Study On Surface-related Multiples Attenuation Of OBC Data In Shallow Water

In seismic prospecting especially on the sea, it is the multiples that cause theSNR of seismic data to reduce greatly, disturb the identifiability of primary reflection,bring enormous difficulty on velocity analysis and excursion before and after stacking,and then affect the seismic imaging authenticity and reliability, sometimes may befalse. Multiples may seriously influence seismic interpretation work also. So it is veryimportant and necessary for multiples suppression. The research of this dissertation isfunded by the National Natural Science Foundation of China（No.41176077and No.41230318）,aimed to study the surface-related multiples suppression of OBC data inShallow Water.Ocean bottom cable technology was introduced into China in the late nineties.OBC is the main technique to obtain high-quality seismic data in the shallow waterand obstacle area（such as offshore drilling platforms, etc.）,and it is more efficientthan traditional seismic towed cable in such areas. Recently with the development ofChina’s offshore oil and gas exploration, OBC data processing technique has beenacquired more and more attention. However, due to the influence of the free surface,submarine and other strong reflection interfaces, reverberation in OBC data is verysevere, and often couples with the effective wave, reducing the quality of the data.Especially in paralic zone, this phenomenon is much more serious, which is the mainobstacle to obtain the high-quality seismic profile.At present, which make use of different characteristics of multiples, methods thatsuppress multiples can be classified into two broad categories：One is based on thedifferences of separability, moveout and multiples periodicity between primary andmultiples. The second one is mainly based on the elastic wave equation theory, whichpredicts, matches it with the original data and then subtracts multiples from the inputdata. This method, which can be called fluctuation predicted subtraction method （mainly include SRME）,has developed in the1990s. It is the main developmenttrend of foreign research of multiples suppression. However, the application of thismethod is mainly concentrated on seismic towed cable in deep water, it is not ideal todeal with seismic data in shallow water. The dual-sensor summation method in OBCacquisition process is the main technique to suppress multiples of OBC data，and isutilized more widely than SRME.Utilizing the differences of polarity, amplitude and frequency between pressuredetector（hydrophone）and velocity detector（geophone）, pressure detector data andvelocity detector data are added with optimum ratio, so as to suppressing ghost andreverberation. This method, which is called OBC dual-sensor summation processingtechnology, is simple without too many assumptions, can be used widely. Based onin-depth research of dual-sensor seismic wave propagating mechanism and matchingfiltering techniques, this paper proposes a new dual-sensor matching summationmethod to suppress reverberation. Compared with traditional matching summationmethod, the dual-sensor matching summation method bases on equipoisepseudo-multi-channel adaptive matched filtering method without calculatingreflection coefficient. By comparing the synthetic data processed by dual-sensormatching summation method and traditional dual-sensor summation method, and theprocessing of actual data, the results have proved that the dual-sensor matchingsummation method is efficient.The essence of dual-sensor summation is to separate up-going and down-goingwave-field of hydrophone and geophone. Normally, the down-going wave-field whichthe major component is multiples is no value. However, according to the study of thepaper, the down-going wave-field separated by dual-sensor summation method canachieve multiples imaging by using the mirror migration technique. Compared withthe traditional primary wave imaging, the multiples imaging method has moreadvantages and practical application value. It can be used to obtain images of seafloorand expand the scope of the underground exploration. The efficient of this multiplesimaging method has been proved by synthetic data of models.In the dual-sensor data acquisition process, geophone cannot couple with seafloor efficiently by the influence of acquisition technique, seafloor topography andocean current. So the signal to noise ratio of geophone data is low, althoughhydrophone data have high SNR because there is no coupling problem, the accuracyof dual-sensor summation data will be reduced. Due to the limitations of currentacquisition condition, especially the low SNR of geophone data, a new method isproposed in the paper to avoid the above phenomenon. In order to remove theinfluence of seawater，the method just processes pressure component（P component）of dual-sensor data by using of conventional migration and mirror migrationtechniques，then utilizes joint deconvolution method to deal with the two kinds ofmigration results. Compared with dual-sensor summation method which suppressesghost before imaging, joint deconvolution method suppresses ghost after imaging, andremoves surface-related multiples of OBC data finally. The paper has proved theefficient of the method with the synthetic data calculated by various models.All the synthetic data and actual processing data show that all the methodsproposed in the paper can suppress multiples more efficiently.