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"ABSTRAKSalah satu metode yang paling cocok digunakan dalam eksplorasi panas bumi adalah metode magnetotelurik (MT). Metode ini memanfaatkan gelombang elektromagnetik yang menjalar secara alami di permukaan bumi untuk menggambarkan struktur resistivitas bawah tanah dengan rentang frekuensi yang luas. Terdapat berbagai macam alat untuk mengukur MT seperti alat keluaran Phoenix dan keluaran Metronix. ADU-07e (Analog Digital Unit) adalah instrumen dari metode magnetotelurik yang merupakan keluaran dari Metronix. Untuk melakukan pengolahan data dari alat tersebut dibutuhkan software dari Metronix itu sendiri yaitu dengan Mapros. Penulis akan menggunakan bahasa pemograman Matlab untuk pemograman software pengolahan data MT dari data MT keluaran Metronix agar lebih user friendly dan menambahkan fitur untuk handling noise seperti kalkulasi robust. Dalam penelitian ini digunakan format time series (.ats) dari instrumen Metronix. Reduksi dari efek outlier pada data impedansi dilakukan dengan kalkulasi robust. Hasil dari penelitian ini menunjukkan kurva resistivitas hasil pengolahan data menggunakan program Matlab dengan fitur kalkulasi robust menujukkan kurva yang lebih smooth dibandingkan dengan hasil pengolahan Mapros dan hasil pemodelan inversi dua dimensi menunjukkan hasil yang signifikan dalam penentuan Base of Conductor sistem panas bumi. Untuk penelitian lebih lanjut, diharapkan dapat mengatasi pengaruh frekuensi deadband pada data dengan menerapkan filter digital, seleksi cross power, dan remote reference pada pengolahan data.

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ABSTRAKOne method that is most suitable for using in geothermal exploration is the magnetotellurics (MT) method. This method utilizes electromagnetic waves radiating naturally in the earth's surface to describe the resistivity structure of subsurface with a wide frequency range. There are various of instrument to measure MT such as Phoenix Instrument and Metronix Instrument. ADU-07e (Analog Digital Unit) is an instrument of Magnetotelluric method which is from the Metronix. To perform the data processing of this instrument, it is needed software from Metronix itself like Mapros. The author will use the Matlab programming algorithm for data processing of the data MT?s Metronix to make it more friendly and add feature for handling noise such as robust calculation. In this study, it is used time series format (.ats) of the Metronix instrument. The reduction from outlier effect on impedance data is handled by robust calculation. The results of this study indicate curve resistivity from data processing using Matlab program with robust calculation feature showed the curve more smooth compared with the Mapros results and two-dimensional inversion model showed a significant result in the Base of Conductor determination for geothermal system. For the next research, it is expected to be able to deadband frequency influenced on data by applying digital filter, cross power selection, and remote reference on data processing. "

"Area prospek panasbumi Lili, Sulawesi Barat, Indonesia merupakan salah satu daerah prospek yang berasosiasi dengan aktivitas vulkanik yang terjadi sejak zaman Tersier. Aktivitas vulkanik ini diperkirakan merupakan aktivitas gunungapi bawah laut yang berkembang menjadi gunungapi darat berumur Kuarter bawah. Area prospek panasbumi ini memiliki mata air panas tipe klorida, dan tipe bikarbonat yang tersebar di sekitar area prospek. Dari hasil perhitungan geotermometer area prospek panasbumi Lili memiliki temperatur 189-201 °C, yang dikategorikan sebagai moderate to high temperature geothermal system. Untuk mengetahui batas, kedalaman, dan geometri dari reservoir yang ada, dilakukan pengukuran dengan metode Magnetotelluric (MT) serta metode geofisika lainnya sebagai pendukung seperti metode gravitasi, geomagnet, dan geolistrik. Pengukuran dilakukan dengan desain gridding agar dapat diketahui penyebaran resistivitas dari arah Utara-Selatan maupun Barat-Timur. Data MT tersebut dikoreksi terlebih dahulu terhadap efek statik dan noise dengan menggunakan Mat-Lab dan Site to Site Reference sebelum nantinya siap diinterpretasi. Pemodelan sistem panasbumi dari data magnetotellurik dengan menggunakan analisa 2-dimensi dan 3-dimensi. Hasil area prospek berada dari arah barat daya hingga ke tengah lokasi pengukuran menerus ke utara. Rekomendasi pengeboran di sekitar daerah outflow dengan kedalaman sekitar 1.8 km dekat dengan zona patahan yang mempunyai permeabilitas yang lebih besar untuk mendapatkan fluida panasnya.

