Kusmita 2022 IOP Conf. Ser. Earth Environ. Sci. 1108 012066

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IOP Conference Series:

Earth and

Environmental Science

PAPER • OPEN ACCESS

Resistivity analysis of geothermal system at cubadak village west sumatera using

magnetoteluric method

To cite this article: T Kusmita et al 2022 IOP Conf. Ser.: Earth Environ. Sci. 1108 012066

View the article online for updates and enhancements.

Resistivity analysis of geothermal system at Cubadak Village West Sumatera using magnetoteluric method

T Kusmita^1*, Y A Adhyaksa², M Z Nasri², and W Joni³

1Physics Department Faculty of Engineering, Universitas Bangka Belitung, Bangka Island, Indonesia

2Geophysical Engineering Department of Science and Technology, Universitas Jambi, Jambi, Indonesia

3Coal Mineral and Geothermal Resource Center, geological Agency, Ministry of Energy and MineralResources, Bandung, Indonesia

*Email: [email protected]

Abstract. Cubadak area was located in depression zone of great Sumatra fault zone which affected by normal faults. Geothermal in this area was characterized by the appearance of hot springs type chloride-bicarbonate on Cubadak, Sawah Mudik and Talu. The manifestation temperature on this area is about 37,1⁰– 74,8⁰ C. This study purpose to recontruct model of subsurface geothermal system in Cubadak area based on MT data. Model 1-D was processed by apply Bostick inversion and NLCG (Nonlinear Conjugate Gradient) for model 2-D. The result showed that lowest resistivity was on Southwest to Northeast research area. Highest resistivity spread at West to Northeast. Geothermal system was composed by caprock (2-10 ohm), reservoar (20-300 ohm) and heat source (>300 ohm).

1. Introduction

Sumatera has a very complex geological structure, both in terms of sedimentology, volcanology, tectonic and natural resource potential [1]. The complexity of the Sumatera geological arrangement, is a result of tectonic changes that developed over time and space, which enables the various direction of vector patterns related to slip-rate and segmentation of Sumatera fault [2]. This would be in consequence of the difference of tectonic condition will make a various reaction to rock towards structure reactivation, also older geological structure will affect capability of younger rock deformation[3].

Cubadak geothermal area extends in a northwest-southeast direction[3]. Normal fault activity is northwest-southeast which is influenced by the Great Sumatran Fault[2]. This fault is filled with material forming sedimentary rocks and alluvium deposits. Tectonic activity in the Pliocene – Pleistocene has resulted in the formation of several normal fault structures that form the Cubadak fault area [4]. This tectonic activity also triggers volcanic activity in the study area which produces pyroclastic flows and lava [5]. So, physical characteristics of geothermal system is crucial study. It can be used to defict presence of geothermal system component such as reservoir, clay cap, heat source and geological structures that control the area [6] [7], [8]. Magnetotelluric (MT) method have a high sensitivity for identify of resistivity variation which delineate subsurface condition of [3], [9], [10].

MT is a passive geophysical survey method which involves the natural variation of the earth's magnetic field as the source [3], [11]. The Magnetotelluric method has a long penetration depth with a much deeper penetration range of ±10km [12], [13]. In this study anomaly resistivity used for recontruct model of subsurface geothermal system in Cubadak area based on MT data.

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2. Data Analysis and Methods

Applied geophysics on geothermal exploration used for getting the physical information in geothermal field. It was related to geothermal system component such a reservoir and cap rock. In MT method it was detected using resistivity value (1-100 Ωm)[14]. This method used to obtain conductive layer based on resistivity anomaly [15]. Resistivity anomaly variations can figure an existence of structure in sub surface [13] MT method use an electromagnetic wave as source such as solar wind and lightning activity (0.001-104 Hz) [12]. Frequency range of magnetotelluric is extensive and worth it for delineate geothermal system component up to 1000 m [16].

Application of magnetotelluric for geothermal system investigation requires inversion theory by apply an inversion algorithm [17]. The selected algorithm must be able to describe conditions of subsurface based on observations data. Inversion modelling done by fitting the data for the model parameters and find the best fits response with the observation data[18]. This study used Bostick algorithm for inversion scheme. This algorithm produces distribution of resistivity anomaly continuously or almost continuous to depth [19]. Resistivity was estimated by mean of apparent resistivity, and the factor of phase observation by eq (1):

𝜌 (𝑧) = 𝜌_𝑎(𝜔) ( 𝜋

2𝜙 (𝜔)− 1)

(1) 𝑧 = (𝜌_𝑎(𝜔)

𝜇𝜔 )

1/2

Where 𝜔 is frequency and z corresponding of depth versus skin depth on half space of apparent resistivity 𝜌_𝑎. Distribution of resistivity on lateral variation deliniated by 2-D model using Nonlinear Conjugate Gradient (NLCG) algorithm inversion (eq. 2). this schematic algorithm used for minimize of residual effect from resistivity anomaly which form of second derivative spatial [20]. NLCG was chosen because minimize the effects of irregularities from object [20]

𝒅 = 𝐹 (𝒎) + 𝒆 (2)

vector of observation data (d) and vector of model (m), F is forward modelling function and e is error vectors. If d rewritten as 𝑑 = [𝑑¹ 𝑑² . . . 𝑑^𝑁]^𝑇, where log of amplitude (𝑑^𝑖) and 𝜌_𝑎𝑝𝑝 phase (eq. 3) for polarization of transverse electric and transverse magnetic in observation site, ω is cutoff frequency.

