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Hayashi K.,Geometrics | Martin A.,Geovision | Hatayama K.,National Research Institute of Fire and Disaster | Kobayashi T.,Oyo Corporation
Leading Edge | Year: 2013

This article summarizes a passive surface-wave method that uses only two sensors and its application to the estimation of deep S-wave velocity structure. Three-dimensional S-wave velocity structure to a depth of several kilometers has a large effect on long-period ground motion in tectonic basins, such as the Los Angeles (LA) Basin. Recent studies of long-period ground motion in the LA Basin (e.g., Hatayama and Kalkan, 2012) show that observed ground motion in some areas cannot be explained by the S-wave velocity models in current use. Most studies of basin velocity structure rely on geologic information, surface and borehole geophysical data, and observed earthquake records to deduce or measure seismic velocities. Geophysical data and seismic stations commonly used for velocity analysis are sparsely distributed and most well data are too shallow to characterize deep S-wave velocity structure. To establish more accurate basin velocity structure, there is a need for more densely distributed deep S-wave velocity data. © 2013 © 2013 by The Society of Exploration Geophysicists.

Goff D.S.,Louisiana State University | Lorenzo J.M.,Louisiana State University | Hayashi K.,Geometrics
28th Symposium on the Application of Geophysics to Engineering and Environmental Problems 2015, SAGEEP 2015 | Year: 2015

Levee foundation soils in New Orleans, USA, are composed of unconsolidated Holocene deltaic sediments. Traditionally, geotechnical tests at point locations can identify the more unstable zones, but cannot predict accurately the laterally heterogeneous facies of the Mississippi delta. Together, electrical resistivity and seismic shear wave studies can aid in the interpretation of different soil types between geotechnical sites. In such highly conductive, coastal soils, resistivity measurements are limited to shallow depths, but remain useful for describing variations in saturation and the presence of clays. Similar studies conducted in Japanese fluvial and Australian calcrete environments do not consider the influence of brackish water in coastal settings. The London Avenue Canal levee flank of New Orleans, which failed in the aftermath of Hurricane Katrina, 2005, presents a suitable site in which to pioneer these geophysical relationships in a coastal setting. Shear wave velocity and resistivity are related to soil properties through Hertz-Mindlin Theory and Archie's Law. Preliminary cross-plots show electrically resistive, high-shear-wave velocity areas interpreted as low-permeability, resistive silt. In brackish coastal environments, low-resistivity and low-shear-wave-velocity areas may indicate both saturated, unconsolidated sands and low-rigidity clays. Published polynomial approximations to similar cross-plots must be modified for use in the near-surface sediments of the Mississippi River Delta. We present new relationships between soil type, resistivity, and shear wave velocity to distinguish the three main sediment groups found in deltaic environments: sand, silt, and clays.

Hayashi K.,Geometrics | Roughley C.,California State University, East Bay | Craig M.,California State University, East Bay
28th Symposium on the Application of Geophysics to Engineering and Environmental Problems 2015, SAGEEP 2015 | Year: 2015

A M6.0 earthquake occurred in South Napa County on August 24, 2014. We recorded aftershocks and conducted S-wave velocity (VS) surveys using active and passive surface wave methods at four sites in Napa. Portable accelerometers were deployed at three locations for aftershock observation on the day after the mainshock. One of the three sites is located directly on the line of the surface rupture. The accelerometers recorded continuous data for two weeks. At least 550 aftershocks with magnitudes between -0.6 and 3.9 were recorded during a one-week period starting the day after the mainshock. Surface wave surveys were conducted at each of the three aftershock observation sites and also at Napa Valley College. Dispersion curves from active and passive methods were combined and phase velocities were obtained to a minimum frequency of 0.3 Hz. VS profiles were determined to a depth of 100 to 2000 m by joint inversion of dispersion curves and horizontal to vertical spectral ratio (HVSR) curves. Depth to bedrock with VS higher than 3000 m/s appears to be at least 2000 m at three aftershock observation sites. The VS profiles obtained by the surface wave surveys are generally consistent with site-specific differences in amplification and S-P time observed in aftershock observations.

Johnson R.,Geometrics
Hydro International | Year: 2013

Geometrics Inc., based in San Jose, CA, USA, designs and manufactures rugged, portable and technologically-innovative geophysical instruments for land, sea and air subsurface investigations. The company's main product lines include high speed cesium magnetometers, exploration seismographs, digital marine streamers and electrical conductivity imaging and resistivity systems. The company introduced commercial proton precession magneto - meters, which for a decade were the only source of high-resolution systems for oil exploration. Eventually the company expanded into other instrument manufacturing including gamma ray spectrometers, data acquisition and processing software, and airborne survey with company owned planes. Today, Geometrics' geophysical instruments, sensors and data processing software are used globally for energy and resource exploration. Geometrics was founded with a mission to promote a program of continuous improvement and to ensure delivery of high-quality and reliable products. Airborne mineral exploration companies such as Fugro and Geotech as well as smaller airborne contractors comprise the gold, iron and base metal exploration groups operating Geometrics magnetometer sensors.

Ikeda T.,Kyoto University | Matsuoka T.,Kyoto University | Tsuji T.,Kyoto University | Tsuji T.,Kyushu University | Hayashi K.,Geometrics
Geophysical Journal International | Year: 2012

Microtremors are usually analysed without any consideration of the higher modes of surface waves. However, recent studies have demonstrated that higher modes contain useful information for improving the inverted S-wave velocity model. In this study, we propose two inversion methods that consider higher modes by using the amplitude response of each mode, which can avoid mode misidentification in the spatial autocorrelation (SPAC) method. One method is to compare the observed phase velocities by the extended spatial autocorrelation (ESPAC) method with the effective phase velocities calculated from theoretical dispersion curves and the amplitude responses of each mode. In the other method, SPAC coefficients are fit directly by comparing theoretical SPAC coefficients determined from dispersion curves and amplitude responses with the observed ones. The latter, direct-fitting approach is much simpler than the method using effective phase velocities. To investigate the effectiveness of these methods, a simulation study was conducted. Simulated microtremors that included higher modes were successfully inverted by the proposed multimode methods. The observed phase velocities and SPAC coefficients determined from field data were also consistent with theoretical ones constructed by the proposed methods except at low frequencies. The inversion using effective phase velocities required prior information about an infinite half-space to obtain a better S-wave velocity model whereas the direct-fitting inversion worked well without prior information, suggesting the direct-fitting method is more robust than the method using effective phase velocities. We conclude that our proposed inversion methods are effective for estimating the S-wave velocity structure even if higher modes of surface waves are predominant in observed microtremors. © 2012 The Authors Geophysical Journal International © 2012 RAS.

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