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"Lapangan geotermal X berada di area gunung A yangmana berdasarkan data geologi ditemukan adanya manifestasi berupa hot spring dan fumarole. Pengukuran MT dilakukan untuk mengetahui persebaran resistivity batuan di bawah permukaan. Pengolahan data MT dilakukan dari analisis time series dan filtering noise kemudian dilakukan Transformasi Fourier dan Robust Processing. Setelah itu baru dilakukan crosspower untuk menyeleksi data sehingga output dari proses ini berupa kurva MT. Setelah didapatkan kurva MT dilakukan koreksi statik dikarenakan kurva TE dan TM terjadi shifting. Untuk proses akhirnya baru dilakukan inversi 2D dan inversi 3D. setelah itu dilakukan perbandingan antara 2D dan 3D. Wilayah interest lapangan X berada di lintasan AA dan lintasan AB. Berdasarkan analisis 3D diidentifikasi bahwa zona alterasi menipis di wilayah upflow dan menebal ke arah outflow yangmana sesuai dengan teori. Wilayah upflow dapat diketahui dengan melihat manifestasi berupa fumarole.

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The geothermal field X is located in the area of Mount A which based on geological data found the presence of hot spring and fumarole manifestations. MT measurements were carried out to determine the distribution of rock resistivity in the subsurface. MT data processing is starts from time series analysis and noise filtering then Fourier Transform and Robust Processing are performed. After that, crosspower is done to select data so that the output of this process is an MT curve. After got the MT curve then a static correction is done because the TE and TM curves are shifting. For the final process are 2D inversion and 3D inversion. After that make a comparison between 2D and 3D. The area of interest in field X is on the line AA and line AB. Based on the 3D analysis, it was identified that alteration zones thinned in the upflow region and thickened towards the outflow which is make sense with the theory."

"Geotermal merupakan salah satu kekayaan alam terbaik di Indonesia yang saat ini masih belum dimanfaatkan secara maksimal. Tercatat dari data tahun lalu, baru 8,9 persen dari total sumber daya yang berhasil dimanfaatkan. Salah satunya berada di wilayah Pegunungan Ijen, Jawa Timur. Pegunungan Ijen ini merupakan pegunungan api aktif yang dahulunya berupa gunung api tunggal bernama Gunung Kendeng yang mengalami erupsi secara eksplosif. Meskipun PLTP ditargetkan akan beroperasi di tahun 2025, namun kondisi medan yang berupa pegunungan aktif dengan banyak patahan didalamnya serta hanya terdapat sedikit mata air panas (hot spring) dan batuan teralterasi sebagai manifestasi geotermal yang terlihat di atas permukaan membuat daerah ini masih dalam tahap penelitian. Salah satu cara untuk melihat kondisi bawah permukaan bumi adalah dengan metode gravitasi satelit. Data yang diambil berada di area perbatasan Kabupaten Bondowoso, Kabupaten Situbondo, dan Kabupaten Banyuwangi. Data gravitasi satelit ini akan menghasilkan peta complete bouguer anomaly. Untuk melihat secara lebih baik dibuat peta regional orde 1 dan 2, dan peta residual gravitasi orde 1 dan 2 dengan metode Trend surface analysis (TSA). Daerah prospek geotermal terdapat di bagian tengah peta disekitar persimpangan Struktur Blawan dan Struktur Cemara-Kukusan. Beberapa stuktur teridentifikasi dengan metode FHD. Diantaranya Patahan Kendeng Merapi, Patahan Djampi, Patahan Cemara-Kukusan, dan beberapa struktur lain yang bisa membantu proses pemodelan. Pemodelan 2D data gravitasi menggunakan peta residual dari Butterworth Filter dengan diikat data gravitasi lapangan dan data MT sehingga dapat membantu melihat keadaan bawah permukaan serta digunakan untuk mengidentifikasi zona permeabel geotermal. Pemodelan menghasilkan beberapa lapisan yaitu Batu Breksi Gunung Api (ρ=2.7-2.8 g/cc), Basaltic Lava (ρ=3.3 g/cc), Clay Cap (ρ=2.0 g/cc), Batuan Reservoir (ρ=2.8 g/cc), dan Basement (ρ=3.3 g/cc).

