WILAYAH KERJA PROYEK

PENDAHULUAN

GEOLOGI

Dalam penelitian pendahuluan, penggunaan alat/teknik remote sensing, terutama Satelite Imagery dan Thermal Infra-Red mapping sangat berguna. Begitu juga dalam Monitoring.

Hal penting yang harus dibahas dalah hubungan antara hydrogeology dari suatu system dengan dengan bentuk morfologi daerahnya. ( gambr 2 dan 3 di buku ). Hal ini sangat relevan sekali di Indonesia, dimana banyak geothermal system terdapat di bentang alam gunung api yang curam. Bentang alam seperti ini akan mempengaruhi tipe2 manifestasi di permukaan, dan mengapachloride water dapat discharge (if at all) hingga beberapa km dari zona upflow.Jelasnya, dengan mempeljari mengenai hal ini dalam suatu perencanaan program eksplorasi geothermal, tidak akan mengalami kegagalan dari sejak awal. Maka mempelajari geohydrologi ini sangat penting , yaitu pengetahuan tentang fluid geochemistry, dan geofisika seperti juga geologi suatu daerah pnas bumi, seperti yang banyak ditemukan di Indonesia.

Hal penting lainnya untuk dipelajari adalah perbedaan dari tipe2 fluida geotermal, genesisnya, dan hubungan satu dengan lainnya, yng biasanya lebih jelas dengan menggambarkannya dalam cross section.

Geolo gi Regional / Fisiografi

Bentuk morfologi : Graben, Caldera, Dome, Mix, untuk memperkirakan complexitas struktur reservoir. Semakin simple, semakin menarik (?).

Geologi daerah Penelitian, Pembahasan mengenai litologi dan struktur geologi.

Geologi keseluruhan dari batuan tertua yang dapat diketahui, dan selanjutnya penelitian/pembahasan diutamakan pada batuan-batuan Kwarter, dengan tujuan untuk menentukan Mother Source Rocknya.

GEOKIMIA

Tugas utama ahli geokimiaadalah meakukan sampling dan menganalisa fluida geotermal di kedalaman, berdasarkan data dari manifestasi permukaan dan/atau dari sumur pemboran. Salah satu tujuannya adalah memberi informasi tentang karakteristik fluida geotermal di kedalaman, sehingga kita dapat membedakan antara fluida yang berasal dari kedalaman atau berasal dari daerah dangkal. Banyak sistem geoterml yang diekspresikan di permukaan oleh dischargenya seperti fumarol dan hot spring. Sebagian dari fluida ini berasal dari reservoirnya, dimana komposisinya dapat digunakan untuk menginterpretasi kondisi di kedalaman. Karena komposisi fluida merupakan refleksi langsung dari ”Flow path”, maka geochemists dapat menginterpretasikan reservoir geotermalnya dalam hal temperatur dan ”plumbing’ (aliran?).

Clasification Of Geotherml Fluids.

Walupun komposisi sangat bervriasi dari satu lpangan dengan lainnya, bahkn bisa berbeda walau dalam satu lapangan (kibat mixing dan boiling), tetapi pengelompokkan air dapat dilakukn berdasarkan ” end member composition ”. Klasifikasi yang paling berguna yaitu klasifikasi berdasarkan ” anion-anion ” Chloride, Sulfate dan Bicarbonate, sebagai guide.

Chloride Water : Merupakan komposisi fluida yang paling banyk dalam geotermal reservoir, dari reservoir inilah Uap diambil untuk Pembngkit Listrik. Merek banyak mengandung Chloride ( CL min ) sebagai anion utama, berkisar antara 0.1 sampai 1.0 % berat (air laut mengndung 1.9 % berat CL, sebagai pembanding ).

Larutan penting lainnya adalah Sodium (Na plus), Potassium (K plus), Calsium (Ca plus2) dan Magnesium (Mg plus2). Larutan tsb. Banyak mengandung konsentrasi Silica (SiO2), dan bervariasi tetapimengndung Sulfate (SO4 min2) dn Bicarbonate (HCO3min) dalam jumlah yang significance. Unsur2 lain yng diukur adalah Boron (B), Lithium (Li),Rubidium (RB) dan Caesium (Cs). Jarang sekali terdapat Fluoride (F), dan Ammonia (NH3).

Arsenic (As) dan Mercury (Hg) sering diukur untuk alas an lingkungan. Carbon Dioxide ( CO2 ) dan hydrogen Sulfide (H2S) merupkan gas yang paling umum, berasosiasi dengan Chloride Water, dengan perbandingan ; CO2 / H2S berkisar 10 – 100. Elemen Iron (Fe) dan Alumunium (AL), banyak terdapat pada batuan (host rock), pada Chloride Water, mereka ditemukan dalam jumlah : trace quantities saja pada chloride water. Chloride Wter mempunyai pH mendekti “netral”, kadng sedikit acidic atau alkaline.

Kenampakan karekteristik dari discharge chloride water adalah banyak silica sinter disekitar mulut spring. Hal ini dikrenakan fluida mengalami satursi dgn amorphous silica.Catatan: Chloride Spring denfan flow rate yang sangt tinggi, merupakan indikasi bahwa fluida tsb naikke permukaan secara cepat dari reservoir, adalah air yang paling berguna untuk dianalisa dalam studi reservoirnya.

Acid Sulfate WatersBanyak mengandung Sulfate, melebihi 1000pp SO4.

Air ini merupkan hasil dari 2 proses yang berbeda :1. Steam-heated Acid Sulfate water, yang terbentuk manakala uap berkondenssi menjadi air permukn. Sulfate berasal dari oksidsi Hydrogen Sulfide (H2S) dalam zonaVadose, yaitu di daerah subsurface, diatas water table. OksidasiH2S, menghasilkan Sulfuric Acid (H2SO4)--------------H2S + 2O2) = H2SO4.

Pembentukan “ Steam-Heated Acid Sulfate Water, berhubungan denganboiling dripada Chloride fluid dlam reservoir geoterml pada temperature < 300 derajt C. Exsolved gas H2S naik ke permukaan dimn oksidsi menghsilkn Sulfuric acid Condensate. Karena Chloride bukan volatile pada temperature < 300, maka Steam-Heated Water mengandung sedikit sekali CL, tidk lebih dari beberapa ppm saja. Selnjutnya, krena air2 ini terbentuk di kedalaman yang paling dangkal daripada system, maka mereka memberikan sedikit sekali indikasi mengenai bagian yang lebih dalam daripada system.

2. Magmatic Acid-sulfte Chloride Water, terbentuk diman volatil2 magma (H2O, CO2, SO2, HCL ) mengalami kondensasi menjadi fas liquid. Hal demikian terbentuk disekitar daerah magma ( >= 800 derajat C.) tetapi dalam kedalaman yang sangat bervarisi dari permukaan ( > 1 km ), misal; pada puncak danau kawah, seperti pada White Islnd Volcano. Keberadaan chloride dalam air ini dan volcanic-hydrologic setting membentu dalam membedakannya dari Steam-Heted Acid-Sulfate Water, walau analisa isotop daripada hydrogen dan oksigen adalah yang paling dapat mendiagnosa originnya.

Komposisi Acid-Sulfate Water merefleksikan “ dissolution “ daripad country rock, akibat dari keasmannya, jadi tidk dapt digunkan sebagai geotermometer. Natural pools (kolam lam) umumnyaberasosiasi dengan collapse crater (runtuhan kawh) seperti yang terdapt di Waiotapu dan Rotokawa. Catatan: jumlah kandungan AL dan Fe dalam air tsb memperlihatkan indiksi danya rock dissolution.

Bicarbonate Water, ( CO2-Rich Water ),mengandung bicarbonate sebagai anion utama, plus variable sulfte dan low chloride. Pada system yang didominasi oleh Volcanic Country Rocks, Cirinya, Bicarbonate water terbentuk pad daerah tepi (marginal) dan shallow subsurface region (dikedalaman yng dangkal), dimana gas CO2 diserap dan uap (steam) terkondensasi kedalam air tanah yang dingin (cool ground water). Proses kondensasi uap memanaskn air tanah, maka sering disebut sebgai “steam-heated “. Sodium umumnya menjadi kation utama, sementara carbonate (calcite) tidak begitu soluble (tidak larut) , dn potassium dan magnesium fixed didalam clay.

Kebalikan dari acid sulfate water, bicarbonate water terbentuk dibawah water table, apabila sedikit cid (weakly acidic) , menyebabkan kerusakan pada casing sumur di lapngan Broadlnds Ohaaki, tetapi kehilangan (loss) CO2 dari fluidnya, keluar (discharging) di permukaan, pHny bertambahmenjdi normal atau sedikit lebih alkaline.

Pada beberapa geothermal system (Yellowstone dan Valles, Usa), pembentukan bicarbonate water dipengaruhi oleh adanya limestone (batugamping) di subsurface, dan spring discharge menghasilkn endapan travertine (CACO3) siner.

Brines dlah air yang mengandung konsentrasi yang tinggi daripd “solute’, (contoh: air laut). Chloride merupkan komponen yang utama, kndungannya berkisar 10 000 sampai >100 000 ppm., dengan konsentrasi yng tinggi pula kndungn Na, K, dan Ca untuk ”chrge balance ” (keseimbangan pemaukan). Ph ir tsb. Sedikit acid, dikrenkan tingginya konsentrasi ”solute ”. Brines pad geothermal system, terbentuk dlam beberapa cara, mislnya : connate brines trapped in sedimentary basin, dissolution daripada sequen evaporate oleh meteoric water. Dalam beberapa kasus, density dripad konsentrasi brines cukuptinggi, sehingga tidk dapt naik ke permukaan (e.g.; Salton Sea, California ).

