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"ABSTRACTDi bidang panas bumi, data magnetotelluric biasanya digunakan untuk mendapatkan informasi resistivitas di bawah permukaan. Salah satu pengolahan data magnetotelurik adalah inversi data, dimana inversi adalah proses mengubah data magnetotelurik menjadi penampang resistivitas. Inversi yang digunakan dalam penelitian ini adalah inversi 1D dan 2D. Salah satu informasi bawah permukaan yang menjadi fokus eksplorasi panas bumi adalah lapisan tudung yang dicirikan oleh nilai resistivitas kecil. Salah satu mineral yang terkandung dalam batu yang diubah adalah smektit. Smektit terbentuk pada suhu 20-180oC, pada suhu di atas 70oC, smektit menjadi tidak stabil dan lapisan smektit-ilit menjadi tidak terdeteksi pada batuan berpori pada suhu di atas 200oC. Untuk mendeteksi smektit ini, teknik sederhana, yaitu titrasi metilen-biru (MeB), digunakan pada batuan pemboran. Teknik ini telah lama digunakan untuk memperkirakan kandungan smektit pada batuan yang dipotong dengan baik di bidang panas bumi. Data penelitian diambil di wilayah kontrak Sarulla, bidang Namora I-Langit yang merupakan bidang panas bumi yang berlokasi di Sumatera Utara. Korelasi antara nilai MeB dengan data resistivitas berbanding terbalik. Ini karena nilai MeB yang tinggi dari konten smektit dalam lapisan juga tinggi, oleh karena itu nilai CEC dalam lapisan juga akan tinggi, akibatnya nilai resistivitas lapisan akan rendah.

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ABSTRACTKata kunci: Magnetotellurik, Inversi 1D & 2D, MethyIn the geothermal field, magnetotelluric data are commonly used to obtain resistivity information below the surface. One of magnetotelluric data processing is data inversion, wherein inversion is a process of changing magnetotelluric data into resistivity cross section. The inversion used in this research is 1D and 2D inversion. One of the subsurface information that is the focus of geothermal exploration is the hood layer which is characterized by a small resistivity value. One of the minerals contained in the altered rock is smectite. Smectites are formed at temperatures of 20-180oC, at temperatures above 70oC, smectites become unstable and smectite-illite layers become undetectable in porous rocks at temperatures above 200oC. To detect this smectite, a simple technique, namely methylene-blue (MeB) titration, is used on the drilling rock. This technique has long been used to estimate smectite content in well-cut rocks in the geothermal field. The research data was taken in the Sarulla contract area, Namora I-Langit field which is a geothermal field located in North Sumatra. The correlation between MeB values ​​with resistivity data is inversely proportional. This is because the high MeB value of the smectite content in the layer is also high, therefore the CEC value in the layer will also be high, as a result the resistivity value of the layer will be low.Keywords: Magnetotellurik, 1D & 2D Inversion, Methy"

"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."

"Eksplorasi panasbumi yang dilakukan pada daerah prospek panasbumi bertujuan untuk mencari zona reservoir. Zona reservoir yang baik bisa dilihat dari 2 faktor yaitu, batuan reservoir memiliki permeabilitas yang tinggi dan fluida reservoir memiliki suhu yang tinggi. Berdasarkan faktor pertama, permeabilitas batuan reservoir yang tinggi memungkinkan reservoir untuk memiliki kandungan fluida panasbumi yang banyak. Pada umumnya batuan memiliki permeabilitas lebih besar disebabkan oleh batuan tersebut memiliki permeabilitas sekunder yang berasal dari struktur geologi berupa patahan. Metode geofisika seperti metode Magnetotellurik (MT) dan Gravitasi diaplikasikan pada penelitian ini untuk memetakan zona reservoir sistem panasbumi. Metode MT digunakan untuk mendeteksi struktur resistivitas bawah permukaan. Analisis metode gravitasi yang melibatkan data anomali bouguer lengkap dan anomali residual dapat digunakan untuk memetakan struktur densitas bawah permukaan. Faktor kedua yaitu temperatur yang didapatkan dari data sumur yang ada. Selanjutnya, proses interpretasi terintegrasi dilakukan dengan melibatkan data penunjang lainnya berupa data geologi, geokimia, dan data sumur yang menghasilkan model konseptual panasbumi.

