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provided by Open Access Jurnal Politeknik Negeri Jember ISBNISBN 978-602-14917-7-5 978-602-14917-7-5

Geoelectrical Survey of Blawan Ijen Geothermal Field, East

Siti Diah Ayu Febriani1, Rizqa Daniyati2 1, 2 Renewable Energy Dept, Politeknik Negeri Jember Jalan Mastrip 164, Jember 68101, Indonesia

[email protected]

Abstract. An geoelectrical survey with Schlumberger configuration of Blawan Ijen geothermal field has been performed. This research to investigations the geothermal system based on resistivity value. In Blawan Ijen be found around 20 hot water manifestations geothermal field with temperature of 510C. Measurement of primary data uses land gravitymeter G 403 type with 8 m up to 80 m spacing, made in 96 points and 5 locations in Blawan Ijen. Data obtained from this research are resistivity. The result of 2D modelling show that rock layer in Blawan Geothermal field composed of 3 rock zone with resistivity that are cap rock zone (>10 ohm meter up to 50 ohm meter), reservoir zone (>50 ohm meter up to 100 ohmmeter) and heat source zone (>100 ohm meter).

Keywords:Blawan, geolectrical survey, geothermal, resistivity

1. Introduction Indonesia Has 40% of the geothermal potential from 80% of volcano geothermal [1]. Geothermal is renewable energy and an alternative energy in energy saving programs. Indonesia still very dependent on non-renewable energy sources such as petroleum. Geothermal potential in Indonesia is actually quite large, including in East Java, which is not used optimum.Characteristics of the geothermal reserves, geothermal potential sources with large energy reserves can be developed into power plants energy reserves that are not too large are utilized in geotourism directly, such as Ijen Volcanoes located in Blawan, Sempol, Bondowoso which is geographically located at 7059’9’’S-114010’32’’ E. The existence of this geothermal energy shown by a pool of water heat with temperature around 390C - 510C [2].Hot water tends to be in rocks with high porosity and permeability or commonly called permeable zones. The value of resistivity anomalies in geothermal permeable zones has a low characteristic character [3]. Based on observations, data literature and discussions conducted that must have more fundamental research on potential of geothermal through various method to further be used as geothermal mining work areas [4]. Several geothermal studies in Blawan-Ijen have been carried out but never been carried out using geoelectrical resistivity with Schlumberger configuration [5, 6]. This research using geoelectrical method with Schlumberger configuration that have very high level of sensitivity to contrast resistivity and conductivity of subsurface structure [7]. This method have been done used in Kanada for mineral exploration and in America for groundwater survey. Geoelectrical

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method used in resistivity survey because it low electromagnetic effects caused by current circuits and potential. [8].

2. Geological Setting This research is located around the hot spring source area in Blawan-Ijen Bondowoso East Java. Geographically the study area is located at coordinates S 07 ° 59 '13.2 "-S 12 ° 62' 15.1" and E 113 ° 08 '27.6 "- E 114 ° 10' 32.8" shown in Figure 1. The hot springs flow in Blawan geothermal field have two different rock types, Old Ijen volcanic rock and Young Ijen volcanic rock [9].

Figure 1. Research area [10]

3. Materials and Methods The research was conducted on 5-8 Mei 2018 located in Ijen area. Data processing in renewable energy laboratory of renewable energy department, Politeknik Negeri Jember. Data acquisition to choose an approach strategy with the distribution of hot air. The research stage starts from surveys, coordination, data collection, acquisition and interpretation of data. All measurements are carried out in various locations close to hot springs. In this study using 5 locations to sample with 4 electrodes to obtain geoelectric resistivity data. Geoelectrical resistivity data are acquired with Schlumberger configuration using gravitymeter G 403 type with 8 meters up to 80 meters spacing and 96 point. The Geoelectric resistivity method consist of injection of two electric current between two electrode potentials [11]. The Schlumberger configuration uses four electrodes consist of two potential electrodes and two current electroodes. The potential electrode is placed in the middle and the current electrode are placed left and right from potential electrode (Figure 2). MN distance have a smaller distance compared to AB electrode. Measurements are made by changing the distance of current electrode with potential electrode distance relative silent.The apparent resistivity equation used in data processing is:

퐴퐵2− 푀푁2 ∆푉 휌 = 휋 ( ) / (1) 4푀푁 퐼

퐴퐵 2 푀푁 2 푀푁 퐾 = 휋 (( ) − ( ) ) /2 ( ) (2) 2 2 2

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Where ρ is apparent resistivity (Ωm), V is the voltage, I is the current, AB is the current electrode, MN is the potential electrode, K is the geometry factor. K used to estimated quality and quantity of resistivity data. If the value is lower then there will be a decrease in the K value. In Determining the actual subsurface resistivity, an inversion of the measured resistivity value uses a computer program software that is RES2DINV. This programme provides inverse pseudosections of resistivity, windows will be determined and shown that two dimensional (2D) subsurface resistivity model from data surveys automatically [12]. RES2DINV using least squares optimization method with lateral and vertical distribution of area resistivity for identification underground hot water [13].

Figure 2. Schlumberger configuration [14]

Figure 3. Land Gravitymeter G 403

4. Result and Discussion Measurements of geoelectrical survey with Schlumberger configuration has been done in geothermal proscpect area of Ijen. Geoelectrical measurements were carried out on 5 lines with each track have 80 meters (Figure 4). Every measurements data taken are voltages, current, Datum point, coordinates. The data from survey was interpreted using RES2DINV computer programme with least square inversion rapidly. Based on data in this study, depth of the resistivity image depend on distance data electrode. Geoelectrical resistivity for 2 dimension have been description of rock layers with different resistivity value in others location. Figure 4 shows that the contour of 5 locations that have been sampled in this study. Location 1 consist of resistivity lines Sequentially Figure 5 are the result of resistivity survey. The result of 2D modelling show that rock layer in Blawan Geothermal field composed of 3 rock zone with resistivity that are cap rock zone (>10 ohm meter up to 50 ohm meter), reservoir zone (>50 ohm meter up to 100 ohmmeter) and heat source zone (>100 ohm meter).