Komentar:Suatu lapangan geotermal dptmempunyi spring sebagi discharge wter dari beragam tipe. Chloride Water adalah yang paling berguna, tetpi kadang tidak ditemukn. Chloride water absent (tidk ada ) sebagai discharge dari “ vapour dominated field “ atau pada diatas water table “liquid dominated system” ( why ? ).Bahkan pad keadaan chloride water bercampur dengn meteoric ground water, mereka tetap masih dpat memberikan informasi yang berguna mengenai kondisi reservoir.

Steam and gas Composition

Analisa gas (Tabel 4.3) dapat membantu dalam pengembangan geothermal dalam penentuan efek /akibat exploitasi terhadap kondisi bawah permukaan. Selain itu, konsentrasi dari dissoleved gas (yang paling banyak : carbon dioxide ), berpengaruh terhadap kedalaman “first Boiling” dan potensi /kemungkinan calcite scaling.

Dalam “ vapour dominated system “, konsentrasi gas dalam sample uap tidak berbeda dengan ”collection Pressure ”(tekanan pada saat pengambilan uap/stam). Tetapi pada ” High temperature Liquid Dominated System ”, boiling terjadi di dalam sumur bor atau pada rekahan/ saluran alamiah, pada saat fluida naik ke permukaan. Setelah pembentukan beberapa persen steam/uap, fasa steam banyak mengandung gas yang tadinya/bersal dari dissolved dengan liquid. Konsentrasi gas2 tersebut (misal: CO2, H2S, CH4, H2, N2 ), berhubungan terbalik (inversely related) dengan persentase dari steam/uap yang memancar (flashed) dari air. Dan demikian pula dengan tekanan separasi (separation pressure) pada saat steam/uap diambil/dikoleksi dari discharge. Hal ini berarti fumarole dapat memberi indikasi kualitas daripada steam yang akan diproduksi oleh sumur bor

dikemudian hari. (lihat: gas2 yang mempunyai konsentrasi rendah). Wlaupun demikian , kita hru berhati hati akan komposisi gas2nya. (CO2, H2S, NH3, CH4) dapat termodifikasi dari titik pemisahannya, dikarenakan oleh reaksi yang terjadi pada kedalaman dangkal (shallow subsurface reaction).

Hal terpenting adalah menguraikan chemical characteristic dari 3 tipe fluida yang ada pada suatu typical geothermal system ( table 3 di buku ) dan memperlihatkan bagaimana hubungan genetic antara satu dengan lainnya., dan terhadap hydrologi daripada systemnya.Yang pling mudah dimulai dengan ”alkali chloride prent water”, diikuti oleh ”acid sulfate water”. Untuk menjelaskan hubungan tsb. Dapat dilihat dari 2 gas utama yang terpish dari alkali chloride water,apabila mencapai kedalaman dimana terjadi boiling, yaitu CO2 dan H2S dan reaksi simpelnya H2S + SO2 --------- = H2SO4, yang merupakn kunci yang memprodiksi ion2 sulphate didalam acid sulphate water, H2SO4 adalah asam yang kut ( sangat asam ), yang menyebabkan pH asam dari air tsb. Sedangkan acid sulphate water membentuk residu silica, kaolin, hematite dan sulphate ( missal: alunite ) apabila bereaksi dengan host rocks.

Air bicarbonate dan kaya CO2 juga sesungguhnya berasal dari parent alkali chloride water yang terbentuk manakala CO2 yang berlebih/ naik ( ascending ), terpisah dari air tsb, lalu larut dengan air meteoric yang berada diatasnya. Pada morfologi yang curam, mereka dapat bermigrasi sampai beberapa kilometer.

Sebagian besar Chloride-Sulphate Water dapat dijelaskan terbentuk akibat bercampurnya Chloride water dan Acid Sulphate Water. Sebagian Sulphate yang terdapat dalam air Acid Sulphate dan Air sulphte-Chloride pada bentang alam volcanic dmungkin berasal dari gas SO2 magmatik. Air2 ini, pada umnya mempunyai pH kecil ( asam ), dan bisa mendapatkan chloride dari gas HCL.

Sebagian besar tipe-tipe air lainnya, missal: Air Chloride-Bicarbonate dapat juga dianggap sebagai campuran dari perjalanan ahir dari ketiga tipe fluida : CL (min), HCO3 (min ), dan SO4 (min ). Catatan : Salinitas daripada Alkali Chloride Thermal Water, dapat berkisar dari sangat rendah ( TDS = 50 mg/kg ) kepada Strong Brine ( TDS=35 %), tetapi salinitasnya bergantung kepada jumlah perbandingan ion2 kunci ( key ions ).

Tabel : komposisi kimia dari 3 tipe umum pada sistem geothermal :

Geothermal Water : ( mg / kg )

Chloride Acid Sulfate Bicrbonate

pH 20derajt C 8.0 1.8 7.0

Na 1070 4 398

K 102 6.2 31

CL 1770 < 2 30

SO4 26 1047 96

HCO3 76 - 8492

SiO2 338 151 190

KLASIFIKASI KENAMPAKAN DISCHARGE DI PERMUKAAN

Hal yang penting harus diperhtikan hubungan antara hydrologi dan bentuk morfologi dengan pengaruhnya terhadap 2 gambaran sistem geotermalnya. Penjelasan mengenai karakteristik daripada kenampakan tipe2 utama discharge, terutama mengenai perbedaan komposisi kimia, flow rate, dan asosiasi mineral2 ubahannya.

Chloride Spring mempunyai flow rate tinggi, bening ( jernih ), dan mungkin terdapat silica sinter pada beberapa bentuk fisik tertentu..

Acid Sulfate spring, mempunyai flow rate yang rendah, kusam (tidak jernih / turbid ), danmembentuk kaolin, minerl2 sulfate dan residu silica, akibat bereaksi dengan host-rock disekelilingnya.

Mixed Acid Sulfate – Chloride dischrge features mempunya kracteristik dari campuran keduanya, tapi biasanya mempunyai pHberkisar antara 2.2 – 5. , kenampakannya kdang bening ( jernih ) tau bisa juga kusam ( turbid ), tapi umumnya dengan flow rate yang rendah.

Dalam membahas mengenai kenampakan discharge bicarbonate spring, penting untuk membedakan antara ” boiling ” dan ” effervescence ”. Dalam ”effervescence”, CO2 semuanya ”bubbling” (bergelembung) melalui/bersama air, pada sub-boiling temperature. Endapan yang biasa terjadi pada discharge tipe ini adalah Calcite, tetapi apabila CO2 menguap secara cepat, maka akan terbentuk Aragonite sebagai gantinya.

Harus dimengerti juga apa perbedaan antara travertine (calsium Karbonate) dengan Silica Sinter. Kesalahan dalam mengidentifikasi keduanya dapat menjadi salah interpretasi hidrilogi daripada suatu geothermal sistem.

Dapat juga terjadi Chloride – Bicarbonate water yang discharge dengan flow rate yang tinggi dan menyebabkan proses silisifikasi, apabila ada pengaruh dari host rocksnya.

Geothermal System dengan convective heat transfer dalam reservoir utama, umumnya mempunyai kenampakan discharge di permukaan. Jumlah total heat discharge di permukaan (Qs (total natural heat loss) dan kenampakan serta locality dan tipe kenampakan discharge, dipengaruhi oleh :

a. Total Heat Input ( Qin) pada bagian bawah reservoir.b. Reservor Parameter, i.e.: vertical permeability, pathlength, dsb.c. Fluid Parameter , i.e.: density, viscosity, temperature, nature of fluids, dsb.d. Faktor2 yang menyebebkan “influx” fluida yang lebih dingin pada berbagai level,

kedalam system, tetapi juga di permukaan, yang dikontrol oleh gross hydrological setting of the system.

e. Factor penyebeb Out Flow fluida panas pada daerah yang lebih dalam.

Walaupun kenampakan discharge di permukaan sangat bervariasi, tetapi kita bisa melakukan studi secara detail mengenai discharge, dalam penelitian daerah prospek, karena :

i. Dalam hubungannya dengan studi kimia daripada fluida discharge, pemetaan lokaliti dan tipe semua kenampakan discharge dapat mendapatkan informasi tentang tipe dan grsoss structure dalam system dan general hidrological setting.

ii. Peta yang melokalisir seluruh discharge features, missal: dengan radius :10 – 15 km, yang berpusat pada kenampakan discharge yang paling aktif.-----------dibuat sebagai peta dasar untuk penyelidikan/survey geofisika.

iii. Pemetaan pada daerah discharge yang intense (aktif dan/atau inactive thermal ground misalnya) sangat penting dilakukan untuk Engineering Geologist (siting of engineering structure); juga untuk mendapatkan informasi tentang structute tektonik dekat permukaan (sesar).

iv. Monitoring :total heat discharged oleh surface features, selama eksploitasi dapat member informasi penting untuk “management” pada suatu proyek.

v. Memahami tentang mekanisme heat transfer daripada kenampakan surface discharge merupakan hal penting untuk pemahaman secara umum tentang geothermal system dan memberikan hubungan yang menarik antara Thermodynamics dengan (surface geology ).

