Mise-Á-La-Masse and Gravity Data Surveys at the Kamojang Geothermal Field

MISE-Á-LA-MASSE AND GRAVITY DATA SURVEYS AT THE KAMOJANG GEOTHERMAL FIELD

Prihadi SUMINTADIREJA1, Sayogi SUDARMAN2, Hideki MIZUNAGA3 and Keisuke USHIJIMA 3

1 Geological Department, FIKTM-Institut Teknologi Bandung, Jl. Ganesha 10 Bandung 40132, Indonesia 2 Pertamina, Geothermal Division, Jl. Merdeka Timur 6, Gd. Kwarnas Pramuka 5th fl., Jakarta 10110, Indonesia 3 Exploration Geophysics Lab., Earth Resources Department, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan

Keywords: Mise-á-la-masse (MAM), Gravity, Kamojang, 812,000 East and 9,205,000–9,214,000 North. Topographic Indonesia elevations range from 1,400-1,800 m above sea level (a.s.l.). The Kamojang field can be reached from two district cities, ABSTRACT Garut and Majalaya. Intensive geological, geochemical, and geophysical investigations started in the Kamojang Numerous geophysical surveys have been performed at the geothermal field in 1972. This project was a collaboration Kamojang geothermal field. In spite of these surveys, the between the Indonesia and the New Zealand governments as success rate of production well drilling is only near 60%. To part of the bilateral aid project, namely the Colombo Plan. In more precisely define areas of permeability within the 1973, the drilling of exploratory and production wells was reservoir and the drilling success rate Mise-á-la-masse and done for a 5 MW capacity pilot power plant. The first stage gravity surveys are performed. These surveys, combined with 30 MW turbine was completed in February 1983. previous geophysical results, have identified a possible Subsequently the second and third steps were completed in productive region in an area previously believed to have low September 1987, for generating 2 x 55 MW or a total of 140 permeability. MW.

1. INTRODUCTION The objective of the Mise-á-la-masse field survey, carried out in October-November 1996, was to confirm the extension of The purpose of this paper is to present an integrated the geothermal field prospect from 14 km2 by the direct geophysical interpretation, with geological constraints, of the current Schlumberger resistivity method in 1972 to 21 km2 Mise-á-la-masse method. The geophysical work is carried out by the CSAMT (Controlled Source Audio Magnetotelluric) in order to develop a realistic geothermal system model of method in 1988 (Figure 1). The MAM field survey was done the ultimate potential and the reservoir boundaries. Although by the Indonesia Oil and Gas State Owned Company the prospect area was already localized by the Schlumberger (Pertamina). The Exploration Geophysics Laboratory, mapping method and a shallow temperature survey, almost Faculty of Engineering, Kyushu University and the ITB- 40% of the wells do not produce commercial amounts of Community Services was responsible for the field survey lay steam. To overcome this problem ongoing research is out design plan, data acquisition quality control and data conducted, to improve drilling success and to give better interpretation. matches between the field performance and the modeling results. 2. WELL DATA

Electrical resistivity is related to the lithology, porosity, The main production zone is confined to the lower section of temperature and fluid content of rocks. The Mise-á-la-masse the Gandapura complex within the depths ranging from 700 survey is a quick method to map the electrical resistivity to 1,200 meters. In 1926, five wells were drilled ranging distribution within the geothermal area. The processed data between 18.5-130 m in depth. Well 3 is still discharging with is interpreted to delineate and locate the promising a temperature of 130oC and 12,400 kg/hr. steam. Pertamina geothermal area in the Kamojang geothermal field. has drilled 66 wells with bottom hole temperatures ranging from 115-245oC. The pressure and temperature logs indicate Gravity prospecting is one of the geophysical methods for a typical vapor dominated convecting geothermal system. obtaining a gross model of the structure of a geothermal The pressure and temperature increase linearly down to the system. This method has also been applied in monitoring top of the steam zone. At greater depths they increase slowly. subsidence and estimating mass recharge, which is reduced However, there are some wells that are not consistent with by fluid withdrawal in the reservoir during long-term this vapor dominated type such as KMJ-9 (at 724 m a.s.l.) exploitation. and KMJ-10 (at 715 m a.s.l.). Some of the well data information from the Kamojang geothermal field are already The Kamojang geothermal field is located in the western part published in international geothermal journal/seminar of Java Island Indonesia, about 42 km SSE of the west Java proceedings and most of the well data information are province capital city Bandung. The field is geographically available in a Pertamina internal report (Pertamina, 1995). situated between 07o 11’ 02”– 07o 06’ 08” South latitude and 107 o 44’ 36”– 107o 49’ 30” East longitude or at UTM The geometry of the Kamojang reservoir is the result of the (Universal Transverse Mercator) zone 48 between 803,000– complex interactions of active volcano-tectonic processes,