Resistivity model calculated using function of parameters vector 𝑚 = [𝑚¹ 𝑚² . . . 𝑚^𝑀]^𝑇. 𝜌_𝑎𝑝𝑝= ^𝑖

ωµ(^〈𝐸^𝑥^〉

〈𝐻_𝑦〉)

2

(3)

〈𝐸_𝑥〉 is average of electric field observation component in x-axis. 〈𝐻_𝑦〉 is average of magnetic field observation component in y-axis. Problem of inversion solved by apply ill-posed solution[20]. Function ψ can minimized using a model with regularized solution on eq. 4. [20].

ψ (𝐦) = (𝐝 − 𝑭 (𝒎))^𝑻𝑽^−𝟏(𝐝 − 𝑭 (𝒎)) + λ𝒎^𝑻𝑳^𝑇𝑳𝒎 (4) with matrixs (V) used for compute error vectors variation (e) and λ is a + parameter. ψ used as a function for stabilize model which chosen. L is a matrix that used as operators in second derivative for model with uniform blocks. Lm applied by Laplacian approximation from log 𝜌_𝑎𝑝𝑝 [20].

3. Result and Discussion

model 1-D generates resistivity anomaly in vertical space using Bostick inversion algorithm. This model used as compactor of model 2-D. 2-D was deliniated a subsurface model in vertical and horizontal space in Cubadak area using NLCG computational algorithm. 1-D model from Bostic inversion used to generate resistivity anomaly variations versus depth based on apparent resistivity pseudo section curve [21].

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3.1. Resistivity Model

Resistivity model show the value of average resistivity from transverse electric and transverse magnetic mode every line (Figure 1 for Line 1). In this line there are 7 points data acquisition start at point MTCB7, MTCB8, MTCB9, MTCB0, MTCB11, MTCB2, and MTCB3. In this line there are four resistivity value with different range. Low resistivity (6-20 ohms) was spread at point MTCB 10 and 11.

It’s related to hot spring manifestation. Low resistivity maybe conducted to clay cap in geothermal system. High resistivity (300 ohm) at -2200 msl associated with igneous rock and also intrusive rock was spread in point MTCB13. Medium resistivity (green to blue sky color) spreading on other point.

Figure 1 given a model 1-D. This model just shown resistivity anomaly in every point. It’s difficult to analysis relationship between points acquisition. It’s mean 1D cann’t deliniated subsurface along the line. 2-D model used for solve that problem.

Figure 1. 1D Inversion Model Based on Resistivity Anomaly

Model of 2D inversion (Figure 2) used combination of transverse electric and transverse magnetic modes (τ=1, rms = 1,5 %). Figure 2 shown a low resistivity anomaly (<20 Ohm-m) speading along the line until 150 m underneath subsurface. Medium resistivity anomaly (20-200 ohm-m) spead to northeast with with a thickness of 800 m indicated as reservoir. Next layer is high resistivity zone (200-300 ohm- m). This layer indicated as a bedrock that spread to northeast with shallower depth. Geothermal system actually compossed of geothermal system component that distributed vertically. Figure 2 shown reservoir distributed at 60 m – 180 m along the line. Distribution of bedrock is more homogeneous about 300 m – 700 m along the line.

Geothermal in this area was controlled by Rantau Panjang Fault (Southwest-Northeast) and Cubadak Fault (North - South). These faults not only control geothermal system component but also surface manifestation (presence of hot spring). Charateristic geothermal system in this area is relative complex.

This geothermal is a combination of outflow an upflow zone from Mt. Marapi. Agrilik alteration zone that rich in clay minerals is upflow zone which heating the rock and made a thermal maturation. Outflow zone is leading to surface manifestation (hot spring) through the Cubadak fault (SC). Top reservoir zone at geothermal of Cubadak be around the surface manifestation of Cubadak about 1000 msl and 1250 msl around surface manifestation of Sawah Mudik [3]. The existence of the reservoir zone is suspected on Sabak rock as base rock in this area. Cap rock also associated with pyroclastic rock which fills almost the entire Cubadak depression zone.

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Figure 2. 2D Inversion Model Based on Resistivity Anomaly

3.2. Cycle of Geothermal on Cubadak

Geothermal of Cubadak has unique conditions compared to other common volcanic geothermal systems. The structure plays an important role in the features of the geothermal system in this field.

Starting from the thermal features that lie in en echelon pull-apart depression of Sumatera fault. Then the reservoir zone is predicted as the fractured formation in fault zone, and also the heat source the appearance of which is contolled by the Great Sumatera Fault (Bukit barisan belt). [14]. Geothermal system of Cubadak have arisen due to the fissure eruption. This eruption produces young intrusions in Cubadak Graben. Residual heat from it process used as heat source in geothermal Cubadak. Cubadak fault, Botung and also Rantau Panjang was controlled the manifestation [9].

Figure 3. Cycle of Geothermal Model based on Resistivity Anomaly

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Geothermal cycle (figure 3) in this area start from meteoric water that flows through the crack and fault (Cubadak, Botung and Rantau Panjang) then heated by reservoir rock (Sabak rock) underneath of subsurface. The hot water trapped under the clay cap rock which has a low resistivity anomaly. Heat source (base rock) in this area reach to 2 km vertically with a large size. Heat transfer from this layer triggers the alteration activity around the fault in this geothermal area.

4. Conclusion

Cubadak area is a combination of outflow an upflow zone geothermal system from Mt. Marapi. It has a conductive layer about 1km as an alteration zone (2-10 ohm-m). Reservoir has a resistvity anomaly about 20-300 ohm-m) along 1 - 1,8 km. Heat source about >300 ohm-m West to Northeast area until depth about 2000 m. This system was controled by Rantau Panjang and Cubadak Fault.

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Acknowledgement

We gratefully acknowledge the funding from Universitas Bangka Belitung through the RKAKL of Physics Department Faculty of Engineering for the publication of this paper.