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Geothermal prospect area in Lili, West Celebes, Indonesia is one of the prospect area in Indonesia associated by volcanic activity which happened since tertiary. This volcanic activity is predicted as undersea volcano which grown become to volcano quartenary. This geothermal prospect area has chloride and bicarbonate hot springs. Based on geothermometry calculation the geothermal prospect area of Lili has temperature 189-201 °C which is categorized as a moderate to high temperature geothermal system. To estimate the boundary, depth, and geometry of the reservoir, Magnetotelluric (MT) and other geophysics methods were used such as gravity, geomagnetic, and geoelectric method as supporting data.

Data acquisition was designed gridding method to delineated resistivity distribution in North-South or West-East orientation. MT data was then corrected for static effect and possible noise using Mat-lab and Site to site reference before comprehensive interpretation. Modeling of the geothermal system was carried out by using 2-dimensional MT resistivity and 3-dimensional visualization. As a result, the prospect area is exist in south west until center of measurement area. Pattern of this zone still continue to the north of measurement area. In addition, drilling recommendation is proposed around the outflow in Lili with depth at 1.8 km near the fault zone to get the hot fluid."

"ABSTRAKMetode Magnetotellurik (MT) merupakan suatu teknik elektromagnetik pasif yang sangat efektif dalam menggambarkan distribusi tahanan jenis batuan bawah permukaan bumi. Namun, metode ini sangat sensitif terhadap noise yang timbul akibat aktivitas mekanik seperti vibrasi dan aktivitas kelistrikan seperti aliran listrik pada powerline yang berada didekat titik pengukuran. Untuk mengeliminasi noise ini digunakan filter digital yang terdiri dari median filter dan notch filter. Median filter digunakan untuk mengeliminasi spike noise yang bersifat irreguler, sedangkan notch filter untuk harmonic powerline noise yang bersifat reguler. Seleksi data time series digunakan untuk mengeliminasi noise yang bersifat iireguler, tetapi tidak dapat dieliminasi oleh median filter. Filter-filter ini dibangun dalam program MTFilter yang dapat mengolah dan memfilter data time series MT hingga didapatkan kurva resistivitas dan fase dalam domain frekuensi. Hasil pengolahan data dan inversi menunjukan bahwa teknik pengeliminasian noise menggunakan filter digital ini dapat meningkatkan kualitas data dan mengurangi resiko kegagalan pemboran.

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ABSTRACTMagnetotelluric (MT) Method is a very effective techniques to describe the distribution of rock resistivity below the earth's surface. However, this method is very sensitive to noise which is arising from mechanical activity such as vibration and electrical activity such as powerline current. Digital noise filter consisting of median filter and notch filter is used to eliminate those noise. Median filter is used to eliminate spike noise which is irregular, while notch filter is used to eliminate harmonic powerline noise which is regular. The selection of time series data is used to eliminate noise which is irregular, but can not be overcome by the median filter. All these filters are built in MTFilter program which is can prosess MT data from time series data untill provide apparent resistivity and phase data in frequency domain. Based on results of data processing and inversion model, we conclude that these techniques can improve data quality and reduce the risk of drilling failures"

