Resistivtiy Structure in Tangkuban Parahu Area Drived from CSAMT Data

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IOP Conf. Series: Journal of Physics: Conf. Series 877 (2017) 012055 doi :10.1088/1742-6596/877/1/012055

Resistivtiy Structure in Tangkuban Parahu Area Drived from

CSAMT Data

  1

  

1

  1

  1 Enjang Jaenal Mustopa , Nurhasan , Wahyu Srigutomo , and Doddy Sutarno

1 Earth Physics and Complex System Research Divison, Faculty of Mathematics and

  Natural Sciences, Bandung Institute of Technology, Indonesia E-mail: enjang@fi.itb.ac.id Abstract. Tangkuban Parahu is located in Bandung, West Java, one of a volcano area.

  Controlled-source audio frequency magnetotelluric (CSAMT) survey has been conducted in the area using transmitter located in 3-5 km from the survey area. The objective of the survey was to determine a subsurface resistivity structures that may correlate to a geothermal reservoir in unknown geothermal potential area. In the paper, we applied a two-dimensional (2D) inversion scheme to interpret of the CSAMT data to obtain several resistivity sections describing the subsurface resistivity structures. The results of 2D inversion indicated several subsurface low resistivity anomaly (1-10 ohm.m) in the depth of 1000-1500 meter that may correlate to the existing a geothermal reservoirir confirmed by several surface manifestations in the area.

1. Introduction

  

Tangkuban Parahu is located in a volcanic region in Bandung, West Java, Indonesia. The area is

o o

geographically situated in the South latitude of 06 7’17”– 6 7’17” and in the East longitude of

o o

  

107 36’12”- 107 40’16”. The topographic elevation of the field is ranging from 800 to 2200 m above

sea level. A Controlled-source audio frequency magnetotelluric (CSAMT) measurement, in the

operating frequency range of 0.25 Hz to 8192 Hz, has been carried out in the area to acquire 42 soundings

with the irregular space stations, varied from 200 to 500 m between sounding, covering the Tangkuban

Parahu and its environs. The location of transmitter was about 3 - 5 km in the south from the survey

area (Figure 1).

  The objective of the survey was to demonstrate the capability of the CSAMT method to determine a

subsurface resistivity structures in the deeper part and to delineate it that may correlate to a geothermal

reservoir in unknown geothermal potential area. Further more, if it is possible in the future, the area can

be exploited to produce a geothermal energy for generating the electricity after doing several

geophysical, geological and geocemical exploration. For the reason, the geothermal energy is one of the

alternatives in Indonesia.

  The configuration of CSAMT measurement is shown in Figure 2. The CSAMT method utilizes a

grounded electric dipole generating electromagnetic (EM) wave source has been developed by Goldstein

and Strangway [1]. The EM wave polarization of the fields can be selected by the orientation of the

transmitting antenna and the signal strength. The CSAMT measurement must be carried out at a distance

greater than 3-5 skin depths from the transmitter site where the plane wave approximation is valid. Since

the measurements were made using the electric field along the transverse, we assumed that the data

correspond to the transverse magnetic (TM) mode [2].

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IOP Conf. Series: Journal of Physics: Conf. Series 877 (2017) 012055 doi :10.1088/1742-6596/877/1/012055

2050 2000 1950 1900 1850 1800 1750 1700 1650 1600 1550 1500 1450 1400 1350 1300 1250 1200 1150 1100 1050 1000 950 900 850 800

  

Figure 1. Map of Tangkuban Parahu area

Transmitter

  3

  2

  

5

~

  1

  4

  4

~ 4 km

  1. Generator o 2 ~ 4 km

  

60

~

  2. Transmitter

  3. Frequency controller

  4. Current electrode

  5. Electric dipole 4 ~ 8 km

Line survey

  Receiver

Rx Hr

Magnetic coil

  Hy

5 ~ 10 m

Receiver

  INPUT SENSOR CH 8

  INPUT SELECTOR

  1 2 3 4 5 6 7 Ground

  Ex 100 ~ 200 m Pb/PbCl

  2 Figure 2. Configuration of CSAMT measurement

  

IOP Conf. Series: Journal of Physics: Conf. Series 877 (2017) 012055 doi :10.1088/1742-6596/877/1/012055

The location of the sounding sites of the CSAMT survey in Tangkuban Parahu is shown in Figure 3.

  

The transmitter (Tx) was located about 3 to 7.5 km in the South from the sounding site area that was in

Cikole, Lembang. The orientation of the electric field was parallel to the electric current dipole in every

sounding site. On the other hand, the magnetic coil is located about 7 meters from the receiver and

perpendicular to the electric field (TM mode).

