The Ischia volcanic island (Southern Italy): Inferences from potential field data interpretation

Journal of Volcanology and Geothermal Research 179 (2009) 69–86

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Journal of Volcanology and Geothermal Research j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j vo l g e o r e s

The Ischia volcanic island (Southern Italy): Inferences from potential field data interpretation V. Paoletti a,⁎, R. Di Maio b, F. Cella c, G. Florio b, K. Motschka d, N. Roberti b, M. Secomandi a, R. Supper c, M. Fedi b, A. Rapolla a,b a

C.U.G.RI., C/O Laboratorio di Geofisica, Complesso Universitario di Monte Sant'Angelo, Via Cintia, I-80126 Naples, Italy Dipartimento di Scienze della Terra, Università di Napoli Federico II, Largo S. Marcellino 10, I-80138 Naples, Italy Dipartimento di Scienze della Terra, Università della Calabria, I-87036 Arcavacata di Rende (CS), Italy d Department of Geophysics, Geological Survey of Austria, Rasumofskygasse 23, A-1031 Vienna, Austria b c

a r t i c l e

i n f o

Article history: Received 2 June 2008 Accepted 15 October 2008 Available online 28 October 2008 Keywords: high-resolution aeromagnetic survey self-potential survey gravity data geothermal system Phlegrean Fields volcanic district

a b s t r a c t In this paper we present a study of the structural setting of the volcanic island of Ischia by the analysis and interpretation of high-resolution aeromagnetic and self-potential data recently acquired over the island. The magnetic data allowed us to locate the main anomaly sources and lineaments of the island and its offshore surroundings, while the self-potential (SP) data provided information on both the structural pattern of the resurgent caldera and the high-temperature fluid circulation. An inversion of the acquired magnetic and SP data, and a joint modelling of the magnetic data and of a previous gravity data set along a SW–NE profile allowed us to build a model of the island. The model is characterized by the presence of an igneous–very likely trachytic–structure, whose top is located at 1200–1750 m b.s.l. Such a body, possibly formed by several neighbouring intrusions, has a density contrast with the pyroclastic cover of about 0.4 gr/cm3 and its centralwestern part, below Mt. Epomeo, seems to be demagnetized. The demagnetization should be connected to the high geothermal gradient measured in this portion of the island and may be due to hydro-chemical alteration processes and/or to the possible presence of partially melted spots within the intrusion. Our outcome is consistent with the results of previous geophysical, geo-volcanological and geothermal studies. © 2008 Elsevier B.V. All rights reserved.

1. Introduction The study of potential field (magnetic, gravitational and selfpotential) data of an active volcanic region can yield useful information about the subsurface distribution of magnetization, density and electrical charge occurrence, allowing significant insights into the geostructural and volcanological features of the area. Magnetic and gravimetric surveys have been shown to be effective in several regional and near surface applications, such as mapping of lineaments and faults and modeling of outcropping or buried structures. This seems to be especially significant in volcanic regions, where loose volcanic sediments often cover prominent volcanic structures. These areas are therefore normally characterized by the overall effects of regional and local potential field anomalies that can be effectively separated and investigated by means of filtering and enhancement techniques. In this framework, the use of aeromagnetic surveys can lead to major advantages with respect to ground magnetic surveying, i.e.: (i) a higher speed in collecting data, which implies lower costs and ⁎ Corresponding author. E-mail address: [email protected] (V. Paoletti). 0377-0273/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jvolgeores.2008.10.008

reduced effects on data of the magnetic field time variations; (ii) a better spatial coverage with respect to ground-based magnetic surveys, allowing surveying of areas inaccessible to ground work; (iii) a variable spatial resolution of the data, as the flight altitude can be tuned to favor imaging of magnetic effects from structures of different sizes and depths. Self-potential surveys, on the other hand, can fruitfully put in evidence the main shallow-sited electrical charge polarisation, most likely caused by rock-water-magma interactions in active areas, and, more generally, by invasion and circulation of hot fluids in porous media and/or along permeable fracture systems. We can mention several examples over the last 30 years of successful applications of aeromagnetic, gravimetric and self-potential surveys to the study of volcanic and geothermal regions. These methods have been used to: infer the structure of volcanic cones and/ or calderas and their magnetic, density and electrical properties (e.g., Ballestracci, 1982; Flanagan and Williams, 1982; Lénat and Aubert, 1982; Finn and Williams, 1987; Davy and Caldwell, 1998; Mickus and McCurry, 1999; Malengreau et al., 1999; Arana et al., 2000; Della Monica et al., 2004; Ueda, 2007), characterize the geological and structural setting of volcanic and geothermal areas (e.g., Mickus and Durrani, 1996; Di Maio et al., 1998; Johnson, 1999; Rybakov et al., 2000; Soengkono, 2001, Chandrasekhar et al., 2002; Ferraccioli et al.,

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2002; Cassidy et al., 2007; Bektas et al., 2007; Loera et al., 2008) and evaluate the hazards of these regions by identifying and mapping hydrothermally altered zones (e.g., Zohdy et al., 1973; Ercan et al., 1986; Massenet and Pham, 1985; Di Maio et al., 1997; Michel and Zlotnicki, 1998; Finn et al., 2001; Finizola et al., 2002; Finn and Morgan, 2002; Okubo et al., 2005). Despite the risk characterizing the Ischia volcanic island (Campanian Plain, Southern Italy) and the complexity of its geo-volcanological and structural setting, only a small amount of information derived from geophysical data is presently available. Therefore, in light of the lack of detailed geophysical data sets in the area, a new highresolution aeromagnetic survey was carried out above the island and its nearby surroundings in October 2005. Moreover, to back-up the information yielded by the magnetic data, an in-land self-potential survey was performed in May 2006. In this paper we present a study of the geo-structural setting of the island of Ischia by analysis, 2D and 3D inversion, and modeling of the newly acquired aeromagnetic and self-potential data. Our interpretations are supported by the modeling of a previous gravity data set and by taking into account temperature data measured in deep thermal wells. A joint interpretation of all the resulting information allows us to build a 2D potential field model of the island. 2. The Ischia island 2.1. Geological and volcanological framework The island of Ischia represents the westernmost active volcanic field of the Campanian Plain (Southern Italy), a K-alkaline volcanic area that includes Phlegrean Fields, the Procida island and the Mt. Somma-Vesuvius complex (Fig. 1a). The Campanian Plain is a PlioQuaternary NW–SE trending depression, about 5 km deep, bordered by Mesozoic limestone platforms (e.g., Fedi and Rapolla, 1987). The volcanic activity in this area, which started in Upper Pleistocene accompanying extensional processes, is controlled by regional strain fields along NE–SW- and subordinately NW–SE-trending fractures (Vezzoli, 1988). The island of Ischia is the emerged top of a large volcanic complex located along this NE–SW trend (Fig. 1b). Bruno, de Alteris, and Florio (2002) hypothesize that Ischia represents only the remnant of a once larger volcanic complex and points out the importance of regional tectonics on the patterns of magma uprising. Acocella and Funiciello (1999) interpret the NE–SW lineaments as transfer faults with sub-vertical fault planes which could be easily intruded by magma. The Ischia offshore area is characterized by several monogenetic edifices aligned along the NE–SW system of faults that connects the South-Eastern sector of Ischia to the Island of Procida and to Phlegrean Fields (Di Girolamo and Stanzione, 1973; de Alteris and Toscano, 2003). These edifices have been mainly formed through subaqueous explosive eruptions (hyaloclastites) of potassic magma with latitebasaltic-latitic composition (Di Girolamo and Rolandi, 1975), but a few lava domes can also be found (Di Girolamo et al., 1984). Ischia extends over a surface of about 42 km2 and is morphologically dominated by Mt. Epomeo (787 m a.s.l.) located in its central part, while a lowland is located in the north-eastern sector of the island (Fig. 2). The island is composed of volcanic rocks (trachytes, alkali-trachytes, trachybasalts, latite, phonolites) (Civetta et al., 1991; Orsi et al., 1991), marine sediments and landslide deposits. The presence of all these products is connected to the interplay of tectonism, volcanism, erosion, sedimentation and slope instability in the course of the history of the island, as pointed out by De Vita et al. (2006). In particular, their study showed that the caldera resurgence has to be considered as a main factor for inducing intense slope instability, as it is accompanied by activation of faults and renewal of