CLASIFICATION OF SURFACE DISCHARGE FEATURES

i. Diffuse discharge (i.e.: warm ground, steaming ground, evaporation at a free water surface ).

ii. Direct ( concentrated ) discharge (i.e.: warm and hot spring, steam vent, fumaroles)

iii. Intermittent Discharge (i.e.: geyser)iv. Catastropic Discharge (i.e.: Hydrothermal Eruption)v. Concealed Discharge (i.e.: seepage, concealed outflow).

DISCHARGE FEATURES :

A. WARM GROUND, heat tersalurkan hanya oleh conduction yang dideterminasi ole “conductivity: K “ dari lapisan yang paling atas, dengan gradient temperature dekat permukaan delta T / delta z. Warm Ground dapat ditemukan disemua system reservoir yang muncul di permukaan, biasanya ditemukan disekitar daerah yang discharge lebih yang lebih intense, spt steaming ground atau hot pools.

B. STEAMING GROUND, Steam dapat berasal dari penguapan air panas dangkal (biasnya pada titik didih, pada daerah yang sangat aktif teaming ground), atau dari “Flashing “ pada daerah yang lebih dalam. Apabila berasal dari penguapan air dangkal, hubungan antara :heatflux, Temperatur dan kedalaman penguapan sudah terbentuk. Temperatur hamper konstan dalam lapisan kecual bila gradient temperature sangat tinggi pada lapisan yang palng atas, dimana kondensasai dari steam mempertahankan kelembaban dan terlihat seperti basah pada active steaming ground.

C. HOT POOLS., Transfer panas terutama terjadi akibat penguapan air permukaan. Losses karena radiasi dan proses2 lain pada batas dapat diabaikan, karena sangat kecil.Hot pools didaerah hot water system (terutama hot lakes) sering merupakan bekas (ancient) pusat erupsi hydrotrhermal yang kecil; di daerah Steam system, hot pools kadang terjadi didaerah fumaroles yang mendingin (quenched fumaroles). Catatan : Pengambilan sample hany dilakukan pada OUT FLOW POOLS, untuk analisa geokimianya, sebab geokimia dari stagnant pool bisa berbeda dengan pools lain yang berada didekatnya. Fluida dalam Hot pool, dapat berasal dari Hot Water System (ph hamper netral), bisa juga berasal dari Steam Heated Ground Water atau dari Steam (Acid ). Keasamannya berasal dari oksidasi traces H2S dalam steam. Pool yang berasal dari steam, sering dijumpai didaerah steam system umumnya : muddy and gray ( berlumpur, abu2 ). Pool yang fluidanya dari Hot Water System biasanya bening, berwarna kebiruan. Acid pool dapat ditemukan juga di daerah Hot Water System, apabila Hot Water Level terdapat di kedalaman yang lebih dalam (contoh: di Tykitere System, dekat Rotorua, NZ). Pool yang besar ( > 1000 m2 )------ disebut Lake, yang terjadi akibat erupsi hydrothermal. Hot and Warm lake, pada geothermal system berbeda dengan Crater Lake pada puncak gunung api aktif, dimana intermittent of hot volcanic gases terjadi di bagian bawah crater lake. Apabila Energi Input dari bawah melebihi Energi Loss di permukaan, crater lake menjadi tidak stabil dan “Flasshing steam” terjadi dibagian bawah kawah dan menerus keatas menjadi erupsi hidrotermal. Mekanisme erupsinya sama saja dengan yang terjadi pada lake diatas geothermal system.

D. MUD POOLS. Kenampakan akibat dari minor steam dan non-condensable gases (biasanya CO2 ) discharge melalui “vent” 2 kecil. Lumpurnya bertahan pada fasa liquid akibat kondensasi dari steam, sedangkan Upwelling (pembumbungan keatas/ letupan) liquid lumpur akibat dari discharge CO2. Mud Volcano, adalah kasus special daripada mudpool, dimana gas discharge terbatas pa pada hanya satu vent. Temperatur lumpur biasanya sekitar dibawah temperature didih dan heat discharge oleh kenampakan ini sedikit.

E. WARM AND HOT SPRINGS. ( WARM < 50 derajat C; HOT > 50 ).. Kenampakan ini berasosiasi dengan semua tipe geothermal system. HOT : slightly acid atau sangat acid springs, discharge yang mencirikan Steam System atau Hybrid System. Neutral Hot Spring umumnya berasosiasi dengan Hot Water System, jarang ditemukan neutral hot spring yang ditemukan didaerah sekitar bagian atas kaki bukit; Steam system dan Hybrid System. Neutral Hot Spring diatas Hot Water System, dimana hamper mengalami saturasi dengan silica, dapat menghasilkan sinter terraces atau sinter platforms.Features demikian dapat pula berhubungan dengan discharge dari carbonate Rich Colder Spring, yang mengandung minor portion of thermal fluids, dan mengalami kondensasi, dan yang berasal dari Dilluted Out Flow dari system yang berada diatasnya.

F. FUMAROLES.

G. GEYSER

H. SEEPAGE

I. HYDROTHERMAL ERUPTION

ASSESSMENT OF NATURAL HEAT LOSS OF DISCHARGE FEATURESa. Warm Groundb. Steaming Groundc. Hot Poolsd. Warm and Hot Springse. Fumarolesf. Geyserg. Shallow Seepage

HYDROLOGY AND THE DISTRIBUTION OF GEOTHERMAL FLUIDS

Proses kimia dan hydrology tidak bisa dipisahkan dalm membentuk komposisi dan distribusi fluida geothermal. Hubungan ini terjadi dlm penentuan btuan2 volkanik yang berhubungan dgn magma sebagai host dari system geothermal, dimanaboiling, mixing, dan kondensasi sering terjadi. Gambar 4.2 dan 4.3 (di buku ) memperlihatkan contoh dari 2 bentang alam volcanic (volcanic terrain: Rhyolite-Caldera dan Andesite-stratovolcano) dimana terdapat istem geothermal. Perbedaan utama adalah dalam ekspresi topografi volcano, yang sangat mempengaruhi permukaan water table.

Gambar 4.2.Topogrfi landai, dan di daerah dengan curh hujantinggi, water table berada didekat permukaan (contoh: Taupo Volcanic Zone, New Zeland). Di daerh yang demikian, Chloride water mendominasi Up Flow Zone, sedangkan Steam Heated Acid Sulfate water terletak diatas Up flow Zone dan terbentuk diatas water table. Bicarbonate Water terkumpul di daerah margin (tepi) yng stagnant. Zona Out Flow dapat terjadi tergantung hydraulic gradient daripada water table.

Gambar 4.3.Topografi mempunyai relief yang tinggi (curam). Disini, Water table berada pada elevasi dekat dengan country side disekitarnya (banyak pada system2 di Indonesia). Bergantung kepada lava sekuen, batuan piroklstik, debris flow dan permeabilitas batuan2 tersebut, Perched Aquifer dapat berkembang dibwah sayap dari volcano . Neutral Chloride Water cenderung untuk berada dibwah sayap volcano, sedangkn Acide Sulfate water (steam-heated dan mgmatic) dapat berada pada daerah puncak hingga beberapa kilometerkebawah, tergantung dari kedalaman magma. Steam-Heated Acid Sulfate water akan cenderung mendominasi aquifer dangkal di daerah puncak, memberi jalan Bicarbonate Water pada aquifer dangkal di daerah sayap bagian bawah dari volcano. Struktur Out Flow yang berkembang di lingkungan demikian, Chloride spring dapat discharge sampai sejauh 20 km dari sumbernya, dengan temperature yang lebih rendah dari boiling, tanpa terjadi pengendapan amorphous silica.

FACTORS AFFECTING THE COMPOSITION OF GEOTHERMAL FLUIDS

Chloride Water merupakan target drpadFaktor2 yang mempengaruhi komposisinyaadalah :1. Origin of water ( meteoric atau ir laut )2. input volatil2 magma (Cl, S, C ), di kedalaman, pada system.3. Fluid-mineral equilibria (dissolution-precipation daripada mineral2)

4. Boiling-dillution

Origins of Geothermal Water

Untuk kebanyakan convecting (pancaran) system geothermal, air meteorik merupakan sumber utama H2O. Tetapi pada system geothermal yang berada di sekitar pantai, air laut dapat juga merupakan recharge fluid dankan mempengaruhi komposisi therml water yang diindikasikan oleh relative tingginya salinitas. Berbeda dengan penentuan konsentrasi chloride daripada fluida, rasio2 isotop stabil (stable isotop) oxygen dan hydrogen merupakan satu2nya cara untuk mendiagnosa, yang dapat membedakan originnya (meteoric/sea water ). Menganalisa ratio2 tsb, memerlukan “ a mass spectrometer “.

Gambar 4.4 ( di buku )Memperlihatkan plot dari Hydrogen Isotop Ratios (D/H) versus Oxygen Isotop Ratios ( 18 O / 16 O ) untuk beberapa tipe air. Isotop Ratio dihitung dalam “per mil value “ ( 0/00 ) bukanpersen ( % ), diukur berlawanan (against ) dengan standar air laut (Standard Mean Ocean Water = SMOW ). Komposisi geothermal water reservoir di Taupo Volcanic Zone System, terlihat bergeser/ berpindah ke kanan, dibandingkan dengan komposisi yang berasal dari local meteoric water . Hal ini diketahui sebagai “ pergeseran/perubahan positive delta 18 O ).Penyebab adanya pergesern ini tidak penting disini, tetapi lebih memperlihatkn kedekatan dripad thermal water dengan garis meteoric water, mengindikasikan “meteoric Origin “. Catatan: Meteoric water diseluruh dunia diplot dlam garis yang disebut : Garis Meteorik Water (gambar 4.4, di buku ).Indikasi komposisi magmtic water, terlihat pad “White Island Volcano” yang berada pada 50 km Off Shore (lepas Pantai) North Island di derah Bay of Plenty.Kandungan dari White Island sangat mirip dengn semua Andesite Volcanos diseluruh dunia. Kemungkinan lain, sumber airnya adalah air formasi atau disebut juga Connte water; yaitu air laut yang terperangkap dlm sediment, dicerminkan oleh tingginya salinitas (i.e. >2 wt % CL ).

