Seismic Student

SEISMIC SURVEYS 1.0 Introduction Seismic exploration provides the engineer with knowledge of subsurfaceconditions, required for the safe and economical design of a project and inform the engineer about the materials and conditions he will encounter. Explorations are normally accomplished in phased sequence as follows: - reconnaissance investigations - exploration for preliminary design - explorations for detailed design - explorations during construction

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Seismic surveys are carried out primarily by the oil and gas industry: - to identify areas (called hydrocarbon traps) where oil and gas may have accumulated - determining a geologic formation’s potential for containing commercial quantities of economically producible oil and/or gas - identifying the best locations to drill an exploratory well - drilling exploration and delineation wells to determine where hydrocarbons are present and to measure the area and thickness of the oil/gas reservoir. 5

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The rock formations may indicate whether oil/gas might be present. These include former sedimentary basins, faults and ancient reefs that can act as underground traps for crude oil/gas. Seismic information helps companies decide whether: - the available information is sufficient to justify drilling an exploratory well - additional surveys are needed to better define the structures before drilling - the features present are not attractive enough to warrant further interest

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A seismic line looks like a cross section through the earth. Initially these are used to map structural traps where hydrocarbons may accumulate – at its simplest at high points of domes known as anticlines, but also places where faults or erosion cut off a reservoir. Stratigraphic traps, where the geology changes laterally from one rock type to another, such as a buried sandy channel, sand bar, or carbonate reef can also be mapped using seismic data.

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1.1 System Configuration i) Acoustic signal processor and graphic recorder

ii) Sound source

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iii) a recording cable called ‘streamer’ which contains hydrophones

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iv) Energy source The energy source required to drive a seismic source must be able to convert regular mains voltage into the high voltage (3500 volt/1500 amperes) necessary to energise the sound source. It must be able to do this safely and reliably, and be sited in a convenient location.

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1.11 Signal Processor and Graphic Recorder - The recorder is the visual interface for any seismic system. - The recorder also include a control panel to allow the operator to select optimum repetition rate of the energy source. - Gain controls are used to vary the intensity of the traced record - By programming appropriate triggering and gating sequences, most of the undesirable background noise may be eliminated

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1.12 Hydrophones Hydrophone is the standard receiver in marine seismic and responds to variations in pressure. It exist of a piezoelectric ceramic disc. A pressure wave effectively bends the piezoelectric disc, thus generating a voltage (sea picture). This voltage is proportional to the variation of the pressure.

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1.13 Streamer Streamer digunakan untuk meletakkan hydrophones bagi merekod tenaga seismik yang dibalikkan dari lapisan dibawah dasar laut.

Biasanya didalam streamer diletak minyak khas bagi mengapungkan streamer secara semula jadi dan membuang udara didalamnya yang memberi kesan kepada sensitivi hidrophone.

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Steramer akan merekod tenaga seismik yang dibalikkan menggunakan hidrophone. Tenaga seismik akan ditukar kepada tenaga elektrik dan disalurkan melalui streamer kepada sistem perekod di kapal. Streamer terdiri dari beberapa komponen penting: - hydrophones, usually spaced 1 m apart - electronic modules, which digitize and transmit the seismic data - stress members, steel or kevlar, that provide the physical strength, allowing the streamer to be towed in the roughest of weather - an electrical transmission system, for power to the streamer electronic modules, and peripheral devices, and for data telemetry - the skin of the streamer in which all the above are housed The streamer is divided into sections, each 50-100 m in length, to allow modular replacement of damaged components. Each section is terminated with a connector unit. Each section is filled with special fluid (synthetic material), which has a specific gravity of less than 1, to make the overall streamer neutrally buoyant.

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Recent advances have led to a new generation of streamers, moving away from the fluid filled to a solid cable, constructed of extruded foam. They are more robust and resistant to damage, don not leak oil, and are less sensitive to weather and wave noise. Streamer tow depths vary from 4-5 m for shallow and in good weather areas, to 8-10 m for deeper penetration in more open waters. In addition to the internal components, there are 3 types of external device, which are attached to the streamer: - depth control units or birds - magnetic compasses - acoustic positioning units

bird

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The wings on the birds are electronically controlled to control the depth in response to the hydrostatic pressure measured by a pressure transducer inside each bird. The birds are normally spaced approximately 300 m apart.

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For 3D seismic, the position of the streamer must be precisely located. The shape of the streamer is determined using compasses, which measure the deviation of the streamer. A computer algorithm is used to derive the true position from these individual compass measurements, which are located roughly every 300 m.