volcanism that produces over-steepening of slopes and generates seismicity that could trigger landslides. An active resurgent block, uplifting about 1000 m in the last ~30 ka (Gillot et al., 1982), characterizes the island. Carlino et al. (2006) proposed that the uplift, the volcanic and seismic activity and the ground deformation of the past 2000 years are connected to the existence of a laccolith. This body, with a hypothesized diameter of 10 km and a depth of up to 1 km in the centre of the island, is thought to have triggered the caldera resurgence after the Mount Epomeo Green Tuff eruption (55 ka years ago) and to have controlled the dynamic of the island afterwards. Ischia is crossed by four main fault systems (Vezzoli, 1988; Molin et al., 2003): i) a ENE–WSW to E–W striking system in the northern and southern sector of Mt. Epomeo; ii) a NE–SW system cutting the deposits of the eastern sector; iii) a NW–SE striking fault in the southwestern corner of the island; iv) a NNW–SSE to N–S system bordering the eastern and western side of Mt. Epomeo. This structural setting seems to result from the interplay between regional and local tectonics as suggested by Acocella and Funiciello (1999), who inferred an interaction between the Mt. Epomeo resurgent block and the NE– SW and NW–SE regional lineaments. The most important phenomena recorded recently on the island are low intensity and shallow seismicity, bradyseismic events and hydrothermal activity. The island has indeed always been characterized by strong heat flows and hydrothermal manifestations (Penta, 1954; Ippolito and Rapolla, 1982; Panichi et al., 1992). The highest temperature (above 200 °C) was measured at 1051 m of depth in a deep well on the western side of the island, while data from shallow wells located on the same side of the island (Penta and Conforto, 1951) suggest that the hydrothermal circulation concentrates in the highly fractured lavas underlying the Green Tuff. Chiodini et al. (2004) hypothesize that the hydrothermal system consists of two main reservoirs: a shallower reservoir, with a temperature of about 250 °C, located at 100–300 m b.s.l., and a deeper one with a temperature of about 300 °C. Most of the hydrothermal activity in the resurgent block area is located along the northern, western and southern borders (Di Napoli et al., 2007) (Fig. 3), where hot springs and fumaroles are aligned along N–S, NE–SW, NW–SE and E–W faults bordering the block (Molin et al., 2003). The main paths for the uprising vapours are the NW–SE striking normal faults bounding the western side of Mt. Epomeo. These faults connect highly fractured lavas, in which the main hydrothermal aquifers are likely located, with the surface and cross the more impermeable pyroclastic cover made of Green Tuff and Citara Tuff (Chiodini et al., 2004). As potential field data are strictly linked to the structural features of the underground and to the inside hydrothermal circulation, their modelling and interpretation could play a key role in validating existing hypotheses derived from geo-structural and geochemical studies. 2.2. Previous geophysical studies As mentioned, the amount of information derived from geophysical data–and in particular potential field data–related to Ischia island is rather poor. So far the only aeromagnetic data available for the Ischia area refer to a survey carried out at regional scale (flight altitude of 2.5 km and line spacing of 2 km) (AGIP, 1981) (see Fig. 4a). The data outlined a prominent magnetic anomaly in the southwestern offshore of the island (partially shown in Fig. 4a) and a smaller anomaly in the south-eastern area of the island. A modeling of these regional data and of in-land gravity and magnetic data sets (Nunziata and Rapolla, 1987) pointed out the presence of a pyroclastic unit that contains local domes and lava flows of higher density and magnetization with a thickness of a few hundreds of meters. These structures lie on a trachytic basement that

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Fig. 1. a) Campanian Plain volcanic area; b) Topographic and bathymetric map of the Phlegrean Fields-Ischia ridge. The main structural features are redrawn from Di Girolamo and Rolandi (1975) and de Alteris et al. (2006): white lines are faults, black lines are ridges, red circles are crater rims, asterisks are latitebasaltic and latite volcanoes. The yellow square shows the area considered in this study. (For interpretation of the references to colour in figure legends, the reader is referred to the web version of this article.)

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can be interpreted as an ellipsoidal structure elongated in the E–W direction and with a shallowest depth of about 1000 m b.s.l. Fig. 4b shows the magnetic field measured by the above cited authors during an in-land survey. Later, in the framework of a study on the crustal structure of the Ischia-Phlegrean geothermal fields, Rapolla, Fedi, and Fiume (1989) modelled the aeromagnetic anomaly observed in the southwestern offshore of Ischia (Fig. 4a) with an igneous body whose top is about 2.5 km deep. The authors attributed the relatively low amplitude of the magnetic anomaly characterizing the island to a demagnetization effect due to the high geothermal gradient connected to the hydrothermal activity of the area. Orsi et al. (1999) carried out a 3D modeling of the regional aeromagnetic field of the whole Phlegrean volcanic district. Taking into account magnetic, heat flow and volcanological information, the authors suggested the existence of a partially solidified magma chamber below Ischia at a depth ranging from 7 km to 10 km b.s.l. This result is in agreement with former hypotheses on the deep structures of the area (Rapolla et al., 1989).

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A recent study of the magnetic field of the whole Neapolitan district, performed by merging different high-resolution data sets, resulted in an overall view of the main features of this volcanic region, including the island of Ischia (Paoletti et al., 2005). However, as the magnetic data regarding Ischia used in all the above mentioned studies were derived either from an in-land survey with an irregular sample spacing or from a low resolution aeromagnetic survey, the information yielded are in many cases not complete and/or sketchy. Geodetic data recorded between 1913 and 1967 show a maximum downward movement of 30 cm in the central and southern areas of the island and a few cm uplift in its northern sector (Maino and Tribalto, 1971). During bradyseismic events of 1967–1984 and 1987– 1994 only downward movements were reported all over the island (e.g., Luongo et al., 1987; Civetta et al., 1998). A recent analysis of the island surface deformation using spaceborne radar interferometry (Manzo et al., 2006) showed that the present-day subsidence of Ischia mainly occurs in areas characterized by active landslides and along faults. The study suggests that the deflation of the island is related to

Fig. 2. Geologic sketch map of the Ischia island and its nearby offshore surroundings showing the lines flown during the helicopter-borne survey of October 2005. The geovolcanological features of the island are from Vezzoli (1988), ABNOC (2002) and De Vita et al. (2006). The lava and/or tuff deposits of the offshore areas are from Budillon et al. (2003), the island structural lineaments (blue lines) are from De Vita et al. (2006). The blue dots show the thermal wells considered in our study (Penta and Conforto, 1951; Penta, 1954; Ippolito and Rapolla,1982).

