The Study On Free Surface-related Multiple Attenuation Of Seismic Data In Shallow Sea

Multiple attenuation of the seismic data which is collected from the shallow seahas always been a tough problem in aspect of multiple wave suppression. In the caseof shallow water, free surface-related multiple and reverberation generated by shallowinterface and water bottom are a kind of short period multiple. These multiplesinterfere with the primary events reflected from the shallow interface mutually.Because of the shallow depth, the incident wave from the source reaches the criticalangle in a near offset in terms of the water bottom. Refractions can be seen clearly inthe near offset on the seismic shot gather. And the most of the incident wave energy ispreserved in the water layer, and then produces the multiples with multi ordersfollowing the refraction. In addition, the moveout between direct wave and theprimary reflection from the seabed is small. Although theoretically these two kinds ofevents do not cross over with each other. But In fact, the waveform already interferedin the near offset, and to the far offset they are almost completely overlapped. And interms of the marine seismic acquisition, minimum offset is often more than100meters, so on the collected data of the near offset the primary reflected wave from thesea bottom is already unable to identify. Although for the oil and gas exploration theseabed is not the layer of interest, but it is a reflection interface which has a bigreflection coefficient and is able to produce multi orders multiples. If the primaryfrom the bottom cannot be obtained from seismic records, both for the multiplesuppression technology based on time difference and the multiples attenuation methodbased on wave equation the multiples attenuation effect are restrained directly orindirectly and what is more the deep layer of interest can not be estimated accurately.Therefore, in this paper, based on the analysis of conventional free surface-relatedmultiple attenuation methods, it is ascertained that the main cause which subjected tothe water depth is the failure of reconstruction of desired near offset data andestablishment of accurate multiple model from the original data. And so the perfecteffection of multiple suppression can not be obtained. Therefore this ariticle study akind of better near offset data reconstruction method, forming a set of technology ofthe shallow water multiple suppression which is economical and practical.In the first place, this paper discusses the characteristic of free surface-relatedmultiple of shallow water seismic data. And then it discussed the key problem and improvement measures in the free surface-related multiple elimination (SRME) whichis proposed by Verschuur. To apply SRME method in shallow water seismic data, thekey is to reconstruct near offset data which contains the full wave field information.Then the paper aimed at tackling the most important problem of shallow seismic data:near offset data missing. It improved a near offset data reconstruction method basedon the theory of seismic interferometry to obtain the missing data, and discusses themain advantages and disadvantages. At the end it proposed the technique of shallowwater multiple depression (SWMD). This technique taking SRME idea canreconstruct the desired near offset data to predict the free surface-related multiple.And further it is combined with Radon domain multiple attenuation method. Thiscombination’s effectiveness is tested by using some simulated data and field data, andthe desired results are achieved.The shallow water multiple depression technique (SWMD) used in this paper hassome advantages as follows:(1) The multiple suppression method of free surface inSWMD is completely data driven, and the signal’s amplitude can be preserved well. Itis a kind of multiple attenuation method which does not need any prior informationand mathematical assumptions. But it requests a high level of data integrity. While thenear offset data reconstruction method based on seismic interference theory is alsocompletely data driven. So that, it’s more convenient to unify process thought andflows in the whole procedure of multiples attenuation processing.(2) According to thebasic principle of the data reconstruction methods, just considered from the dataintegrity after reconstruction, the more shallow the water depth is, the moreinformation from the deep water the reconstruction tract contains. So from this pointthe reconstruction method is more suitable for the shallow sea seismic data.(3)Because the short period residual multiples in the far offset distance has a poorattenuation effect and the more obviously periodic feature in the linear Radon domain,predictive deconvolution method in Radon domain taken to attenuation residual has abetter effect than in time domain.