1777 Sumintadireja, Sudarman, Mizunaga, and Ushijima older stratigraphy, and structure. Generally the caprock is Mizunaga and Ushijima (1991) have given a thorough 500-600 m thick but seems to be only 200-300 m thick description of the 3-D resistivity numerical computation. The toward the northern and eastern parts. This caprock consists subsurface illustration of the line source and the block model of prophylitic altered volcanic rock. In the Kamojang are given in Figure 3. The calculation of average apparent geothermal field the resistivity is more sensitive to reservoir resistivity within each block is permeability than the temperature or rock alteration type. M 1 (1) The productive geothermal reservoir, which usually has high log m = log rr ai M å porosity, high permeability, high temperature, and adequate i=1 size with sufficient fluid, is located between 600-2,000 m in where rm = average apparent resistivity within each block ( depth. The reservoir consists of strongly altered andesitic Wm), rai = inversion apparent resistivity (Wm), M = value rocks and some volcanic pyroclastics. Permeability is is the slope of the apparent resistivity curve on a log-log plot produced by structural events such as faults, joints and which is approximated using numerical differentiation fractures or by stratigraphic characteristic such as method. intergranular porosity in lapilli. The Jacobian of the homogenous model is expressed as ? ¶f 3. GEOPHYSICAL METHODS = r -2 Ñ f Ñ f ©dW (2) ?¶r òòò Vol 3.1 Mise-á-la-masse where f = electrical potential, r = resistivity. The potentials due to the current source and potential electrode The Mise-á-la-masse field data acquisition consists of two are fixed current electrodes C1 and C2, which are located about I 1 rr I ll 1 (3) 5 to 6 km apart and are assumed not to influence each other. f == 22 2 2p rc 2p ++ -()-()-( z ) The fixed potential electrode, P2, is located about 3 to 4 km yyxx cc z c distances away in the opposite direction of the current electrode C . The potential electrode P is distributed around r 1 r 1 2 1 f' == the charged well C as shown in Figure 2. Design of the 222 1 2p r 2p yyxx )-()-( ++ z Mise-á-la-masse field survey is based on maximum coverage p pp from the current electrode at the production well within the (4) study area. The KMJ-48 directional well and the KMJ-63 vertical well were selected for the charged current electrode The inversion procedure of the numerical model for the 3-D C1, because they were not yet connected to the geothermal Mise-á-la-masse inversion based on the least square production pipeline. Both wells are good steam producers at deconvolution of the apparent resistivity is TTT 89.7 ton/hour and 76.1 ton/hour respectively. The C2 and the JJ l CC )( Dp =+ J D g (5) P are almost 5 km and 3 km away from the well 2 where J = Jacobian matrix of partial derivatives, l = respectively and are used together at the same location for damping factor, g = differences between measured and both wells. Based on the Mise-á-la-masse survey results in 1996, the further Mise-á-la-masse study in 1998 was carried calculated apparent resistivity values, p = correction to the out to identify the geothermal prospect distribution to the model parameter, C = smoothness constraint. In this north and north-east by using KMJ-47 as a charged current. computation, the inversion involves the calculation of the An advanced application of this MAM method in fluid flow apparent resistivity value from the model and from the monitoring of the Enhanced Oil Recovery processes has been Jacobian matrix of partial derivatives and then solves the developed by Ushijima et.al., (1997). linear equation.