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Fig. 3. Location of the SP in-land survey performed in the Ischia island. Measure points are marked by yellow dots. Structural lineaments of the island (green lines - De Vita et al., 2006) and temperature trends coming from thermal wells and hot springs (Di Napoli et al., 2007) are also shown.

the de-pressurization of its hydrothermal system. Sepe et al. (2007) report levelling data collected on the flanks of Mt. Epomeo from 1990 to 2003 that show a subsidence of the mount. Basing their hypothesis on dislocation models constrained by geological and geochemical information, the authors ascribe this subsidence to crack closure processes along two main ENE-WSW and E-W faults, induced by a decrease in the fluid pressure within the hydrothermal system. As regards the offshore surroundings of the island, Bruno et al. (2002) and de Alteris et al. (2002) analysed 600 km of recent seaborne seismic and magnetic profiles in the western offshore area. This study highlighted unknown E–W and NE–SW volcanic ridges that follow the regional structural patterns. The authors suggest the presence of a large cooled magnetic system with deep dykelike intrusions along the NE–SW direction.

3. Survey layouts and data preprocessing 3.1. Aeromagnetic survey The high-resolution helicopter-borne magnetic survey, carried out in October 2005, covered the entire area of the island of Ischia, extending also into the offshore zones (Fig. 2). The flight lines, with a N–S azimuth, were spaced 300 m apart, while the cross-track tie lines, with an E–W azimuth, were spaced about 4 km apart. The sample spacing along each flight line was about 5 m. The survey was flown at a constant terrain clearance of about 300 m. In order to avoid numerical problems associated with the different sampling rates along the N–S and E–W directions, we re-sampled the data along each line with a spacing of 50 m.

Fig. 4. a) Airborne magnetic data collected in the Ischia area by Agip (1981) during a regional survey; b) Magnetic field measured by Nunziata and Rapolla (1987) during an in-land survey; c) New high-resolution aeromagnetic data of the Ischia island and its offshore surroundings acquired in 2005.

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The instrumentation used for the survey was supplied by the Geological Survey of Austria and consisted of a ground and a flight section. The ground section contained a magnetometer used to monitor the external field activity during the flights and a GPS reference station used for the differential correction of satellite data. The flight section consisted of: (1) a caesium magnetometer having a sensitivity of 0.01 nT, which was contained in a “bird” hung 30 m below the helicopter, (2) a GPS sensor for the horizontal positioning of the helicopter, having a precision of ± 5 m after correction by satellite data, (3) a laser-altimeter for the vertical positioning of the helicopter, (4) a computer for data acquisition and (5) a video-camera. The pre-processing of the aeromagnetic data set measured over the island of Ischia included the following steps: (1) removal of spikes and gaps in the data; (2) flight path check and repositioning, which consisted of the removal of wrong coordinates, correction of the GPS data, and check of the flight altitude; (3) Earth's magnetic field diurnal variation correction, which was performed using the local base station data; (4) removal of the International Geomagnetic Reference Field (IGRF), performed using the Italian Geomagnetic Reference Field updated for 2000 (De Santis et al., 2003); (5) statistical levelling, consisting of a minimization of the differences between the field values measured at the crossing points of flight lines and tie lines. 3.2. Self-potential survey The self-potential (SP) survey was carried out in-land in May 2006, along the circuits indicated in Fig. 3. The data were collected by measuring the potential difference between two consecutive positions (leapfrog technique) of grounded copper rod electrodes. The mutual distance between every two consecutive electrodes was taken equal to 100 m. A KEITHLEY digital multimeter, equipped with a 20 channel multiplexer, was used. A total of 365 SP drops, distributed over a linear pattern about 37 km long, were measured. The adopted acquisition technique involved averaging the data set sampled at a frequency of 1 Hz over a period of 15 min. In order to eliminate drift and/or locally spurious signals, the closed circuits have been surveyed by turning back to the initial point and the mis-tie has been equally distributed over all dipolar SP drops. In the case of open circuits, the SP measurements have been carried out in forward and backward directions and for each dipole the corrected SP drop has been calculated by averaging the forward and backward values. Spurious effects due to the possible presence of electrode polarization have been removed following the procedure described in Di Maio et al. (1996). As Fig. 3 points out, the survey was designed in such a way to assure at least one common point between adjacent circuits, in order to evaluate, from the SP drops across the dipoles, the point by point SP values. In practice, these values were calculated by attributing the SP zero value to a reference point, corresponding, in this case, to the position of the first electrode of the measurement network. Then, for a better visualization of the anomaly contrasts that characterize the surveyed area, we preferred to generate a new SP data set by subtracting from each value of the previously calculated set its average value. 4. Data presentation, analysis and inversion 4.1. Aeromagnetic data The aeromagnetic data obtained after the data correction procedure were gridded to an interval of 150 m by using the Kriging algorithm. The map obtained (Fig. 4c) is characterized by a number of prominent anomalies with amplitude of 400–600 nT, both onshore and offshore. Some of the anomalies onshore seem to correlate with the main geovolcanological characteristics of the island, such as domes and lava flows, and of the offshore areas (see Fig. 1b and 2 for reference). Other

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anomalies, onshore and offshore, seem instead uncorrelated to both geological and topographical/bathymetric features. A comparison between the new data and two previous data sets measured during regional air-borne (AGIP, 1981) (Fig. 4a) and in-land (Nunziata and Rapolla, 1987) (Fig. 4b) surveys shows how the new aeromagnetic data allow a much more detailed definition of the characteristics–amplitude and shape–of the anomalies of the island and its offshore areas. In order to locate the position of the magnetic sources of the observed anomalies, we computed a pole-reduced map (Fig. 5), using the direction of the present inducing field in the area (inclination = 60°; declination = 0°) for the induced and total magnetization vectors. As we can see, the pole-reduced anomalies correlate fairly well with the volcanological features of the island. More specifically, we notice that the main maxima of the field are placed in correspondence with most of the lava deposits younger than 10 ka, such as Zaro, Mt. Rotaro, Mt. Trippodi and the deposits on the coast between Pt. Imperatore and Capo Negro. These anomalies show an amplitude of about 400 nT, 750 nT, 300 nT and 450 nT, respectively. The Arso lava flow on the other hand, seems not to be associated with any relevant anomaly. This fact could be related to a low thickness of this lava deposit. Smaller anomalies, showing amplitudes of about 60 nT, 100 nT and 280 nT are located over the lava deposits older than 55 ka of Ischia Ponte, Sant'Angelo and Mt. di Vezzi, respectively. The area of Mt. Epomeo is characterized by an anomaly of about 340 nT, connected to the tuff deposits of the area. Regarding the offshore areas, we notice only a few relevant anomalies: one is located in correspondence with the shoal of Forio, exhibiting an amplitude of 280 nT; other two maxima are placed in the offshore area west of Procida. These anomalies are very likely connected to the Formiche di Vivara volcano (amplitude of 160 nT) and to the newly discovered Punta Serra submarine volcano (amplitude of about 500 nT) (see Paoletti et al., in press). The cited authors found that the two anomalies are connected to two dyke-like igneous bodies extending to a depth of several hundred meters b.s.l. These bodies have a maximum magnetization ranging from 2 to 2.4 A/m for Formiche di Vivara volcano and from 3.3 to 3.6 A/m for the Pt. Serra submarine volcano. To locate the lateral boundaries limiting the magnetic sources of the Ischia island, we used the location of maxima of the horizontal gradient of the pole-reduced field that, as shown by Cordell and Grauch (1985), are located above changes of magnetization. The horizontal gradient method assumes that the boundaries can be approximated as single, near-vertical, sharp boundaries. Therefore, when the boundaries are not represented by a vertical contact or when several boundaries are close together, the location of the gradient maximum can be offset from the boundary (Grauch and Cordell, 1987). The horizontal gradient map of Ischia (Fig. 6) clearly shows the boundaries of the lava deposits younger than 10 ka that characterize the NW and SW sides of the island, as well as the areas of Mt. Rotaro and Mt. Trippodi. The limits of the lava deposits older than 55 ka in the areas of Ischia Ponte, Sant'Angelo and Mt. di Vezzi are highlighted as well. Finally we see a number of circular or sub-circular structures in the western and eastern offshore areas; among them the most meaningful anomalies are those connected to the Formiche di Vivara and Punta Serra submarine volcanoes. It's worthwhile to note that in several cases, such as in the areas of Zaro, Mt. Trippodi and Mt. Vezzi, the boundaries highlighted by the maxima of the horizontal gradient correspond to the known faults crossing the island. The magnetic field components related to the larger and deeper sources of Ischia were studied by filtering the pole-reduced data to remove both the long-period components connected to crustal sources (with a wavelength larger than 8.6 km) and the short-period ones connected to local sources (with a wavelength smaller than 3.3 km). The cut-off wavelengths were chosen according to the spectral analysis performed by Nunziata and Rapolla (1987) on

