A scheme to process passive surface waves is briefly described. It transforms wavefields of horizontal plane-wave propgation, measured with receiver spreads along the two orthogonal axes (x and y), into dispersion images. The scheme first transforms the measured wavefields of a particular frequency (ω) into the energy in phase velocity (c)-azimuth (θ) space where multiple sources and modes of surface waves are represented as energy peaks at different azimuths and phase velocities, respectively. The scheme, then, stacks all the energy through the azimuth axis to produce an energy distribution along the phase-velocity (c) axis only. A final image is then created by repeating these steps for different frequencies and displaying the energy in frequency (ω)-phase velocity (c) space. This process greatly alleviates general complications with the passive method such as disturbance of phase velocities due to multiple sources and modes as well as the spatial aliasing.

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Shallow shear wave reflection surveys using high frequency vibroseis techniques provides information about the Tertiary bedrock surface and Pleistocene sediments on the Fraser River delta, in B.C., Canada. Besides the clear advantage of shear wave profiling in this shallow gas environment, the added resolution potential and ability to measure the shear wave velocity field enhances the fusion of this surface seismic data into earthquake site response estimations. Surface materials ranging from undisturbed, native delta sediments to clay/ rubble fill used in dike construction seemed amenable to the generation and recording of shear waves when using a small (6,000 kg) vibrator and engineering recording systems. The thickness of Pleistocene sediments and depth to Tertiary bedrock were mapped at three locations within the delta where gaps existed in the geologic record. These three sites provided cultural and near-surface settings that uniquely tested this shallow imaging technique.

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