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The objective of geothermal exploration which was concluded at geothermal prospects area is to find the reservoir zone. Good reservoir zones can be seen from two factors, reservoir rocks which have high permeability and reservoir fluid has high temperature. Under the first factor, high permeability of reservoir rocks allows the reservoir to contain much geothermal fluids. In general, great permeability of the rock is caused by secondary permeability derived from geological structures like faults. Geophysical methods such as magnetotelluric (MT) and gravity were applied in this study to delineate the reservoir zone. MT method was used to detect subsurface resistivity structure. Analysis of gravity data to complete bouguer anomaly map (CBA) and residual anomaly can figure subsurface density structures. Under the second factor, the temperature can be obtained from well data. Furthermore, the integrated interpretation is done by involving other supporting data such as geological, geochemical, and well data which produces geothermal conceptual model."

"Terdapat dua prospek panas bumi di sekitar Gunung Slamet, yaitu prospek Guci di sebelah barat laut dan prospek Baturaden di sebelah selatan dari Gunung Slamet. Menjadi sangat menarik untuk mengetahui hubungan kedua prospek tersebut, apakah prospek tersebut merupakan dua daerah prospek yang dipisahkan oleh tinggian low permeability barrier sehingga tidak akan terjadi interferensi diantara kedua prospek? Dengan melakukan deliniasi zona permeabel berdasarkan analisis data magnetotelurik dan data gravity dikorelasikan dengan data struktur geologi permukaan dan data manifestasi permukaan yang ada, diharapkan dapat mengetahui hubungan diantara kedua prospek tersebut. Dalam penelitian ini dilakukan pemrosesan dan pemodelan data geofisika menggunakan metode magnetotelurik inversi 2-D dan metode gravity 2-D forward. Pemodelan ini sangat efektif dalam mendeteksi zona-zona dengan kontras resistivitas tinggi untuk mendeliniasi zona permeabel lapangan panas bumi. Daerah prospek panas bumi Gunung Slamet dapat terdeliniasi dengan jelas berdasarkan beberapa penampang lintasan yang dibuat, yang menunjukkan daerah prospek berada di sisi sebelah barat Gunung Slamet dengan luas berdasarkan peta BOC sekitar 13 km2, dan berdasarkan peta resistivitas pada elevasi 0 meter yang dikombinasikan dengan peta struktur geologi luas daerah prospek sekitar 22 km2. Dan hasil akhir dari penelitian ini adalah memberikan rekomendasi dalam menentukan lokasi pemboran, dengan sebelumnya membuat model konseptual prospek panas bumi Gunung Slamet.

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There are two geothermal prospects in the vicinity of Mount Slamet, the prospect of Guci in northwest and prospects Baturaden in the south of Mount Slamet. Be very interesting to know the relationship between the two prospects, whether the prospect of two regions separated by low permeability barrier heights so that there will be no interference between the two prospects?

By doing permeable zone delineation based on data analysis magnetotelluric and gravity, correlated with surface geological structural data and existing surface manifestations, are expected to know the relationship between the two prospects.

In this research, processing and modeling of geophysical data using magnetotelluric inversion method 2-D and 2-D method of gravity forward. Modeling is very effective in detecting zones with high resistivity contrast to delineate the permeable zone geothermal field. Geothermal prospect areas of Mount Slamet can be delineated clearly based on some of the tracks that made cross-section, showing the prospect area is located on the west side of Mount Slamet with broad based map BOC about 13 km2, and resistivity maps based on elevation of 0 meters, combined with the structure geological maps, the prospect area about 22 km2.