Input Of Magmatic Volatiles (halaman. 37) di buku

Fluid Mineral Equilibria (dissolution-precipitation rections)------ hal 38

Boiling-Dilution ---------------- hal 38

Concentration Units ( Henley et al., 1984. p. 1-2 ) ----------hal 40

Boiling point for Depth Relations --------------- hal 40

CHEMICAL GEOTHERMOMETER

Konsentrasi unsur2 kimia yang berhubungan dengan temperatur dikedalaman disebut Chemical Geothermometer. Geothermometer digunkan tergantung dari keberadaan pda kedalaman diman terjadi kesetimbangan mineral – fluid ( mineral-fluid equilibria) yang terjadi selama perjlanan fluida ke permukaan.

Komposisi fluid geothermal merefleksikan reservoir temperaturnya., terutama Silika dan Alkali geotermometer. Yang harus diperhatikan terutama dalam analisa pH, ionic balance, flow rate (bila ada), sebelum melakukan numerical manipulation dalamperhitungan temperature yang significance.

Aplikasi yang sukses dalam silica geotermometer memerlukan bahwa discharge air dari spring atau well, datang dari kedalaman padasaat Thermal equilibrium dengan kwarsa atau pengecualian dengan Cristobalite. Demikian juga dalam penggunaan alkali Geotermometer mengasumsikan bahwa fluid yang berasal dari kedalaman, dalam keadaan Thermal Equilibrium dengan Feldspar.Pada kenyataannya, keadaan tidak demikian, apabila fluidanya Acidic, sebab feldsfar tidak stabil atau bahkan absent.

Menghitung Keseimbangan temperature Acidic water, kenyataannya “tidak Berguna”. Perlu dicatat juga bahwa Geotermometer kimia berdasarkan ” Kation Ratio ” sangat menguntungkan daripada ”silica Geotermometer yang dipengaruhi oleh ” Steam Loss and Mixing ” ( hilangnya uap dan percampuran ).

Maka dalam semua perhitungan ” temperatur ”, kita harusbetul2 mengerti ” the concept of error assessment ”

HYDROTHERMAL ALTERATION

Utamanya perhtian tertuju kepada : bagaimana caranya batuan2 berubah karena berinteraksi dengan fluida geothermal. Hal tersebut terjadi karena mereka berada pada lingkungan yang berbeda dengan lingkungan pda saat pembentukannya. Sebagai contoh, lava andesit terbentuk pada temperatur 1100 derajat C, pada keadaan ” Anhydrous ”. Tidak mengagetkan bila kemudin berubah karena adanya sistem geotermal dan menjadi ”saturated” (jenuh) dengan air atau uap pada sekitar suhu 250 derajat C. Batuan akan merespon lingkungan barunya dengan 3 cara :

1. Perubahn physical properties.2. perubahan kimianya.3. Perubahan mineraloginya.Ketiganya sangat jelas berhubungan satu dengan lainnya., dan yang paling penting dilkukan pada tahap awal adalah ” perubahnkiminya “.

Juga perlu diperhtikan perbedaan antara “ rank” dn “ Intensity “ of Alteration.

Berdasarkan hydrothermal mineral, kadang kita bisa memperkirakan subsurface temperature, permeability, fluid composition, boiling zone. Berdsarkan studi detil Fasa2 key minerals, kita bisa menentukan termal history reservoirnya, apakah sedang ” heating Up ”atau ” Cooling Down ”. Fluid Inclusion Geothermometry sangat baikdigunakan untuk mempelajari perubahan temperatur di dalam geotermal reservoir.

Tujuan: Untuk mengetahui temperatur (T, derajat C) dan sistem fluidanyaManifestasi Hydrotherml di Permukaan.Indikasi daerah yang bertemperatur tinggi (T tinggi) : Fumarol, hot spring, mud pots. Moderate (Tmoderate, T> 250): fumroldan silica Silica sinter. Rendah (T rendah T<150 derajat C.) : TravertineGeokimia hasil laboratorium

Silika --------------- T tinggi, tapi permeabilitas Drop.

Enthalpi ( kj / kg ), sebanding dengan tempertur (T).

Mineral-mineral yang menunjukkan temperatur (T) tinggi : Epidot, Wairakie, Adularia

Mkin tinggi Gas Content -------------- makin banyk Scaling

Chlorite ---------------- (ppm) ------T tinggi

GEOFISIKA

Tujuan: untuk menentukan Geometri, dan karakteristik batuan.- Area (km2) dari data geolistrik dan TG- Untuk mengukur Lose Circulation Zone----------Geolistrik- Depth of Top Reservoir (m)

100 meter sebelum Top of Reservoir ---------Stop Cementing- Diteksi Clay ------------Geolistrik :

Clay Caprock TIPIS ------------------- SteamClay Caprock TEBAL -----------------Water

Biasanya : Clay Caprock : 10 Ohm-m, tapi diluarnya :50 Ohm-m, jadi perbedaannya sekitar 5X lipat.

- Pseudo resistivity : Shlamberger, MT ------------memprediksi Clay Content

GEOPHYISICAL EXPLORTION FOR GEOTHERMAL FIELD

Extensive experiences with geophysical exploration for measuring physical quantities of the earth have indicated that such methods are effective in locating area and the site of production drilling where hot water and steam may be produced in economic quantities. It will be recognized from geophysical surveys done in a number of countries that geophysical exploration used in the development of geothermal resources.

The assumed model of subsurface structure in geothermal region is that, at and near surface, compact rock formation exists, and the formation plays an important role of cap rock, and below of the formation consist of permeable formation rich in porosity. The ground water circulating is heated by ascending heat flow, through channels or faults and fissures, from magma. This formations called geothermal reservoir. Cap rocks and reservoirs are not always only one formations.

Methods of geophysical exploration for geothermal field are classified by 6 groups:1. Gravity2. Magnetic3. Electrical4. Seismic5. Temperature methods6. Logging

1. In gravity methods, measurement are made of anomalies in gravity attraction produced by differences in densities of formations and structures. The density of compact rock is greater than that of porous rock. In general, iso-gal lines in gravity map become dense arrangement at the place where the fault like structure exists. Strictly speaking, contour lines of gravity anomalies reduces to discontinuous lines at the fault line.At present, gravitational exploration is useful for reconnaissance survey of geothermal resources, but it is impossible to discuss the subsurface structure of geothermal region from a view point of great or small value of gravity anomalies only. However, the evident is that gravity anomaly reveals the existence of caldera in volcanic area.

2. In magnetic exploration, measurements are made of anomalies in the earth’s magnetic field due to geologic bodies of different degrees of para- or dia-magnetism. The main functional property is susceptibility, which is very different in common rocks and formations. In general, the value of susceptibility of igneous rocks including magnetite is greater than that of sedimentary formations.In the course of the geologic history of a formation, a number of physical forces are likely to affect its magnetic properties. They are of thermal and mechanical nature and occur in connection with igneous intrusion, regional metamorphism, tectonic movements etc. The remanent magnetism of ferromagnetic substances decreases with an increase in temperature. In fact all magnetite parameters (coercive force, remanent magnetization, susceptibility) are individually depend on temperature. A correct analysis of thermal relations is difficult. The intensity of magnetization decreases first slowly and the more rapidly with temperature until the Curie point is reached (3480 for pyrrhotite, 5250 for magnetite, and 6450 for hematite).Magnetic survey may apply for mapping altered zone in geothermal field. Modern techniques of air-borne magnetic survey are more effective for prospecting wide area in efficiency.

In Gravity, magnetic and self-potential method of electrical exploration, the reaction of geologic bodies are permanent, spontaneous and unchangeable; the operator cannot control the depth from which they are received.

In Seismic exploration, energy is supplied by dynamite explosions, and the travel times of reflected and refracted waves are measured. It is impossible to apply the refraction method in the case that elastic wave velocity of cap rock is greater than that of porous formation, but the application of reflection method is valid. In this case, seismic reflection measurements with magnetic recording systems are used for eliminating the surface noise of elastic wave in geothermal region.Recording re divided into 3 types:1. the wiggle-type recording2. the variable-area recording3. the variable –density recording

The existence of faults and cracks is indicated by the out of phase in seismic wave records. In general, seismic wave velocity decreases as temperature increases, and the so called S-waves do not exist in a fluid such as magma. These interesting features play an important role in the exploration of geothermal resources in the near future.In the field of geophysics, seismic wave have been utilized to investigate physical properties of the interior of the earth.

In resistivity and seismic methods, energy is applied to the ground for the purpose of prospecting measurable reaction of geologic bodies. This gives the possibility of spacing transmission and reception points in such a manner that the depth range can be controlled.In temperature method for geothermal field, new category, the unit of geothermal gradient, 0C/10 m depth is introduced. The area showing geothermal gradient of

exceeding 10C/10 m corresponds to powerful geothermal region. This is the method worth carrying out, but it is limited by economical cost. The important problem on the effect of shallow water bearing formation to geothermal gradient is left in future.

