Refraction microtremor and optimization methods as alternatives to
boreholes for site stength and earthquake hazard assessments

John N. Louie
Robert E. Abbott
Seismological Laboratory, Mackay School of Mines, University of
Nevada 174, Reno, NV; 775-784-4219; fax 775-784-1833;
louie@seismo.unr.edu

Satish Pullammanappallil
Optim LLC, University of Nevada 174, Reno, NV; 775-784-6613;
fax 775-784-1833; satish@optimsoftware.com

A thorough assessment of shallow shear velocity is important to both
earthquake-hazard assessment and efficient foundation design.  The only
standard procedure for determining shear velocity, crosshole seismic
(ASTM D4428), is not much used as it requires two boreholes with
high-precision positional logs.  Downhole shear-wave profiles in a
single hole are adequately accurate, but still too expensive for many
projects.  We tested two methods for inexpensively estimating shallow
shear velocities with seismic refraction equipment, at the sites of
several boreholes in California and Nevada. The sites ranged from hard
to soft (NEHRP hazard classes A to D). The first method, refraction
microtremor, records ambient ground noise on simple seismic refraction
equipment (as in ASTM D5777). Wavefield analysis of the noise allows
picking of Rayleigh-wave phase velocities.  It works well in dense
urban areas and transportation corridors.  The second method,
SeisOpt(R)@2d by Optim LLC, takes standard (ASTM D5777) seismic
refraction arrival picks and finds an optimized 2-d P-velocity model.
The shear velocities estimated from both methods are just as effective
as borehole velocities for two purposes: estimating 3-meter
depth-averaged shear velocity for foundation design; and estimating the
seismic spectrum input for earthquake-hazard evaluations of sites.