24. Schmucker-Weidelt-Kolloquium Neustadt an der...

Magnetotelluric Exploration of the Sipoholon Geothermal Field, Indonesia

Sintia Windhi Niasari1, Gerard Muñoz

1, Kholid Muhammad

2, Edi Suhanto

2, Oliver Ritter

1

1GFZ German Research Centre for Geosciences, Telegrafenberg, 14473 Potsdam, Germany

2Pusat Sumber Daya Geologi (PSDG), Badan Geologi, Jl. Soekarno-Hatta 444, 40122 Bandung,

Indonesia

Abstract

The Sipoholon geothermal field is located in North Sumatra, Indonesia. The geothermal field

is characterized by 15 hot springs situated in the Tarutung pull-apart basin and an additional 8

hot springs outside of the basin. The main difficulty in understanding the geothermal system

is the temperature distributions of the hot springs, which appears to be random, based on the

occurrence of 3 inactive volcanoes around the basin.

Here we report on preliminary results of two MT experiments that were carried out in

Sipoholon, in December 2010 and July 2011. Data quality is generally good with the

exception of sites from the populated basin area, which is noisier. Preliminary modeling

results indicate a shallow high conductivity layer in the pull-apart basin area is generally

caused by the sedimentary fill. Deeper conductive structure occurs east of the basin. Further

data processing and modeling is necessary to decide if zones of high conductivity are related

with a clay cap or hydrothermal fluids.

Introduction

Indonesia has an abundance of geothermal resources mostly associated with the volcanic arc,

including Sumatra Island. On Sumatra, there are 84 known geothermal areas, but only 6 high

temperature areas are under development (for electricity production). Many of the lower

temperature areas have not been studied in detail and energy production from the latter is nil

(Hochstein and Moore, 2008). The low-enthalpy Sipoholon geothermal field is located along

the Sumatra Fault, in the Tarutung pull-apart basin, North Sumatra. Although there are

several publications about this area, the geothermal system is still poorly understood. The

Martimbang, Imun, and Helatoba Tarutung volcanoes are near the basin, but not inside it. It is

still debated which, or whether, one of them could be a heat source for the hot springs. Other

factors of the hydrothermal system are still enigmatic, for example whether faults play a role

as fluid pathways, whether a sealing clay cap exists, and how water recharge into the

hydrogeological system occurs.

Existing geophysical measurements carried out in this area include DC electrics, magnetic,

and gravity methods. Since the spatial distribution of these measurements was insufficient,

the results cannot answer how the Sipoholon hydrothermal system works. The initial MT

measurements showed high resistivity beneath the graben and low resistivity east of the

basin. However, only lines 4 and 5 were crossing the basin (see Figure 1), so to continue our

previous research, we did additional MT field measurements covering a larger area than the

first measurements. Lines A, B, C, and D cross the basin and cover the hot springs area. The

24. Schmucker-Weidelt-KolloquiumNeustadt an der Weinstraße, 19.–23. September 2011

172

southernmost hot springs (Namora I langit) correspond to the Sarulla high enthalpy

geothermal system.

The MT data of the first experiment often showed poor data quality above 4 s, to improve the

situation for the second experiment, we recorded for a longer time (three days per site instead

of one) and we installed a permanent remote reference site.We present data and preliminary

2D inversion result from both experiments, and briefly discuss the resistivity structure of the

Sipoholon geothermal field.

99°5'0"E99°0'0"E98°55'0"E

2°1

0'0

"N2°5

'0"N

2°0

'0"N

1°5

5'0

"N

0 5 10 15 km

Line A

Line B

Line C

Line D

Line 1

Line 2

Line 3

Line 4Line 5

Figure 1. Location map of the study area. 26 MT sites were measured by PSDG in December 2010

(black dots). In July 2011, 71 MT sites were measured by GFZ (blue dots). Red dots mark hot springs.

Red triangles are volcanoes.

Niasiri et al., MT Exploration of the Sipoholon Geothermal Field, Indonesia

173

Geological background

The dextral strike-slip Sumatra Fault accommodates oblique convergence between the

Eurasian and Indo-Australia plates (Yeats et al, 1997 on Sieh and Natawidjaja, 2000). This

1900-km long fault consists of 19 segments (Sieh and Natawidjaja, 2000). There are thirteen

pull-apart basins along the fault which occur near clusters of volcanoes, but volcanoes are

rarely found inside the basins (Muraoka et al., 2010; Bellier and Sébrier, 1994). The

Sipoholon pull-apart basin is dominated by major strike-slip faults along its longitudinal axis

and bound by normal faults along its short-axes (Hickman, 2004). Figure 2 shows the

Sipoholon and Sarulla pull-apart basins. Major fluid discharges are observed along the NW

boundary normal fault in the Sipoholon pull-apart basin and the SE boundary normal fault in

the Sarulla pull-apart basin. The NW boundary normal fault of the Sipoholon pull-apart basin

at Ria-Ria-Sipoholon is situated on top of a travertine terrace. Muraoka et al. (2010)

suggested that the NW boundary normal faults of the Sipoholon pull-apart basins play an

important role as major discharge or fluid up flow zone.