Geophysical logging for geothermal exploration may be divided into 4 groups :1. S.P logging2.resistivity logging3.temperature logging4. pressure loggingIn practice, the reappearbility of data obtained by this methods should be discussed sufficiently.

GRAVITY EXPLORATION

Although there is a wide variation in the design of different kinds of gravimeters, there are fundamentally 2 types :1. stable gravimeter2. unstable gravimeter

The stable gravimeter contains are sponsive element (such as a spring) with a displacement from equilibrium position proportional or approximately proportional to the change in gravity from its equilibrium value. Since such a displacements are always extremely small, they must be great magnified by optical, mechanical, or electrical means.

Unstable gravimeters are so designed that any change in gravity from its equilibrium value bring other forces into play which increase the displacement caused by gravity change alone. How this is accomplished in practice should become more clear when consider some specific examples.

Corrections of gravity resultIn gravity survey, gravity is reduced to gravity of the sea level. This is called the “free-air correction”, and is added to observed gravity.The “bouguer correction” accounts for the attraction of the rock material between sea level and th station at elevation.

These 2 correctionsare both proportional to elevation above sea level.The topographic corrections takes care of the attraction of land higher than the gravity station and also corrects for any depressions below the level of the station which make infinite-slub assumption, used in the bouguer correction, in correct. This correction is always added whether the future is a hill or valley.

AnomaliesThe departure of a corrected gravity value from the theoretical value of gravity on the spheroid at the latitude and longitude of the station is designated as the gravity anomly

associated with this location.The type of anomaly depends on the corrections that have been applied to the observed value. If the free-air, bouguer and topographic correction are applied, the bouguer anomaly is :

Obs.grav. + free-air corr. – bouguer corr. + topog. Corr. – Theoritical grav.

Considering the free-air correction, then the free-air anomaly is :

Obs. Grav. + Free-Air Corr. – Theoritical grav.

Interpretation of gravity anomaliesGravity anomaly may be represented by contours or profiles in connection with geologic sections. This interpretation is largely qualitative and is given in terms of structural and lows of presence or absence of heavier or light bodies. If some information is available bout the subsurface section and dimensions and nature of geologic bodies to be expected, more quantitative interpretation methods may be applied by calculating attraction and by varying the assumptions regarding dimensions, shape, differences in density, and depth until a reasonable agreement between fields curves and theoretical curves is obtained.This method of interpretation is of a trial and error nature and generally referred to as indirect interpretation.

ApplicationsSome case histories in gravity survey re as follows :

- Salt domes are investigated by the geophysical indication of the characteristic gravity lows associated with them.

- In the investigation of anticlines, if beds of less than normal density predominate, gravity minimum should be associated with the anticlinal axis. If the formation having greater than average density are brought nearer the surface at the crest of an anticline, its crest line should be the axis of a gravity maximum.

- Limestone reefs and ore bodies are discovered by gravitational methods.- Gravity survey are also carried out to investigate the geological problem and

possibilities beneath the offshore waters.

Gravity anomalies in geothermal fields

Gravity anomalies in geothermal fields are divided into 2 kinds :1. Negative gravity anomalies2. Positive gravity anomalies

Negative gravity anomalies which appear customarily in the caldera were found beneath the Larderello and Mt. Amiata geothermal regions and beneath the Myacmas uplift. These anomalies are within the range of 10 to 40 mgals, and approximately 15 X 30 km

in extent. It is found that these anomalies are related to underlying low density geologic bodies, such as intrusive geologic bodies and existence of caldera.

Positive gravity anomalies re observed at the wairakei and Niland geothermal fields. At Wairakei, the positive gravity anomaly is caused by the horest block of dense basement rock, which protrudes into the less dense material filling the region of subsidence. The faults bounding this block are the fissure through which the thermal fluids reaches the wairakei reservoir. The caused of the positive gravity anomaly at Niland, which striking coincides with a positive magnetic anomaly and the position of the five rhyolite domes, is not known.

On the south-western flank of somma of the Aso caldera, there is found a region where the positive gravity anomalies amount to +22 mgal (in case when D=2,67 gr/cm3 is assumed ). A detailed geological survey on this area of H. Matsumoto (1963) has disclosed the outcrops of basalt.

A narrow belt of relatively high anomaly is connected to a positive anomalous region. Along this belt, the tectonic line has been supposed to run among the geologists. The gravity high anomaly was recorded at the strip zone of faults already known. It also seems that the geothermal spots and hot springs are located in contact with the gravity high anomaly zone, which is believed to have an important relation to thegeothermal sources of the field.

In most cases genetic relation between gravity anomalies and associated thermal fluids has not been demonstrated, and this anomalies may be useful for empirically defining the limits of an area which should be prospected in detail.

The accuracy of gravity anomaly is strongly controlled by how accurately the heights of the observation points are measured.

Some of the gravity highs in Imperil Valley have been drilled by oil and geothermal explorations companies over the years. The evidence from drill hole data tends to support the assertion that many if not all, of the residual anomaly highs are due to metamorphism of the loosely consolidated sediments by the rising plumes of hot water, resulting in increased density of the sediments and hence in increased gravitational attraction (Koenig, 1967; Rex, 1968).

It has been known some time that an empirical correction exists between high heat flow area and gravity high in the Imperial Valley (Rex, 1968 in Meidav, 1970).

The gravity method is a cheap and rapid means of monitoring the net mass loss from geothermal field under exploitation and can also give an indication of the area from which the water has been drawn. In using this method, however, it is important that the initial gravity survey be made before large steam production or a substantial mass loss from the geothermal field occurs.

Gravity measurements have been used in New Zealand to determine the depth of greywacke basement beneath the volcanic cover rocks at Kawerau (Studt, 1958). The positive residual gravity anomalies (of up to 10 mgal), found to be associated with these geothermal area, have been attributed to basement uplift. Density measurements of cores from drill holes suggested that hydrothermal alteration causes lateral density variations within a single rock unit. A plot of mean density against rank of hydrothermal alteration for the best sampled rock units shows that there is significant increase in density with increase in rank of hydrothermal alteration, and that there is a density contrast of 0.3 to 0.4 g/cm3 between hydrothermally altered and unaltered rocks of the same geological rock unit.

The residual gravity can be separated into 2 components :

First order residual anomalies reflecting mainly changes in depth of basement, and second order residual anomalies reflecting marked lateral density variations resulting from hydrothermal alteration within the volcanic rocks. It is of interest to note that although the gravity method is insensitive to such local variations in depth of the basement, it is nevertheless, sensitive to changes in the angle of dip of the average basement surface.

GEOPHYSICAL CASE HISTORY

Gravity anomaly on the Aso Caldera by Yokoyama.The gravity anomaly begins to decrease gradually at the caldera rim and amounts to about 20 mgal relatively low at the center. This profile means that the caldera rim does not correspond to the vertical fault or vertical discontinuous boundary encircling the caldera from the viewpoint of density contrast and that light material is deposited beneath the caldera bottom.

The prominent features of calderas especially from the geophysical standpoint :

1. Low gravity anomalies mounting to a few score of milligals are almost concentric with the centers of calderas and indicate existence of coarse material to a depth of a few kilometers beneath the calderas.

2. The faults or discontinuous boundaries at the caldera rim do not stand vertically but incline aslant towards the center of the caldera.

3. Mass-deficiency observed at a caldera is just compatible with the total mass of ignimbrite found around the caldera.

4. In order to interpret the transportations of ignimbrite as far s 60 km from the craters, one must suppose extraordinary and, in some cases, lateral explosive energy whether the material was carried in the air or it was flowed over the earth surface.

The above 4 features support a theory concerning origin of caldera formation of low anomaly type that calderas were formed by both violent ejection of ignimbrite material and the simultaneous subsidence in fragments of the fore-caldera volcanoes.

MAGNETIC EXPLORTION

Magnetic prospecting utilizes a natural and spontaneous field of force, with fields of geologic bodies superimposed upon a normal terrestrial field. Coulomb’s law controls the attraction of magnetic bodies, and integral effects of all bodies within range are observed and depth control is lacking. Magnetic bodies frequently owe their magnetization to the magnetic field of the earth. For this reason, magnetic anomalies are often subject to change with latitude. More-over, rocks my have magnetism of their own whose direction may or may not coincide with that induced by the terrestrial magnetic field. An important factor in the interpretation of magnetic methods is that rock magnetism, contrary to rock density, is a bipolar nature. In magnetic prospecting, measurements of the total factor are exception rather than the rule; it is usually resolved into horizontal and vertical components. Magnetic fields are generally expressed in gauss ; in magnetic exploration it is more convenient to use 1/100,000 part of this unit, called the gamma. Most widely used in magnetic prospecting are the Schmidt magnetometers.

Corrections

The following corrections are required in magnetic exploration:1. Correction for temperature of instrument, arising from the fact that the magnets used for comparison with the earth’ magnetic field lose their strength with an increase in temperature.2. A “base” correction which allows for errors of closure when checking back to base station.3. A correction for variation which may be determined by visual observation or recording of a second magnetometer.4. A Planetary correction, which eliminates the normal variations of the earth’ magnetic field with latitude. Fence, bridges, pipe lines, tanks, derricks, well casing, and the like, are a serious handicap to magnetic exploration and must kept at sufficient distance, as it is difficult to correct them.