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Geothermal is one of the best natural resources in Indonesia which is currently not being fully utilized. From last year's data, only 8.9 percent of the total resources were successfully utilized. One of them is in the Ijen Mountains area, East Java. The Ijen Mountains are active volcanoes that were once a single volcano called Mount Kendeng which erupted explosively. Although the geothermal power plant is targeted to operate in 2025, the terrain conditions in the form of active mountains with many faults in it and there are only a few hot springs and altered rocks as geothermal manifestations visible above the surface make this area still in the research stage. . One way to see the conditions below the earth's surface is the satellite gravity method. The data taken are in the border area of Bondowoso Regency, Situbondo Regency, and Banyuwangi Regency. This satellite gravity data will produce a complete bouguer anomaly map. For a better view, regional maps of order 1 and 2 were made, and residual gravity maps of order 1 and 2 were made using the Trend surface analysis (TSA) method. The geothermal prospect area is located in the center of the map around the intersection of the Blawan Structure and the Cemara-Kukusan Structure. Several structures were identified by the FHD method. Among them are the Kendeng Merapi Fault, the Djampi Fault, the Cemara-Kukusan Fault, and several other structures that can assist the modeling process. 2D modeling of gravity data using residual maps from Butterworth Filter with field gravity data tied and MT data so that it can help see subsurface conditions and is used to identify geothermal permeable zones. The modeling produces several layers, namely Volcanic Breccia Rock (ρ=2.7-2.8 g/cc), Basaltic Lava (ρ=3.3 g/cc), Clay Cap (ρ=2.0 g/cc), Reservoir Rock (ρ=2.8 g/cc). cc), and Basement (ρ=3.3 g/cc)."

"[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]"

"Manifestasi permukaan pada sistem panas bumi dapat menjadi indikator adanya zona permeabel dibawah permukaan. Zona permeabel dalam sistem panas bumi memiliki dua kegunaan yaitu sebagai frakture atau struktur patahan yang dibutuhkan untuk penyedia zona recharge dan discharge fluida ke dalam reservoir dan memiliki peran sebagai pengontrol mobilitas fluida di reservoir. Zona permeabel ini umumnya terbentuk dari proses konduksi dan konveksi termal dari heat source. Zona ini dapat dideteksi dengan indikasi adanya curve splitting data magnetotelurik. Curve splitting terjadi karena adanya perbedaan resistivitas semu kurva sounding TE dan TM. Gelombang elektromagnetik pada mode TE merambat lebih cepat jika medan listriknya sejajar dengan arah strike. Hal ini menyebabkan impedansi batuan akan lebih kecil dibandingkan dengan mode TM serta nilai resistivitas semu kurva TM akan lebih besar. Hasil kurva TE dan TM dari data lapangan dianalisis untuk mengestimasi zona permeabel yang mungkin ada dibawah permukaan. Korelasi antara TE-TM split curve dengan zona permeable selama ini hanya sebatas analisis kualitatif. Penelitian ini mencoba melakukan analisis kuantitatif keberadaan struktur bawah permukaan tersebut. Kami peroleh zona permeabel dengan strike dan dip dari struktur patahan. Hal ini dapat membantu kita dalam merekonstruksi konseptual model sistem panas bumi lapangan 'J'.

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The Surface manifestation on the geothermal systems is expected to be indicator of the permeable zone. These permeable zone in the geothermal system has two important roles as a fracture or the fault structure that is needed to recharge and discharge zones provider of fluid into the reservoir and as a controller the mobility of fluid in the reservoir. Permeable zone is generally formed by thermal conduction and convection from the heat source. This zone can be analyzed due to the splitting of magnetotelluric data curve. Curve splitting occurs because of the differences in the apparent resistivity from sounding of TE and TM curves. Electromagnetic waves in the TE mode propagating more rapidly if the electric field parallel to the direction of the strike. This causes the impedance of the electromagnetic field will be smaller than the TM mode and the value of apparent resistivity from TM curve will be bigger. The results of the TE and TM curves of field data were analyzed to estimate the permeable zone that may exist below the surface. The correlation between TE TM split curve and permeable zone is only qualitative analysis so far. This research try to do a quantitative analysis the existence of subsurface structures. We have obtained the permeable zones with strike and dip of the fault structure. The analysis may help us to be reconstructed the conceptual model of the geothermal system at J field."