Nukman and Moeck (2011) identified hot springs situated within and around the basin which

are regionally controlled by NW-SE strike slip. The hot springs west of the Tarutung pull-

apart basin are dominated by NE-SW trending fractures, while a NW-SE trend prevails north

of the basin. The highest temperatures of 62°C (Ria-Ria Hot spring) were measured during

the rainy season. However, there is no simple correlation between temperature distribution of

hot springs and distance from volcanoes (Figure 3). Figure 3 also shows lineaments which

were derived from SRTM (Shuttle Radar Topography Mission) high resolution digital

topographic data of the Earth.

Sarulla pull-apart basin

Toba caldera volcanic cluster

Sipoholon pull-apart basin

Lubukraya volcanic cluster

Martimbang volcanic cluster

NW edge discharge zone

SE edge discharge zone

99 20' E° 99 40' E°

02 20' N°

02 00' N°

01 40' N°

Figure 2. Hot spring discharges from the northwestern boundary normal fault of the Sipoholon pull-

apart basin (Muraoka et al., 2010).


174

Figure 3: Tectonic lineaments derived from SRTM digital topography, drainage patterns, and field

observations. Coloured dots mark locations of hot springs, temperatures after Nukman and Moeck

(2011).

Magnetotelluric data processing and interpretation

The first measurements were carried out by Badan Geologi in the Sipoholon area in

December 2010 using MTU 5A Phoenix instruments. Time series of Hx, Hy, Ex, and Ey

were measured at 26 MT sites in the frequency range 320-0.0034 Hz for approx. 1 day. The

data were processed using MT Editor from Phoenix. Some data points with larger error bars

(very noisy data above 4 s) were removed by manual inspection.

For the second measurements time series of Hx, Hy, Hz, Ex, and Ey were recorded in the

frequency range 1000-0,0001 Hz for approx. 3 days. For the duration of the experiment we

Mt. Imun

Mt. Martimbang

99°00' E98°50' E

2°10' N

2°00' N

5 02,5 5 Kilometers

Temp. Manifestation22,3 - 27,6 deg. C

27,7 - 40,0 deg. C40,1 - 45,9 deg. C46,0 - 49,2 deg. C49,3 - 62,0 deg. C

Mt. Helatoba

Sumatra fault

Taru

tung p

ull-apart ba

sin


175

operated a Remote Reference site to improve the data quality. The remote reference

technique can help to remove noise at sites which are close to houses, power lines or other

electromagnetic noise sources. To remove noise in the short period range, particularly 50 Hz

and harmonics, we applied a delay line filter.

After performing geo-electric strike analysis (Becken and Burkhardt, 2004), the data from

first measurements were rotated to N47°E so that the xy polarization corresponds to the TE

mode. Secondary electromagnetic fields which are elliptically polarized are generated in the

presence of a regional 2-D conductivity anomaly. The regional strike direction can be then be

identified from vanishing ellipticities of electric and magnetic fields, because the ellipticity

remains unchanged even if the electric field at the surface is distorted by inhomogeneities. If

the minimal ellipticities are close to zero, then the telluric vectors can be rotated to the

coordinate system of the regional strike. We found an angle of -43° in which the ellipticity

vectors are generally minimal for all periode (0,001-10 s) and for all sites (Figure 4). The 90°

ambiguity can be solved as the direction seems to coincide with the direction of the Sumatra

Fault which is N40±2°W in this area (Genrich et al. 2000). Some sites show deviations from

the regional strike directions by more than 30°; likely these sites are affected by local shallow

structures.

Figure 4. Rose diagrams showing the strike angle distribution of line 1, 2, 3, 4, and line 5, and the best

fit regional strike angle for all lines. These were calculated for period 0,001 – 10 s.

Resistivity models were obtained along five lines (Figure 5) from 2D inversion of the rotated

data, using the code of Rodi and Mackie (2001) which is included in the Winglink software

package. The inversions were performed setting an error floor of 100% for TE apparent

resistivity (i.e. down-weighting this component), 10% for TM apparent resistivity and 1.5°

1 2 sites

N

S

EW

Regional strikeLine 1

1 2 sites

N

S

EW


1 2 sites

N

S

EW


N

S

EW

Best fitRegional strike

All lines

1 2 sites

N

S

EW


1 2 sites

N

S

EW


30 sites 10

20


176

for the phases of both modes. After an L-curve analysis, the optimal value of the smoothing

parameter (τ) was found to be 10.