And the end result of this study is to provide recommendations in determining the location of drilling, with previous a conceptual model of geothermal prospects Mount Slamet."

"Tahapan eksplorasi masih menyimpan tantangan terbesar dan memiliki resiko yang tinggi bagi para pelaku industri bidang geothermal. Oleh karena itu, diperlukan pemahaman yang baik mengenai kondisi bawah-permukaan dengan mengintegrasikan data geosains yang memiliki kualitas yang bagus. Target utama dari eksplorasi yaitu penentuan lokasi pemboran dengan tingkat kepastian yang lebih tinggi. Pemboran diarahkan pada area yang memiliki temperatur dan permeabilitas yang tinggi. Distribusi temperatur bawah permukaan dapat didekati dari nilai resistivitas yang diperoleh dari data MT. Sementara zona dengan permeabilitas yang tinggi, berasosiasi dengan struktur geologi. Pemetaan geologi hanya dapat menggambarkan struktur geologi di permukaan, sementara kemenerusannya di bawah-permukaan menjadi kesulitan tersendiri untuk dideteksi. Penelitian ini difokuskan pada identifikasi struktur geologi bawah-permukaan menggunakan data Magnetotellurik (MT) dan Gravitasi. Analisis pola spliting kurva, arah elongasi polar diagram, serta pencitraan struktur di bawah-permukaan dengan melihat hasil inversi 3-dimensi, yang diperoleh dari data MT, serta didukung oleh hasil pemodelan data Gravitasi, merupakan metodologi yang digunakan dalam penelitian ini. Data geologi dan geokimia, dilibatkan sebagai data pendukung untuk membuat analisis keberadaan struktur geologi bawah-permukaan ini menjadi lebih komprehensif. Tahap akhir dari penelitian ini adalah memberikan rekomendasi dalam menentukan lokasi pemboran, dengan sebelumnya membuat model konseptual dan mendelineasi daerah prospek. Hasil analisis struktur, model konseptual, dan delineasi daerah prospek, menghasilkan rekomendasi tiga buah sumur eksplorasi. Dua sumur mengarah pada upflow di Gunung ?X?, dan satu sumur mengarah pada upflow di scoria cone.

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Exploration stage still holds the biggest challenges and have a high risk for the geothermal industry. Therefore, required a good understanding of subsurface conditions by integrating the geoscientific data that has a high quality. The main target of exploration is the determination of drilling trajectory. The subsurface drilling target is actually directed to high temperature and high permeability zone. Subsurface temperature distribution can be approximated from the resistivity values obtained from the data MT. While the zones with high permeability, associated with geological structures. Geological mapping could only figure out geological structures indicated at the surface. However, continuation of the geological structure into the subsurface is difficult to detect. This study focused on the identification of subsurface geological structure using Magnetotelluric (MT) and gravity data. Splitting pattern analysis from MT curve, the elongation of orientation of polar diagrams, as well as imaging of subsurface structures by looking at the results of 3-dimensional inversion, the data obtained from MT, and supported by the results of Gravity data modeling, a methodology used in this study. Geological and geochemical data, were included as supporting data to make the analysis of the presence of subsurface geological structure has become more comprehensive. And the final stage of this research is to provide recommendations in determining the location of drilling, by first making a conceptual model of the geothermal system and delineating the prospect area. The result of structure analysis, conceptual model, and prospect delineation, provide three exploration wells for recommendation. The first two will be directed to upflow at Mount ?X?, and the other one to upflow at scoria cone."