"Telah dikembangkan program inversi dan program forward-modeling data MT 2D. Program inversi telah dites menggunakan data sintetik (dari program forward-modeling) dan data lapangan (daerah geotermal Sibayak) dengan hasil terbukti mampu memetakan bawah permukaan. Program inversi yang dibuat digunakan untuk menginversi data MT lapangan geotermal Sibayak, Sumatra Utara. Hasil inversi tersebut digunakan untuk membuat penampang 2-D distribusi resistivitas bawah permukaan. Model yang diperoleh kemudian diinterpretasi dengan bantuan data sumur dan data geologi. Hasil interpretasinya adalah zona up-flow terdapat di sebelah utara, dekat Gunung Sibayak, sedang zona out-flow berada di sebelah selatan. Rekomendasi pengeboran diberikan untuk daerah di sebelah utara. Rekomendasi reinjeksi fluida diberikan untuk daerah di sebelah selatan.

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A 2-D MT Software for inverse and forward-modeling has been developed. The inversion program was tested using both synthetic data (from forward-modeling software) and real data (from Sibayak geothermal area) resulting conclusion that the inversion program was capable reconstructing subsurface model. The inversion program was used to invert Sibayak geothermal MT data. The inversion result was used to produce cross-section model of subsurface resistivity distribution. The model derived was then interpreted by incorporating borehole and geology data. Interpretation results are: up-flow zone is situated in the northern side near Mount Sibayak, while out-flow zone is situated in the southern side. Drillings are recommended to be located in northern area. Geothermal brine reinjection is recommended to be located in southern area."

"Lapangan geothermal Sibayak terletak di kawasan utara Great Sumatra Fault Zone (GSFZ) yang memiliki topografi yang tinggi di dalam kaldera Singkut. Ditinjau dari kondisi geologinya, lapangan ini memiliki prospek geothermal yang ditandai dengan keberadaan manifestasi panas berupa solfatara, fumarole, chloride springs dan silica sinters. Untuk menginvestigasi struktur bawah permukaan secara lebih detail, maka dilakukan reinterpretasi data magnetotellurik dan gravitasi. Dari pemodelan 2-Dimensi MT yang menggunakan software MT2Dinv dan 3-Dimensi MT menggunakan software GeoSlicer-X maka dapat diketahui clay cap mempunyai nilai resistivitas 5-10 Ωm. Zona reservoir diindikasikan dengan harga resistivitas 50- 200 Ωm yang terdapat di bawah zona clay cap dan berada pada kedalaman sekitar 1600m. Pusat reservoir terdapat pada daerah yang meliputi Gunung Sibayak dan Gunung Pratektekan dengan luas yang diperkirakan sekitar 4 km². Pemodelan data gravitasi mendukung gambaran stuktur utama yang berupa kaldera Singkut dan sesarsesar yang berarah barat laut-tenggara. Berdasarkan studi ini dapat direkomendasikan sumur produksi diarahkan pada pusat reservoir, sedangkan reinjeksi ditempatkan di daerah dekat reservoir tetapi yang diduga memiliki hubungan permeabilitas, yaitu di sekitar batas kaldera sebelah selatan.

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Sibayak geothermal field is situated in the northern Great Sumatra Fault Zone (GSFZ), which has high topography inside Singkut caldera. From the geological point of view, Sibayak field is a potential geothermal area supported by the occurrence of surface manifestations such as solfataras, fumaroles, chloride springs and silica sinters. To investigate subsurface geological structure, reinterpretation of the Magnetotelluric and gravity data were carried out. Two-dimensional modeling of MT data using MT2Dinv software and 3-D visualization of the MT data using GeoSlicer-X have delineated clay cap with resistivity of 5-10 ohm. Reservoir zone is indicated by slightly higher resistivity (50 - 200 ohm-m) below the clay cap located in the depth of about 1600m. Center of reservoir is probably located in the area between Mt Sibayak and Mt Pratektekan covering about 4 km². The gravity data modeling supports the existence of main structures, those are Singkut caldera and faults zone oriented in the northwest - southeast direction. Based on this study, it is recommended that the production wells shoud be located to the central of reservoir and reinjection wells should be sited to the area close to the main reservoir which has permeability connection, that is in the southern caldera boundary."