"Proses identifikasi dalam kegiatan eksplorasi Geotermal menjadi salah satu upaya dalam meminimalisasi risiko ketidaksuksesan. Pemanfaatan suatu lapangan Geotermal  seperti pada lapangan X memiliki proses yang panjang dan berisiko tinggi. Setelah diidentifikasi dengan baik pada kasus-kasus ketidaksuksesan eksplorasi, kualitas data, dan tipe sistem Geotermal, selanjutnya hasil identifikasi tersebut diaplikasikan dalam proses pengolahan data Magnetotellurik (MT) pada lapangan X. Hasil yang diperoleh ialah lapangan X teridentifikasi sebagai lapangan Geotermal bersistem Hidrotermal Temperatur Tinggi Natural-2 Fase, dengan estimasi suhu reservoir 245o C dan estimasi energi 238 MW. Hal tersebut menggambarkan bahwa lapangan X dapat dikembangkan lebih lanjut, salah satunya dengan proses pengeboran sumur eksplorasi dengan rekomendasi titik pada koordinat 11400.00 m N dan 63100.00 m E sekitar 4 km dari puncak gunung L.

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The identification process in geothermal exploration activities is an effort to minimize the risk of unsuccessfulness. The use of a Geotermal field such as in field X has a long and high-risk process. After being well identified in cases of exploratory success, data quality, and geothermal system type, the identification results were then applied in the Magnetotelluric (MT) data processing on the X field.

The results obtained were that the X field was identified as a Hydrothermal Geothermal field High Temperature Natural-2 Phase, with an estimated reservoir temperature of 245 oC and an estimated energy of 238 MW. This illustrates that the X field can be further developed, one of which is the process of drilling exploration wells with recommendations for points at coordinates 11400.00 m N and 63100.00 m E about 4 km from the peak of Mount L."

"Untuk menentukan zona prospek diperlukan beberapa kriteria yaitu struktur yang berasosiasi dengan permeabilitas tinggi, temperatur tinggi dan zona upflow. Berdasarkan peta lineament density serta metode FFD menunjukkan zona yang memiliki densitas tinggi berasosiasi dengan zona depresi yang merupakan struktur utama pada lapangan panasbumi ldquo;Q rdquo; dan munculnya beberapa manifestasi berupa fumarol, dan mata air panas APWO, APWK, APLM dan APBK. Dari diagram ternary Cl - SO4 - HCO3 menunjukkan APWO memiliki tipe steam heated water, APWK memiliki tipe bicarbonate water sedangkan APBK dan APLM memiliki tipe chloride water. Berdasarkan tipe fluida tersebut maka dapat diklasifikasikan bahwa lapangan panasbumi ldquo;Q rdquo; merupakan sistem hidrotermal. Penentuan estimasi temperatur reservoir menggunakan geothermometer silika dan geothermometer gas yang masing - masing berkisar antara 141 C - 212 C dan 250 C. Sehingga manifestasi APWO, FWO-1 dan FWO-2 menunjukkan berada pada zona upflow dan APWK, APBK, APLM berada pada zona outflow. Hasil dari inversi 3D MT memperlihatkan sebaran resistivitas rendah < 10 ohm-m memiliki ketebalan 1000 m sampai 1500 m yang diduga sebagai clay cap, sebaran zona resistivitas tinggi > 100 ohm-m terlihat berbentuk updome pada kedalaman -500 sampai -4000 m yang diduga sebagai heat source. Dari hasil proses upward continuation dan reduce to pole, nilai magnetik yang berkisar -250 nT sampai -350 nT berada di bawah manifestasi FWO-01, FWO-02, APWO dan daerah dimana alterasi tersingkap. Sehingga dapat disimpulkan bahwa area yang memiliki densitas tinggi berkorelasi dengan sebaran resistivitas yang bernilai 10 ohm-m dan daerah yang memiliki nilai anomali magnetik rendah -250 nT sampai -200 nT dimana mengindikasikan zona reservoir berasosiasi dengan batas bawah clay cap atau BOC.

