Ambient Rayleigh wave field imaging of the critical zone in a weathered granite terrane

A 200 m wide square array of 400 vertical-component geophones was deployed three times to record the ambient seismic field at a 500 Hz sampling frequency. A geophone spacing of 10 m was used to target the near surface velocity structure and each array recorded for 3–4 days. Cross-correlation of the ambient seismic field produced cross-correlation functions from which fundamental-mode Rayleigh wave group delay times where picked in 59 narrow-bands from 14 to 130 Hz. The group delay times were inverted to make a set of 59 group velocity maps from 14 to 130 Hz. From the group velocity maps, dispersion curves were interpolated at 400 10 m spaced grid points to make data for 400 one-dimensional shear-velocity inversions. To find a best starting velocity model for the inversions, both linear gradient and half-space velocity models were explored. Our conclusion was that the half-space velocity models were the least prejudicial to the inverted form of our shear-velocity models. To find the best half-space velocity at each array inversion point, shear-velocity inversions were performed for a range of regularization values and half-space velocities from 500 to 2000 m/s. Joint inspection of the model-norm versus data-residual norm curves (‘L-curves’) and the shear-velocity models was performed to find smooth monotonically increasing velocity models that also have sharp L-curve corners. The resulting shear-velocity volumes find about 400 m/s near-surface velocities increasing to 700–800 m/s at 30 m depth which is our maximum depth of good resolution. Critical zone research generally defines the near-surface using three layers: the saprolite that includes the soil layer, the fractured bedrock, and the non-permeable bedrock also called the protolith. This work was able to image the base of the saprolite layer but not the base of the fractured bedrock layer. The saprolite thickness varies from 5–20 m and is generally thicker along the ridge with respect to the valleys. In summary, this work shows that high-frequency fundamental mode Rayleigh waves can be extracted from multi-day ambient seismic field recording to make images of near-surface shear-wave velocity.