Methodologies

VSSP involves the measurement of variations in seismic compressional waves (P-waves) and shear waves (S-waves) velocities with depth. Seismic P-waves and S-waves are generated at the surface, these propagate through the subsurface and are recorded by 3 components geophone (3W-R1) placed at various depths within a suitably prepared borehole. The data is processed and interpreted for the generation of P- and S- wave seismic velocities and when correlated with soil density can be used to calculate dynamic modulus values including Shear modulus, Young’s modulus and Poisson’s ratio.

Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.

VSSP is carried out by lowering a self-clamping tool that houses a 3-component geophone, into a suitable prepared borehole. Seismic P-waves are generated by hitting a sledgehammer on a metal plate near the borehole, and seismic S-waves are generated by horizontally hitting a wood plank from either side to obtain an opposite response. The process is repeated at every interval along the borehole as required by the investigation.

Sample of tabulated geotechnical parameters: Poisson’s Ratio, Young’s Modulus and Shear Modulus.