Diffraction stacking with stacking velocity analysis - Its application to a surface seismic survey in an active fault area

A new type of prestack time migration method is presented. This method is a kind of prestack Kirchhoff time migration. One of the biggest advantages of this method is that data processing can be performed without a priori information about the velocity structure. The optimum constant stacking velocity can be determined at each image point from a stacking velocity analysis based on primary diffraction patterns. This proposed data processing method does not require multiples iterations for achieving the velocity structure in the case of prestack migration.

Both the conventional CDP stacking method and the conventional stacking velocity analysis are modified as follows.

1. In the case of the CDP stacking method, the amplitudes of observed data are stacked along the reflection pattern in a CDP gather. On the other hand, in the case of diffraction stacking method with stacking velocity analysis, the amplitudes of observed data are stacked along the diffraction pattern in a common source gather or common receiver gather.
2. Stacking velocity analysis can be a useful tool for detecting characteristic patterns. Conventional stacking velocity analysis is modified to detect patterns caused by diffraction events. Final stacked records can be obtained based on results of stacking velocity analysis.

The proposed data processing procedure was applied to surface seismic field data obtained in an active fault area and it was found that the active fault was imaged more adequately by application of diffraction stacking than CDP stacking. The reason was discussed from the viewpoint of S/N ratio. Our numerical experiments revealed that diffraction stacking generally has the ability to produce a seismic reflection image with higher S/N ratio than CDP stacking even for a flat and horizontal reflector. This can be explained by Huygens’ principle. And also, stacking coverage, defined as an angle range of collecting seismic traces, is an important factor in obtaining the highest S/N ratio. This can be explained in terms of Fresnel zones.