This is a two part project that will make use of a set of flexible, small scale seismic arrays. The design of the arrays will be optimized to estimate the spatial gradients of regional and teleseismic wavefields. In the first portion of the project, a single, fourteen-element array will be deployed with the goal of determining the optimal array geometry and aperture to estimate wavefield gradients, as well as to quantify the uncertainty of these measurements. The second portion will consist of two, simultaneous deployments of seismic arrays in two adjacent locations. The first location is a former floodplain, with approximately 700m of unconsolidated alluvium overlying bedrock. The second location has bedrock within one meter of the surface. The goal of this portion of the project is to rigorously document the effects of near-surface geology on parameters estimated from seismic spatial gradients (such as strain, rotation, P-S energy partitioning, etc). Up to now, there has been no study to quantify the accuracy and/or precision of seismic wavefield gradients using small-scale seismic arrays (termed gradiometers). However, previous studies suggest that gradiometers are highly sensitive to uncertainties in instrument location, noise in the recorded seismograms, and local subsurface geology. This study will investigate these issues in detail and attempt to 1) produce a set of guidelines for deploying seismic gradiometers, and 2) quantify the effects of local geologic conditions on the estimation of seismic attributes derived from gradient estimations. By developing a standard of seismic array design, future gradient measurements will be uniform allowing a more direct comparison from one experiment to another. Furthermore, by rigorously quantifying the effects of subsurface geology on parameters derived from seismic gradients, our interpretations of Earth structure will be more accurate, as well as our interpretations of the nature of the seismic source.
