IMAGING OF NEAR SURFACE VELOCITY HETEROGENEITIES OF THE MEDIUM IN WAVE PATTERN OF ACOUSTIC MODELING

Abstract

Shallow refraction seismics has long been used for the determination of the near surface layer structure. The most common goal of this investigation in seismic prospecting for gas and oil was the definition of the static corrections . The model of near surface layer was the result of refraction interpretation allowing to derive estimates of the thicknesses and velocities of the near-surface layers by analyzing the first breaks of head waves. Several methods have been proposed for the interpretation of refraction data . However, all these methods have certain drawbacks restricting their range of applications. Nowadays the CDP method (Common Depth Point method) is the dominating method of surface acquisition. This information may be used to estimate static corrections. Additionally we can realize special velocity surveys in deeper shot holes and use their results (traveltimes) to derive velocity model of LVL ( Low Velocity Layer ). Their is no such a possibility in the case of Vibroseis method. In this case we must practically obtain all the information about LVL only from reflection field records. This information is mainly inherent in first breaks of refraction arrivals. In the presented paper the results of imaging near surface velocity heterogeneities of the medium in the wave pattern of finite difference acoustic modeling have been described. The main goal of this modeling was to define the effect of near surface layer heterogeneities on the breaks of head waves connected with shallow refractors. The effect of heterogeneities dimensions, velocity distributions in the near surface layer as well as the seismic signal parameters on the wave pattern of head waves breaks of reflection records has been estimated. The analysis has been undertaken to assess the possibility of applications these breaks for recovering near surface velocity distributions by means of head wave tomography. The seismic modeling has been performed using computer program of 2-D finite difference modeling available in the processing system ProMAX.