Obtaining Shear-Wave Velocity Structure to 100 Meters Depth From Microtremor Recordings on Seismic Refraction Equipment

John N. Louie
Seismological Laboratory and Dept. of Geological Sciences
Mackay School of Mines, The University of Nevada, Reno, NV 89557-0141
Phone: 775-784-4219 Email: louie@seismo.unr.edu Web: www.seismo.unr.edu

Abstract:

Current techniques of estimating shallow shear velocities for assessment of earthquake site response are too costly for use at most construction sites. They require large sources to be effective in noisy urban settings, or specialized independent recorders laid out in an extensive array. I propose that microtremor noise recordings made on 200-m-long lines of seismic refraction equipment can estimate shear velocity with 20% accuracy, often to 100 m depths. The combination of commonly available equipment, simple recording with no source, a wavefield transformation data processing technique, and an interactive Rayleigh-wave dispersion modeling tool exploits the most effective aspects of the microtremor, SASW, and MASW techniques. The slowness-frequency wavefield transformation is particularly effective in allowing accurate picking of Rayleigh-wave phase-velocity dispersion curves despite the presence of waves propagating across the linear array at high apparent velocities, higher-mode Rayleigh waves, body waves, air waves, and incoherent noise. I illustrate application of this technique at many urban and rural locations in Nevada, Southern California, and New Zealand. With no source, densely urbanized locations gave better results than quiet rural settings. Thirty-meter average velocities estimated from P-wave hammer refraction agreed with surface-wave results to better than 10%, under the assumption that soils have Poisson's ratios of 0.25-0.30. This technique duplicated microtremor array results above 3 Hz, but could not estimate velocities deeper than 100 m. Surface-wave dispersion modeling cannot duplicate the detail in the velocity profile yielded by a suspension logger, but was able to match the average velocity of 10-20 m depth ranges and suggest structure below the 100 m logged depth of a hole in Southern California.