Resolution Estimation of Reflections and Refractions in Cross-Hole Seismic Imaging by Fresnel Volume Modeling

The propagation and recording principles of seismic acoustic signals in the 3D space are considered with the case of cross-hole imaging near the boundary with abrupt changes in the elastic wave velocity. The 2D and 3D modeling of constructive interference has been carried out during a seismic survey to estimate the first Fresnel zone volume and the resolution of refracted and reflected waves in boreholes. The Fresnel volume of head waves is compressed along the boundary plane the greater, the further the oscillation source and receiver are from this boundary. The calculated Fresnel volume is given for various relative positions of sources and receivers. It is concluded that the projection of the Fresnel volume of head waves onto the plane of the high-velocity refractive layer can be obtained by drifting the source and receiver points along the normal to this layer. As a consequence, the boreholes are perpendicular to the refractive layer during cross-hole imaging, whereas the projection of the Fresnel volume of head waves onto this layer is almost independent of the source and receiver positions in the borehole. Quantitative estimates of the seismic resolution of the borehole seismoacoustic methods for studying the rock massif have been obtained.