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Effects of Energy Directionality on ReMi Analysis
AuthorNaphan, Dustin O.
AdvisorLouie, John N.
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An obstacle shared by all passive surface wave analysis methods is the unknown source-receiver relationship, and possible adverse effects on apparent velocities. Particularly in the case of linear array geometry, the risk is that velocities will be overestimated if the direction of energy propagation is not approximately parallel to the array. Refraction Microtremor (ReMi™) is a passive method that utilizes a linear array geometry. Our experimental approach to directional analysis makes use of a 2-D array configuration consisting of two linear arrays, arranged orthogonally in an “L” configuration. 3-D synthetic wave modeling using this configuration explores a variety of source-receiver orientations for directional effects. The analysis demonstrates the most extreme cases of directional preference and how they present during processing, as well as the ideal case where the wave field is omni-directional. Based on the addition of omni-directional source energy to the worst-case synthetic scenario, the synthetic modeling helps make a determination of the minimum proportion of energy necessary to have propagated in-line with the array to achieve an accurate result. This approach indicates such a proportion to be 15% of total rms amplitude, or 2.25% of the total energy. In the case of several different experimental surveys collected in the Reno-Sparks area using this “L” array configuration, directional effects similar to those exhibited in synthetic models are not apparent. A statistical evaluation from over 10,700 ReMi™ surveys, collected by Optim for the Clark County Parcel Map, demonstrate similar statistical distributions between geotechnical estimates of Vs30, regardless of predominant array orientation. These and other empirical results suggest that in and near urban sites rich in microtremor noise, the proposed 2.25% of required omni-directional energy is likely present, and ReMi™ will accurately estimate Vs profiles given proper collection and processing techniques.