Tomographic Inversion Of Near-surface Absorption Structure And Its Application Software Development

In the process of seismic wave propagation in stratum,the attenuation of seismic wave energy is caused by the viscoelastic property of stratum.This part of the attenuated energy is converted into heat energy,and the quality factor Q is usually used to describe the absorption attenuation of seismic wave by absorbing medium.Especially for near surface strata,because of shallow burial and weak compaction,seismic wave energy has a strong absorption effect,which seriously affects the resolution of seismic data.It is of great practical significance to obtain accurate Q value and establish near-surface absorption structure model for Q value compensation and improving the resolution of seismic records.This paper combines the characteristics of near-surface exploration.Combining spectral ratio method and centroid frequency shift method with improved microlog observation system,a method of attenuation tomography inversion based on ray tracing is proposed.Attenuation tomography is established on the basis of velocity tomography,which is mainly divided into three modules,the calculation of ray path,the calculation of attenuation travel time and the solution of inversion equation,respectively.Because the single well location of microlog covers a small scale and the lateral variation can be neglected,we assume that the medium is horizontally stratified.In ray path tracing,the dichotomy method can obtain high accuracy,and the method is simple to calculate.In the calculation of attenuation travel time,direct wave is picked up according to the characteristics of direct wave,and the interference of noise is removed as far as possible.The attenuation travel time is calculated within the effective frequency band to obtain credible attenuation travel time.In the selection of inversion methods,more accurate inversion results can be obtained by using algebraic reconstruction method,joint iterative reconstruction technology and minimum damping binary method.The inversion software of near-surface absorption structure tomography is designed and developed.The software first loads micro-log seismic data and corresponding observation system information.After pretreatment of the data,users can select direct wave travel time tomography,conventional spectral ratio method to calculate Q value,centroid frequency shift method to calculate Q value,spectral ratio method to attenuate tomography inversion,centroid frequency shift method to invert data processing.The software’s first break time picking,window selection of first break wave and spectrum analysis improve the efficiency of the processing process,and its interactive processing mode is more conducive to operators to extract more accurate data based on experience.In conclusion,the development of inversion software for near-surface absorption structure tomography provides a useful tool for the investigation of near-surface velocity and absorption structure.Finally,the software is used to process the actual microlog seismic data in Dagang Oilfield.A total of 232 microlog data were processed in Cangxian uplift,Cangdong sag and Dongguang-Xuhei buried hill.Three-dimensional models of near-surface velocity and Q value were obtained,which provided credible data for near-surface absorption structure investigation in these areas.