"[Kegiatan eksplorasi geothermal bertujuan mengetahui sistem geothermal daerah penyelidikan yang meliputi model dan batas prospek, karakteristik dan potensial reservoir dan hidrogeologi, untuk penentuan target pemboran, dilanjutkan dengan pemboran eksplorasi.Hasil inversi 3-dimensi data MT akan menyajikan distribusi struktur resistivitas bawah permukaan.Pemboran eksplorasi geothermal bertujuan untuk membuktikan adanya sumber daya geothermal dan menguji model sistem geothermal yang telah dibuat. Kriteria target pemboranadalah area yang memiliki temperature dan permeabilitas yang tinggi. Pada waktu pemboran sumur panas bumi ditembusnya zona bertemperatur tinggi yang disertai atau diikuti dengan terjadinya loss of circulation sangat diharapkan (permeabilitas tinggi), karena merupakan suatu indikasi telah ditembusnya rekahan-rekahan yang diharapkan merupakan zona produksi (feed zone).Untuk menguji model sistem yang dibuat dilakukan korelasi antara data hasil pemboran dengan inverse 3D data MT, khususnya nilai resistivity lapisandengan data temperatur, kandungan mineral alterasi, geokimia dari data pemboran.Dari hubunganantar parameter akan terlihat karakteristik sistem geothermal di daerah penyelidikan, yang memperlihatkan zona prospek yang berhubungan dengan temperature dan permeabilitas yang tinggi. Dari hasil evaluasi akan dilakukan rekonstruksi system geothermal daerah penyelidikan, yang lebih mendekati kondisi bawah permukaan dan dapat dipergunakan untuk membuat rekomendasi pemboran selanjutnya dan arah pengembangan di masa yang akan datang;Geothermal exploration activities aimed at knowing the geothermal system that includes model and boundary the prospects, potential and reservoir characteristics and also hydrological system. By using 3D inversion of MT data, subsurface resistivity distribution structure can be obtained and with the addition of other geosciences data, LumutBalai geothermal system can be constructed. Futhermore, drilling targets zone can be identified from geothermal system which then followed by exploration drilling .Geothermal exploration drilling is carried out to verify the existence of geothermal resources and test the geothermal systems which previously has been made. Drilling target criteria is the area which consist of high temperature and permeability. During geothermal drilling, it is expected that high temperature zone shall be encountered. It will be followed by loss circulation zone which indicates that fractures have already been penetrated and confirm that feed zone has been discovered.In order to test constructed model, correlation between drilling data and 3D MT inverse is carried out, particularly values of resistivity layer with temperature data, alteration mineralcomposition, and geochemical data derived from drilling.Parameter correlation will explain geothermal system characteristics in study area which delineates prospect zones and its association with high temperature and permeability. The evaluation results of this study will reconstruction geothermal system the investigation area, which can be used to develop a recommendation of subsequent drilling and further development direction;Geothermal exploration activities aimed at knowing the geothermal system that includes model and boundary the prospects, potential and reservoir characteristics and also hydrological system. By using 3D inversion of MT data, subsurface resistivity distribution structure can be obtained and with the addition of other geosciences data, LumutBalai geothermal system can be constructed. Futhermore, drilling targets zone can be identified from geothermal system which then followed by exploration drilling .Geothermal exploration drilling is carried out to verify the existence of geothermal resources and test the geothermal systems which previously has been made. Drilling target criteria is the area which consist of high temperature and permeability. During geothermal drilling, it is expected that high temperature zone shall be encountered. It will be followed by loss circulation zone which indicates that fractures have already been penetrated and confirm that feed zone has been discovered.In order to test constructed model, correlation between drilling data and 3D MT inverse is carried out, particularly values of resistivity layer with temperature data, alteration mineralcomposition, and geochemical data derived from drilling.Parameter correlation will explain geothermal system characteristics in study area which delineates prospect zones and its association with high temperature and permeability. The evaluation results of this study will reconstruction geothermal system the investigation area, which can be used to develop a recommendation of subsequent drilling and further development direction, Geothermal exploration activities aimed at knowing the geothermal system that includes model and boundary the prospects, potential and reservoir characteristics and also hydrological system. By using 3D inversion of MT data, subsurface resistivity distribution structure can be obtained and with the addition of other geosciences data, LumutBalai geothermal system can be constructed. Futhermore, drilling targets zone can be identified from geothermal system which then followed by exploration drilling .Geothermal exploration drilling is carried out to verify the existence of geothermal resources and test the geothermal systems which previously has been made. Drilling target criteria is the area which consist of high temperature and permeability. During geothermal drilling, it is expected that high temperature zone shall be encountered. It will be followed by loss circulation zone which indicates that fractures have already been penetrated and confirm that feed zone has been discovered.In order to test constructed model, correlation between drilling data and 3D MT inverse is carried out, particularly values of resistivity layer with temperature data, alteration mineralcomposition, and geochemical data derived from drilling.Parameter correlation will explain geothermal system characteristics in study area which delineates prospect zones and its association with high temperature and permeability. The evaluation results of this study will reconstruction geothermal system the investigation area, which can be used to develop a recommendation of subsequent drilling and further development direction]"

