Shear and Compression Wave Measurements

The P- and S-wave velocity measurements are obtained by excavating exploratory borings and then installing a PVC casing, grouted into place with cement slurry. A downhole 3-component geophone is then placed in the borehole and clamped into place against the wall of the casing. P-waves are generated by using a sledgehammer and aluminum plate, which is placed on the ground surface at a measured distance (offset) from the borehole. The plate is then impacted several times to produce stacked P-wave arrivals. Horizontal S-waves are generated using a sledgehammer and weighted, wood beam, placed on the ground surface at a measured offset. The beam is then impacted on each end several times to produce stacked, reverse polarity horizontal S-waves.

Data are collected and stored using an EG&G Geometrics StrataView Exploration Seismograph. Measurements are typically conducted at 3 feet intervals throughout the depth of the borehole. First arrivals for the P- and S-waves are selected from the data and average P- and S-wave vertical velocities, and P- and S-wave interval velocities are calculated. From this data, values for Poisson’s ratio can be determined for each interval.