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5. Conclusions

References

Figure 4. Contour of study area

The total length of the geoelectric resistivity is around 80 m and range of the dept anomaly is up to 68 m for location 1 to location 5 in this research. Figure 5, it can be interpreted that cap rock with that contain hot water, reservoir and heat source. The result of 2D modelling show that rock layer in Blawan Geothermal field composed of 3 rock zone with resistivity that are cap rock zone (>10 ohm meter up to 50 ohm meter), reservoir zone (>50 ohm meter up to 100 ohmmeter) and heat source zone (>100 ohm meter) with error 1.69 %. Based on geothermal gradient theory that getting into center of earth then the temperature get hogher too, so should be getting lower the water resistivity value get smaller because the water conductivity is getting more big. The presence of hot spring in the hydrothermal area in Ijen area is an indication of possibility a geothermal system. Changes in rock physics reservoirs such as porosity and permeability can occur due to hydrothermal alteration while the effect of hydrothermal alteration is a decrease in porosity. Characteristics and abundande of hydrothermal minerals formed during fluid and rock interactions depending on several factors, especially temperature, fluid composition (Ph), permeability, and boiling temperature. The composition of hot water in Ijen area is a chemical content of Sulfur.

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13.6 23.6 35.8 50.4 58.9

68.2

Figure 5. 2D resistivity mapping

5. Conclusion Based on the result of data processing shows that the geoelectrical survey can be found rock layer in Blawan geothermal field composed of 3 rock zone with resistivity that are cap rock zone (>10ohm meter up tp 50 ohm meter), reservoir zone (>50 ohm meter up to 100 ohm meter) and heat source zone (>100 ohm meter).

References [1] Iguchi. M, Nishimura. T, Rosadi.U, Ohkura.I, Triastuty. H, Basuki.A, Loeqkman. A, Maryanto. 2012. Methods For Eruption Prediction and Hazard Evaluation at Inndonesia Volcano. Journal Disaster Res.7,26-36. [2] Afandi. A, Maryanto. S, Rahmansyah. A. 2013. Identifikasi Reservoar Daerah Panas Bumi Dengan Metode Deometri Daerah Blawan Kecamatan Sempol Kabupaten Bodowoso. Jurnal Neutrino.Vol 6.2. pp-33-39. [3] Suhanto. E dan Bakhrun. 2005.Penyelidikan Geolistrik Tahanan Jenis Pada Daerah Panas Bumi Pincara Kabupaten Masamba Utara. Pemaparan Hasil Kegiatan Lapangan Subdit Panas Bumi. [4] Kashani, Suhanto. E, Dahlan. 2005. Kesiapan Data Potensi Panas Bumi Indonesia dalam Mendukung Penyiapan Wilayah Kerja Kelompok Program Penelitian Panas Bumi. Pusat Sumber Daya Geologi. [5] Raehanayati and Maryanto, S. 2013. Study Of Blawan-Ijen Geothermal Energy Potential, East Java Based on Gravity Method. Jurnal Noutrino. Vol.6 No.1 PP. 31-39. [6] Dewi C.N, Maryanto. S, and Rachmansyah. A. 2015. Blawan Geothermal System East Java Based On Magnet of magnetotuluuric Survey. Jurnal Riset Geodesi dan Pertambangan. Vol 25 no 22 P.111 [7] Nurchasan. Y, Ogawa. N, Ujihara. S, Tank. Y, Honkura. S, Onizawa. T, Mori and Makino. 2016. Two Electrical Conductors Bereati Kusatsu-Shirane Volcano, Japan, Image by audio magnetotuluric and their Implication for Hydrotermal Syste. Eart Planet space. [8] Loke. M.H. 1997. Elctrical Imaging Surveys for environmental and engineeringStudies. A Practical Guide to 2D and3 d Surveys. Panang. Malaysia. [9] Sidarto, S.T and Sudana, D. 1993. Geological Map of the Quadrangle Java 1707-4. Scale 1:100.000. Geological Research and Development Center.Bandung, Indonesia. [10] Maryanto. S, Suciningtyas. I.K.L, Dewi. C.N, and Rachmaniyah. A. 2016. Integragreted Resitivity And ground Penetrating Radar Observation of Underground Seepage of Hot water at Blawan–Ijen Internation Jurnal Geohphysics.1-14.

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[11] Claude. N.P, Ophile. A, Patrick. S, and Crepin. K.T. 2014. Evidence of Mineralization Channels in the Messodio area (centre-cameroon) using geoelectrical (DC&IP) Method, A case study. International Journal of Geosciences, Vol.5 No.3 pp 346-361. [12] Griffith. D.H and Barker R.D. 1993. Two dimentoral Resitivity Imaging and Modelling in areas of complex geology. Journal of Aplied Geophysics vol 29. No.3-4-pp 211-226. [13] MetWaly and Alfouzan. 2013. Aplication of 2-D Geoelctrical Resistivity Tomography for Subsurfacecavity detection in the estern of Saudi Arabia. Geoscience Frontless Vol 4 No 4 pp 469- 476. [14] Telford. W.M, Beldart. L.P and Sheriff. R.E. 1990. Applied Geophysics Second edition. Newyork;Camridge University Press.

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