Deniz sismiğinde kaynak dalgacığının biçimlendirilmesi

Abstract

Denizdeki çalışmalarda en çok kullarımı yüzdesine sahip kaynak hava tabancasıdır. Hava tabancası kullanım aşamasında mühendislik kısıtlamalarına bağlı olarak gemiden oldukça uzaklara yerleştirilemez. Bu sebeple deniz çalışmaları asımda geniş çaplı bir çalışma imkanı sağlayamamaktadır. Diğer bir taraftan, hava tabancasının kontrolü gemi üzerinden oldukça rahat sağlanabilmektedir. Bu tabancaların bir dezavantajı, oldukça uzun bir kabarcık etkisi (bubble effects) bırakmasıdır. Bu türlü bir soruna karşı da GI (jeneratör-enjektör) tabancası kullanılır. Bu iki bölme farklı zamanlarda ateşlenerek kabarcık etkisinin giderilmesine çalışılır. Ayrıca hava tabancası kontrolünün etkin olduğu parametreler üzerinde durulmaktadır. Bu konumda tabanca basıncı, hacmi, derinliği ve gecikme zamanı oldukça önemlidir. Tabanca hacmi ve basıncı arttıkça gecikme zamanı da artmaktadır. Tabanca derinliği arttırıldıkça da gecikme zamanı azalmaktadır. Hava tabancası dizilim biçimleri, amaca göre, tek tabanca, birleşik tabanca ve tabanca düzeni şeklinde oluşturulur. Tabanca düzenleri birden fazla tabancanın belirli bir geometri ile dizilimiyle oluşturulur. Tabanca kabarcık sinyalini bastırmada oldukça etkili bir yoldur. Zira düzen içindeki her bir tabancanın faz farklılıkları sebebiyle kabarcık sinyali zayıflatılmaktadır. Tabanca düzen sinyali, düzendeki tabanca sayışma, tabancaların uzaysal konumlarına, hacimlerine, ateşleme zamanlarına, başlangıç basınçlarına, hava boşaltma odası alanlarına, düzen geometrisine ve çalışma doğrultusu gibi faktörlere bağlıdır. Kabarcık etkisi kaynak dalga biçiminin belirlenmesinde oldukça önemlidir. Yapılan çalışmalara göre, suda salman bir hava kabarcığının, kendi denge yarıçapının 300 katı bir dalga boyu yaydığı söylenebilir. Hava tabancası çalışmalarında en çok kullanılan kriterler, birincil sinyalin uzunluğu ve genliği ile birincil-kabarcık genlik oranıdır. Başlangıç sinyalinin genliği kabarcık sinyalinin genliğinden daha büyüktür. Bununla beraber, kabarcık sinyalinde içerilen enerji başlangıç sinyalinde içerilen enerjiden daha fazladır. Peryot, tabanca derinliğinin 5/6 kuvvetiyle ters, oda basıncının ve oda hacminin 1/3 kuvvetiyle doğru orantılıdır. Genlik ise, oda basıncının 3/4 kuvvetiyle, oda hacminin de 1/3 kuvvetiyle doğru orantılıdır. Hava tabancası derinlik değişim aralığı çok fazla olmadığından dolayı genlikle olan ilişkisi bağımsız kabul edilebilinir. Tabanca hacmi ve basıncı azaldıkça, sinyal zamanda kısalır, spektrum yüksek frekanslarca zenginleşir, dolayısıyla temel peryot küçülür ve genlikte de azalma gözlenir. Hacim ve basınç arttıkça, spektrumda düşük frekanslara doğru bir kayma olur, dolayısıyla temel peryot artar ve genlikte de artma görülür. Tabanca derinliği azaldıkça, sinyalin boyu zamanda uzar, spektrum düşük frekanslara doğru kayar, dolayısıyla temel peryot büyür. Genlikte de küçük artışlar izlenir. Tabanca derinliği arttıkça, spektrum yüksek frekanslarca zenginleşir, dolayısıyla temel peryot küçülür. Genlikte de küçük azalmalar izlenir.

