Vertical Seismic Shear-wave Profiling

Applications

Method

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, propagate through the subsurface and are recorded by 3 component geophone 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.

A sledgehammer is typically used to generate the seismic energy needed for near surface VSSP investigation with P-wave energy generated by impacting a metal plate placed next the borehole and S-wave energy by impacting a weighted wooden plank at a horizontal angle. The borehole receiver is composed of three geophones oriented in an x-y-z pattern that record three data channels and is moved up the borehole at regular intervals and secured for each shot. By measuring the direct surface to borehole receiver travel time at various depths

Data quality and results from VSSP are generally higher resolution than surface seismic methods and can be used to correlate surface seismic data in order to provide an improved subsurface model than from surface seismic methods alone.

Data Analysis & Presentation

P-wave and S-wave velocities are calculated from the picked travel times and from the known source to receiver distance at regular depth intervals. The calculated P-wave and S-wave velocities are used to generate Poisson's ratio and with additional density information a number of geotechnical parameters including the shear modulus, bulk modulus and Young's modulus can also be calculated.

The results of the VSSP method are provided in table and graphical format giving the calculated parameters including the P-wave and S-wave seismic velocity, and dynamic modulus values at depth increments for each VSSP test site. The results can also be correlated with the geological logs for each borehole.