The equipment used in this survey has the capability of 3.2 Gravity charging up to 2 Ampere current using 1.5 K.Volt of direct current voltage. The surveyed area is measured along the The gravity data acquisition in the west part of Kamojang radial lines for almost 20 km total length for each charged was carried out in 1997 by Lemigas (R & D Gas and Oil Technology of Indonesia). The project was initiated by well. The electric potential P1 on the ground surface was measured at 100 m intervals along the survey lines. Pertamina to support the geoelectrical mapping. Observation Observation points at 100 m intervals were determined by points are distributed randomly and along existing access the topographic survey team prior to the potential roads. measurement by the Mise-á-la-masse team. The potential The gravity data processing involves data reductions such as electrodes, P1 and P2, are immersed with a saturated copper sulfate solution in the porous pots, to eliminate the corrections for drift, tides, ellipsoid earth gravity reference, polarization effect during the Mise-á-la-masse potential free air, bouguer, and terrain. The Bouguer anomaly map is derived from the calculation of Complete Bouguer Anomaly measurement. The current electrode C2 is a 1.5 m long copper rod 3 cm in diameter, buried and covered with CBA = gobs - (gj - FAC + BC -TC) mgal (6) bentonite to keep good earth contact. where Ellipsoid earth gravity reference (IUGG, 1979), gj = 978.03185(1 + 0.005278895 sin2 + 0.000023462 sin4 ) 3.1.1 Resistivity Inversion j j mgal; where gj = theoretical gravity as a function j/latitude of the gravity observation point; Free Air Correction (FAC) = - 0.3086 h mgal; Bouguer Correction (BC) = 0.04187 h mgal;

1778 Sumintadireja, Sudarman, Mizunaga, and Ushijima where h = elevation of gravity station (m). Terrain Correction increases toward the ENE. The geological model for the (TC) mgal; consists of inner zones (A,B,C,D) and outer WSW-ENE gravity profile was controlled by the CHR-1 zones (E,F,G,H,I,J). Gravity anomalies typically arise from vertical well at an elevation 1,483 m with a total depth of relatively small density difference between different rock 1,804 m. formations and quantitative interpretation is very sensitive to these values. The quantitative gravity data interpretation is Bouguer gravity anomalies consist of those due to shallower carried out by the 2-D gravity modeling by the Talwani mass inhomogeneities superimposed on an undisturbed method. regional field. A regional gravity field is typically a long wavelength anomaly caused by deep seated mass inhomogeneties. 4. PROCESSING RESULTS Such a field can slightly distort the observed anomalies and 4.1 Mise-á-la-masse must be subtracted from the observed values to obtain residual anomalies of shorter wavelength which are caused Theoretically the potential value will always have a circle by mass inhomogeneities in the upper few kilometers of the shape and decrease gradually away from the current source crust. C1. The data processing consists of plotting the observed field data of V/I in mV/A, the theoretical V/I in mV/A, the The Bouguer anomalies indicate that the gradient of the apparent resistivity ra in Wm, and the difference values of regional field must be small in the NS direction the observed and the theoretical between the potential DV/I and the apparent resistivity Dra. From the resulting maps, we can interpret a general distribution of the low resistivity 5. DISCUSSION area and the discontinuity of the contour pattern, which correlates with the producing geothermal area and structural Sudarman et.al., (1996) and Sumintadireja et.al., (1997 and features. The apparent resistivity (ra) and the potential 1998) discussed of the application of various geophysical differences (DV/I) show the low resistivity area of less than 8 methods in geothermal resource exploration. However, Wm covering almost all of the radially measurement points. geothermal exploration is very complex with high uncertainty The exceptions lie along lines A15-18, B11-18, E12-13, of steam production from development wells, even if the H11-14, L19-20, M12-13 and M17-18, S21-26, T19-20, location is already surveyed by various geological and U19-20. The extension of the main prospective anomaly geophysical methods. Based on the experience in Kamojang, around the KMJ-63 well is bigger than the KMJ-48 well where only 60% of the wells produced commercial steam, we (Figure 4). have to improve the drilling success ratio by doing integrated studies. At Kamojang, the MAM survey has optimum 4.1.1 Resistivity Inversion Results of the Mise-á-la-masse detection coverage for a 1.6 km radius from the charged Method casing well.