Figure 5. Resistivity models obtained from 2D inversion of the MT data of the first experiment along

five lines. The black line indicates the location of the Tarutung pull-apart basin.

The results (Figure 5) indicate a shallow high conductivity layer extending down to approx. 1

km in all lines. In lines 4 and 5 the shallow conductors could correspond to unconsolidated

sedimentary fill of the Tarutung pull-apart basin. This basin formed by discontinuities in the

Sumatra strike slip fault and was filled with thick pyroclastic flow deposit of Toba volcano.

Lines 1, 2 and 3 do not cross the graben area and hence, the 2D inversion models of those

lines appear quite different to the other lines. The southernmost lines 3, 4 and 5 indicate the

presence of a deeper conductive anomaly located slightly off to the east of the graben starting

at 3 km depth.

Figure 6 shows 2D inversion model result from lines A, B, C, and D which include all data

from the first and second measurements. Recalculation of the geo-electric strike analysis

(Becken and Burkhardt, 2004) based on the entire data set, resulted in a more complicated

situation for the regional strike. Data rotation depends on the distance of a site from the basin

area: sites east of the basin were rotated to N327°E, sites within the basin were rotated to

N343°E, and to N324°E for sites west of the basin. After rotation the xy- polarization

corresponds to the TE mode. The inversion models were obtained using the code of Rodi and

Mackie (2001) which is included in the Winglink software package and setting error floors to

100% for TE apparent resistivity (i.e. down-weighting this component), 10% for TM

apparent resistivity and 1.5º for the phases of both modes. After an L-curve analysis, the

optimal value of the smoothing parameter (τ) was found to be 3.

The 2D inversion results are still preliminary. High resistive bodies to the west of the basin

area could correspond with Permian granites. Shallow high conductive zones found in the

basin area could be caused by (unconsolidated) sedimentary fill. Lines B and D show these

-6-4

-2

0

2

4

6

-6

-5

-4

-3

-2

-1

De

pth

[km

]

Line 1 (rms: 1,29)

-6-4

-2

0

2

4

6

-6

-5

-4

-3

-2

-1

-6-4

-2

0

2

4

6

-6

-5

-4

-3

-2

-1

-6-4

-2

0

2

4

6

-6

-5

-4

-3

-2

-1

-6-4

-2

0

2

4

6

-6

-5

-4

-3

-2

-1

Distance [km]

Line 5 (rms: 1,29)

Line 4 (rms: 1,74)

Line 3 (rms: 1,70)

Line 2 (rms: 1,12)

210502001000

Resistivity (Ohm.m)

Tarutungpull-apart basin


177

shallow conductive zones quite clearly while their expression is weaker in lines A and C.

More conductivity anomalies appear east of the Tarutung pull-apart basin. However, 3-D

modeling is needed to find out if and how these conductors are connected.

Figure 6. Resistivity models obtained after 2D inversion of the all MT data (four lines). The green

bars indicate the location of the Tarutung pull-apart basin.

0

5

10

15

-10

-8

-6

-4

-2

0

Dep

th

[km

]

0

5

10

15

20

25

-10

-8

-6

-4

-2

0

0

5

10

15

20

25

-4

-2

0

Distance [km]

02

46

810

1214

-10

-8

-6

-4

-2

0

?

?

Tarutungpull-apart basin

Resistivity [Ohm.m]2105020010005000


178

Acknowledgment

This work would not have been possible without field support and the cooperation of

numerous colleagues from Germany (GFZ) and Indonesia (Badan Geologi, ITB, Unila). The

project is funded by the German Federal Ministry of Education and Research (BMBF,

03G0753A)., the Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences

and Badan Geologi. MT Instruments for the field campaign in 2011 were provided by the

Geophysical Instrument Pool Potsdam.

References

Becken, M. and Burkhardt, H. (2004) - An elipticity criterion in magnetotelluric tensor

analysis, Geophys. J. Int., 159, 69-82.

Bellier, O. and Sébrier, M., 1994. Relationship between tectonism and volcanism along Great

Sumatran fault Zone deduced by SPOT image analyses. Tectonophysics, 233:215-231

Genrich, J.F., Bock, Y., McCaffrey, R., Prawirodirdjo, R., Stevens, C.W., Puntodewo, S.S.O.,

Subarya, C., and Wdowinski, S. (2000) - Distribution of slip at the northern Sumatra fault

system, Journal of Geophysical Research, 105, 28327-28341.

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Tectonic and stratigraphic evolution of the Sarulla graben geothermal area, North Sumatra,

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Rodi, W. and Mackie, R.L. (2001) - Nonlinear conjugate gradients algorithm for 2-D

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Sieh K. and Natawidjaja D., 2000. Neotectonics of the Sumatran Fault, Indonesia. J.

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