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Fig. 5. Pole-reduced map of Ischia overlapping the geo-volcanological features of the area. For the legend of the geo-volcanological units refer to Fig. 2.

magnetic in-land data of the island. The magnetic field obtained (Fig. 7) is characterized by two maxima, a bigger one in the central-eastern sector of the island and a smaller one in the south-western offshore of the island. These two maxima are separated by a minimum located approximately in correspondence with Mt. Epomeo. In order to have an estimation of the geometric and magnetization characteristics of the anomaly source, we carried out a three-dimensional inversion of the filtered magnetic field of Fig. 7 and computed Generalized Singular Value Decomposition (GSVD) solutions (see, e.g., Hansen, 1998). We used a nonparametric discretization of the inverse problem and assumed a source volume of specified depth and horizontal extent, in which the solution is piecewise constant within a three-dimensional grid of prisms (Fedi et al., 2005; Fedi et al., 2007). In the adopted inversion approach the a priori information is incorporated in the regularization matrix in the form of high-order derivatives in the x, y and z directions. Provided that enough SVD components are available for the solution–i.e., the problem is not strongly under-determined and the data noise level is not too high–and a suitable regularization matrix is used, depth resolution can be obtained without resorting to any subjective choice of depth weighting. The discretization used for the inversion is composed of 11 × 11 × 11 prisms in the x, y, and z directions respectively, i.e., the solution consists of N = 1331 prisms. The dimension of each prism is 1459 m in the x and y direction and 500 m in the z direction. For data points, we used P = 11449 data points arranged in a 107 × 107 grid, covering an area of 16.05 × 16.05 km2. The data points are located 300 m above the terrain. The reconstruction obtained with 500 GSVD components (Fig. 8) shows the presence of a magnetic body in the eastern sector of the island characterized by a maximum magnetization of 1.8 A/m.

This value corresponds to a susceptibility of 0.004 c.g.s. and is compatible with the presence of igneous material. The body's depth-to-the-top is about 1500 m b.s.l. and the structure extends down to about 3.5 km b.s.l. As mentioned, Nunziata and Rapolla (1987) and Carlino et al. (2006) hypothesize the existence of an ellipsoidal trachytic laccolith elongated in the E–W direction with a shallowest depth of about 1000 m b.s.l. in the center of the island. We can therefore argue that the magnetic body in the eastern sector of Ischia, shown by the inversion, represents the most magnetized part of this laccolith. On the other hand, the absence of distinct and meaningful magnetic anomalies in correspondence with Mt. Epomeo (see Figs. 4, 5, 7) and the outcome of the inversion seem to indicate that this laccolith is not magnetized in the central part of the island. The inversion solution highlights also two deeper sources in the west and east offshore areas of the island whose depth-to-the-top is about 2.5 km and that could be related to crustal structures. 4.2. Self-potential data Fig. 9 shows the elaborated SP final map obtained by the Kriging algorithm with a space interval of 100 m. It outlines long wavelength anomalies all over the island, and significant high frequency anomalies in the Northern sector. A qualitative interpretation of the data was performed comparing the observed SP field with the structural lineaments of the resurgent caldera (De Vita et al., 2006) and with the geothermal anomalies identified by Di Napoli et al. (2007) (see Fig. 3). The comparison seems to show a correlation between the low frequency SP anomalies and the structural discontinuities associable to the caldera boundaries. We also notice,

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Fig. 6. Horizontal gradient of the pole-reduced magnetic data of Ischia. White lines mark the boundaries of the magnetic structures highlighted by the maxima of the horizontal gradient signal.

in the Northern sector, an area characterized by a high frequency SP positive anomaly and a high geothermal gradient. It is worth pointing out that the low frequency SP anomaly patterns observed in other sectors of the island, in particular in the central and westernmost parts, could be influenced by a coarse measuring network. Although the SP measurement network would not be enough for a 3D data inversion, we have tentatively inverted our data in order to have a rough estimate of the in-depth distribution of the polarisation field sources. We adopted a tomographic inversion procedure (Di Maio and Patella, 1994; Patella, 1997a) taking in account the topographic effects (Patella, 1997b). Briefly, the 3D tomographic approach aims at the recovering of the underground electric charge distributions through a cross-correlation procedure between the observed natural electric field and a theoretical scanner function, representing the synthetic SP response on surface of an elementary positive charge with unitary intensity, located in any generic point of the subsoil. Then, by a normalisation procedure of the cross-correlation operator based on Schwarz's inequality, it is possible to define a charge occurrence function, constrained to vary inside the interval [−1,1]. Positive values are the result of a major influence from positive charge accumulations, while negative values result from negative charge concentrations. Finally, by a suitable 3D grid distribution of the retrieved values in the investigated volume, the tomographic image can be drawn for inspecting the pattern of the in-depth polarisation source centres. Fig. 10 shows the 3D tomographic inversion of the SP data displayed in the map of Fig. 9, and here reported again at the top of the figure. The tomography refers to images between 300 m a.s.l. and 1500 m b.s.l. Starting from about 300 m b.s.l., a double dipolar trend, approximately NE–SW oriented, seems to outline, whose maximum

positive charge accumulation appears localised at a depth of about 1200 m b.s.l. in correspondence with Mt. Epomeo relief. The trend of the positive and negative clusters could be ascribable to induced polarisation charges along the nearly NW–SE fracture systems (see Fig. 3) characterized by a conspicuous fluid circulation. Moreover, the lack of primary charge nuclei in the northern area, attributable to geothermal fluid flows, suggests their occurrence below about 300 m a.s.l. 5. 2D analysis of magnetic and SP data The outcome of the inversion of the magnetic and SP data (see Figs. 8 and 10, respectively) shows that the central-western part of the Ischia island, corresponding approximately to Mt. Epomeo, is characterized by a minimum in the magnetization values and a maximum in the positive electric charge occurrence. In order to investigate this result, we considered a gravity data set previously measured on the island and performed a joint 2D modeling of magnetic and gravity data. The magnetic data used in the modelling are shown in Fig. 7. The gravity field was obtained taking the data set quoted in Nunziata and Rapolla (1987) that had already been cleared of a regional trend, and filtering out the anomalies related to the shallowest sources. We chose a cut-off wavelength λ N 2.58 km, following the spectral analysis performed by Nunziata and Rapolla (1987). The filtered data are characterized by a maximum located in the south-western side of the island (Fig. 11). The forward modelling was performed along a SW–NE profile (AA', see Figs. 7 and 11) crossing both the magnetic and gravity maxima. The 2D model (Fig. 12) was built with GMSYS (Grav/Mag Modeling Software, www.nga.com) using the typical density and magnetic

