ORCID Profile
0000-0001-8699-0786
Current Organisation
US Geological Survey
Does something not look right? The information on this page has been harvested from data sources that may not be up to date. We continue to work with information providers to improve coverage and quality. To report an issue, use the Feedback Form.
Publisher: Springer Science and Business Media LLC
Date: 23-06-2022
Publisher: Society of Exploration Geophysicists
Date: 05-2019
Abstract: The spatial autocorrelation and spatially averaged coherency (SPAC) methods of processing Rayleigh-wave microtremor noise observations for estimation of S-wave velocity profiles traditionally require a circular or triangular array symmetry to allow spatial (azimuthal) averaging of interstation coherencies over a constant station separation. Common processing methods allow for station separations to vary by typically [Formula: see text] in the azimuthal averaging before degradation of the SPAC spectrum is excessive. Transformation of a set of frequency-coherency spectra to wavenumber-coherency spectra (kr spectra) allows spatial averaging of spectra from multiple pairs of sensors irrespective of differences in spatial separation of the pairs. The method is called krSPAC and is implemented by iterative direct fitting of observed and model kr spectra to determine an optimal layered-earth S-wave velocity model. The observed kr spectra are updated with each iteration of the velocity model because the wavenumber is a function of model phase velocity that varies with each iteration of the modeling process. The method proves applicable when modeling either with the assumption of fundamental mode Rayleigh-wave propagation or with a summation of fundamental and higher modes. The method proves robust when compared with alternative methodologies using symmetric and asymmetric arrays on a s le of synthetic data and on field data in which station spacings vary from 70 to 800 m side lengths.
Publisher: Springer Science and Business Media LLC
Date: 2022
DOI: 10.1007/S10950-021-10059-4
Abstract: Site response is a critical consideration when assessing earthquake hazards. Site characterization is key to understanding site effects as influenced by seismic site conditions of the local geology. Thus, a number of geophysical site characterization methods were developed to meet the demand for accurate and cost-effective results. As a consequence, a number of studies have been administered periodically as blind trials to evaluate the state-of-practice on-site characterization. We present results from the Consortium of Organizations for Strong Motion Observation Systems (COSMOS) blind trials, which used data recorded from surface-based microtremor array methods (MAM) at four sites where geomorphic conditions vary from deep alluvial basins to an alpine valley. Thirty-four invited analysts participated. Data were incrementally released to 17 available analysts who participated in all four phases: (1) two-station arrays, (2) sparse triangular arrays, (3) complex nested triangular or circular arrays, and (4) all available geological control site information including drill hole data. Another set of 17 analysts provided results from two sites and two phases only. Although data from one site consisted of recordings from three-component sensors, the other three sites consisted of data recorded only by vertical-component sensors. The sites cover a range of noise source distributions, ranging from one site with a highly directional microtremor wave field to others with omni-directional (azimuthally distributed) wave fields. We review results from different processing techniques (e.g., beam-forming, spatial autocorrelation, cross-correlation, or seismic interferometry) applied by the analysts and compare the effectiveness between the differing wave field distributions. We define the M index as a quality index based on estimates of the time-averaged shear-wave velocity of the upper 10 ( V S10 ), 30 ( V S30 ), 100 ( V S100 ), and 300 ( V S300 ) meters and show its usefulness in quantitative comparisons of V S profiles from multiple analysts. Our findings are expected to aid in building an evidence-based consensus on preferred cost-effective arrays and processing methodology for future studies of seismic site effects.
Publisher: Springer Science and Business Media LLC
Date: 19-04-2022
DOI: 10.1007/S10950-021-10051-Y
Abstract: Microtremor array measurements, and passive surface wave methods in general, have been increasingly used to non-invasively estimate shear-wave velocity structures for various purposes. The methods estimate dispersion curves and invert them for retrieving S-wave velocity profiles. This paper summarizes principles, limitations, data collection, and processing methods. It intends to enable students and practitioners to understand the principles needed to plan a microtremor array investigation, record and process the data, and evaluate the quality of investigation result. The paper focuses on the spatial autocorrelation processing method among microtremor array processing methods because of its relatively simple calculation and stable applicability. 1. A summary of fundamental principles of calculating phase velocity from ambient noise 2. General recommendations for MAM data collection and processing using SPAC methods 3. A discussion of limitations and uncertainties in the methods
No related grants have been discovered for William Stephenson.