Research On Reflection Seismic Data's Response And Related Processing Methods In Complicated Ocean Acoustic Environment

The complex ocean acoustic environment's influence has recently received much attention with the oil targets' expansion from shallow to deep water and the popularization of time-lapse seismic exploration.Many real data and numerical studies have shown that the undulating sea surface scatters the waves complexly and repeatedly,and the SOFAR's velocity changes the wave's direction and travels time.If we still treat the real ocean acoustic environment as an approximated flat-surface uniform medium from classic seismic exploration theory,it will introduce the approximate errors and influences in the following data processing,e.g.,migration,deghosting et al.In underwater acoustics and seismic oceanography,there are many studies about the above errors and influences.However,they have not been eliminated effectively in seismic exploration and remain the following problems.(1)Owing to different research targets,processing methods,and preconditions,it is difficult to directly apply the conclusions from underwater acoustics and seismic oceanography into seismic exploration.(2)In addition to the reflection and transmission,the seismic exploration often needs many complex wavefields,e.g.,multiple reflections,et al.,which lead to more complex impacts from the ocean acoustic environment.(3)Owing to the errors' time-varying and unpredictability,one cannot eliminate them empirically in seismic exploration,which is more cumbersome and has some limitations.A fundamental way to solve the above problems is to systematically study the influence of time-vary undulating sea surface,SOFAR's velocity distribution,and related treating methods,respectively,from the formation of seismic data.Thus,we propose a multistage algorithm that can calculate the multiple scatterings flexibly and accurately by improving and combining different numerical simulation algorithms.Then we simulate the multiply scattered waves in a stage-by-stage way under the ocean acoustic environment models.Overcoming the angular limitation of parabolic approximation,we used the multi-area multi-axis parabolic approximation technique to extrapolate the scattered waves at large azimuth accurately and test its effectiveness in representing the Green's function for seismic imaging.Based on the above methods and the simulated data,we studied the influence of the undulating sea-surface and sea-water velocity distribution(SOFAR)qualitatively and quantitatively.Based on the given influence of undulating sea surface,we used the shaping filter of deconvolution theory and designed a corresponding correction method,which can suppress the sea surface's undulating effect in seismic data without the sea surface shape information.Based on the given influence of SOFAR,we obtained a non-uniform water sound velocity profile through the FWI(Full Waveform Inversion).Then we established a velocity model(water body),including SOFAR,to improve deep targets' imaging accuracy.Through the above studies,we reported the following conclusions.(1)The multi-area multi-axis parabolic approximation can effectively overcome the angular limitation of classical parabolic approximation.It can calculate the multiply scattered waves(forward and back)accurately at large azimuth and image the underwater steeply inclined targets validly by representing the Green's function.Therefore it has good application potentials.(2)The multistage algorithm can extrapolate the multiply scattered waves flexibly,effectively,and stage-by-stage.Therefore,we can simulate the multiple scatterings in the complex ocean acoustic environment models.(3)The influence of undulating sea surface and SOFAR cannot be ignored under certain conditions.It mainly affects the seismic data by in the kinematic and dynamic characteristics of waves.(4)Without the sea surface morphology information,the designed method can effectively eliminate the influence of undulating sea surface on the seismic data and subsequent data processing.(5)Theoretically,one can use the FWI to obtain the velocity distribution of SOFAR and establish a more accurate velocity model for migration,thereby improving the imaging accuracy of deep-water targets.