The inversion results of the MAM data using the charged The geothermal reservoir boundaries and depth can be current of KMJ-48 and KMJ-63 is represented by the block identified or inferred by correlating the observed resistivity of 500x500 m size in x and y axis with 4 layers in z axis at anomaly from various geophysical methods. The MAM result depth of 300 m, 900 m, 1,500 m and 2,300 m, respectively gives higher resolution compared to the previous (Figure 5). The low resistivity is located SSE of KMJ-63 and Schlumberger mapping. SSW of KMJ-48. The low resistivity block moves from south to north starting near to the KMJ-48 at depth of 1,500 m. The The caldera shape in WNW part of the study area is clearly inversion results can be completed much faster than the identified by the MAM survey. The gravity survey also forward interpretation. The minimization of the error supports the MAM data in west area. The prospect boundary between observed and calculated data was automatically in west area is not clearly identified, but the reservoir can be done by iteration. The low resistivity area is wider at the predicted in the altered auto brecciated andesite rocks. In the 1,500 m level, but the extension does not continue to the Kamojang geothermal field the resistivity change is more SSW direction from KMJ-63. This can result from multiple sensitive to the reservoir permeability than the temperature causes such as the presence of the Ciharus lake, a uniformly or rock alteration type. Electrical conductivity is mainly a tight volcanic formation, or an absence of structures within function of the total ionic mobility of the ions in solution this area. whereas the pH is a function of the hydrogen ion concentration. The higher chloride springs are generally sub- 4.2 Gravity alkaline, are hotter and have a greater discharge.

The gravity data are mostly from the in western area between Based on the experience of the power plant operational in 800,000-808,000 East and 9,205,000-9,215,000 North Kamojang, an hydrothermal system will decrease its (Figure 6). The Bouguer gravity map with density of 2.5 permeability with time by pressure solution or by deposition gr./cc, consists of regional and residual anomalies trending of minerals from the water as they cool and loose CO2. SW-NE in southern area. In north area the orientation is Therefore the permeability maintenance must be carried out more SE-NW. Interpretation of the WSW-ENE gravity by periodically fracturing or by re-injection under the profile is shown on Figure 7, where the Bouguer anomaly direction of the Mise-á-la-masse and gravity data

1779 Sumintadireja, Sudarman, Mizunaga, and Ushijima interpretation result. Re-injection of the geothermal water is ACKNOWLEDGEMENTS necessary to recharge the reservoir in order to maintain power plant output and to prevent chemical pollution. The We would like to express our gratitude to the management of main reservoir area is represented by the low/negative Pertamina for their permission to publish this paper. Dr potential different (DV/I) value, therefore the location of re- Benoit and Dr. David D. Blackwell are gratefully injection well is suggested in the limit of the main reservoir. acknowledged for their thorough reviews.

We have to reevaluate the area which already considered as a REFERENCES low permeability zone, in other words there is a promising area located within the previously stated as low permeability Mizunaga, H and Ushijima, K., (1991), Three-dimensional zone, such as the area between line of L8-17, line of K13-18 numerical modeling for the Mise-á-la-masse method, and line of Q4-12. BUTSURI-TANSA (SEGJ), vol.44, no.4, pp.215-226.

6. CONCLUDING REMARKS Pertamina (Internal report, team Pokja 53), 1995, Feasibility evaluation of Kamojang geothermal area development, 53 pp. The geophysical investigation of mapping and VES Schlumberger, CSAMT, MAM, gravity survey has already Sudarman S., Sumintadireja P., and Ushijima K., 1996, been completed at Kamojang. However, all the survey results Exploration of geothermal resources in Lahendong area, give slightly different results in delineating the reservoir. North Sulawesi, Indonesia, Memoirs of the Faculty of The combination of several geophysical methods is the best Engineering Kyushu University, vol.56, no.3, pp.171-186. way for defining the reservoir boundaries (Figure 1). Sumintadireja P., Sudarman S., Mizunaga H., and Ushijima The MAM method can be performed following the K., 1997, Mise-á-la-masse study of Kamojang geothermal completion of the first exploratory well. The MAM method is field, Garut-West Java, Indonesia, Geothermal Research a quick mapping method and can be use for locating Report of Kyushu University, no.6, pp.69-81. fracture/fault zones, conductive areas and reservoir extensions. Gravity data gives subsurface information, which Sumintadireja P., Ushijima K., and Sudarman S., 1998, can be combined with other geophysical data. Mise-á-la-masse resistivity modeling of Kamojang geothermal field, Indonesia, Proceeding of the 99th SEGJ Interpretation of the Mise-á-la-masse results at the Kamojang Conference-Tokyo, pp.118-120. geothermal field need detailed elaboration of other geophysical and geological data. The Mise-á-la-masse results Ushijima K., Mizunaga H., Tanaka T., and Masuda K., define the reservoir boundaries. These results indicate a 1997, Fluid flow monitoring of EOR process by electrical need to reevaluate a potential production area located in a prospecting, SEG Expanded Abstract, vol., pp.651-653. low permeability zone.