"ABSTRAKInversi data magnetotellurik merupakan suatu proses mengubah data magnetotellurik menjadi penampang resistivitas. Salah satu metode inversi yang digunakan adalah inversi 3D. Inversi 3D magnetotellurik mengasumsikan bahwa bumi memiliki variasi resistivitas baik arah vertikal maupun lateral. Inversi tersebut menghasilkan model yang paling mendekati keadaan lapisan bumi yang sebenarnya. Akan tetapi, inversi 3D dimensi membutuhkan memori serta waktu yang lama dalam prosesnya. Untuk mengatasi masalah tersebut, digunakan variasi model awal sebagai pengontrol proses inversi. Model awal yang dapat digunakan adalah resistivitas hasil inversi 1D dimana hasil inversi tersebut memiliki kemiripan dengan hasil inversi 3D. Pada penelitian ini, penulis melakukan inversi data riil magnetotellurik dengan memvariasikan beberapa model awal. Variasi 'inversi dengan menggunakan model awal 1D menunjukkan bahwa model awal 1D mampu mengontrol proses inversi 3D dilihat dari kesesuaian hasil inversi 3D dengan model awal yang digunakan. Selain itu, hasil inversi dengan menggunakan model awal data inversi 1D menunjukkan hasil yang lebih baik pada model yang menggunakan lebih banyak mesh grid. Hal tersebut dapat dilihat dari RMS error model terhadap data observasi.

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ABSTRACT Inversion of Magnetotelluric data is a process to obtain resistivity variation from magnetotelluric data. 3D Inversion of magnetotelluric data is a method that usually used. Those method assume that earth has resistivity variation along vertical and lateral direction. It can produce the most similliar earth resistivity model to the real earth. However, 3D inversion method need high amount of CPU memory and calculation time. In order to cover that weakness, initial model is used to control the inversion process. The initial model used is resistivity variation from 1D inversion of magnetotelluric data. Resistivity variation of 1D inversion has simmiliar pattern with resistivity variation of 3D inversion. 3D inversion is done on real magnetotelluric data with variation of initial model. The variabels which are used initial model are resistivity variation and number of mesh grid blocks. The results of 3D inversion using 1D resistivity initial model show that initial model can control the inversion process. The result of 3D inversion have similiar pattern with the inisial model which is used. The results of 3D inversion using 1D resistivity initial model show better result than 3D inversion using homogenous resistivity initial model on larger number of mesh grid, it can be proven by its RMS errors."

"Sekarang ini metode MT cukup berkembang dan seringkali digunakan sebagai metode geofisika yang mampu memetakan kondisi bawah permukaan dengan baik, khususnya sistem panasbumi. Namun di sisi lain, keberadaan software atau program yang dapat digunakan untuk melakukan pengolahan data MT masih terbatas dan harganya relatif mahal. Dengan demikian penulis berupaya untuk melakukan penelitian dalam pembuatan program pengolahan data MT tersebut, terutama yang dapat mengolah data mentah MT berupa time series sampai menjadi data resistivitas semu dan fase. Dalam penelitian ini, penulis memfokuskan pada pembuatan program menggunakan MATLAB yang dapat melakukan pengolahan data time series menjadi resistivitas semu dan fase. Ada beberapa tahapan penting yang perlu dilakukan dalam melakukan proses pengolahan data time series, yaitu proses transformasi Fourier dengan teknik Fast Fourier Transform (FFT) yang bertujuan untuk mentransformasi data dari domain waktu menjadi domain frekuensi. Selanjutnya dilakukan penentuan interval frekuensi yang nantinya akan diproses pada tahapan selanjutnya. Kemudian dilakukan teknik robust processing yang tujuannya adalah untuk membuat data menjadi lebih smooth. Setelah itu dapat dihitung nilai tensor impedansinya untuk perhitungan resistivitas semu dan fase. Adapun hasil pengolahan data MT dari program yang telah dibuat sangat baik, dimana terdapat adanya kesesuaian antara kurva resistivitas semu dan fase yang dihasilkan dari program yang dibuat dan yang dihasilkan dari software komersial (SSMT2000). Perbandingan dengan menggunakan hasil inversi 2-D dengan input berupa data resistivitas semu dan fase dari kedua program pun menunjukkan adanya kesesuaian.

