Richard E. Warrick

Comparison of phase velocities from array measurements of Rayleigh waves associated with microtremor and results calculated from borehole shear-wave velocity profiles

Shear-wave velocities (VS) are widely used for earthquake ground- motion site characterization. VS data are now largely obtained using borehole meth- ods. Drilling holes, however, is expensive. Nonintrusive surface methods are inex- pensive for obtaining VS information, but not many comparisons with direct borehole measurements have been published. Because different assumptions are used in data interpretation of each surface method and public safety is involved in site character- ization for engineering structures, it is important to validate the surface methods by additional comparisons with borehole measurements. We compare results obtained from a particular surface method (array measurement of surface waves associated with microtremor) with results obtained from borehole methods. Using a 10-element nested-triangular array of 100-m aperture, we measured surface-wave phase veloci- ties at two California sites, Garner Valley near Hemet and Hollister Municipal Air- port. The Garner Valley site is located at an ancient lake bed where water-saturated sediment overlies decomposed granite on top of granite bedrock. Our array was deployed at a location where seismic velocities had been determined to a depth of 500 m by borehole methods. At Hollister, where the near-surface sediment consists of clay, sand, and gravel, we determined phase velocities using an array located close to a 60-m deep borehole where downhole velocity logs already exist. Because we want to assess the measurements uncomplicated by uncertainties introduced by the inversion process, we compare our phase-velocity results with the borehole VS depth profile by calculating fundamental-mode Rayleigh-wave phase velocities from an earth model constructed from the borehole data. For wavelengths less than 2 times of the array aperture at Garner Valley, phase-velocity results from array measure- ments agree with the calculated Rayleigh-wave velocities to better than 11%. Mea- surement errors become larger for wavelengths 2 times greater than the array aper- ture. At Hollister, the measured phase velocity at 3.9 Hz (near the upper edge of the microtremor frequency band) is within 20% of the calculated Rayleigh-wave veloc- ity. Because shear-wave velocity is the predominant factor controlling Rayleigh- wave phase velocities, the comparisons suggest that this nonintrusive method can provide VS information adequate for ground-motion estimation.

THE SPECIFICATION AND TESTING OF A SEISMIC-REFRACTION SYSTEM FOR CRUSTAL STUDIES;

A new seismic-refraction system built for the U. S. Geological Survey for crustal studies has been tested in the laboratory and shown to meet strict performance specifications for broad frequency response, low noise, high gain, and high dynamic range. The inherent advantages of magnetic recording, with selective filtering on playback, were demonstrated in field tests by the recovery of weak events that otherwise would be obscured by high seismic noise.

A preliminary evaluation of the shallow reflection seismograph

Following successful tests of a specially constructed shallow reflection seismograph in early 1954, new experimental work has been conducted on the Colorado Plateau, in the Upper Mississippi Valley zinc‐lead district, and in Portage County, Ohio. Although reflections were recorded in all of these areas, they were not continuous and correlatable on the Colorado Plateau and in the Upper Mississippi Valley. In Ohio, on the other hand, reflections were recorded from horizons within the Pleistocene overburden as well as from bedrock and horizons within the consolidated rock section. Conditions favoring good reflections are similar for both shallow‐ and deep‐reflection work; they differ only in scale.

APPLICATION OF SEISMIC METHODS TO A GROUND‐WATER PROBLEM IN NORTHEASTERN OHIO

Valleys cut in the bedrock in northeastern Ohio by Tertiary and Pleistocene streams have been filled by Pleistocene glacial drift so that there is little surface evidence of their existence. Some of these buried valleys are good sources of ground water, so information regarding the location, depth, and cross‐section of the buried valleys is important in water‐supply investigations. Detailed knowledge of the configuration of the buried valleys is useful also in interpreting the glacial history of the region. Test surveys in Portage and Summit Counties, Ohio, by the U.S. Geological Survey indicate that seismic methods can be used to determine depth to bedrock and thus to be applicable to the study of the buried valleys.

Effect of cable capacitance on in‐situ borehole geophone calibration

Using 2-Hz electromagnetic moving‐coil geophones as sensing elements, we have constructed and deployed three‐component seismometers in boreholes at various sites for wave‐propagation studies associated with earthquake hazards (Liu et al., 1991). For example, one such seismometer has been deployed in a 88-m deep borehole reaching bedrock in the Marina District of San Francisco since 1990 (Liu et al., 1992) for the purpose of comparing ground motions in the bedrock and those at the surface. Periodic calibrations for such geophones are necessary to check if the geophone parameters have changed because of decreased magnetization of the geophone ferro‐magnet. For example, the coil transductance of the vertical‐component geophone of the borehole seismometer mentioned above was calibrated to be 121 V-s/m using phase‐ellipse test and step test before deployment. Sixty six months after the deployment, the coil transductance, when calibrated in situ and with a 100-m intervening cable between the geophone and the ca...

Integrated Surface and Borehole Strong-Motion, Soil-Response Arrays in San Francisco, California

An integrated set of four borehole arrays and ten surface installations is installed in the city of San Francisco, California to measure the response of soft-soil deposits to strong earthquake ground motions. The borehole arrays extend through thick layers of soft water-saturated soils of Holocene age and older more consolidated soils of Pleistocene age into bedrock at depths up to 90 m. The surface installations are configured in pairs to provide simultaneous comparative surface measurements of soft soils and nearby rock. The rock locations also permit comparative measurements of rock as observed at the surface and in nearby boreholes. The arrays are designed to address a wide variety of scientific and engineering issues, and especially the issue of anelastic and nonlinear soil response at high strain levels as might be recorded during a large regional earthquake. Recordings of ground motions from the largest regional earthquakes which have occurred since the installation of the arrays show marked evidence of amplification as measured on the borehole and surface arrays. Implications of the results for low-strain site coefficients in present U.S. building codes are discussed.