Resolution of Crosshole Shear-Wave Testing

This research project investigates the ability of crosshole shear-wave testing to resolve low-velocity layers of varying thickness. The resolution of crosshole shear-wave testing, as currently implemented by Reclamation, is investigated by generating and analyzing synthetic waveform data for seven shear-wave velocity models. The models include velocity layers of varying thickness and velocity contrast. Three experienced geophysicists from the Bureau of Reclamation, U. S. Geological Survey, and U. S. Army Corps of Engineers independently estimated the shear-wave arrival times on the synthetic waveforms. Velocity-depth profiles were computed from the arrival times using Reclamation’s standard crosshole processing procedures.  The thin-bed resolution limit of the crosshole shear-wave method is controlled mainly by the frequency content of the propagated seismic energy. Tests conducted during this investigation indicate that incorporation of ray bending into the data processing procedures has little effect on the resolution of velocities within layers less than two feet thick. The most effective way to improve the resolution of Reclamation’s crosshole shear-wave surveys would be to use a higher-frequency borehole shear-wave source.  These tests also demonstrate that computed crosshole velocities are smeared at layer interfaces, with velocities being overestimated in the lower-velocity layer near an interface and velocities underestimated in the adjacent higher-velocity layer. The smearing at layer interfaces is caused by refraction of seismic energy and the finite frequency range of the propagated seismic energy. Because of this smearing near layer interfaces, the most accurate crosshole velocities are those computed near the centers of velocity layers. The exact depths of layer interfaces are better determined from geologic drill logs and geophysical borehole logs rather than from crosshole velocity profiles.