Estimating shallow compressional velocity variations in California’s Central Valley

Abstract

SUMMARY A theory for modelling the evolution of elastic moduli of grain packs under increasing pressure is combined with a method that accounts for the presence of fine-grained particles to develop a new conceptual framework for computing the seismic velocities of compacting sediments. The resulting formulation is then used to construct a seismic velocity model for California’s Central Valley. Specifically, a set of 44 sonic logs from the San Joaquin Valley are combined with soil textural data to derive the 3-D velocity variations in the province. An iterative quasi-Newton minimization algorithm that allows for bounded variables provided estimates of the nine free parameters in the model. The estimates low- and high-pressure exponents that resulted from the fit to the sonic log velocities are close to 1/2 and 1/3, respectively, values that are observed in laboratory experiments. Our results imply that the grain surfaces are sufficiently rough that there is little or no slip between grains. Thus, the deformation may be modelled using a strain energy function or free energy potential. The estimated Central Valley velocity model contains a 27 per cent increase in velocity from the surface to a depth of 700 m. Lateral variations of around 4 per cent occur within the layers of the model, a consequence of the textural heterogeneity within the subsurface.