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Fig. 7. Magnetic field of Ischia island obtained after pole-reduction and filtering. We filtered out the components with wavelength λ N 8.6 km and wavelength λ b 2.58 km. The black line shows the position of the profile interpreted in Figs. 12 and 13.

susceptibility values of the deposits of the area (see, e.g., Carrara et al., 1974; Nunziata and Rapolla, 1987) and assuming a model compatible with those derived from the 3D magnetic and SP inversions. The resulting 2D model shows the presence of a pyroclastic cover with low magnetization and of a structure whose depth-to-the-top ranges from 1200 to 1750 m b.s.l. This structure is characterized by a density contrast with the pyroclastic cover of 0.4 gr/cm3 and by a variable magnetization distribution. More specifically, while in the easternmost and westernmost areas of the profile the structure's magnetization values are compatible with those of an igneous body, in the central-western part of the profile the structure seems to be characterized by a demagnetized region. The contemporary presence of a magnetic minimum and a gravimetric maximum in the band-passed data could therefore be explained with the existence of an intrusion–or several intrusions of different ages–characterized by hydro-chemical alteration processes and/or by partially melted spots. This hypothesis is supported by the high temperature gradient measured in the central-western sector of the island (Penta and Conforto, 1951; Penta, 1954; Ippolito and Rapolla, 1982; Panichi et al., 1992) and is consistent with the results of the 3D magnetic inversion. Further support for the quoted model is provided by the 2D tomographic inversion of the SP data observed along the AA' profile, whose results are overlaid to the grav-mag model in Fig. 12. The most evident correlation between the 2D SP tomography and the density and magnetization distribution is the trend and the location in depth of the deep negative and positive electric charge zones, whose highest occurrence values appear to correspond closely to the structure

coming from the grav-mag modelling. Moreover, the hypothesis that only the central portion of the outlined structure is demagnetized could be supported by a physical modelling of the observed SP source centres in terms of Onsager's theory of coupled flows (Onsager, 1931; Landau and Lifšits, 1978, 1986), originally introduced in geophysics by Nourbehecht (1963), then examined by Mizutani et al. (1976), Sill (1983) and Di Maio and Patella (1991). In particular, following the discussion reported in Di Maio et al. (1998) for interpreting the electric charge accumulations observed at the Mt.Somma-Vesuvius volcanic complex, and considering the remarkable hydrothermal circulation and the high geothermal gradient observed in the central-western sector of the island (Chiodini et al., 2004), we can argue that the primary sources responsible of the observed negative and positive nuclei are generated by fluid mass and heat primary flows, under the action of pressure and temperature gradients, respectively, and that their divergences may be considered ineffective. In fact, in a steady state condition the primary flow divergences are different from zero only in the zone where mass and heat flows are continuously generated, i.e. a magma chamber. This hypothesis is to be excluded as at present no observation supports such evidence. These primary sources very likely induce secondary charge distributions along both structural and physical discontinuities. Indeed, the maximum values of positive electric charge occurrence are located approximately in correspondence with the top of the demagnetized portion of the outlined structure. On the other hand, the distribution of the maximum values of negative charge occurrence correlates with the pattern of the physical boundaries between the magnetized and the de-magnetized sectors of the structure.

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Fig. 8. 3D inversion of the filtered magnetic data of Fig. 7 (see text for details).

Fig. 9. Self Potential anomaly map of Ischia Island. The black dots are the measured points. The black line indicates the profile interpreted in Fig. 13.

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Fig. 10. 3D tomographic inversion of the SP data of Fig. 9 (see text for details).

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Fig. 11. Gravity field of the Ischia island obtained by filtering the data quoted in Nunziata and Rapolla (1987) (already cleared of a regional trend) with a cut-off wavelength λ N 2.58 km.

Concerning the shallow positive charge accumulations shown in the central part of the profile (see Fig. 12), we can tentatively attribute their origin to primary sources connected to the circulation, along superficial fracture systems, of hot fluids coming from the shallow reservoir hypothesized by Chiodini et al. (2004). 6. Conclusions We carried out a study of the geo-structural setting of the island of Ischia by the analysis, inversion and modeling of newly acquired potential field–aeromagnetic and self-potential–data. Our interpretation shows that the central-western part of the Ischia island, corresponding approximately to Mt. Epomeo, is characterized by a minimum in the magnetization values and a maximum in the positive electric charge occurrence. This outcome suggests the existence of a structure (Fig. 12) whose top is located at 1200–1750 m b.s.l. that seems to be demagnetized in its central portion, very likely in connection with the high geothermal gradient measured in the central-western sector of the island. In order to investigate this hypothesis, in Fig. 13a–b we report the thermal well temperatures measured at depths of 20–150 m b.s.l., within the sea-water shallow aquifer, and at depths greater than 200 m b.s.l. within the sea-water deep aquifer (Penta and Conforto, 1951; Penta, 1954; Ippolito and Rapolla, 1982). As we can see, the temperature values remain basically constant with depth, except for an increase in the L6 well and a decrease in the L1 and L4 wells. The comparison between Figs. 12 and 13a–b shows a good correspondence between the minimum of the magnetic field, the maxima of the gravity and self-potential data and the pattern of the temperature data. Taking into account this pattern and all the information derived from the potential field data inversion and modelling, we built a 2D interpretative model along profile AA' (Fig. 13c). In Fig. 13c we also report the pattern of the temperatures throughout the section. The model shows a structure rising to about 1200 m b.s.l. that could be interpreted as the ellipsoidal laccolith hypothesized by Nunziata and Rapolla (1987) and Carlino et al. (2006) and, longer before, by Rittmann (1930) only on the basis of volcanological inferences. This structure, which might possibly be formed by several neighbouring intrusions placed at different ages, is characterized by a density of 2.4 gr/cm3 and variable magnetization values. More specifically, the magnetization values of the easternmost and westernmost areas of the structure suggest the presence of igneous–very likely trachytic–