The objective of this study is the generation and shaping of the source wavelet in marine seismics. In order to accomplish this task, the validity of the effective parameters is investigated. These parameters are airgun volume, pressure and firing depth. In addition, the study provides a basis for suppressing noises that originate from the out of profile plane. For this reason, the receiver system, the airgun source and its related parameters are reviewed. Then, the design of airgun arrays and airgun source wavelets are examined. Next the travel time equations for ghost reflections are derived. Finally, the control and shaping of the airgun source wavelet are discussed. In Chapter I, marine seismic is compared with the land seismic in various aspects. The scope is restricted in marine seismic. In another step, shallow and deep marine seismic surveys, and shallow and deep marine seismic energy sources are compared. Marine seismic reflection profiles have two significant advantages over similar land profiles. They are economical and in general they give better quality data. The second and scientifically more significant advantage of marine profiling is in the quality of the final sections produced. There are a number of contributing factors, the most important of which is the increased signal to noise ratio obtainable with marine data. A repeatable, high energy, well coupled seismic source gives strong signals and the ambient environmental noise can be very low at sea. There are several disadvantages of working at sea. The most common accusation made against marine profiling is that the surface geology is not well known for marine areas. A second and more important disadvantage of marine acquisition is that many geologically interesting features occur on land far from the coast. These are clearly not open to direct investigation by marine techniques. In Chapter II, the marine seismic energy sources, namely airguns and detectors are examined in more detail. 'Airgun' a type of sources are especially examined. Airguns have a high percent applicability in the marine surveys since the 70's. Land source-receiver array designs, by their very nature, easily allow any source-receiver array patterns. Conversely, marine source-receiver array designs are somewhat limited in comparison with the land seismic data acquisition. In land surveys, the choices on geophone spacing is very flexible. In marine surveys, however, each hydrophone unit has to reside in their fixed positions as they were initially planted within the streamer during their construction at the factory. Due to the engineering restrictions, the airguns cannot be placed too much farther away from the vessel. Therefore, marine surveys do not provide as large spatial distribution flexibility of the marine seismic sources and receivers as their counterpart on land. On the other hand, when explosive seismic sources are considered, control mechanism over the airguns is more developed than the land-site dynamite applications. Airgun is a device which discharges air under very high pressure into the water. Pressures up to 10000 psi are used although 2000 psi is most common. A very important matter of seismic sources xi would be neglected. Thus, it can be concluded that the amplitude is independent of airgun depth. Some properties of the amplitude spectrum of the signals are: - As the volume and pressure decrease, the main period gets shorter due to the high frequency content of the spectrum with a following decrease in amplitude, resulting in high resolution seismic sections. Although the resolution of the shallow seismic data is high, the penetration depth is low. - As the volume and pressure increase, the main period gets longer due to the lack of high frequencies resulting in larger amplitudes. - As the gun depth increases, the main period gets shorter due to the high frequency content of the spectrum. The signal amplitudes slowly decrease. - As the gun depth decreases, the main period gets longer due to the low frequencies. The signal amplitudes slowly increase. xv is the difficulty of producing a desirable seismic source pulse. Once the pulse emitted by an airgun, it is followed by one or more bubble oscillations after the initial impulse. Much of the energy remains to propagate successive bubble pulses. The actual reason for it is the prevention of bubble effects occuring in the water. An example to those kind of airguns is the so-called GI (Generator/Injector) guns. The GI guns possess two separate pressure chambers: G and I, which are individually controled by the operator as far as the time-breaks (shooting times) are concerned. The technical information is provided from the MTA Sismik- 1 research vessel. Each one of those chambers has a volume capacity of 45, 75, 105 in3. The change in volume is arranged by volume reducers. The operation pressure of the airgun changes between 1000- 3000 psi. There are two major disadvantages of the airgun system when used as a seismic source. These are its low efficiency and its fairly long pressure bubble pulse. The latter is especially disadvantageous in high resolution seismic surveys. There are two methods currently used in marine prospecting for overcoming this disadvantage. One method, known as the wave shape kit (WSK) technique, involves the dissipation into heat of the seismic energy which would have been radiated as a bubble pulse. Another method, known as the array technique, is based on the use of an array of variable sized airguns placed at the same depth and fired simultaneously. Knowledge of airgun sources can be obtained in two ways: by field experiments and theoretical studies. Both approaches have advantages and disadvantages. Field experiments - including the manufacturing and testing of new gun designs, as well as far-field signature test mesurements- can provide definitive answers, if done carefully, but are time-consuming and expensive. Theoretical calculations -based on computer models of airguns- are relatively easy and inexpensive to perform, but their validity depends on the adequacy of the model, something that can be difficult to assess. The distance between the airguns is very important. If this distance is more than 2 meter, then the signatures of individual airguns don't effect each other. Minimum