1780 Figure 1. Geothemal Field Prospect in Kamojang Area (Compiled from PERTAMINA)

(A)

9213000 14 L8 13 Meters 12 11 10 0 2000 L9 9 9212000 13 12 11 8 10 7 9 6 5 8 10 9 8 7 6 5 4 3 2 1 L7 4 7 9211000 6 N 3 5 18 2 4 16 O 1 3 Y A 2 14 16 1 B M 16 14 12 14 16 16 9210000 12 14 C 12 10 10 14 14 5 4 3 2 1 1 12 18 13 12 11 10 10 9 8 8 78 6 12 2 L6 12 16 8 L 18 10 10 X 6 6 16 10 14 8 6 14 8 8 8 12 6 4 P 3 4 6 8 12 4 14 4 10 6 6 10 D 2 2 2 6 5 8 12 C1 (KMJ-63) 8 4 6 10 12 10 8 6 4 2 4 6 8 10 4 4 8 9209000 6 4 6 6 W 2 63 2 2 7 3 2 2 4 Q 4 2 4 5 21 2 8 7 4 2 4 6 18 16 14 11 10 4 2 4 6 8 10 12 3 8 6 2 9 E 11 9 4 10 8 2 48 5 6K 2 2 4 10 13 5 4 6 4 7 6 1314C1 (KMJ-48) 15 8 1 6 4 4 8 17 5 7 16 4 10 19 6 9 8 2 10 18 8 6 6 7 9 1 11 9208000 V 8 11 3 12 10 6 8 F 10 9 12 8 12 13 4 11 14 2 8 10 11 5 14 R 13 15 13 16 10 14 12 6 3 16 10 13 17 12 15 14 7 15 18 418 G 19 12 20 5 14 L2 L5 17 20 U 6 16 9207000 19 J 24 H 7 18 T Remarks: 8 8 S I Caldera Strct. 9 9 10 8 6 10 MAM Line 11 9206000 12 11 11 12 CSAMT Line 13 L4 12 L3 P2 804000 805000 806000 807000 808000 809000 810000 (B) Figure 2. Field Layout of the Mise-á-la-masse Survey

1781 y

C*(x ,y ,-z ) cc c P(x ,y ,0) pp

O x

C(x,y,z) ccc

B(x,y,z)

