Bear, L.K., Pavlis, G.L., Bokelmann, G.H.R., 1999, Multi-wavelet analysis
of three-component seismic arrays: application to measure effective anisotropy
at Pinon Flats, California, Bulletin of the Seismological Society of
America, 89, 3, 693-705
We develop and apply a new technique to determine array-averaged particle motions from three-component
seismic array data. The method is based on multi-wavelets, which are an extension of multi-taper
spectral methods, and is a hybrid of Fourier and time-domain methods of array processing. Particle motions
are determined by a time-domain principal-component method. A complex singular value decomposition
is used on wavelet transformed signals assembled 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 procedure is made more stable by the redundancy inherent in the
multi-wavelets and by M-estimators applied to individual phase factors in the complex plane. The
method is applied to data from three-component array experiments conducted at Pinon Flats,
California, in 1990 and 1991. We find remarkable departures of P-wave particle motions from
the pure longitudinal motion expected for an isotropic media. Anomalies as large as 40 deg are
measured from some azimuths. The azimuthally varying particle-motion anomalies are frequency
dependent, generally increasing in magnitude as frequency increases. Borehole measurements from sensors
at 153 and 274 m depth below the array show a pattern indistinguishable from the surface sensors.
The data are fit with a dipping, transversely isotropic medium with a symmetry plane having
a strike of 70 deg and a dip of 30 deg to the northwest. We attribute our results to three
superimposed effects: (1) an anisotropy of the near surface induced by preferential weathering
of the granodiorite bedrock along joints, (2) a larger scale anisotropy induced by structural
and intrinsic anisotropy related to the Santa Rosa mylonite, and (3) near-surface scattering.
Back to My Homepage , Stanford University or Department of Geophysics