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Recently, Magnetotelluric (MT) Method has been developed and often used as geophysical method which has good ability for subsurface mapping, especially geothermal system. However, software and program that could be used to carry out MT data processing is limited and expensive. Accordingly, the author attempted to do research in developing MT data processing program, especially time series data processing to be apparent resistivity and phase data. In this research, the author focuses on developing the computer program using MATLAB to proces the time series data transformation to be apparent resistivity and phase. There are several important steps to do in time series data processing, firstly Fourier transformation using Fast Fourier Transform (FFT) technique to transform the data from time domain to frequency domain. The next step is determination of frequency interval to be used for the next step. After that, a robust processing technique is performed to make the data smoother. Then, further step is calculation of tensor impedance for calculating apparent resistivity and phase. The MT data processing result produced from the computer program is excellent, where there is similarity between the apparent resistivity and phase curve produced from the computer program and those produced from the commersial software (SSMT2000). Comparison using 2-D inversion by inputting the apparent resistivity and phase data produced from both computer programs shows good agreement."

"[Daerah prospek panas bumi Gunung Arjuno dan Gunung Welirang berada pada jalur vulkanik yang dikenal dengan jalur ring of fire, yaitu rentetan gunung api, baik yang aktif, maupun gunung api yang tidak aktif. Gunung tersebut berasosiasidengan pembentukan sistem panas bumi yang ditandai dengan kemunculan manifestasi yang terdiri dari mata air panas Padusan, Coban dan Cangar serta adanya fumarol yang terdapat di komplek Gunung Welirang. Dari hasil perhitungan geothermometer daerah prospek panas bumi Gunung Arjuno danGunung Welirang memiliki temperatur 250o C dan masuk dalam kategori high temperature (>225 oC). Untuk mengetahui batas, kedalaman, dan geometri dari reservoir yang ada, dilakukan pengukuran dengan metode Magnetotellurik (MT), Time Domain Electromagnetic (TDEM) dan gaya berat. Dari hasil pengukurantersebut, dilakukan pemodelan pada 138 data MT, 103 data TDEM dan 253 data gaya berat. Selanjutnya hasil pemodelan dianalisa dengan menggunakan penampang 1 dimensi, 2 dimensi dan visualisasi 3 dimensi. Karakteristik reservoir berada pada kisaran 10-30 Ohm-m dengan nilai densitas rata-rata 2.2gr/cc dan menghasilkan prospek panas Gunung Arjuno dan Gunung Welirang sekitar 40 km2 dengan potensi yang dapat dikembangkan untuk pembangkit tenaga listrik sebesar 140 MWe, rekomendasi penentuan titik bor eksplorasi berada di 2 km baratlaut dari komplek Gunung Welirang.

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The geothermal prospect areas Mount Arjuno and Mount Welirang is on track which is known as volcanic ring of fire, which is a series of volcanoes, both active and inactive volcanoes. The mountain is associated with the formation of geothermal systems that are characterized by the appearance of manifestations consisting of Padusan, Coban and Cangar hot springs and their fumaroles located

in Mount Welirang complex. From the calculation geothermometer, the geothermal prospect areas Mount Arjuno and Welirang has a temperature of 250°C and in the category of high temperature (190 oC-236 oC). To determine the

boundary, the depth, and the geometry of the existing reservoir, measured by the method of magnetotelluric (MT), Time Domain Electromagnetic (TDEM) and gravity. From the results of these measurements, modeling performed on the 138