Figure 3. Line Source Model and the 3-D Resistivity Block

N-19 17 N 16 15 O-17 A-18 14 16 Y-15 17 B-18 13 15 16 14 12 14 M-18 17 13 17 15 16 13 11 15 C-20 9210000 12 12 16 14 19 11 10 11 15 13 14 18 10 9 10 14 12 13 17 Ohm-m 13 16 9 8 9 11 12 15 8 7 8 L-20 12 10 11 14 7 6 7 19 11 10 13 6 18 10 9 12 X-10 6 5 P-101716 8 9 44 9 5 5 9 15 9 8 11 8 4 4 8 14 8 7 10 7 6 4 3 7 13 7 9 5 3 56 12 7 6 40 4 3 2 4 11 6 6 8 1314 3 2 2 3 109 5 5 7 12 2 111 2 8 5 4 6 1011 D-15 9209000 W-13121110 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 91011Q-12 4 4 5 9 18 1 7 6 3 3 3 4 7 8 21 111 2 5 3 5 6 3 2 2 3 4 3 2 2 2 4 4 2 4 2 11112 3 16 6 5 3 3 5 1 1 2 7 4 3 4 K-206 719181716151413 11 109 5 4 3 2 1 1 2 3 4 5 6 7 8 9101112E-13 10 8 4 8 11 1 11 9 5 5 9 8 7 6 2 11 2 3 14 6 5 6 10 3 22 2 4 12 11 3 5 13 7 6 7 13 4 3 3 6 1514 8 7 8 14 5 4 4 7 12 15 6 4 8 9 1716 9 8 9 16 7 5 5 10 18 10 10 17 6 5 6 19 9 18 8 6 F-11 10 V-20 11 11 R-2019 9 7 7 9208000 10 12 10 7 8 12 11 11 8 13 13 12 9 8 9 8 14 12 14 13 10 9 10 15 13 15 14 11 10 G-11 6 16 14 16 15 17 17 16 12 11 15 17 13 12 18 16 18 18 4 19 19 19 14 13 U-20 17 20 20 15 H-14 18 22J-21 16 2 9207000 19 23 17 T-20 24 18 0 25 19 S-26 I-20 805000 806000 807000 808000 809000 810000 ρ Figure 4. Combined Apparent Resistivity ( a ) Map of KMJ-48 and KMJ-63

1782 300 m Caldera Structure KMJ-63 KMJ-48 No rth

600 m Ohm-m

40 20 10 2300 m 8 6 4 600 m 2 1 KMJ-63 (Vertical Well) Elevation: 1,490 m Measured Depth: 1,500 m

KMJ-48 (Directional Well) Elevation: 1,483 m Kick of Point: 630 m Measured Depth: 1,375 m 800 m True Vertical Depth: 1,314 m Direction: S 790 E

Figure 5. Resistivity Inversion Result

Figure 6. Bouguer Anomaly Map of West Kamojang Area (density=2.5 gr/cc)

1783 WSW ENE ( A ) Bouguer Anomaly Profile 32 32

x x 26 x 26 x x x x x x ( mgal ) 20 x x x x 20 x x x x x x x x

14 14

8 8 2000 ( m ) 17500

( B ) Gravity Model 1800 1800

ρ = 2.5 gr / cc ρ = 2.1 gr / cc ρ = 2.35 gr / cc ρ = 2.5 gr / cc 1000 ρ = 2.2 gr / cc 1000

c c ( m ) 0 / 0 r ρ = 2.15 gr / cc g .5 2 = ρ

-1000 -1000

-2000 ρ = 3.18 gr / cc -2000 2000 ( m ) 2000 17500

Cibitung Ciharus Caldera structure ( C ) Geological Model fault fault 1800 1800 CHR-1

2000 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^ ^^ ^ ^ ^ 1000 ^ ^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ 1000 ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ ^ + + + + ^ ^^ ^ ^ ^ ^ ^ ^ ^^ ^ ^ ^ ^ ^ + + + + + ^ ^ ^^^ ^ ^ ^ ^ ^ ^ ^ ^ ^ + + + + + + ^ ^ ^ + + + ^ ^ ^ ^ ^ ^ + + + ^ + + + + + + + ( m ) 0 ^ ^ ^ 0 + ^ ^ ^ + + + + + ^^^ + + + + ^ ^ ^ + + + + + + + + + ^ ^ ^^ + + + + + + +^ ^ ^ + + + + + + + + + + ^ + + + + + + ^ ^ ^ ^ + + + ^ ^ ^ ^ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ^ ^ ^^^ + + + + + + + + + + + + ^ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ^ ^^ ^ + + + + + + + + + + + + + ^ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ^ ^ + + + + + + + + + + + + + + + + + + + + + + + -1000 ^ ^ -1000 + + + + + + + + + + + + + + + + + + + + + + + + + + ^ ^^ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + ^^ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + -2000 -2000 2000 ( m ) 17500 Legend : + + Basaltic-andesitic Altered (auto)- Pyroclastic Volcanic clastics + rock brecciated andesite rocks rock ^ ^ Altered andesit Slightly altered Volcanic ash ^ pyroxene andesite and sand

Figure 7. WSW-ENE Gravity Profile, Model and Interpretation

1784