Generating direct-S modes with simple, low-cost, widely available seismic sources

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-Smodes 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.