MT data, 103 TDEM data and 253 gravity data. Furthermore, the modeling results were analyzed using 1 dimensional cross-section, 2 dimensional and 3 dimensional visualization. The position of the reservoir is in the range of 10-30 Ohm-m with an average density value 2.2 g/CC3 to generate hot prospects Mt.Arjuno and Mount Welirang approximately 40 km2. with potential developed for power plants of 140 MWe, recommendations exploration drill point

determination located at 3km north-west of the mountain complex Mount Welirang.;The geothermal prospect areas Mount Arjuno and Mount Welirang is on track

which is known as volcanic ring of fire, which is a series of volcanoes, both active

and inactive volcanoes. The mountain is associated with the formation of

geothermal systems that are characterized by the appearance of manifestations

consisting of Padusan, Coban and Cangar hot springs and their fumaroles located

in Mount Welirang complex. From the calculation geothermometer, the

geothermal prospect areas Mount Arjuno and Welirang has a temperature of

250°C and in the category of high temperature (190 oC-236 oC). To determine the

boundary, the depth, and the geometry of the existing reservoir, measured by the

method of magnetotelluric (MT), Time Domain Electromagnetic (TDEM) and

gravity. From the results of these measurements, modeling performed on the 138

MT data, 103 TDEM data and 253 gravity data. Furthermore, the modeling results

were analyzed using 1 dimensional cross-section, 2 dimensional and 3

dimensional visualization. The position of the reservoir is in the range of 10-30

Ohm-m with an average density value 2.2 g / CC3 to generate hot prospects

Mt.Arjuno and Mount Welirang approximately 40 km2. with potential developed

for power plants of 140 MWe, recommendations exploration drill point

determination located at 3km north-west of the mountain complex Mount

Welirang.;The geothermal prospect areas Mount Arjuno and Mount Welirang is on track

which is known as volcanic ring of fire, which is a series of volcanoes, both active

and inactive volcanoes. The mountain is associated with the formation of

geothermal systems that are characterized by the appearance of manifestations

consisting of Padusan, Coban and Cangar hot springs and their fumaroles located

in Mount Welirang complex. From the calculation geothermometer, the

geothermal prospect areas Mount Arjuno and Welirang has a temperature of

250°C and in the category of high temperature (190 oC-236 oC). To determine the

boundary, the depth, and the geometry of the existing reservoir, measured by the

method of magnetotelluric (MT), Time Domain Electromagnetic (TDEM) and

gravity. From the results of these measurements, modeling performed on the 138

MT data, 103 TDEM data and 253 gravity data. Furthermore, the modeling results

were analyzed using 1 dimensional cross-section, 2 dimensional and 3

dimensional visualization. The position of the reservoir is in the range of 10-30

Ohm-m with an average density value 2.2 g / CC3 to generate hot prospects

Mt.Arjuno and Mount Welirang approximately 40 km2. with potential developed

for power plants of 140 MWe, recommendations exploration drill point

determination located at 3km north-west of the mountain complex Mount

Welirang.;The geothermal prospect areas Mount Arjuno and Mount Welirang is on track

which is known as volcanic ring of fire, which is a series of volcanoes, both active

and inactive volcanoes. The mountain is associated with the formation of

geothermal systems that are characterized by the appearance of manifestations

consisting of Padusan, Coban and Cangar hot springs and their fumaroles located

in Mount Welirang complex. From the calculation geothermometer, the

geothermal prospect areas Mount Arjuno and Welirang has a temperature of

250°C and in the category of high temperature (190 oC-236 oC). To determine the

boundary, the depth, and the geometry of the existing reservoir, measured by the

method of magnetotelluric (MT), Time Domain Electromagnetic (TDEM) and

gravity. From the results of these measurements, modeling performed on the 138

MT data, 103 TDEM data and 253 gravity data. Furthermore, the modeling results

were analyzed using 1 dimensional cross-section, 2 dimensional and 3

dimensional visualization. The position of the reservoir is in the range of 10-30