"Keberadaan struktur geologi merupakan salah satu parameter penting dalam menentukan zona permeabel pada suatu sistem geotermal. Penelitian ini dilakukan di salah satu area prospek geotermal di zona Sistem Sesar Sumatera (GSF) yang termasuk dalam segmen Angkola dan Barumun yang bertujuan untuk mengidentifikasi kemenerusan fitur permukaan hingga bawah permukaan terutama struktur geologi yang berkaitan erat dengan zona permeabel dengan mengintegrasikan data geologi, geokimia, dan geofisika. Teknologi remote sensing digunakan untuk mengidentifikasi struktur geologi yang terobservasi di permukaan yang dikorelasikan dengan persebaran manifestasi permukaan. Namun, tidak semua struktur geologi yang terobservasi di permukaan dapat diamati dan kemenerusannya dari permukaan hingga bawah permukaan dilakukan dengan pendekatan geofisika menggunakan data magnetotelurik (MT) dan gravitasi. Interpretasi struktur geologi permukaan berdasarkan analisis remote sensing dan persebaran manifestasi permukaan memiliki korelasi yang positif dengan hasil gravitasi adanya struktur graben dari zona GSF yang memiliki orientasi baratlaut-tenggara. Kelurusan dan karakteristik (arah dan kemiringan) struktur ditandai dengan adanya kontras nilai gravitasi, nilai Horizontal Gradient Magnitude (HGM) maksimum, dan nilai zero Second Vertical Derivative (SVD) serta analisis Multi Scale-Second Vertical Derivative (MS-SVD). Hasil interpretasi struktur bawah permukaan gravitasi berkorelasi positif dengan analisis parameter MT (splitting curve MT) yang dapat mengindikasi zona struktur bawah permukaan. Gabungan interpretasi struktur permukaan dan bawah permukaan teridentifikasi adanya 5 struktur (F1, F2, F3, F4, dan F5) yang diklasifikasikan sebagai Struktur Pasti (F1, F2, F3, dan F4) dan Struktur Diperkirakan (F5) yang memiliki orientasi baratlaut-tenggara. Struktur F3 yang berorientasi baratlaut-tenggara merupakan struktur utama yang berperan sebagai fluid conduit (zona permeabel) yang dibuktikan dengan adanya manifestasi mata airpanas bertipe klorida. Berdasarkan hasil pemodelan inversi 3-D MT dan pemodelan kedepan 2-D gravitasi dapat mendelineasi zona reservoir pada kedalaman 1500 – 2000-meter yang dikontrol oleh struktur F3 dan zona reservoir berasosiasi dengan batuan metasediment yang nantinya dapat menentukan lokasi sumur pengeboran. Untuk memvisualisasikan sistem geotermal secara komprehensif, maka dikembangkan model konseptual dengan mengintegrasikan model geofisika yang memiliki kualitas data optimum dengan data geologi dan geokimia yang saling berkorelasi, sehingga dapat dijadikan dasar dan acuan dalam menentukan lokasi pengembangan sumur produksi dan reinjeksi dan menurunkan resiko kegagalan dalam well targeting.