InterpretationMagnetic results are generally represented in the form of lines of equal magnetic anomaly (iso-anomalic lines). Interpretation of magnetic anomalies is usually qualitative. Depth determinations are the exception rather than rule, because magnetic anomalies may be do not only to variations in the relief of magnetic formation but also t changes in magnetization; the ratio of induced and remanent magnetization is frequently subject to unpredictable variations. In quantitative interpretation magnetic effect of assumed bodies

are calculated, compared with the field curves, and assumption changed until a geologically reasonable agreement is obtained.Direct methods of interpretation are applicable when the magnetic anomaly is simple and arises from one geologic body only; in that case, approximate calculations of depth may be made directly from the anomaly curves by assuming that the magnetic bodies are equivalent to single poles, magnetic doublets, single magnetized lines, and line doublets.

ApplicationsMagnetic methods of exploration are used for searching for magnetic deposits, in oil exploration, to determined the structure by the mapping of magnetic sedimentary, mapping of basement topography and of igneous intrusions, and in civil engineering.

Magnetic anomalies in geothermal field.

At the place where local geologic conditions are favorable, magnetic measurements reveal a buried volcanic or intrusive rocks, and is useful for checking the distinguishing gravity anomalies. Similar to gravity anomalies, magnetic anomalies are generally too large or to poorly defied to be useful for locating individual production wells. Some times, however, magnetic survey reveal magnetic lows over geothermal fields due to hydrothermal alteration of magnetite to pyrite. If the surface magnetic measurements can be interpreted in the light of subsurface information, including lithology, alterations, and fluid movement, it may be possible to locate area in which the thermal fluid is being fed into a reservoir. These areas seem to be characterized by more intense alteration of magnetite to pyrite than other parts of the reservoir, and consequently they produce near-surface negative magnetic anomalies.

ELECTRICAL EXPLORTION

Mineral deposits and geological structures may be mapped by their reaction to electrical and electromagnetic fields. In respect to surveying procedure nd the field measurement, 4 main groups of electrical methods of prospecting may be distinguished :1. self-potential2. resistivity3. induced polarization4.electromagnetic methods

1. Self potential methodThe self potential method s the only electrical method in which a natural field is observed; Its causes are spontaneous electrochemical phenomena. This phenomena occur on ore bodies and on metallic minerals and placers; they are produced by corrosion of pipe lines and on formation boundaries in wells by differences in the conductivity of drilling fluid and formation waters. Ore bodies whose ends are composed of materials of different solution pressure and are in contact with solution of different ion concentration,

act as wet cells equipotential lines or potential profiles. For the mapping of equipotential lines, a potentiometer is connected to two non-polarizable current vanishes. At that point the electrodes are on an equipotential line. Interpretation of self-potential is qualitative; the negative potential center may be taken with sufficient accuracy to be highest location of an ore body.

2. Resistivity methodsIn resistivity procedures not only the potential differences between two points but also the current in the primary circuit I observed. The ratio of potential difference and current, multiplied by factor depending on electrode spacing, give the resistivity of the ground.True resistivity o observed in homogeneous ground only; the presence of horizontal or vertical boundaries in the range of the instrument give what is known as apparent resistivity. The arrangement in most frequent use I the four-terminal wenner-Gish-Rooney method. Resistivity methods my be applied in 2 ways : 1. With constant electrode separation, that is constant depth penetration, called resistivity mapping. 2. with fixed center point and progressively increasing electrode separation, called resistivity sounding, Whereby the apparent resistivity is observed as a function of electrode separation and therefore of depth.

A modification of the resistivity mapping method is used in electrical logging and field survey. The interpretation of resistivity data may be qualitative and quantitative.

In resistivity method of electrical explorations, compact rock formation has great value in resistivity as compare with porous rock formation. Especially, the resistivity value of formation including water rich in Cl ions reduces to very small. The experiment has demonstrated that the data fall on a straight line, when the logarithm of the conductivity of rocks, rock forming minerals, and other ionic crystal is plotted against the reciprocal of the absolute temperature.The result of resistivity survey is represented in the resistivity block diagram relative to surveying lines. If, in in geothermal region, value of resistivity of the formation involved is in the order of several 10 ohm-meter, then we may conclude that the formation corresponds to a geothermal reservoir. Further, if it is impossible to correlate adjacent resistivity formation, then the existence of a fault may be surmised. Further more, from the value of resistivity in reservoir, the temperature of reservoir will be estimated. As described above, resistivity measurements are of great use to make clear the subsurface structure in geothermal region.

The result of resistivity measurements in geothermal field.

Electrical resistivity explorations have been very helpful in locating major faults in the Italian geothermal fields. In which the electrically resistant anhydrite reservoirs re overlain by an impermeable and conductive shale. In such case, the resistivity survey is successful because the thickness of the shale cover is characteristic of ech fault block, so that resistivity measurements how the position of the faults as well as the comparative

thickness of the shale, and therefore, the depth to the reservoir. The resistivity method is useful not only for determining geological structure it can also indicate the presence of geothermal heated zone. It has been found in Italy that ground heated from 170C to 1500C decreases in resistivity by a factor of 5, and, if heated from 170C to 2800C, its resistivity decreases by a factor of 9. The effect of temperature on resistivity has been successfully used for natural stem exploration at Kawerau, new Zealand. In that area, Studt found that at the water table ( 3 – 6 meters deep ) the resistivity drops abruptly to several hundred ohm-m in cold country, and to several tens of ohm-m in hot. Resistivity surveys utilizing this heat effect are best applied in area having uniform surface geology and uniform depth to the water table.

In the Otaka geothermal field, the observed resistivity values of pyroxene adesite and augite andesite are in the range of 6000-8000 ohm-m and 2000-3000 ohm-m respectively. While calculated values of low resistivity layer which corresponds to altered zone is of order 4-50 Ohm-m. Consequently it is possible to investigate the subsurface distribution of altered zone by resistivity method of exploration.

Results of resistivity explorationIt is the aim of resistivity exploration to determine the shape and the distribution of altered zone in close relation to heat sources.

The interpreting process of resistivity data in the following 2 steps :

The first step deals with the interpretation of resistivity mapping curves, which are prepared from a series of resistivity sounding curves of the same line. Discussing the tendency of these curves accompanied with the increase of L/2. We get such the knowledge of information as the part of the area showing the stratified earth, which indicated by horizontal forms of these curves, the existence of faults, which is indicated by step forms, and of high or low resistivity zone.

The second step is to apply the curve matching method of interpretation to the resitivity sounding curves showing the layered earth. Owing to the effect of topography, the existence of faults, complex geological conditions characterized by volcanic area, ground water and so on, the observed curves in many cases deviated from the theoretical curve. However, resistivity measurements indicated clearly low resistivity layer corresponding to altered zone because of very high resistivity contrast between altered zone and formation above and below it.

In general the application of resistivity method to the detection of natural steams is to determine the distribution of formations storing superheated waters. Resistivity measurements indicated low resistivity layer which correspond to altered zone, but it is not always reservoirs. Further resistivity data offered such many informations as new areas to be developed, the existence of faults, the depth. The shape and distribution of reservoirs and limit of drilling depth for the development of geothermal resources.

3. Induced Polarization methodInduced electrical polarization appears in the presence of metallic ore particles. We may observe low decay of the voltage at the potential electrodes, when a direct current introduced by to current electrodes is interrupted. In the case of alternating current of rectangular waves, the effective conductivity of the medium containing the disseminated particles is a complex function of frequency. This is called a ‘trangent method ‘ of induced polarization method. Secondly, the conductivity of various types of minerals changes with frequency. This phenomena is applied to mineral prospecting and is called a variable frequency method. In the former, metal factor is defined, and this quantity is plotted against the distance. In the latter, frequency effect is defined, and the quantity is plotted against the distance. These values of metal or frequency effects are used for the interpretation of induced polarization methods.

4. Electromagnetic MethodElectromagnetic methods of electrical prospecting differ from potential methods in that the electromagnetic field of ground currents and not their surface potential is measured. They fall into 2 major groups : 1. Electromagnetic Galvanic methods, in which the primary energy is supply by contact as in the potential methods. 2. Electromagnetic inductive methods in which the ground is energized by inductive coupling with insulated loops.

SEISMIC EXPLORATIONSeismic methods of geophysical exploration are in the category of indirect geophysical methods, in which the reactions of geologic bodies to physical fields ate measured. Since the depth of penetration of such fields depend upon the spacing of transmission and receiving points, variation of physical properties with depth may be measured by nothing how certain physical quantities change in horizontal direction. In seismic exploration a change of dynamite is afire at or near the surface and the elastic impulse are picked up by vibration detectors, likewise at the surface. The time which elapses between generation and reception of the elastic impulse (travel time) is measured by recording also the instant of the explosion and time marks (usually at 1/100 sec. interval). Seismic explorations are divided into 2 methods :

1. Refraction method, in which travel times of first arrivals are observed along a profile. The variation of this travel time with distance or the travel time curve makes it possible to determine true velocities and depths of the refracting formations. 2. Reflection method, in which the time required for elastic impulse to travel to and from a reflecting bed is measured. From the travel times it is possible to make a direct calculation of the depths of the reflecting surfaces but not an evaluation of the elastic wave speeds within the reflecting formations.

1. Refraction MethodIn refraction shooting, the travel times of first impulses are determined and plotted as functions of the distance of receptors arranged in a profile. If the medium between source and reception points is homogeneous in horizontal and vertical direction, the arrival times will be proportional to distance, and thereof are the travel time curve will be straight line, its slope giving the velocity in the medium. From a certain distance on, wave that have taken a detour through the lower high-speed medium will overtake and therefore arrive ahead of the wave through the upper medium. The simultaneous arrival of the two waves will be indicated by a break in the travel time curve; the slope of the second part of the travel time curve will correspond to the velocity in the lower medium. From these two velocities and the abscissa of the break in the travel time curves, the depth of the interface may be calculated.