material. This hypothesis is supported by the finding of trachytic material in a drilling located near Cetara (Nunziata and Rapolla, 1987). The central-western area of the structure, below Mt. Epomeo, seems instead to be demagnetized. This could be due to alteration processes of the igneous materials connected to the high temperatures measured in the wells of the central-western sector of the island (Penta, 1954) and/or to the presence of partially melted material. Regarding this last hypothesis, Nunziata and Rapolla (1979) show that for a body of X = 1000 m, Z = 3500 m and Y = ∞, after ~ 30,000 years, the temperature at a distance of 1000 m from the surface of the body is about 150 °C. Given the bigger dimension of the modeled intrusion(s), we note that this computed temperature is in agreement with the temperature actually measured in the deepest well of the island (about 200 °C at 1150 m b.s.l., see Fig. 13). This evidence suggests that most of the structure should be cool enough to generate a magnetic field, even though smaller portions of it may be still melted to some extent and therefore demagnetized. Moreover, our model highlights that the westernmost portion of the outlined structure is fairly magnetized, suggesting a temperature decrease in this sector. The lack of wells in such an area can't obviously confirm this hypothesis, even though the relatively low temperatures observed in the L4-L5 wells and in the deeper section of the L1 well (Fig. 13a–b) seems to indicate a temperature decreasing trend westward. We can therefore hypothesize the following qualitative model that accounts for the different potential field and temperature evidence: after the emplacement of a trachytic intrusion(s), responsible for the raising of Mt. Epomeo, a hydrothermal system was generated by the cooling of the igneous material. Due to the lithology and the variability of the hydrothermal alteration caused by the circulation of hot fluids and their chemical composition, the system may be considered as divided into different–partially isolated–sub-systems, each one characterized by its own average temperature. Obviously, the deeper the sub-system, the hotter the temperature; even though lateral temperature variations may be encountered depending on the local permeability and the presence of descendent or ascendant convective cell branches. This system could be further complicated by the presence, all over the island, of fracture systems where fluids circulate, which are here and there totally or partially self-sealed. The outcome of our study is in agreement with those of Penta and Conforto (1951), who hypothesize that the hydrothermal circulation of the island concentrates in the highly fractured lavas underlying the

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Fig. 12. 2D joint forward modelling of magnetic and gravity data along profile AA' overlapped with the outcome of the 2D inversion of SP data along the same profile.

Mt. Epomeo Green Tuff, and of Chiodini et al. (2004), who suggest the existence of two distinct hydrothermal reservoirs located at different depths.

Acknowledgements This work was carried out with the financial support of “Istituto Nazionale di Geofisica e Vulcanologia” (INGV) and of “Dipartimento per la Protezione Civile”, Italy. The authors acknowledge “ICARUS Linee Aeree” for proving with flight devices and are grateful to Roberto Scandone and an anonymous reviewer for their constructive comments.

References ABNOC - Autorità di Bacino Nord Occidentale della Campania, 2002. Piano Stralcio per l'Assetto Idrogeologico. DVD. (in Italian). Acocella, V., Funiciello, R., 1999. The interaction between regional and local tectonics during resurgent doming: the case of the island of Ischia, Italy. J. Volcanol. Geotherm. Res. 88, 109–123. AGIP, 1981. Carta aeromagnetica d'Italia scala 1:5000000: fogli G, H, L, M. Attivita Mineraria, Direzione Esplorazione Idrocarburi, S. Donato Milanese, Italy (in Italian). Arana, V. , Camacho, A.G., Garcia, A., Montesinos, F.G., Blanco, I., Vieira, R., Felpato, A., 2000. Internal structure of Tenerife (Canary Islands) based on gravity, aeromagnetic and volcanological data. J. Volc. Geother. Res. 103, 43–64. Ballestracci, R., 1982. Self-potential survey near the craters of Stromboli volcano (Italy). Inference for internal structure and eruption mechanism. Bull. Volcanol. 45, 349–365.

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Fig. 13. a) Well temperature data measured at depths of 20–150 m b.s.l. For well locations see Fig. 2. The temperatures, labelled with the names of the wells, are not interpolated but are projected on the AA' profile; b) Well temperature data measured at depths greater than 200 m b.s.l. The Mt. Tabor, Arso and IREM wells do not reach the deep aquifer and are shown only in Fig. 13a; c) Geophysical model of the section AA' derived from the joint interpretation of magnetic, gravity, self-potential and well temperature data. The hatched area delineates the demagnetized region of the trachytic intrusion(s). For the magnetization and density values of the model see Fig. 12. The pattern of the temperatures throughout the section is also shown.

Bektas, O., Ravat, D., Buyuksarac, A., Bilim, F., Ates, A., 2007. Regional geothermal characterization of East Anatolia from aeromagnetic, heat flow and gravity data. Pure Appl. Geophys. 164, 975–998. doi:10.1007/s00024-007-0196-5. Bruno, P.P.G., de Alteris, G., Florio, G., 2002. The western undersea section of the Ischia volcanic complex (Italy, Tyrrhenian sea) inferred by marine geophysical data. Geophys. Res. Lett. 29 (9), 57 (1–4). Budillon, F., Violante, C., De Lauro, M., Ferraro, L., Molisso, F., 2003. Carta geologica dei fondali delle isole flegree. Ambiente marino costiero e territorio delle isole flegree (Ischia, Procida e Vivara) e Golfo di Napoli. Risultati di uno studio multidisciplinare, Monografia Accademia di Scienze Matematiche, Fisiche e Naturali. Liguori Ed., Naples (in Italian). Carlino, S., Cubellis, E., Luongo, G., Obrizzo, F., 2006. On the mechanics of caldera resurgence of Ischia Island (southern Italy). In: Troise, C., De Natale, G., Kilburn, C.RJ. (Eds.), Mechanisms of Activity and Unrest at Large Calderas. Geological Society, vol. 269. Special Publications, London, pp. 181–193. Carrara, E., Iacobucci, F., Pinna, E., Rapolla, A., 1974. Interpretation of gravity and magnetic anomalies near Naples, Italy, using computer techniques. Bullettin Volcanologique XXXVIII (2), 1–10. Cassidy, J., France, S.J., Locke, C.A., 2007. Gravity and magnetic investigation of maar volcanoes, Auckland volcanic field, New Zealand. J. Volcanol. Geotherm. Res. 159, 153–163. Chandrasekhar, D.V., Mishra, D.C., Poornachandra Rao, G.V.S., Mallikharjuna Rao, J., 2002. Gravity and magnetic signatures of volcanic plugs related to Deccan volcanism in Saurashtra, India and their physical and geochemical properties. Earth and Planet. Sci. Lett. 201, 277–292. Chiodini, G., Avino, R., Brombach, T., Caliro, S., Cardellini, C., De Vita, S., Frondini, F., Granirei, D., Marotta, E., Ventura, G., 2004. Fumarolic and diffuse soil degassing west of Mount Epomeo, Ischia, Italy. J. Volcanol. Geotherm. Res. 133, 291–309. Civetta, L., Gallo, G., Orsi, G., 1991. Sr- and Nd-isotope and trace element constraints on the chemical evolution of the magmatic system of Ischia (Italy) in the last 55 ka. J. Volcanol. Geotherm. Res. 46, 213–230.