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The existence of geological structures is one of the important parameters in determining the permeability zone in a geothermal system. This study was conducted in one of the geothermal prospect areas in the Sumatera Fault System (GSF) zone included in the Angkola and Barumun segments which aims to identify the continuity of surface to subsurface features, especially geological structures that are closely related to permeability zones by integrating geological, geochemical, and geophysical data. Remote sensing technology is used to identify geological structures observed at the surface that are correlated with the distribution of surface manifestations. However, not all surface-observed geological structures can be observed and their continuity from the surface to the subsurface is done with a geophysical approach using magnetotelluric (MT) and gravity data. Interpretation of surface geological structures based on remote sensing analysis and the distribution of surface manifestations has a positive correlation with the gravity results of the graben structure of the GSF zone which has a northwest-southeast orientation. The alignment and characteristics (direction and slope) of the structure are characterized by the contrast of gravity values, maximum Horizontal Gradient Magnitude (HGM) values, and zero Second Vertical Derivative (SVD) values as well as Multi Scale-Second Vertical Derivative (MS-SVD) analysis. The results of gravity subsurface structure interpretation are positively correlated with MT parameter analysis (splitting curve) which can indicate subsurface structure zones. The combined interpretation of surface and subsurface structures identified 5 structures (F1, F2, F3, F4, and F5) classified as Certain Structures (F1, F2, F3, and F4) and Estimated Structure (F5) that have a northwest-southeast orientation. The northwest-southeast oriented F3 structure is the main structure that acts as a fluid conduit (permeability zone) as evidenced by the manifestation of chloride-type hot springs. Based on the results of 3-D MT inversion modeling and 2-D gravity forward modeling, it can delineate the reservoir zone at a depth of 1500 - 200 meters controlled by the F3 structure and the reservoir zone is associated with metasedimentary rocks which can later determine the location of drilling wells. To visualize the geothermal system comprehensively, a conceptual model was developed by integrating geophysical models that have optimum data quality with geological and geochemical data that are correlated, so that it can be used as a basis and guide in determining the location of production well development and reinjection and reduce the risk of failure in drilling targets."

"Geothermal System Modeling has been illustrates Using Magnetotelluric Method. This method utilize a natural source Waves which are come from solar Wind, or the other Electromagnetic Waves in an ionosphere. Earlier data has been process with frequency sorting for gets the right signal points and remove noises. Then the Cokriging method utilized to remove the distortion effect With Static shift correction. After Data processing phases are finished, 2D Inversion, and 3D visualization of MT data are needed. And then for illustrates good geothermal system modeling, We must integrated MT and the others data, like geology or geochemistry data.

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Dilakukan pemodelan sistem geothermal dengan menggunakan metode Magnetotelluric (MT). Metode ini menggunakan sumber alami gelombang (natural Source) yang berasal dari solar Wind, ataupun gelombang elektromagnetik lainnya yang ada di ionosfer. Pengolahan data dilakukan dengan melakukan pemilahan frekuensi yang tepat untuk mendapatkan sinyal yang dapat merepresentasikan keadaan subsurface serta untuk menghilangkan noise. Dilakukan pula koreksi pergeseran static menggunkan metode cokriging untuk menghilangkan efek distorsi. Setelah tahap pengolahan data selesai dilakukan proses inversi data MT, Visualisasi 3-Dimensi, serta dilakukan integrasi terpadu terhadap data-data yang lain, baik geologi ataupun geokimia guna mendapatkan pemodelan dari suatu system geothermal."

