Multi-wavelet analysis of three-component seismic arrays: Application to measure effective anisotropy at Pinon Flats, California

We develop and apply a new technique to determine array-averaged particle m otions from three-component seismic array data. The method is based on mult i-wavelets, which are an extension of multi-taper spectral methods, and is a hybrid of Fourier and time-domain methods of array processing. Particle m otions are determined by a time-domain principal-component method. A comple x singular value decomposition is used on wavelet transformed signals assem bled into multiple matrices (one for each wavelet). The eigenvector of the largest singular value of each matrix is used to estimate the phase between individual signals. We determine the relative phase between components to estimate an average particle motion ellipse for the array,The estimation pr ocedure is made more stable by the redundancy inherent in the multi-wavelet s and by M-estimators applied to individual phase factors in the complex pl ane. The method is applied to data from three-component array experiments c onducted at Pinon Flats, California, in 1990 and 1991, We find remarkable d epartures of P-wave particle motions from the pure longitudinal motion expe cted for an isotropic media. Anomalies as large as 40 degrees are measured from some azimuths, The azimuthally varying particle-motion anomalies are f requency dependent, generally increasing in magnitude as frequency increase s. Borehole measurements from sensors at 153 and 274 m depth below the arra y show a pattern indistinguishable from the surface sensors. The data are f it with a dipping, transversely isotropic medium with a symmetry plane havi ng a strike of 70 degrees and a dip of 30 degrees to the northwest. We attr ibute our results to three superimposed effects: (1) an anisotropy of the n ear surface induced by preferential weathering of the granodiorite bedrock along joints, (2) a larger scale anisotropy induced by structural and intri nsic anisotropy related to the Santa Rosa mylonite, and (3) near-surface sc attering.