in the profile direction and 80 m long perpendicular to the profile direction. Profile 2, having a NE–SW direction, shows a residual minimum of −0.5 mGal located between 60 and 200 m. Three additional local minima with lower wavelengths were identified at 30, 120 and 155 m in length. The main minimum
Fig. 6. Microgravity models along the study profiles showing the galleries marked in Fig. 4. (a) NW–SE profile and (b) SW–NE profile.
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Fig. 7. Collapse susceptibility map over (a) residual anomaly map and (b) orthophoto. The red line highlights the main gallery and the yellow line marks the area having shallower voids. Purple circles mark the areas where major collapse occurred. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
was adjusted with a void located 50 m-deep, 80 m-length and 90 m perpendicular; the secondary minimum was modeled with a shallow void, 20–40 m-length and having a perpendicular extension of 10–20 m. 5. Discussion 5.1. Microgravity studies in void location Microgravity enables one to detect underground voids in different contexts. This method was applied to cross-sections of maps elaborated from gravity data that highlight cavities as gravity minima, showing different values and wavelengths depending on the gallery dimensions and depth. Equidistant spacing of the measurement stations is necessary to obtain useful maps fitting the analysis of an anomaly produced by voids (Leucci and De Giorgi, 2010). It is furthermore possible to obtain cavity morphologies as well as depth estimations through forward modeling (Beres et al., 2001; Mochales et al., 2008; Martínez-Moreno et al. 2014), ideally in conjunction with other geophysical methods. Our microgravity study resulted in a residual anomaly map (Fig. 5) highlighting minima associated with the position of voids. Still, the morphology, depth and dimensions were unknown. Independent and simultaneous 2.5D forward modeling of the anomaly along the two profiles served to obtain approximate real morphologies. In both models, the perpendicular lengths of the mine are concordant, the cave points coincide, and the observed and calculated anomaly match. These models reflect contours of the residual anomaly map that fit the surface projection of the mine area. 5.2. Analysis of different densities applied to the terrain correction A detailed terrain correction in this research study was deemed essential to obtain reliable results (Debeglia and Dupont, 2002). Such calculations call for an adjusted DTM due to the fact that nearby
topography substantially influences the final anomaly maps (Martínez-Moreno et al., 2015a). In this case we used a DTM of 5 × 5 m spacing improved with acquired differential GPS measurements. In addition, the density value applied in the terrain (Hammer, 1939, 1982) and Bouguer anomaly corrections are determinant for the reliability of results. While in typical study areas a standard density value of 2.67 g/cm3 can be applied as the initial approximation, we applied different values, from 2.5 g/cm3 to 2.7 g/cm3, as the possible densities associated with a hill of the given lithologies (Fig. 4). Firstly, results show that the terrain correction is well calculated, because the features do not only correlate with the main topography of the hill. The most pronounced minima are not located at the highest topographies, but rather to the NW. The contour anomaly shapes are likewise not restricted to the topography shape. In most of the resulting maps (Fig. 4), it is seen that high densities applied in the terrain correction imply more highlighted and expanded minima for the residual gravity anomaly, pertaining to the mine gallery location. Whereas the topographic correction gives higher values for higher densities in gravity calculation, the Bouguer anomaly shows a reversed and more pronounced sense. Accordingly, the higher density values correspond to more pronounced minima obtained in the residual anomaly map. However, as the average density of unaltered limestones and dolostones is close to 2.67 g/cm3, we applied this standard density to obtain the most suitable residual anomaly map (Fig. 5).
5.3. Collapse susceptibility map based on microgravity results Susceptibility maps for construction collapse may be based on Geographic Information System (GIS) (Yilmaz, 2007), geomorphic factors (Papadopoulou-Vrynioti et al., 2013) or sinkholes (Galve et al., 2009b). It is less common for susceptibility maps to be derived from microgravity to highlight areas where collapse might occur owing to the presence of voids.