2. Reflection MethodReflection impulses always appear in a seismic record after the first arrivals. Important factors controlling the appearance of reflections in a seismogram are the placement (depth) of the charge and the distance between the shot point and receiver locations. Depths are calculated from reflection records by timing the reflections for a mean receptor distance, and multiplying the time one-half of the average velocity. This is true for nearly vertical incidence. For greater distances a spread correction is applied. Although reflection rays are curved, it is usually satisfactory to calculate depths on the basis of straight ray propagation. If beds are dipping, at least two profiles must be shot and down dip. For determination of dip and strike, two profiles at an arbitrary angle with each other, shot up and down dip, are required. Relative depth determinations the average velocity must be known. It may be determined by recording reflection from known depths, by shooting in wells, or by surveying a long reflection profile at the surface. If squares of travel times are plotted against squares of distances, the square of the average velocity follows from the slope of such curve. Because of the delay affecting primarily the return ray in the low-velocity surface zone, a weathered layer correction must be applied. Data for the correction are obtained by the refraction procedure described above. Elevations are considered by a topographic correction to shot datum; by reducing to a regional datum, variations in the geology of surface beds may be allowed for. Sometimes it is necessary to make Correction for horizontal velocity variations.

SEISMIC PROSPECTING AT GEOTHERMAL AREA by Hochstein et al.

The principle of seismic method is to find out the underground structure by utilizing the differences of seismic velocities of underground velocity layers. In general, the seismic

velocity of rock depend upon its elastic property and density. Since rock is more or less porous and contain underground water and natural state, water contain is one of factor to determine seismic velocity of rock. Temperature and pressure of underground rocks re also important factors to take into consideration.The velocity of rock-specimen can be measured by using ultrasonic pulse in the laboratory. Though this ultrasonic velocity of each rock differs generally from seismic velocity encountered actually at field when seismic prospecting is carried out, it is considered that such laboratory test by using rock specimens obtained from bore-holes or suitable outcrops is quite useful for interpreting seismic data.

At Matsukawa geothermal Field, Japan, was conducted about 2 km length. They started with laboratory experiment including density, porosity, and ultrasonic wave velocity measurements by using the specimens of outcrops at and adjacent area of this geothermal field. From these results, tentatively they presumed as follows by combining geological data.

- Matsukawa Andesite might correspond to the first cover-rock because of the high velocity and low porosity.

- The subsequent dacite tuff formation probably might be the first reservoir of hot water because of low velocity and high porosity.

- Likewise dacite lava beneath the dicite tuff might correspond to the second cover rock.

- The underlaid marine sediments correspond to the second reservoir.

Consequently, they rather preferred the reflection seismic method instead of refraction method because the seismic velocity might not increase with depth.

Several years later, more extensive and detailed seismic survey was carried out at Matsukawa geothermal field. The common depth points horizontal data staking techniques of 4-fold multiplicity were adopted. The total length of surveyed line was about 12.6 km. The recorded magnetic tapes were processed by seismic data processing analogue system, static correction, dynamic correction, staking and filtering. The results were expressed by the wiggle and variable area recordings, and they were interpreted to find out underground structure.

The similar reflection seismic surveys were carried out at Onikobe and Otake geothermal areas. Though the seismic method is, in this way, applied to exploration of geothermal areas in Japan, they had still many unsolved problems, from both practical and theoritical points of view.

The structure of a few geothermal areas in New Zealand has been investigated by reflection methods, and some results of these studies have been published for the Kawerau field (Studt, 1958) and for the Wairakei field (Modriniak, Studt, 1959). In both cases the measurements were not very successful a result which was attributed to the disturbing ground noise and the unusually high attenuation of seismic waves in these areas. On the other hand, seismic refraction measurements in the same test area showed

that at lest one interface could be traced, and mapping of various interfaces by the refraction method was started in 1967.

The aim of this survey was to determine the structure of the volcanic rocks, to locate major faults, and to find out whether the attenuation and velocity of seismic waves in volcanic rocks within a thermal are differ from those outside.

PEMODEIAN 3D RESERVOAR PANASBUMI KAMOJANG DENGAN MENGGUNAKAN METODA GAYABERATMIKRO 4D UNTUK MENINGKATKAN SISTEM MONITORING PRODUKSI-REINJEKS1

Oleh: Ahmad Zaenudin, S.Si., M.T. Lembaga PenelitianDibuat: 2008-01-03 , dengan 1 file(s).

Keywords: PRODUKSI-REINJEKS1,SISTEM MONITORING,Subject: Panas BumiCall Number: 621.47 Zae p c.1

RINGKASAN

Penelitian ini adalah untuk memodelkan secara 3D reservoir panasbumi Kamojang berdacarkan data gayaberatmikro 4D. Kegiatan-kegiatan ini meliputi : (1) Studi literatur yang menyangkut reservoar Kamojang, geologi, geohidrologi, pengukuran geofisika yang pemah dilakukan dan subsidence, (2) Simulasi respon anomali gayaberat dan gayaberatmikro 4D, (3) Pengukuran gayaberatmikro dalam dua periode, (4) Pengukuran tide presisi, perubahan muka airtanah, tekanan barometrik, temperatur, dan curah hujan, (5) Prosesing data gayaberat lanjut untuk mendapatkan besaran-besaran fisik dengan teknik inversi, (6) Interpretasi berdasarkan model 3D reservoir dari anomali gayaberatmikro 4D, peta distribusi rapat massa, dalam upaya evaluasi keseimbangan produksi dan reinjeksi, dan (7) Usulan tata cara penggunaan teknologi gayaberatmikro 4D untuk monitoring panasbumi.

Keberhasilan monitoring ditujukan sebagai upaya penghematan dan perpanjangan usia cadangan sumberdaya energi panasbumi terbarukan dengan metoda gayaberatmikro 4D sebagai alternatif. Pada penelitian ini telah dikembangkan : (a) tata cara pengukuran gayaberatmikro untuk pemantauan, (b) teknik pemisahan anomali benda dangkal dan dalam menggunakan filter stripping, dan (c) inversi liner untuk mendapatkan besaran

perubahan rapat-massa dari anomali gayaberatmikro 4D.

Metoda gayaberatmikro antar-waktu dapat digunakan untuk memantau perubahan massa pada reservoar akibat eksploitasi dengan baik. Depisit massa sebagai refresentasi pengurangan rapat-massa (negatif) dan penambahan massa atau penambahan rapat-massa (positif) dapat dibedakan dengan baik dengan

menggunakan teknik inversi liner.

Pemodelan 3D dapat menunjukan sebaran zona-zona perubahan rapat-massa negatif dan positif dalam arah horizontal maupun vertikal. Pengetahuan in dapat dijadikan rujukan dalam memperbaiki sistem monitoring lapangan panasbumi, yaitu dengan reposisi atau pengalih fimgsian sumur produksi dan reinjeksi. Pada zona rapatmassa negatif hams ditempatkan/difungsikannya sumur reinjeksi dengan kedalaman tertentu dan sebaliknya, dengan memperhatikan struktur geologi pengontrol.

Metoda gayaberatmikro antar-waktu dapat dijadikan alternatif dalam pemantauan reservoar panasbumi disamping metoda pemantauan lainnya seperti gempa mikro (MEQ) dan tracer isotof. Karena metoda gayaberatmikro antar-waktu selain dapat memperkirakan posisi dan kedalaman perubahan massa juga dapat memperkirakan jumlah massa yang harus diinjeksikan, yang oleh metoda lainnya tidak dapat diprediksi.

Translation:

SUMMARY In a geothermal reservoir, water mass is heated by the magmatic rocks and transformed to boiled water and high pressured steam. These high temperature fluids are extracted to the surface and results in the decrease of mass at the reservoir. Gravity measurement at the surface associated with this process may indicate decreasing values. These changes are very small and require Microgravity method to monitor. For this purpose, gravity data acquisition must be done periodically over the production period. In this research, mass change was modeled by using 3-D prisms forming a reservoir layer. With constant prism geometries, model parameters to be determined for are densities in each prism. In this case, model response is kernel matrix multiplied by density distribution matrix. Kernel matrix is formed by array of prism geometry. Linear inverse method was applied to recover the density change from gravity observation data. The method was tested with synthetic data corresponding to a reservoir layer.

PEMBORAN DANGKAL,

dilakukan selektif apabila diperlukan, karena mahal.Tujuan: untuk mengetahui gradient temperatur., dilakukan apabila Propose Well, di boundary yang diffuse, temperature meragukan .

PETROFISIKA:

Tujuan : Untuk menentukan Porositas (%), untuk :Kontrol Potensial Permeabilitas (mD), Untuk: Kontrol Output Sumur (Q)

5 – 7 % porosity-------- Steam7 – 10% porosity ------- 2-fase10 – 15 % porosity ----- Water

SISTEM PANAS BUMI ( dari data integrasi )

MENURUT Sudarman, S. (2009), Temperatur reservoir dapat dikelompokkan enjadi:- Tinggi ( >230 derajat C), Kuarter; Uap, 2-Fasa, Air Panas- Moderate ( 150 – 160 ), Tersier; air panas- Rendah ( 100 – 120 ), Tersier; Air Panas

Jenis Fluida ( T Tinggi ) :- Dominasi Air Panas (Water Dominated System ) ( 230 – 320 derajat C )- 2-Fasa ( 270 – 300 derajatC )

- Dominasi Uap ( Steam Dominted System ) (240 derajat C )

Reservoir Geometry: A, h D.