  

Figure 3. Location of the sounding sites of the CSAMT survey

The typical structure of geothermal system is schematically presented in Figure 4. The cooler upper

zones are characterized by alteration of electrical conductive layer that is formed at temperatures above

o

  70 C. At higher temperatures, illite of less conductive layer becomes interlayered with smectite. The

proportion of illite increases with the temperature, forming about 70% of the mixed-layer clay at 180

o

C. Above this temperature, the smectite content continued to decrease, and pure illite commonly

  o appears at greater than 220 C[3]

  < 10 ohm .m TITE SMEC MIXED TE

ECTI LAYE

R

  E-SM

  ILLIT LITE < 10 oh CLAY m.m

   IL CAP

  

10 - 60 ohm.m

RESERVOIR

(PROPYLLITIC)

  

Figure 4. A generalized geothermal system [3]

  

IOP Conf. Series: Journal of Physics: Conf. Series 877 (2017) 012055 doi :10.1088/1742-6596/877/1/012055

2. Basic theory

  The basic concept used in the CSAMT method can be derived from the solution of Maxwell’s equations, these are ¶

  H (1)

• Ñ ´ =

  µ E

  ¶ t

  ¶ D (2)

• Ñ ´ H J = ¶ t ×

  (3) Ñ D =

  × (4)

  Ñ B = where E is the electric field intensity (V/m), B is the magnetic induction (tesla), H is the magnetic field intensity (A/m), w is the angular frequency, µ is the magnetic permeability (H/m).

o

  The general solutions of a tangential component of electric field (E ) and a radial component of f magnetic field (H r ) are expressed in cylindrical coordinates as [4]

  Idl sin f

  E (5)

  »

  3 f r ps

i

• - p

  Idl sin f

4 H e

  (6) » r

  3 r

  µsw p The apparent resistivity on the earth surface derived from Maxwell’s equation in a far-field zone is

  

2

E 1 x

  (7) r = axy

  µw H

y

  Although the CSAMT measurements were carried in a far-field zone, the data still contain some near

field and transition zone data when the frequency from transmitter signal is low (deep survey).

Therefore, it needs to make a correction technique for a near-field and transition effect to use the

CSAMT data before applying the MT interpretation scheme [5].

3. Interpretation of CSAMT data

  

A 2D inversion technique based on a non-linier conjugate gradient method [7] was applied to invert the

selected CSAMT data. Since the measurements were made using the electric field along the transverse

and the magnetic field normal to the traverse, it is assumed that the data correspond to the TM-mode.

  Employing a 2D inversion scheme to the CSAMT data in Tangkuban Parahu, we constructed the 2D

resistivity section for several profiles. The resistivity section obtained from 2D inversion of Line-2

indicates that the distribution of the low resistivity anomaly (<10 ohm.m) in the depth of 700-1000 m is

distributed in the northeast from t-19 to t-28 (Figures 5). This anomaly is may considered as

hydrothermal alteration zone in a geothermal reservoir structure. On the contrary, the low resistivity

anomaly is not found in the southeast. From the figure also shows that the electrical discontinuity is

found below the sounding site t-07, it may correspond to the existing fault in the area.

  It is identified from the 2D resistivity section of Line-8 of NW-SE direction (the position of line

shown in the left of the Figure 6) that the distribution of the low resistivity anomaly is also found below

sounding sites t-17 to t-35. It is also considered as the hydrothermal alteration zone. Furthermore, the

electrical discontinuity also discovered below the sounding site t-10, it is considered as a fault in this

zone.

  Finally, from the resistivity section of Line-9 in NW-SE direction (Figure 7) indicates that the

subsurface resistivity of this line is mainly composed of 3 layers. The first layer (overburden) is high

resistivity (50 to 500 ohm-m) that is between 800 to 1000 meters thick overlying the lower resistivity

zone (1 - 20 ohm.m) in the second layer that is approximately 1000-1500 meters thick. These anomalous

features may be due to faulting, fracturing, and hydrothermal alteration along the significant fault .

• 500 500 1000 t12 t13

  t07

  t04 t19 t21 t28
>1500
• 500 500 1000 t39 t10 t15 t17 t19 t16 t35

  790 000 7 910 00 79 200 0 793 000 794 000 7 950 00 925 000 0 925 100 0 925 200 0 925 300 0 925 400 0 925 500 0 925 600 0

  40

  60

  80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 ohm.m

  NW SE

  Tx1 Tx2

  Tx T01 T02 T0 3 T04 T05 T06 T07 T08 T0 9 T1 0 T11 T1 2 T13 T14 T1 5 T16 T17 T1 8 T19 T20 T2 1 T2 2 T23 T24 T25 T2 6 T2 7 T2 8 T2 9 T30 T31 T3 2 T3 3 T34 T35 T36 T37 T38 T39 T40 T41 T42

  Line 9

  500 1000 1500 2000 2500 3000 3500

  500 1000 1500 2000 2500 3000 3500 4000

  20

  40

  60

  80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 ohm.m

  NW SE E le v a ti o n ( s e a l e v e l) ( m )

  20

  Line 8

  Distance (m)

  500 1000 1500 2000 2500 3000 3500 4000 4500 5000

  

IOP Conf. Series: Journal of Physics: Conf. Series 877 (2017) 012055 doi :10.1088/1742-6596/877/1/012055

Figure 5. Resistivity section derived from 2D inversion result at Line-3

Figure 6. Resistivity section derived from 2D inversion result at Line-8

Figure 7. Resistivity section derived from 2D inversion result at Line-9

  To understand the resistivity distribution in term of 3D geological structure, we constructed the 3D

model of the low resistivity anomaly (<10 ohm.m) as shown in the Figure 8. The figure shows that the

conductive (second) layer, which correlates to the layer of a hydrothermal alteration zone, is distributed

in the northeastern part of the Tangkuban Parahu area. The region is probably a hydrothermal alteration

zone where the geothermal reservoir may exist in that area.