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An acoustic ranging units are also used to provide additional position information. These units are attached to the hull of the vessel, the source floats, the streamers themselves and the tailbuoys. A taibuoy is connected to each streamer to provide hazard warning of the submerged towed streamer and positioning infromation. The tailbuoy is used to house DGPS receivers that are used in the positioning solution for the hydrophone groups in the streamers.

Tailbuoy

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Kedudukan streamer sepertimana perancangan pengukuran juga boleh dikawal mengguna alat yang bernama ‘Fin’. Jika berlaku perubahan kedalaman, sudut ‘fin’ akan diubah untuk menetapkan kembali ke kedalaman asal. Fin diletak pada sela 400-800 m. Setiap fin dikawal berasingan bagi menggerak streamer ke atas, ke bawah, ke kiri dan ke kanan.

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1.2 Punca Tenaga/Gelombang Seismic Punca tenaga seismik terawal adalah bahan letupan, antaranya termasuklah dinamit dan gas peletup (gas exploder). Pada masa sekarang teknologi ini telah diganti dengan non-exsplosive technology. - echo sounder - pinger - boomers - sparker - air gun

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Frekuensi tenaga seismik yang digunakan memainkan peranan penting dalam aplikasi seismik. Frekuensi rendah dapat menembusi sedimen dengan lebih dalam dan sebaliknya. Frekuensi melebihi 40 kHz menyebabkan keseluruhan tenaga akustik akan dibalikkan oleh dasar. Jarak pulse yang pendek pula akan memberikan resolusi yang lebih baik. Walau bagaimana pun, frekuensi yang rendah akan menghasilkan jarak pulse yang panjang. Oleh yang demikian nilai frekuensi dan panjang pulse tidak selari, menyebabkan kompromasi mestilah diambil bagi mendapatkan keputusan terbaik. Jadual di bawah menunjukkan panjang pulse dan kadar ulangan pulse bagi jenis-jenis sistem seismik di atas

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Penggunaan tenaga yang tinggi juga menimbulkan masaalah dimana lebih banyak balikan akan berlaku dalam lapisan kolum air. Ini akan menyebabkan data cerapan tidak begitu jelas. Masaalah lain berkaitan dengan kuasa tenaga ialah kadar ulangan pulse. Bagi menyelesaikan isu-isu yang dibincangkan di atas, terdapat berbagai jenis sistem seismik dipasaran yang dapat digunakan. a. Sparkers Merupakan sparker yang menggunakan tenaga tinggi bagi menghasilkan tenaga bunyi berfrekuensi rendah (25-2000 Hz). Tenaga bunyi dipancarkan dari elektrod yang disediakan. Tenaga elektrik yang dipancarkan biasanya dalam kadar yang rendah. Penembusan dalam julat 200 m hingga 1000 m.

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Sparker melepaskan “steam bubble” dengan menyalurkan elektrik ke electrode yang dikelilingi oleh ‘conducting fluid’ iaitu air laut yang masin. Ini akan menghasilkan getaran tekanan dalam air. Sparker memerlukan punca tenaga elektrik yang dapat menukarkan voltage biasa (contoh 24 volts) kepada voltage yang tinggi (3500 volts/1500 amperes) yang diperlukan untuk energise sparker

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b. Boomer Tenaga elektrik yang hantar kepada coil yang dilekatkan kepada plate secara magnetik untuk menghasilkan tenaga akustik. Tenaga elektrik yang diterima oleh coil akan menghasilkan arus elektrik yang bertindak dalam arah keluar. Ini menyebabkan getaran pada plate yang menghasilkan tenaga bunyi. Boomer menghasilkan ‘sharp pulse’ pada frekuensi 500Hz hingga 10 kHz.

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c. Pinger Pinger menggunakan transducer piezoelectric untuk menghasilkan tenaga akustik pada frekuensi sehingga 12 kHz. Kedalaman penembusan bergantung kepada jenis sedimen dimana dasar berpasir mempunyai julat penembusan dari 0 hingga 3 meter dan sehingga 30 meter bagi jenis selut. d. Air Gun Kaedah ini menggunakan tenaga bunyi bertenaga tinggi tetapi berfrekuensi rendah yang dihasilkan melalui perlepasan udara bertekanan tinggi didalam chamber kedalam air. Tenaga bunyi dilepaskan dengan sela 10-15 saat pada kedalaman 4-8 m. Frekuensi diantara 10-120 Hz.