A = Area (km2) , ditentukan dari dta geolistrik dan TGh = D =

Karakteristik Batuan : permeabilitas, Porositas, AlterasiKarakteristik Fluida : NCG, pH, T, y.Hidrogeologi : Recharge, Upflow, Outflow.

MODEL RESERVOIR

Tujuan : menentukan/memperkirkan :Area pemboran (dari dta Golistrik), Depth of Top Luas Area / Well ------------------------- 3 km2Reservoir (m) (daridata geofisika), Temperature (dri data geokimia), Well Output (MW/WELL) , ditentukan dari data K dan T.

Geotherml system, i.e. Dryness or Wetness (%), dari data lab. Geokimia.

Non Condensable Gas in Steam (% by wt) didapat dridtalab. Geokimia fumarol/solfatar.

POTENSIAL PROSPEK

- Estimasi Potensial :

------- Potensi akurt --------------- effective porosity

----- Korelasi pakai Gravity sampi surface-------- : - Porositas di permukaan = 30% porositas reservoir - Permeabilitas di permukaan = 1/6 X permeabilitas Reservoir - Density di permukaan = 6 X densitas Reservoir

ASUMSI KRUSIAL GREENFIELD:

-Potensial Reservoir ( recharge ) :

0.22 X A (km2) X ( T res – T cut off / inlet derajat C )

- Konstanta 0.22 ( porositas 10%, ketebalan reservoir = 2000 meter, 30 thn )- RMS 57% ( dominasi porositas 5 – 15 % )

Well Output, tergantung : Geological Site Specifics ( Grben, Cldera, Dome, Mixing )Drilling Succes Ratio :

- Dunia : 67 % (Workshop API 2008)- Wayng Windu best field 75 % ( tidak termasuk Blok Malbar )

Sistem Fluida Reservoir, i.e. Dryness :- T reservoir- Kandungan khlorida- porositas

Model reservoir siap bor eksplorasi:P recharge = 0.22 X A (km2) X ( T res. – T inlet )

G. PAPANDAYAN PROSPOCT AREA

Reservoir:

Dibawah antara G.Puntang - G.Papandayan dan dibawah lereng Tegal

Alun-Alun (didasarkan pada zona low resistivity hasil survey MT,

Resistivity dan struktur geologi daerah Gunung Papandayan - Puntang

dan Tegal Alun-alun

Diperkirakan berada pada frakturasi batuan berumur Kwarter Muda -

Kwarter Tua

Puncak reservoir diperkirakan pada kedalaman 700-1100 m dari

permukaan (dari data MT, diperkirakan berdasarkan kontak antara

lapisan/lapisan konduktif dan lapisan resistif)

Fluida reservoir:

Temperatur reservoir : > 240oC (geothermometer gas)

Sistem panasbumi di daerah prospek diperkirakan sistem satu fasa

(sistem uap kering)

Terdapat di sekitar/di bawah Tegal Alun-Alun dan Puntang

Potensi Energi Panasbumi:

Berdasarkan luas daerah pengukuran geofisika, perhitungan panasbumi

dilakukan dengan menghitung cadangan panas (heat storage) dengan memakai

beberapa asumsi, antara lain:

Harga rata-rata densitas bahan yang terlarut (saturated density = 2.5 x 0

E3 (kg/m3)

Harga rata-rata panas spesifik (specific heat) = 1 x 10 E3 (kj/kgoC)

Tebal rata-rata reservoir, 2 x 10E3(m)

Daerah Panasbumi Puntang-Papandayan- Tegal Alun-Alun mempunyai

luas daerah prospek (A) sebesar 15 km2, suhu reservoir 240oC,

maka cadangan panas (J) sebesar = T (oC) x A (km2) x 10 E6 x 2 x 10

E3(m) x 2,5 x 10 E3 (kg/m3) x 1 x 10 E3 (kj/kgm3)

= 5 x 10 E15 x A (km2) x T (oC)

= 13.5 x 10 E18 J

Untuk menghitung potensi listrik digunakan asumsi sebagai berikut:

Suhu dasar sumber panas utuk pembangkit listrik minimum 180oC

30 tahun masa eksploitasi (=1 x 10E9 s)

Harga efisiensi ekstraksi sumber (resource extraction efficiency) = 0.035

Potensi listrik (Sumberdaya Spekulatif) untuk luas`area 10 km2 (daerah

pengukuran MT):

= 5 x 10E15 x A (km2) x (T-180oC) x 1 x 10E-6 x 0,035 x 1 x 10E-9 (/s)

= 0,175 x A (km2) x (T-180oC)

= 0,175 x 10 x 60

= 105 Mwe.

Potensi listrik (Sumberdaya Spekulatif) untuk luas`area 15 km2 (perkiraan

luas dari analisis) :

= 5 x 10E15 x A (km2) x (T-180oC) x 1 x 10E-6 x 0,035 x 1 x 10E-9 (/s)

= 0,175 x A (km2) x (T-180oC)

= 0,175 x 15 x 60

= 157,5 Mwe.

• Bila menggunakan perhitungan di atas, Sumberdaya Spekulatif

panasbumi daerah Papandayan (Papandayan – Puntang) adalah ± 157,5

Mwe. Dengan demikian Sumberdaya Spekulatif panasbumi daerah

Papandayan (Papandayan – Puntang) berkisar antara 151,62 Mwe -

157,5 Mwe.

• Bila dikaji lebih jauh dari analisis hasil pengukuran resisitivity, gaya berat

yang dilakukan oleh Direktorat Volkanologi dan gayaberat serta MT. yang

dilakukan, tidak menutup kemungkinan daerah prospek akan menerus ke

utara yaitu blok di diutaranya yaitu di sekitar Gunung Jaya yang dibatasi

oleh sesar yang berarah baratlaut – tenggara dengan blok Puntang-

Papandayan. Dengan demikian sumberdaya spekulatifnya akan

bertambah besar ( lebih dari 150 Mwe). Tetapi hal ini perlu ditunjang oleh

penelitian pendahuluan untuk daerah blok Gunung Jaya.

G. TANGKUBAN PERAHU GEOTHERMAL PROSPECT

ESTIMASI POTENSI ENERGI PANAS BUMI

Potensi energi panas bumi daerah Tangkuban Parahu dihitung berdasarkan dua

zona prospek yaitu Ciater seluas 10 km2 dan Kancah 8,5 km2 (kompilasi data eks

Pertamina dan Distamben Jabar). Temperatur reservoir Kancah dan Ciater dari hasil

penghitungan geotermometer air diasumsikan sama yaitu ±250 oC.

Penghitungan potensi panas bumi dilakukan dengan cara menghitung cadangan

panas (heat storage) dengan memakai beberapa asumsi. Asumsi-asumsi tersebut antara

lain :

- harga rata-rata densiti bahan yang melarut (saturated density) = 2,5x 0E3 (kg/m3)

- harga rata-rata spesifik panas (spesific heat) = 1 x 10E3 (kJjkg DC)

- rata-rata tebal reservoir, 2 x 10E3 (m)

Untuk daerah panas bumi Tangkuban Parahu mempunyai luas prospek (A) sebesar 10

km2 dan 8,5 km2, suhu reservoir 2500C, maka didapatkan cadangan panas sebesar

Cadangan panas (J) = T(0C) x A(km2) x 10E6 x 2 x 10E3(m) x 2,5 x 10E3(kg/m3) x 1 x

10E3(J/kg m3)

= 5 x 10E15 x A(km2) x T(0C)

Ciater = 12,5 x 10E18 J

Kancah = 10,5 x 10E18 J

Untuk menghitung potensial listrik digunakan asumsi berikut.

- suhu dasar sumber panas untuk pembangkit minimum 180°C

- 30 tahun masa eksplotasi (= 1 x 10E9 s)

- harga efisiensi ekstraksi sumber (resource extraction efficiency) dan efisiensi

pembangkit (generation efficiency) = 0,035

Potensial Listrik (MWe):

= 5 x 10E15 x A(km2) x (T-1800C) x 1 x 10E-6 x 0,035 x 1 x 10E-9 (/s)

= 0,175 x A(km2 ) x (T-1800C)

Ciater = 120 MWe

Kancah = 100 MWe

Jadi estimasi total potensi daerah panas bumi Tangkuban Parahu untuk prospek Ciater

dan Kancah sekitar 220 MW.

KONSEP HARGA LISTRIK

* Capex (capital expenditure) ( $ / MW ) + Financial Cost - Geologically site specifics dan Unit scale

* Operation and Maintenance Cost- Fluid Characteristics, i.e.: scalling and corrosion

* Require Rate of Return ( 30% Equito, 70% Debt )- 16 – 17 %

DAFTAR PUSTAKA

1. Akbar Nikmatul, 1995, Pengantar Geothermal, UNPAD, Bandung, tidak diterbitkan, 57 hal.

2. Armstead, HCH, 1973, Geothermal Energy (Ed), Earth Science. UNESCO.3. Chemical and Geothermal System, Prentice Hall, New York.4. Mahon, 1982, Thermal Manifestation Related To Active Volcanism and the

Energy Potential, Volkanologi, Bandung.5. Rybach, L & Maffler, L., 1981, Geothermal System : Principles and Case

Histories, (Ed), John Wiley and Sons Ltd.6. Wohletz, K., & Heskner, G., 1992, Vulcanology and Geothermal Energy,

University of California Reiss Oxford.7. Henley, R.W., 1983, Fluid Mineral Equilibria in Hydrothermal System, Science

of Volcanik Geologist, University of Texas, El Paso.