Near Surface Traveltime Tomography And Seismic Wavefield Migration

Near surface imaging and inversion play an important role in the seismic exploration.The complexity and uncertainty of near surface structure are challenging for the shallow seismic exploration.Meanwhile,the accuracy of near surface imaging shall directly determine the earth’s interior.The quality of seismic data is significant in seismic processing and imaging.Meanwhile,the seismic trace editing is a tedious task in the data preprocessing.It may require a great deal of manpower and time cost,especially for a large 3D dataset.In this thesis,we propose a method based on machine learning to conduct the process automatically.This study also combines the Hough transformation to improve feasibility in processing the real data.Real data test demonstrates that our method is promising in automated trace editing.Seismic first arrivals offer unique near surface information.The first-arrival traveltime tomography is a standard and effective imaging method in seismic data processing.By applying the traveltime tomography,we can obtain the near-surface velocity structures and the long-wavelength statics.However,the traditional traveltime tomography is difficult to invert the low velocity structures or high velocity contrasts accurately.The inversion procedure is also affected by the different magnitude of the traveltime data.In this thesis,we propose improvements for the first-arrival traveltime tomography.Instead of inverting slowness,this method inverts the natural logarithm value of the slowness along with continuity condition,by which this method can balance the magnitude of the inverted data and generate a higher-resolution velocity image.We apply the method to both synthetics and real data,and the results are much better than the conventional traveltime tomography.With the rapid development of geophysics,the seismic interferometry has become one of the popular researches in the world.Seismic interferometry is widely used in data reconstructions.By using the seismic interferometric method to reconstruct the wavefield,the positions of shot points and receivers can be transformed to a new position closer to the target of interest.However,due to the limitation of seismic interferometry technology,it is necessary to use a large number of seismic shots to ensure the accuracy of the reconstructed data.In this thesis,we combine the seismic interferometry with prestack depth migration technology and develop an improved seismic interferometry migration method.We also apply it to Freecable data and refraction data,respectively.Firstly,the seismic interferometric migration is applied to the refraction dataset.At present,refraction waves are mainly used for the near surface structural tomography.In this thesis,we propose an imaging method for both shallow and deep refraction waves,we name it as refraction wavefield migration method.The method involves preprocessing two opposite(reciprocal)shot sets and generating several new virtual reflection traces through interferometry.Using the velocity above the refractor to migrate these processed virtual traces is similar to migrate the critical reflection waves,which shall generate the refractor images.This method does not require picking the reciprocal time,and can be applied to image shallow near surface structures with a traditional survey geometry,as well as the imaging of deep subsurface with a long offset survey geometry.Finally,the seismic interferometry migration method is also applied to the Freecable data.The Freecable survey system is a new type of marine geometry,which is designed by the French geophysical company Kietta.Its cable receiver can maintain a horizontal state in the sea water,while the shot point is flexibly excited by the surface seismic source vessel.This approach correlates the direct wave and the reflected wave in the Freecable record,which indirectly extends the survey system to the seafloor and makes it closer to the seabed,thus improving the imaging resolution.At the same time,this study combines the seismic interferometric wavefield reconstruction with the prestack depth migration,which eliminates the limitation of shot numbers in traditional interferometry.Moreover,we can even generate the imaging for a single shot.Therefore,we develop a series of studies on the near surface traveltime tomography and the waveform migration.Hoping to make several contributions for the near surface geophysics exploration in the future.