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Bagi exsplorasi minyak dan gas, punca tenaga seismic adalah dalam bentuk array bagi meningkatkan lagi resolusi imej. Array mengandungi beberapa air gun yang memancar tenaga seismic serentak pada sela tertentu.

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An airgun is a hollow metal cylinder that generates pulses of sound by releasing bursts of highly pressurized air into the water. In effect, an airgun is like an underwater “pop-gun”. Airguns are usually combined into an array, and fired in unison to make a louder pulse that penetrates deeper into the seabed. Airgun noise is broadband, and is made up of low frequencies, from about 10 Hz to 1000 3000 Hz. Airguns are designed to emit low frequency pulses, because only very low frequency sounds can penetrate deep into the earth.

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Antara kemudahan lain yang penting bagi airgun adalah compressor room yang mengandungi compressor engine dan compressor. Compressor berupaya untuk recharging airguns dengan sela yang kerap dan berterusan.

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1.3 Principle of Seismic Survey Seismic wave are used to give a picture of deep rock structures. The seismic wave travels through the water and strikes the seafloor. Some of the energy of the wave is reflected back to the hydrophones. The rest of the wave carries on until it reaches another rock layer. The time taken for the waves to travel from the source to the hydrophones is used to calculate the distance traveled - hence the thickness of the rock layers. The strength of the reflected wave gives information about the density of the reflecting rock.

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Each time the seismic pulse meets a change in rock properties, for example from a shale to a sand layer, part of the pulse will be reflected back to the surface. This is called an event. By measuring precisely the difference in arrival time of a given event from the nearer and further hydrophone groups, the velocity of the rock material can be measured. The seismic measurements are made in time, so if the velocity and time are known, geophysicists can workout the depth of the event.

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In marine seismic surveys, reflected sound waves, called signals, are combined and interpreted electronically or reproduced on graphic paper recorders. This data gives information on the depth, position and shape of underground geological formations that may contain crude oil or natural gas.

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Signal Processing The sound sources release seismic waves every 10-15 seconds . The signals are short, sharp pulses with most of the energy at 10-1000 Hz. The pressure waves travel to the ocean floor and subsurface layers. They are reflected back to the surface and recorded by the hydrophones. Different surfaces reflect the waves differently. These differences are recorded and transmitted to the vessel. The structure of the seabed and subsurface is then determined by analyzing the reflected signal.

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To reach the desired depths, seismic surveys use high energy, low frequency sound that can penetrate more than 6000 m below the sea floor. The survey results (rock formations/geological structures) do not show definitely whether oil/gas is present, but indicate where they are likely to be found and can help narrow the search area. How Are The Seismic Data Collected? As the vessel moves along the line, computers control the simultaneous discharge of seismic waves from the sound sources, usually every 10 seconds. The waves travel down through the rock formations. When they encounter a boundary between diff. formations, some sound waves are reflected back to high-capacity computers, check and store the data collected. The collected data go through several processing steps to improve the quality of the signals and filter out background noise. Geophysicists then interpret the information to develop a detailed picture of the structures and rock formations. 40

Sela garis pengukuran seismic bergantung kepada tujuan pengukuran, bagi tujuan penyelidikan, sela mungkin beberapa km, kawasan minyak, sela lebih rapat, dan 3D seismic melibat kawasan kecil dengan beberapa hidropon array pada sela beberapa puluh meter. Sela tembakan bunyi bergantung kepada kedalaman, kawasan dalam memerlukan masa balikan yang lebih lama, menjadikan sela tembakan lebih panjang, 9-20 saat, kadangkala 30-60 saat dengan halaju bot 4-5 knots. Kawasan cetek, bot bergerak lebih laju (sehingga 10 knot) kerana masa balikan tenaga lebih cepat, membolehkan sela tembakan lebih singkat. Bagi seismic cetek dengan kualiti yang baik, sistem seismic menggunakan punca bunyi yang kecil saiznya dengan frekuensi tinggi dan streamer yang pendek. Sparker dan air gun seringkali digunakan. Bagi imej yang jauh dibawah dasar, beberapa airgun yang besar digunakan bersama streamer yang panjang. Sparker tidak beberapa sesuai kerana masaalah ‘low power’, air gun sahaja yang sering digunakan. 41

1.4 Jenis Ukur Seismik Boleh diklasifikasikan kepada 2 iaitu i) ukur seismic 2D ii) ukur seismic 3D i) Ukur Seismik 2D

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When exploring a new area where little is known of the subsurface geology, a 2D survey is usually performed. This consists of survey lines spaced one, two, five or more kilometres apart.