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To determine the prospek area in geothermal field, must have the structure associated with high permeability, high temperature and upflow zone. Based on the map of lineament density and the FFD method shows a high density zone associated with the depression zone which is the main structure of the filed Q and the emergence of several manifestations of fumaroles and hot springs APWO, APWK, APLM and APBK. From the ternary diagram Cl SO4 HCO3 shows that APWO has steam heated water type, APWK has bicarbonate water type while APBK and APLM are chloride water type. Based on the obtained geochemical manifestations the geothermal system is a hydrothermal system. Prediction of reservoir temperature using silica geothermometer and gas geothermometer each ranging between 141 C 212 C and 250 C. So the manifestations of APWO, FWO 1 and FWO 2 indicate that they are in the upflow zone and APWK, APBK, APLM are in outflow zone.

The result of 3D MT inversion shows low resistivity 100 ohm m appears to be an updome at depths of 500 to 4000 m suspected as heat source. From the upward continuation and reduce to pole results, magnetic values ranging from 250 nT to 350 nT are under the manifestations of FWO 01, FWO 02, APWO and the areas where alteration is exposed. It can be concluded that areas with high densities are correlated with 10 ohm m resistivity distributions and regions having low magnetic anomaly values 250 nT to 200 nT which indicate the reservoir zone associated with the lower limit of clay cap or BOC."

"Analisis Splitting Curve Data Magnetotellurik untuk Mengidentifikasi Zona Permeabel Pada Lapangan Geothermal XEmir Ghufron1, Syamsu Rosyid11Departemen Fisika, FMIPA, Universitas IndonesiaEmail : ghufronemir@gmail.com Abstrak Dalam eksplorasi geothermal, zona permeabel merupakan salah satu parameter yang diperhitungkan. Zona permeabel tersebut dipengaruhi oleh rekahan atau patahan yang terbentuk di bawah permukaan daerah prospek geothermal. Dengan melihat kondisi geologi daerah prospek, zona permeabel dapat diidentifikasi. Pada kenyataanya, metode geologi yang digunakan hanya mampu mengetahui kondisi struktur di permukaan bumi. Mengetahui kondisi geologi yang ada di bawah permukaan bumi sangat sulit di perhitungkan. Berdasarkan hal tersebut dilakukan analisis Splitting Curve data MT untuk mengetahui kondisi geologi di bawah permukaan. Penelitian ini dibantu dengan membuat forward modeling data sintetik untuk memperkuat analisis Splitting Curve dan kemudian diimplemetasikan pada data rill MT. Hasil dari forward modeling menunjukan adanya perbedaan nilai resistivitas yang menghasilkan percabangan kurva TE dan TM, hal ini memberi informasi dekat atau jauhnya suatu stasiun pengukuran MT terhadap batas kontras resistivitas atau batas suatu struktur. Hasil akhir dari penelitian ini adalah penentuan zona permeabel daerah prosek geothermal, harapanya dapat mengetahui infornasi struktur geologi bawah permukaan.

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Permeability zone is one of the most important parameter. The permeability zone is affected by fracture or fault that occurs in the subsurface of geothermal prospect area. By studying on the geological condition, we can identify the permeable zone. The fact is, the geological method is limited to the structure on the surface only. To learn more about geological condition in subsurface is very difficult.

Splitting curve analysis of MT data to learn about the continuity of subsurface geological condition. This research is assisted by making forward modelling of synthetic MT data to strengthen spliting curve analysis and then implementing it into the real MT data.

The result from forward modelling showed the difference of resistivity value which produce shifting in TE and TM curve. Thus, this information is letting us know the distance between MT station to resistivity contrast boundary or limit of the structure. The final result for this reasearch is to determine the permeable zone at the subsurface, hopefully the geological structures from subsurface of geothermal prospect area can be determined."

