More robust seismic interpretation can be done when an interpretation project uses both compressional (P) and shear (S) data rather than using only one seismic mode, whether that mode be a P mode or an S mode. Unfortunately, this fundamental interpretation principle is frustrated by the cost and difficulty of deploying S-wave sources and by the limited availability of direct-S sources. We introduce a new seismic interpretation option based on direct-P and direct-S modes generated by vertical-force sources. To explain the potential of this new method for acquiring direct-S data, we evaluate real-data examples that illustrate the physics of P and S body-wave radiations generated at vertical-force-source stations. First, a 3D model of direct-S radiation by a vertical-force source is tested. Next, we discuss a field experiment in which a horizontal vibrator create a series of radially oriented SV displacements at small azimuth increments to simulate the full-azimuth distribution of SV displacements created by a vertical vibrator. The resulting data are recorded by a VSP seismic array and show that for a far-field sensor, some source-generated SV displacements are received as a radial-S wavefield and other SV displacements are received as a transverse-S wavefield. We use data from a walkaround VSP to create map views of direct-P and direct-S radiations from a vertical vibrator. We then use data from a walkaway VSP to illustrate cross-section views of the illumination lobes of direct-P and direct-S propagating into the subsurface from a vertical-vibrator source station.
- Received June 25, 2013.
- Accepted August 11, 2013.