Civetta, L., De Vita, S., Di Vito, M.A., Orsi, G., 1998. Ischia. Acta Vulcanol. 10, 85–196. Cordell, L., Grauch, V.J.S., 1985. Mapping basement magnetization zones from aeromagnetic data in the San Juan basin, New Mexico. In: Hinze, W.J. (Ed.), The utility of regional gravity and magnetic anomaly maps, Society of Exploration Geophysics, pp. 181–197. Davy, B.W., Caldwell, T.G., 1998. Gravity, magnetic and seismic surveys of the caldera complex, Lake Taupo, North Island, New Zealand. J. Volcanol. Geotherm. Res. 81, 69–89. de Alteris, G., Toscano, F., 2003. Introduzione alla geologia dei mari circostanti le isole flegree di Ischia, Procida e Vivara. Ambiente marino costiero e territorio delle isole flegree (Ischia, Procida e Vivara) e Golfo di Napoli. Risultati di uno studio multidisciplinare, Monografia Accademia di Scienze Matematiche, Fisiche e Naturali, Liguori Ed., Naples, pp. 3–24 (in Italian). de Alteris, G., Bruno, P.P.G., Di Fiore, E., Florio, G., Passaro, S., Tonielli, R., 2002. Magnetic marine geophysical data (multibeam bathymetry, magnetics and seismics) in active volcanic area: the case of Ischia volcano and its offshore (Italy, Tyrrhenian Sea) Boll. Geofis. Teor. Appl. 42, 45–47. de Alteris, G., Tondelli, R., Passaro, S., De Lauro, M., 2006. Isole Flegree (Ischia e Procida). Liguori Ed., Naples. 73 pp (in Italian). Della Monica, G., Di Maio, R., Scandone, R., Cecere, G., Del Negro, C., De Martino, P., Santochirico, F., 2004. Electrical modelling of the shallow structural setting of the Cisternazza-Montagnola area (Mt.Etna). Quad. Geofis. 35, 39–44. De Santis, A., Gaya-Piqué, L.R., Dominici, A., Meloni, A., Torta, J.M., Tozzi, R., 2003. Italian Geomagnetic Reference Field (ITGRF): update for 2000 and secular variation model up to 2005 by autoregressive forecasting. Ann. Geophys. 46 (3), 491–500. De Vita, S., Sansivero, F., Orsi, G., Marotta, E., 2006. Cyclical slope instability and volcanism related to volcano-tectonism in resurgent calderas: the Ischia island (Italy) case study. Eng. Geol. 86, 148–165. Di Girolamo, P., Stanzione, D., 1973. Lineamenti geologici e petrologici dell'isola di Procida. Rend. Soc. Ital. Mineral. Petrol. 29, 82–125 (in Italian). Di Girolamo, P., Rolandi, G., 1975. Vulcanismo sottomarino latitebasaltico-latitico (serie potassica) nel Canale d'Ischia (Campania). Rend. Accad. Sci. Fis. Mat., Accademia Nazionale Scienze Lettere Arti in Napoli 42, 561–596 (in Italian).

86

V. Paoletti et al. / Journal of Volcanology and Geothermal Research 179 (2009) 69–86

Di Girolamo, P., Ghiara, M.R., Lirer, L., Munno, R., Rolandi, G., Stanzione, D.,1984. Vulcanologia e petrologia dei Campi Flegrei. Boll. Soc. Geol. Ital. 103, 349–413 (in Italian). Di Maio, R., Patella, D., 1991. Basic theory of electrokinetic effects associated with earthquakes. Boll. Geofis. Teor. Appl. 33, 145–154. Di Maio, R., Patella, D., 1994. Self-potential anomaly generation in volcanic areas. The Mt.Etna case-history. Acta Vulcanol. 4, 119–124. Di Maio, R., Di Sevo, V., Giammetti, S., Patella, D., Piscitelli, S., Silenziario, C., 1996. Selfpotential anomalies in some Italian volcanic areas. Ann. Geophys. 39 (1), 179–188. Di Maio, R., Mauriello, P., Patella, D., Petrillo, Z., Piscitelli, S., Siniscalchi, A., Veneruso, M., 1997. Self-potential, geoelectric and magnetotelluric studies in Italian active volcanic areas. Ann. Geophys. 40 (2), 519–537. Di Maio, R., Mauriello, P., Patella, D., Petrillo, Z., Piscitelli, S., Siniscalchi, A., 1998. Electric and electromagnetic outline of the Mount Somma-Vesuvius structural setting. J. Volcanol. Geotherm. Res. 82 (1–4), 219–238. Di Napoli, R., Aiuppa, A., Bellomo, S., Brusca, L., D'Alessandro, W., Gagliano Candela, E., Longo, M., Pecoraino, G., Valenza, M., 2007. Chemical and isotopic characterization of groundwater discharges on the Ischia Island (Southern Italy). GeoItalia 2007. VI Forum Italiano di Scienze della Terra, Rimini. September 12–14, 2007. Ercan, A., Drahor, M., Atasoy, E., 1986. Natural polarization studies at Balcova geothermal field. Geophys. Prospect. 34, 475–491. Fedi, M., Rapolla, A., 1987. The Campanian volcanic area: analysis of the magnetic and gravimetric anomalies. Boll. Soc. Geol. Ital. 106, 793–805. Fedi, M., Hansen, P.C., Paoletti, V., 2005. Analysis of depth resolution in potential field inversion: a tutorial. Geophysics 70, A1–A11. Fedi, M., Hansen, P.C., Paoletti, V., 2007. Ambiguity and depth resolution in potential field inversion. Communications to SIMAI Congress (Electronic Journal), vol. 2. doi:10.1685/CSC06155. Ferraccioli, F., Bozzo, E., Damaske, D., 2002. Aeromagnetic signatures over western Marie Byrd Land provide insight into magmatic arc basement, mafic magmatism and structure of the Eastern Ross Sea Rift flank. Tectonophysics 347, 139–165. Finizola, A., Sortino, F., Lénat, J.F., Valenza, M., 2002. Fluid circulation at Stromboli volcano (Aeolian Islands, Italy) from self-potential and CO2 surveys. J. Volcanol. Geotherm. Res. 116, 1–18. Finn, C.A., Williams, D.L., 1987. An aeromagnetic study of Mount St. Helens. J. Geophys. Res. 92, 10194–10206. Finn, C.A., Morgan, L.A., 2002. High-resolution aeromagnetic mapping of volcanic terrain, Yellowstone National Park. J. Volcanol. Geotherm. Res. 115, 207–231. Finn, C.A., Sisson, T.W., Deszcz-Pan, M., 2001. Aerogeophysical measurements of collapse-prone hydrothermally altered zones at Mount Rainier volcano. Nature 409, 600–603. Flanagan, G., Williams, D.L., 1982. A magnetic investigation of Mount Hood, Oregon. J. Geophys. Res. 87 (B4), 2804–2814. doi:10.1029/0JGREA0000870000B4002804000001. Hansen, P.C., 1998. Rank-deficient and discrete ill-posed problems: numerical aspects of linear inversion. Society of Industrial and Applied Mathematic (SIAM). Gillot, P.Y., Chiesa, S., Pasquare, G., Vezzoli, L., 1982. b 33,000 yr K-Ar dating of the volcano-tectonic horst of the isle of Ischia, Gulf of Naples. Nature 229, 242–244. Grauch, V.J.S., Cordell, L., 1987. Limitations of determining density or magnetic boundaries from the horizontal gradient of gravity or pseudogravity data. Geophysics 52, 118–121. Ippolito, F., Rapolla, A., 1982. L'energia geotermica in Campania. Fonti Energetiche Alternative, Fondazione Politecnica per il Mezzogiorno. Franco Angeli Ed., Milan, pp. 57–106 (in Italian). Johnson, A.C., 1999. In interpretation of new aeromagnetic anomaly data from the Central Antarctic Peninsula. J. Geophys. Res. 104 (B3), 5031–5046. Landau, L.D., Lifšits, E.M., 1978. Fiziceskaja kinetika. Nauka, Moscow. Landau, L.D., Lifšits, E.M., 1986. Elektrodinamika splošnych sred. Nauka, Moscow. Lénat, J.F., Aubert, M., 1982. Structure of Piton de la Fournaise volcano (La Reunion Island, Indian Ocean) from magnetic investigations. An illustration of the analysis of magnetic data in a volcanic area. J. Volcanol. Geotherm. Res. 12, 361–392. Loera, H.L., Aranda-Gómeza, J.J., Arzate, J.A., Molina-Garza, R.S., 2008. Geophysical surveys of the Joya Honda maar (México) and surroundings; volcanic implications. J. Volcanol. Geotherm. Res. 10, 135–152. Luongo, G., Cubellis, E., Obrizzo, F., 1987. Ischia – Storia di un'isola vulcanica. Liguori Ed., Naples. (in Italian). Maino, A., Tribalto, G., 1971. Rilevamento gravimetrico di dettaglio dell'isola d'Ischia (Napoli). Boll. Serv. Geol. Ital. 92, 109–123 (in Italian). Malengreau, B., Lénat, J., Froger, J., 1999. Structure of Réunion Island (Indian Ocean) inferred from the interpretation of gravity anomalies. J. Volcanol. Geotherm. Res. 88, 131–146. Manzo, M., Ricciardi, G.P., Casu, F., Ventura, G., Zeni, G., Borgstrom, S., Berardino, P., Del Gaudio, C., Lanari, R., 2006. Surface deformation analysis in the Ischia Island (Italy) based on spaceborne radar interferometry. J. Volcanol. Geotherm. Res. 151, 399–416. doi:10.1016/j.jvolgeores.2005.09.010. Massenet, F., Pham, V.N., 1985. Mapping and surveillance of active fissure zones on a volcano by the self-potential method, Etna, Sicily. J. Volcanol. Geotherm. Res. 24, 315–338.