"Seperti hal nya lapangan geotermal yang sudah diekstraksi lebih dari 25 tahun, Salak juga mengalami penurunan produksi akibat cooling atau penurunan suhu pada reservoir. Analisis 3G, yaitu Geofisika, Geokimia, dan Geologi yang dilakukan seringkali tidak sesuai dengan data sumur, karena kesalahan mendasar dari interpretasi zona resistivitas rendah, barrier sesar atau marjin reservoir terutama pada reservoir di bawah sumur produksi dan injeksi lapangan Salak. Semua data yang dianalisis adalah data yang diperoleh pada waktu sumur awal diproduksi. Analisis penyebaran zona resisitivitas dengan metode seleksi dan re-prosessing yang tepat menjadi hal dasar dari penentuan batas area prospek dan zona uplow dan outflow. Sebaran dan kedalaman zona konduktif mendeskripsikan penyebaran mineral alterasi, kontak litologi, barrier patahan, batas TOR dan BOC (updome shape), perubahan suhu karena perbedaan kontras dari jalur permeabilitas dan arah aliran fluida. Pola ketebalan resistivitas yang relatif sama di atas reservoir atau batas low resistivity anomaly mungkin menghasilkan kesalahan interpretasi dari noise yang perlu dikoreksi. Penelitian ini diharapkan dapat memberikan informasi model konseptual yang lebih konsisten, representatif dan terintegrasi serta menunjukkan kesesuaian dengan data pendukung lainnya, sehingga dapat mengidentifikasi masalah yang dihadapi, dan selanjutnya memberikan hipotesis untuk menjaga keberlangsungan performa reservoir.

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Like most geothermal field which has been extracted for more than 25 years, Salak has been also experienced the production decline, which is quite likely caused by change of well pressure and cooling in reservoir. The existing 3G analysis was frequently contradicted with well data, especially caused by misinterpretation of noisy and low deep resistivity. Furthermore, all corrected 3D inversion MT data is compared with well data, geology , and geochemical data, to produce updated and integrated conceptual model, which can be expexted to re-evaluate analysis of changes in the distribution of resistive zones at initial reservoir conditions that give indication of distribution of alteration minerals, TOR and BOC (updome shape) boundary, lithology contacts, variation of pressure and temperature, and give identification of permeability zone contrasting and fluid pathway, fault orientation corresponds to good aggrement of interpetation of low resistivity zone and well data. The powerfull and developed 3D MT Inversion and observation of anomalous resistivity feature interpreted as clay alteration, fault barrier, upflow and or the expansion of neutral spring water in ouflow zone near Awi 20 and 15. This anomaly is strongly correlated to the temperature changes in hydrothermal mineral. This variation shows the deep and shallow up-dome shaped of geothermal system below Awi 9, 10 and 14, confirmed by Parabakti and Cibeureum fumarol analysis, thin clay cap, fault intersection map and especially high temperature in well data. To test this hypothesis, writer recast all supporting data with revised resistivity model. This research is expected to provide information on a representative conceptual model and accurate analysis to the current problems respectively, hence improved approachs can be taken to implement further recommendation on how to hypothesize a strategic solution to maintain reservoir performance. "

"Eksplorasi panas bumi merupakan tahapan yang sangat penting pada kegiatan pengusahaan panas bumi karena memiliki tingkat resiko kegagalan pemboran yang sangat tinggi serta biaya yang dikeluarkan cukup besar. Oleh karena itu, diperlukan interpretasi terpadu berdasarkan data geosains untuk mendelineasi prospek sistem geothermal Gn. Lawu sehingga penentuan lokasi pemboran mempunyai tingkat kepastian yang lebih tinggi. Dalam mendelineasi sistem geothermal sangat ditentukan dengan sebaran batuan resistivitas rendah yang mengindikasikan adanya lapisan penudung (caprock) yang didominasi oleh material lempung. Selain itu, target utama dari eksplorasi panas bumi adalah temperatur dan permeabilitas batuan yang tinggi. Distribusi sebaran temperatur bawah permukaan dapat diperkirakan melalui data manifestasi pada lokasi penelitian. Sedangkan permeabilitas tinggi berasosiasi dengan zona patahan dimana fluida dapat mengalir ke permukaan. Magnetotelluric (MT) merupakan metode geofisika pasif yang melibatkan pengukuran fluktuasi medan listrik dan magnet alam sebagai sarana untuk menentukan resistivitas batuan di bawah permukaan bumi dimana pemodelan data MT dapat dilakukan menggunakan inversi 3D. Untuk memahami lebih lanjut mengenai pemodelan MT tersebut, maka penelitian ini difokuskan pada inversi 3D dengan MT3Dinv-X yang kemudian diintegrasikan dengan data dukung gravitasi, geologi dan geokimia untuk mendelineasi sistem geothermal Gn. Lawu. Hasil akhir penelitian ini adalah memberikan rekomendasi pola sebaran resistivitas batuan serta membuat model konseptual untuk menentukan area target pemboran pada daerah penelitian.