  Tx1 Tx2

  Tx T01 T02 T0 3 T04 T05 T06 T07 T08 T0 9 T1 0 T11 T1 2 T13 T14 T1 5 T16 T17 T1 8 T19 T20 T2 1 T2 2 T23 T24 T25 T2 6 T2 7 T2 8 T2 9 T30 T31 T3 2 T3 3 T34 T35 T36 T37 T38 T39 T40 T41 T42

  790 000 7 910 00 79 200 0 793 000 794 000 7 950 00 925 000 0 925 100 0 925 200 0 925 300 0 925 400 0 925 500 0 925 600 0

  Line 3

  20

  790 000 7 910 00 79 200 0 793 000 794 000 7 950 00 925 000 0 925 100 0 925 200 0 925 300 0 925 400 0 925 500 0 925 600 0

  40

  60

  80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400 420 440 460 480 500 ohm.m

  SW NE E le v a ti o n ( s e a l e v e l) ( m )

  Distance (m)

  Tx1 Tx2

  Tx T01 T02 T0 3 T04 T05 T06 T07 T08 T0 9 T1 0 T11 T1 2 T13 T14 T1 5 T16 T17 T1 8 T19 T20 T2 1 T2 2 T23 T24 T25 T2 6 T2 7 T2 8 T2 9 T30 T31 T3 2 T3 3 T34 T35 T36 T37 T38 T39 T40 T41 T42

• 2000
• 1500
• 500 500 t24 t42 t09 t21 t19 t20
>2000
• 1500
• 1000

  

IOP Conf. Series: Journal of Physics: Conf. Series 877 (2017) 012055 doi :10.1088/1742-6596/877/1/012055

1000 100

  10

  1 Figure 8. 3D view of resistivity structure in Tangkuban Parahu area

4. Conclusions

  

The CSAMT method applied at Tangkuban Parahu area was successful to locate an anomalously low

resistivity zone that may indicate the existing of a potential geothermal reservoir. Employing the 2D

inversion of CSAMT data generates several geoelectrical cross sections are found in the some locations

that the resistivity structure is similar to a geothermal reservoir structure. The subsurface resistivity

structures composed mainly by three types of resistivity feature obtained from 2D inversion results of

CSAMT data. The overburden (first layer) has a resistivity of 30 – 150 ohm-m and thickness of 800 to

1000 m. The intermediate (second) layer has an extremely low resistivity of 3 – 15 ohm.m with 1000 –

1500 m thick. The low resistivity layer as an anomalous feature can be considered as a hydrothermal

alteration zone (impermeable layer/cap rock). Finally, the basement (third) layer that is relatively more

resistive than the second layer with resistivity of 30 - 100 ohm-m.

  Further more, if it is possible in the future, the area can be exploited to produce a geothermal energy

for generating the electricity after doing several geophysical, geological and geocemical exploration.

For the reason, the geothermal energy is one of the alternatives in Indonesia.

  Acknowledgment

The authors are grateful to Penelitian Unggulan Perguruan Tinggi Ristek-Dikti for funding of the

research in the year of 2016.

  References Goldstein M A and Strangway D W 1975 Audio frequency magnetotelluric with grounded electric [1] dipole source Geophysics vol. 40 669-683. Sasaki Y, Yoshihiro Y and Matsuo K 1992 Resistivity imaging of controlled-source [2] audiofrequency magnetotelluric data Geophysics 57 952-955.

  E. Anderson, D. Crosby and G. Ussher, Bulls-eye! -Simple Resistivity Imaging to Reliably Locate [3] the Geothermal Reservoir, Poc. WGC 2000, 909-914, 2000. Kaufman A A and Keller G V 1983 Frequency and transient sounding method Elsevier Science [4] Publ. Co. Inc. Yamashita M, Hallof P G and Pelton W H 1985 CSAMT case histories with a multichannel [5] th

  CSAMT system and near-field data correction 55 SEG Annual Convention 276-278 Rodi W and Mackie R L 2001 Nonlinear conjugate gradients algorithm for 2-D magnetotelluric [6] inversion Geophysics 66 174-187.

  Sasaki Y, Yoshihiro Y and Matsuo K 1992 Resistivity imaging of controlled-source audio [7] frequency magnetotelluric data Geophysics 57 952–955.