Michel, S., Zlotnicki, J., 1998. Self-potential and magnetic surveying of La Fournaise volcano (Réunion Island): correlations with faulting, fluid circulation, and eruption. J. Geophys. Res. 103 (B8), 17 845–17,857. Mickus, K.L., Durrani, B., 1996. Gravity and magnetic study of the crustal structure of the San Francisco Volcanic Field, Arizona, USA. Tectonophysics 267, 73–90. Mickus, K.L., McCurry, M., 1999. Gravity and aeromagnetic constraints on the structure of the Woods Mountains volcanic center, southeastern California. Bull. Volcanol. 60, 523–533. Mizutani, H., Ishido, T., Yokokura, T., Ohnishi, S., 1976. Electrokinetic phenomena associated with earthquakes. Geophys. Res. Lett. 3, 365–368. Molin, P., Acocella, V., Funiciello, R., 2003. Structural, seismic and hydrothermal features at the border of an active intermittent resurgent block: Ischia island (Italy). J. Volcanol. Geotherm. Res. 121, 65–81. Nourbehecht, B., 1963. Irreversible thermodynamic effects in inhomogeneous media and their application in certain geoelectric problems. Ph.D. thesis, M.I.T., Cambridge. Nunziata, C. and Rapolla, A., 1979. Analysis and synthesis of the geological, geophysiscal and volcanological data about the Neapolitan area and its geothermal potential (I parte). Rapporto EUR 6152 E/F, MF, Commissione delle Comunità Europee, Direzione Generale per la Ricerca, Scienza ed Educazione, pp. 22 Nunziata, C., Rapolla, A., 1987. A gravity and magnetic study of the volcanic island of Ischia, Naples (Italy). J. Volcanol. Geotherm. Res. 3, 333–344. Okubo, A., Nakatsuka, T., Tanaka, Y., Kagiyama, T., Utsugi, M., 2005. Magnetization of the Unzen graben determined from aeromagnetic data. Annuals of Disas. Prev. Res. Inst., Kyoto Univ. 48B. Onsager, L., 1931. Reciprocal relations in irreversible processes. I. Phys. Rev. 37, 405–426. Orsi, G., Gallo, G., Zanchi, A., 1991. Simple-shearing block resurgence in caldera depressions. A model from Pantelleria and Ischia. J. Volcanol. Geotherm. Res. 47, 1–11. Orsi, G., Patella, D., Piochi, M., Tramacere, A., 1999. Magnetic modeling of the Phlegrean volcanic district with extension to the Ponza Archipelago, Italy. J. Volcanol. Geotherm. Res. 91, 345–360. Panichi, C., Bolognesi, L., Ghiara, M.R., Noto, P., Stanzione, D., 1992. Geothermal assessment of the island of Ischia (southern Italy) from isotopic and chemical composition of the delivered fluids. J. Volcanol. Geotherm. Res. 49, 329–348. Paoletti, V., Secomandi, M., Florio, G., Fedi, M., Rapolla, A., 2005. The integration of remote sensing magnetic data in the Neapolitan volcanic district. Geosphere 1, 85–96. Paoletti, V., Rapolla, A. and Secomandi, M., in press. Magnetic signature of submarine volcanoes in the Phlegrean Fields-Ischia Ridge (North-Western side of the Bay of Naples, Southern Italy). Annals of Geophysics. Patella, D., 1997a. Introduction to ground surface self-potential tomography. Geophys. Prospect. 45 (4), 653–681. Patella, D., 1997b. Self-potential global tomography including topographic effects. Geophys. Prospect. 45 (5), 843–863. Penta, F., 1954. Ricerche e studi sui fenomeni esalativi-idrotermali e il problema delle forze endogene. Ann. Geofis. 8, 1–94 (in Italian). Penta, F., Conforto, B., 1951. Risultati di sondaggi e di ricerche geominerarie nell'isola d'Ischia dal 1939 al 1943, nel campo del vapore, delle acque termali e delle 'forze endogene' in generale. Ann. Geofis. 4, 159–191 (in Italian). Rapolla, A., Fedi, M., Fiume, M.G., 1989. Crustal structure of the Ischia-Phlegrean geothermal fields, near Naples, Italy, from gravity and geomagnetic data. Geophys. J. 97, 409–419. Rittmann, A., 1930. Geologie der Insel Ischia. Zeitschrift fur Vulkanologie. Erganzungsband VI, Berlin. 268 pp. (in German). Rybakov, M., Goldshmidt, V., Fleischer, L., Ben-Gai, Y., 2000. 3-D gravity and magnetic interpretation for the Haifa Bay area Israel. J. Appl. Geophys. 44, 353–367. Sepe, V., Atzori, S., Ventura, G., 2007. Subsidence due to crack closure and depressurization of hydrothermal systems: a case study from Mt Epomeo (Ischia Island, Italy). Terra Nova 19 (2), 127–132. doi:10.1111/j.1365-3121.2006.00727.x. Sill, W.R., 1983. Self-potential modeling from primary flows. Geophysics 48, 76–86. Soengkono, S., 2001. Interpretation of magnetic anomalies over the Waimangu geothermal area, Taupo volcanic zone, New Zealand. Geothermics 30 (4), 443–459. Ueda, Y., 2007. A 3D magnetic structure of Izu-Oshima volcano and their changes after the eruption in 1986 as estimated from repeated airborne magnetic surveys. J. Volcanol. Geotherm. Res. 164, 176–192. Vezzoli, L., 1988. Island of Ischia. Quad. de La ricerca scientifica, 114 (10). CNR, Progetto Finalizzato Geodinamica 10, Rome. 133 pp. Zohdy, A.A.R., Anderson, L.A., Mufler, L.J.P., 1973. Resistivity, self-potential and inducedpolarization surveys of a vapour-dominated geothermal system. Geophysics 38, 1130–1144.