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Geothermal exploration is crucial step in geothermal business because it has uncertainty drilling result and high cost. Therefore, an integrated interpretation based on geosciences data is needed to delineate the prospect of Gn. Lawu geothermal system so the location of drilling could be more convince. The distribution of low resistivity rocks that indicating the presence of a caprock which is dominated by the material of clay. Moreover, the main target of geothermal exploration is the high temperature and permeability of rocks. The distribution of subsurface temperature can be estimated through the manifestation data on site. High permeability is associated with a fault zone where fluid can flow to the surface. Magnetotelluric (MT) is a passive geophysical method that involves measuring the fluctuations of electric and natural magnetic fields as a means of determining the resistivity of rocks beneath the Earth's surface where MT data modeling can be performed using 3D inversion. To understand more about the MT modeling, this research is focused on 3D inversion with MT3Dinv-X which is then integrated with gravity, geological and geochemical support data to delineate Gn. Lawu geothermal system. The final result of this research is to recommend the pattern of distribution of rock resistivity as well as to create conceptual model to determine drilling target area in research area."

"Pulau Sumatera memiliki potensi panas bumi terbesar di Indonesia yang tersebar di sepanjang zona subduksi antara lempeng Hindia-Australia dan lempeng Eurasia, salah satunya adalah lapangan geothermal ldquo;A rdquo;. Secara umum, litologi di wilayah penelitian didominasi oleh batuan vulkanik yang berumur kuarter dengan manifestasi berupa fumarol dan mata air panas. Struktur geologi berupa patahan dan pendugaan intrusi batuan yang diidentifikasi sebagai heat source menjadi target dalam penelitian ini. Metode penginderaan jauh dengan analisis Fault Fracture Density FFD dilakukan untuk mengidentifikasi gejala struktur patahan di permukaan yang berasosiasi dengan manifestasi dan metode gravitasi dengan analisis First Horizontal Derivative FHD dan Second Vertical Derrivative SVD dilakukan untuk mengidentifikasi patahan di bawah permukaan. Hasil dari penelitian ini menunjukkan bahwa kemunculan manifestasi berada pada zona FFD tinggi dengan kerapatan sebesar 4 km/km2. Analisis data FHD dan SVD dapat mengkonfirmasi patahan berarah Barat Daya-Timur Laut, Barat Laut-Tenggara, dan struktur kaldera dengan jenis patahan keseluruhan berupa patahan normal. Hasil inversi 3D gravitasi mengidentifikasi batuan clay cap memiliki densitas 2.015 gr/cc sampai 2.24 gr/cc, batuan reservoir memiliki densitas 2.3 gr/cc sampai 2.4 gr/cc dan batuan heat source memiliki densitas 2.5 gr/cc sampai 2/8 gr/cc. Zona upflow terletak di bagian Barat wilayah penelitian dengan suhu reservoir berkisar antara 200°C-220°C.

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Sumatra Island has the largest geothermal potential in Indonesia spread along the subduction zone between the Indies Australian plate and the Eurasian plate. ldquo A rdquo geothermal field is one of them. In general, lithology in the study area is dominated by quaternary volcanic rocks and it has some manifestations such as fumaroles and hot springs. This study is focus on identify the structure and intrusion that identified as a heat source.

Remote sensing methods with Fault Fracture Density FFD analysis were performed to identify symptoms of surface fractures associated with manifestations and gravity methods with First Horizontal Derivative FHD and Second Vertical Derivative SVD analyzes performed to identify subsurface fractures.

The results of this study indicate that the appearance of manifestation is in the high FFD zone with a density of 4 km km2. Analysis of FHD and SVD data can confirm the Southwest Northeast, Northwest Southeast fault, and caldera structure with the overall fracture type are normal fault.

The result of gravity 3D inversion identifies clay cap rock has density 2,015 gr cc to 2,24 gr cc, reservoir rock has density 2,3 gr cc to 2,4 gr cc and heat source rock has density 2.5 gr cc to 2 8 gr cc . The upflow zone is located in the west of the research area with reservoir temperatures ranging from 200°C 220°C."