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In this case, the residual anomaly map was considered together with the forward models of the mine galleries to arrive at a collapse susceptibility map underlining key areas related to the mine galleries (Figs. 6 and 7). The extension and position of the galleries along the profiles determine the anomaly contour (Fig. 7a): that of − 0.15 mGal (marked with red line in Fig.7) fits the mine limits and restricts the high collapse susceptibility area. Further, shallow voids in the hill define a lower collapse susceptibility area (−0.05 mGal, yellow line in Fig.7). These contours can be highlighted over the orthophoto to verify the marked areas (Fig. 7b). The mine is seen to lie mainly E of the construction area. However, on the W and NW sides of the hill the red line is located over some buildings. The areas marked with purple circles in Fig. 7b, matching the red line and constructions, indicate major collapses and destruction on the hill. 6. Conclusions The presence of a disused mine beneath Candado hill originated construction collapse along the W side of the hill. The gypsum mine comprised several galleries, but the lack of outside access impedes determination of its morphology and position. Accurate microgravity research to the hill provided new data for assessing its collapse susceptibility. Density analysis, applied to the terrain correction and Bouguer anomaly, revealed that the optimal values of the host rocks is close to the standard 2.67 g/cm3. Two profiles were modeled crossing each other to derive the approximate gallery morphology, dimensions and depth. Altogether, our results produce a collapse susceptibility map supported by the residual gravity anomaly contours, highlighting areas where collapses could occur. Moreover, a second wide area is marked, where shallower and smaller voids might also originate local collapse. The area undergoing the principal construction collapse is underlined, matching the highly susceptible area. This study serves to confirm that gravity research is an adequate tool for detecting voids and elaborating collapse susceptibility maps that condition future construction in areas where subsoils may contain anthropic or natural voids. Acknowledgements We thank Jean Sanders for reviewing the English grammar. Our research was supported by projects CGL2010-21048, P09-RNM-5388 and RNM148. In addition, we are grateful with the positive comments and reviews made by the two anonymous reviewers who significantly helped to improve the manuscript. References Al-Rawi, F.R., Rezkalla, J.S., 1987. The application of microgravity survey for cave detection in a karstic area. Large Rock Caverns, Proceedings of the International Symposium, Helsinki, Finland. 2, pp. 25–28. Azañón, J., Puertas, E., Ureña, C., Gallego, R., Romero-Gómez, F., 2012. Collapse in a rock massif induced by lateral confinement loss: the case of Montegolf (Málaga, Spain). EGU General Assembly Conference Abstracts, p. 12527. Azema, J., 1961. Etude géologique des abords de Málaga (Espagne). Estud. Geol. 17, 131–160. Bell, F.G., Cripps, J.C., Moorlock, B.S.P., Culshaw, M.G., 1992. Subsidence and ground movements in chalk. Bull. Int. Assoc. Eng. Geol. 45, 75–82. Bell, F.G., Donnelly, L.J., Genske, D.D., Ojeda, J., 2005. Unusual cases of mining subsidence from Great Britain, Germany and Colombia. Environ. Geol. 47, 620–631. Bell, F.G., Stacey, T.R., Genske, D.D., 2000. Mining subsidence and its effect in the environment: some differing examples. Environ. Geol. 40, 135–152. Beres, M., Luetscher, M., Olivier, R., 2001. Integration of ground-penetrating radar and microgravimetric methods to map shallow caves. J. Appl. Geophys. 46, 249–262. Butler, D.K., 1984. Microgravimetric and gravity gradient techniques for detection of subsurface cavities. Geophysics 49, 1084–1096. Calaforra, J.M., Pulido-Bosch, A., 1999. Gypsum karst features as evidence of diapiric processes in the Betic Cordillera, Southern Spain. Geomorphology 29, 251–264. Chalouan, A., Michard, A., 1990. The Ghomarides nappes, Rif coastal range, Morocco: a Variscan chip in the Alpine belt. Tectonics 9, 1565–1583.
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