Ohm-m with an average density value 2.2 g / CC3 to generate hot prospects

Mt.Arjuno and Mount Welirang approximately 40 km2. with potential developed

for power plants of 140 MWe, recommendations exploration drill point

determination located at 3km north-west of the mountain complex Mount

Welirang.;The geothermal prospect areas Mount Arjuno and Mount Welirang is on track

which is known as volcanic ring of fire, which is a series of volcanoes, both active

and inactive volcanoes. The mountain is associated with the formation of

geothermal systems that are characterized by the appearance of manifestations

consisting of Padusan, Coban and Cangar hot springs and their fumaroles located

in Mount Welirang complex. From the calculation geothermometer, the

geothermal prospect areas Mount Arjuno and Welirang has a temperature of

250°C and in the category of high temperature (190 oC-236 oC). To determine the

boundary, the depth, and the geometry of the existing reservoir, measured by the

method of magnetotelluric (MT), Time Domain Electromagnetic (TDEM) and

gravity. From the results of these measurements, modeling performed on the 138

MT data, 103 TDEM data and 253 gravity data. Furthermore, the modeling results

were analyzed using 1 dimensional cross-section, 2 dimensional and 3

dimensional visualization. The position of the reservoir is in the range of 10-30

Ohm-m with an average density value 2.2 g / CC3 to generate hot prospects

Mt.Arjuno and Mount Welirang approximately 40 km2. with potential developed

for power plants of 140 MWe, recommendations exploration drill point

determination located at 3km north-west of the mountain complex Mount

Welirang., The geothermal prospect areas Mount Arjuno and Mount Welirang is on track

which is known as volcanic ring of fire, which is a series of volcanoes, both active

and inactive volcanoes. The mountain is associated with the formation of

geothermal systems that are characterized by the appearance of manifestations

consisting of Padusan, Coban and Cangar hot springs and their fumaroles located

in Mount Welirang complex. From the calculation geothermometer, the

geothermal prospect areas Mount Arjuno and Welirang has a temperature of

250°C and in the category of high temperature (190 oC-236 oC). To determine the

boundary, the depth, and the geometry of the existing reservoir, measured by the

method of magnetotelluric (MT), Time Domain Electromagnetic (TDEM) and

gravity. From the results of these measurements, modeling performed on the 138

MT data, 103 TDEM data and 253 gravity data. Furthermore, the modeling results

were analyzed using 1 dimensional cross-section, 2 dimensional and 3

dimensional visualization. The position of the reservoir is in the range of 10-30

Ohm-m with an average density value 2.2 g / CC3 to generate hot prospects

Mt.Arjuno and Mount Welirang approximately 40 km2. with potential developed

for power plants of 140 MWe, recommendations exploration drill point

determination located at 3km north-west of the mountain complex Mount

Welirang.]"

"ABSTRAKAnalisis dimensionalitas merupakan parameter yang kuat untuk memilih pemodelan mana yang sesuai disetiap kondisi bawah permukaan. Hal ini karena sudah dikembangkannya teknologi inversi 1-D, 2-D dan 3-D. Selain itu analisis dimensionalitas dapat digunakan untuk mengetahui arah dari struktur utama. Pada penelitian ini digunakan dua parameter analisis dimensionalitas untuk dipelajari sebagai tahapan awal penelitian analisis dimensionalitas. Parameter diagram polar dan impedansi skew digunakan untuk menganalisis kondisi dimensionalitas model sintetik dan data riil lapangan panas Bumi. Selain itu juga dilakukan pembandingan inversi 1-D, 2-D dan 3-D pada setiap kondisi dimensionalitas bawah permukaan. Perbandingan menunjukkan inversi 3-D dapat menggambarkan kondisi dimensionalitas ideal 1-D, 2-D dan 3-D dengan baik sehingga analisis dimensionalitas untuk memilih pemodelan mana yang tepat tidak perlu lagi dilakukan. Namun, analisis dimensionalitas masih efektif untuk dilakukan dalam mengidentifikasi struktur bawah permukaan dan menentukan arah dari struktur sesuai dengan hasil yang telah ditunjukkan pada data sintetik dan data riil dari penelitian ini.

