The Study Of Reflection Fresnel Zone Travel-time Tomography

The accuracy of the surface velocity model is directly related to the quality of the seismic processing and interpretation.If the speed is not accurate,the effect caused to the imaging and interpretation cannot by using high precision processing methods to compensate for,so established surface velocity model accurately is a key step in seismic data processing.In this paper,we study travel time tomography using the information of seismic reflection wave to inverse velocity.Conventional ray theory based on the assumption of high frequency approximation,hold the opinion that seismic wave in the surface along a zero-width raypath,which does not exist in reality just imaging to describe the seismic wave kinematics process.The seismic wave travel time based on conventional ray theory is the slowness integral of the ray path from the source point to the receiving point.Meanwhile,the conventional ray distribute unevenly which is deviated from the low velocity zone,and gathered at the high velocity zone,so it affects the resolution and precision of the inversion results seriously.Since the seismic waves propagate along zero-width raypath,the conventional ray only cover a limited space in the media,which results in the sparseness of the tomography inversion matrix.The sparseness cause the tomography inversion matrix to be poorly conditioned and affect the imaging resolution.In view of the limitation of the conventional ray tomography,we study Fresnel zone travel time tomography using the information of seismic reflection wave.Compared to conventional ray,the Fresnel zone extend the coverage of velocity model.The reflection Fresnel zone not only contains the conventional ray,but also taking the contribution made by the points nearby the ray path into consideration,thus the reflection Fresnel can increase the scope of the ray coverage.So reflection Fresnel zone can improve the coverage of the conventional ray and reduce the sparseness of the tomography inversion matrix.With these modification of the ray path,the outcome is much more stable and accurate.The slowness perturbation of each point in the reflection Fresnel zone have different impact on the travel time perturbation,if the point is closer to the center ray,the impact is greater,and if it is closer to the reflection Fresnel zone boundary,the impact is less,even if it is out of the reflection Fresnel zone,there is no impact.To quantify the influence of each point in the media,we use the weight factor modifying kernel function,thereby giving a more realistic representation of the relationship between slowness perturbation and travel time perturbation.By the definition of the reflection Fresnel zone,Frequency is an important factor determining zone.Since the shape and width of the reflection Fresnel zone have no relationship with the width of gridding,frequency is an important factor determining resolution of the travel time tomograph.We first use screening method to accurately calculate the reflection Fresnel zone,and then using the reflection Fresnel zone travel-time tomography to inverse the velocity and Synthetic data tests show this method has a better inversion result.Finally,according to the advantages and disadvantages of using different frequency,during the iterative inversion process,we have studied the multi frequency inversion strategy.