depth of the airgun should be 0.6 meter so that some of the energy doesn't escape from the water surface to the out. The parameters which effects the control of the airgun is also very important. For example; gun pressure, gun volume, gun depth and delay time. Delay time is associated with the firing time. - If the gun pressure and volume increase, -because the signal gets longer in time (period gets longer)- the delay time also increases. - If the gun depth increases, -because the static pressure also increases, so the signal in time gets shorter- the delay time decreases. The receiver system consists of hydrophones which are mounted in a long streamer. The hydrophones are placed inside a neoprene tube which is filled sufficient lighter than water liquid to make the streamer neutrally buoyant. Thus, the streamer would get stay on the sea surface. Besides, the depth of the streamer can be controlled easily with the computer on the vessel. One channel is equivalent to the one hydrophone group. Aboard R/V MTA Sismik- 1, one hydrophone group contains 8 hydrophone units. The distance between each hydrophone unit is 78 cm. The length of a hydrophone group is 6.25 m. One module consist of 12 hydrophone groups. The length of one module is 75 m. The number of modules is dependent on the streamer length and the number of channels. For instance, MTA Sismik- 1 vessel has a 480 channel seismic recorder and its streamer length is 1500 m. In Chapter HI, the airgun array design and wavelet are discussed. This design is arranged in three forms: Single gun, coalesced guns and array of guns. For single xu is the difficulty of producing a desirable seismic source pulse. Once the pulse emitted by an airgun, it is followed by one or more bubble oscillations after the initial impulse. Much of the energy remains to propagate successive bubble pulses. The actual reason for it is the prevention of bubble effects occuring in the water. An example to those kind of airguns is the so-called GI (Generator/Injector) guns. The GI guns possess two separate pressure chambers: G and I, which are individually controled by the operator as far as the time-breaks (shooting times) are concerned. The technical information is provided from the MTA Sismik- 1 research vessel. Each one of those chambers has a volume capacity of 45, 75, 105 in3. The change in volume is arranged by volume reducers. The operation pressure of the airgun changes between 1000- 3000 psi. There are two major disadvantages of the airgun system when used as a seismic source. These are its low efficiency and its fairly long pressure bubble pulse. The latter is especially disadvantageous in high resolution seismic surveys. There are two methods currently used in marine prospecting for overcoming this disadvantage. One method, known as the wave shape kit (WSK) technique, involves the dissipation into heat of the seismic energy which would have been radiated as a bubble pulse. Another method, known as the array technique, is based on the use of an array of variable sized airguns placed at the same depth and fired simultaneously. Knowledge of airgun sources can be obtained in two ways: by field experiments and theoretical studies. Both approaches have advantages and disadvantages. Field experiments - including the manufacturing and testing of new gun designs, as well as far-field signature test mesurements- can provide definitive answers, if done carefully, but are time-consuming and expensive. Theoretical calculations -based on computer models of airguns- are relatively easy and inexpensive to perform, but their validity depends on the adequacy of the model, something that can be difficult to assess. The distance between the airguns is very important. If this distance is more than 2 meter, then the signatures of individual airguns don't effect each other. Minimum depth of the airgun should be 0.6 meter so that some of the energy doesn't escape from the water surface to the out. The parameters which effects the control of the airgun is also very important. For example; gun pressure, gun volume, gun depth and delay time. Delay time is associated with the firing time. - If the gun pressure and volume increase, -because the signal gets longer in time (period gets longer)- the delay time also increases. - If the gun depth increases, -because the static pressure also increases, so the signal in time gets shorter- the delay time decreases. The receiver system consists of hydrophones which are mounted in a long streamer. The hydrophones are placed inside a neoprene tube which is filled sufficient lighter than water liquid to make the streamer neutrally buoyant. Thus, the streamer would get stay on the sea surface. Besides, the depth of the streamer can be controlled easily with the computer on the vessel. One channel is equivalent to the one hydrophone group. Aboard R/V MTA Sismik- 1, one hydrophone group contains 8 hydrophone units. The distance between each hydrophone unit is 78 cm. The length of a hydrophone group is 6.25 m. One module consist of 12 hydrophone groups. The length of one module is 75 m. The number of modules is dependent on the streamer length and the number of channels. For instance, MTA Sismik- 1 vessel has a 480 channel seismic recorder and its streamer length is 1500 m. In Chapter HI, the airgun array design and wavelet are discussed. This design is arranged in three forms: Single gun, coalesced guns and array of guns. For single xu would be neglected. Thus, it can be concluded that the amplitude is independent of airgun depth. Some properties of the amplitude spectrum of the signals are: - As the volume and pressure decrease, the main period gets shorter due to the high frequency content of the spectrum with a following decrease in amplitude, resulting in high resolution seismic sections. Although the resolution of the shallow seismic data is high, the penetration depth is low. - As the volume and pressure increase, the main period gets longer due to the lack of high frequencies resulting in larger amplitudes. - As the gun depth increases, the main period gets shorter due to the high frequency content of the spectrum. The signal amplitudes slowly decrease. - As the gun depth decreases, the main period gets longer due to the low frequencies. The signal amplitudes slowly increase.