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ABSTRAKDevelopment of 1-D, 2-D and 3-D inversion has causing dimensionality analysis as a powerful parameter that select which type of approach is more suitable to accomplish modeling, or interpretation : one dimensionality, two dimensionality and three dimensionality. It because 1-D, 2-D and 3-D inversion already developed. Moreover, dimensionality analysis can be used to know the path of geoelectrical strike. This thesis use two parameters of dimensionality analysis as the beginning of dimensionality analysis research. Polar Diagram and impedance skew parameter are used for analizing the dimensionality condition from syntetic model and real data of geothermal field. In this thesis, comparison of 1-D, 2-D and 3-D inversion has been made in each subsurface dimensionality condition. The comparison result show 3-D inversion could imaged the proper condition of ideal dimensionality 1-D, 2-D and 3-D, so the dimensioanlity analysis is not strictly necessary for selecting the more suitable inversion modeling. Otherwise, dimensionality analysis is still recommended in order to identify the subsurface structure, as likes the application of syntethic data and real data in this thesis."

"Lapangan Geotermal Salak merupakan lapangan geotermal terbesar di Indonesia dengan kapasitas terpasang sebesar 377 MW. Dari awal beroperasinya pada Februari 1994 sampai dengan Desember 2014 lapangan ini telah memproduksi 421.759.106,78 Ton uap. Dengan produksi sebesar itu, diperlukan manajemen reservoar yang baik untuk menjaga keberlangsungan produksi jangka panjang. Manajemen reservoar sangat penting dalam upaya mengatasi masalah yang terjadi akibat kegiatan produksi dan reinjeksi, oleh karena itu strategi reinjeksi sebaiknya memperhatikan karakteristik reservoar lapangan geotermal. Penelitian ini menggunakan metode geofisika yaitu 3D MT, Microearthquake dan Microgravity dengan dukungan data sumur dan data produksi serta reinjeksi untuk memprediksi kondisi reservoar sebagai upaya mengantisipasi terjadinya penurunan tekanan reservoar yang berpotensi menurunkan produktifitas sumur produksi. Hasil penelitian ini menyimpulkan bahwa strategi reinjeksi di Awi 9 memegang peranan penting sebagai heat and pressure support di sumur ? sumur produksi. Namun, terdapat indikasi kompaksi pada reservoar sejalan dengan peningkatan kapasitas produksi, hal ini diperkuat dengan terjadinya penurunan permukaan tanah dan peningkatan kejadian gempa mikro pada daerah resevoar dangkal, terjadi penurunan medan gravitasi pada reservoar produksi yang diidentifikasi berhubungan dengan penurunan tekanan reservoar. Hasil ini digunakan sebagai dasar usulan untuk mempertahankan eksistensi sumur - sumur reinjeksi di Awi 9 dan penempatan sumur reinjeksi brine di zona reservoar produksi.

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Salak Geothermal Field is the biggest geothermal field in Indonesia with 377 MW installed capacity. From its commersial operation in February to December 2015, this field has produced 421.759.106,78 Tonnes steam. With these huge production, good reservoir management are necessary to sustain long term production. Reservoir management becomes very important to overcome the problems caused by production and reinjection. Therefore, reinjection strategy should be implemented by considering reservoar characteristic in geothermal field.

This study are using geophysical methods, there are : 3D MT, Microearthquake and Microgravity combined to geological well data support, production and reinjection data to predict reservoir condition as an attempt to anticipate decreasing of reservoir pressure which potentially reduce production.

This study conclude that reinjection strategy in Awi 9 took important part as heat and pressure support to production wells. However, there are some indication of creep compaction in reservoir in line with production capacity escalation, this was supported by land subsidence and increasing of microearthquake event in the shallow part of reservoir, decreasing of gravitational field in production reservoir associated with reservoir pressure drops, this results are used as the basis for the proposals to maintain the existance of reinjection wells in Awi 9 and brine reinjection wells placement in the production reservoir zone."