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- Cuma satu streamer sahaja digunakan dan melibatkan kawasan liputan yang luas - data tegak dan mendatar sahaja (2D) diperolehi iaitu hanya menggambarkan keratan rentas - kos yang lebih rendah berbanding ukur seismik 3D - seismik 2D memberi imej yang kasar (coarse)sahaja untuk meramal kawasan yang mungkin ujud minyak/gas. - kapal yang digunakan lebih kecil dengan streamer panjang 8-12 km digunakan - sela garis pengukuran lebih jauh berbanding 3D seismik.

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ii) Ukur seismic 3D 3D teknik bukan sahaja menggambarkan struktur bawah tanah, tetapi dapat memberi maklumat mengenai cecair yang berada didalamnya. cerapan berulangkali pada masa berbeza dapat mengesan perubahan cecair, contohnya kepada minyak, gas, air, pergerakan cecair dan tekanan.

Oleh itu imej berbeza masa (3D/4D) boleh diguna mengesan perubahan cecair, terutama telaga minyak, berguna bagi menentukan isipadu, masa yang di perlukan bagi mengambil hasil minyak, gas dan lain-lain langkah keselamatan.

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Pengukuran seismik 3D diperkenalkan dalam tahun 1993. Biasanya sumber tenaga seismik berbentuk ‘multiple arrays’ dan juga juga multiple hydrophone streamer dengan panjang beberapa kilometer bagi membentuk lebar liputan sehingga 1 km. Ukur 3D biasanya dijalankan bagi penentuan kawasan minyak atau gas. 46

- merangkumi kawasan yang kecil, biasanya didalam kawasan seismic 2D yang telah dijalankan - intrepretasi data yang lebih baik dengan data cerapan dikeseluruhan kawasan pengukuran - 3D seismic mengguna banyak streamer yang ditarik selari antara satu sama lain. - data yang diperolehi adalah dalam bentuk 3 dimensi - kos pengukuran adalah lebih mahal berbanding 2D. Walau pun begitu dapat mengurangkan resiko kegagalan terhadap projek yang dirancangkan

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Punca tenaga terdiri daripada beberapa array dengan 6-9 airgun setiap array. Lebih dari satu kapal mungkin terlibat bagi menghasilkan tenaga seismic dalam bentuk array tersebut. Array biasanya berada dalam keadaan selari antara satu sama lain pada jarak 50-200 m dibelakang kapal. Dibelakang punca tenaga seismik adalah beberapa streamer (4-12 unit) pada jarak 100-200 m dari punca tenaga. Setiap streamer panjangnya 3-8 km tersebar dalam kawasan 400-900 m. Beberapa kapal punca tenaga (compressors) mungkin diperlukan jika 1 kapal tidak berupaya untuk recharging airgun arrays dalam masa yang diperlukan untuk sela letupan pengukuran. Kapal akan bergerak beriringan dan menghasilkan letupan secara selang-seli (sela 10-15 saat).

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Pengukuran 3D dilakukan dalam bentuk ‘racetrack’, supaya garisan bersebelahan diukur dalam arah yang sama dan mengurangkan masa untuk berpusing kepada garisan jika bersebelahan, oleh itu kerja lebih efisien. Ukur 3D memerlukan komputer berkuasa tinggi untuk memproses isipadu data yang besar. 3D seismik semakin popular dan digunakan dalam hampir keseluruhan exsplorasi mintak/gas, dari peringkat awal, mengenalpasti tempat penggerudian hinggalah mengawasi isipadu minyak dari masa kesemasa.

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1.5 Seismic Image Quality There are a number of factors that affect the success of a seismic survey. These may be grouped into 3 classes i) external ii) vessel iii) instrument limitations i) External Limitations These are those that cannot be controlled. The major is weather: rough seas not only create discomfort and difficult conditions aboard ship, but cause an increase in background noise created by breaking waves. This may reduce the quality of the records. ii) Vessel Limitations These can be generally overcome by evaluating the survey requirements and by throughly checking the vessel’s capabilities before the survey. In some cases, there may be electrical or mechanical interference between the ship’s equipment and seismic instruments. 66

iii) Instrument Limitations These may vary with individual systems, but in general the following factors are inherent in all seismic devices: - the seismic records show layer thickness as a function of time, thus the true thickness may only be determined if the speed of sound is known. Accurate velocity data are seldom available, therefore layer thickness must be considered approximate

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