"The Cinnamon Geothermal prospect is located in the northeastern ofMayonnaise Island, Indonesia. It is located in quaternary volcanic terrainabout 750 m asl. The various types of surface manifestations e.g. solfatara,fumarole, steaming ground, mud pools and hotsprings are concentrated nearCinnamon village. Few reconnaissance surveys had been conducted sinceyear 1977. To model the Geothermal Prospect of Cinnamon, one can useMagnetotelluric and Gravity method.Magnetotelluric method uses telluric current as natural source toidentify the resistivity properties of subsurface rocks. This resistivity sectioncan be used to identify the occurrence of clay cap. Gravity method can beused to map the density response from the subsurface to detect a massivedensity contrast that match the geological condition.The integrated model of Cinnamon combining geophysical,geochemical and geological data can be used to locate the reservoir, outflow,and upflow for further drilling purpose and exploitation."

"Gunung Arjuno-Welirang merupakan gunung api yang berada di Provinsi Jawa Timur yang memiliki potensi energi panas bumi. Secara geologi, batuan penyusun pada umumnya berjenis andesit-basaltik yang berasal dari beberapa pusat erupsi seperti gunung Arjuno, Welirang, Kembar I – II, gunung Bakal, gunung Pundak dan gunung Bulak. Untuk mengetahui keberadaan struktur patahan di bawah permukaan, dilakukan analisis data gayaberat. Zona struktur patahan dapat diketahui dari peta kontur anomali residual, yang ditunjukkan dari adanya nilai anomali positif dan negatif yang dibatasi dengan kontur yang rapat. Identifikasi daerah panas bumi dengan data gaya berat perlu dilakukan untuk mengestimasi kedalaman sekitar 4400 m menggunakan analisis spektrum. Berdasarkan hasil pemodelan 2 dimensi yang telah dikorelasikan dengan data geologi, penampang inversi 3D Magnetotellurik dan hasil analisis second vertical derivative digunakan untuk mengidentifikasi keberadaan serta jenis patahan dan struktur naik yang kemungkinan ada di daerah Padusan dan patahan turun yang berada pada struktur kaldera Arjuno-Welirang. Sistem panas bumi dicirikan dengan munculnya manifestasi berupa air panas dengan temperatur sekitar 50ºC, pH netral, solfatara dan fumarol dengan temperatur hingga 137 ºC dan alterasi batuan.

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Arjuno-Welirang Mountain, the volcanoes which located in East Java, had the potential of geothermal energy. In geology, rocks constituents in general had basaltic andesite type that derived from several eruption centers, such as mountain Arjuno, Welirang, Kembar I–II, Bakal mountain, Bulak mountain and Pundak mountain. To identify the presence of a fault under surface, gravity data analysis was done. Fault zone structures can be seen from the residual anomaly contour map, that show the presence of positive and negative anomalous values that constrained by a tight contours. Identification of geothermal areas with gravity data was important to be done in order to estimate depth around 4400m by using spectrum analysis.

Based on Two-dimensional modeling results that has been correlated with geological data, the cross-sectionals 3D magnetotelluric inversion and vertical second derivative analysis was used to identify the presence and type of fracture and also the ascended stuctures that could be exist on Padusan area and the descended faults that exist in Caldera’s stucture on Arjuno-Welirang. Geothermal system was characterized by the existance of hot water’s manifestations with temperature about 50º C, neutral acidity, Solfatara and Fumaroles that have temperature up to 137 º C and rock alteration."