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Direct P-wave anisotropy measurements at Homestake Mine: implications for wave propagation in continental crust

Atterholt, James and Brownlee, Sarah J. and Pavlis, Gary L. (2021) Direct P-wave anisotropy measurements at Homestake Mine: implications for wave propagation in continental crust. Geophysical Journal International, 224 (1). pp. 121-137. ISSN 0956-540X. doi:10.1093/gji/ggaa416.

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We measured anisotropic seismic properties of schists of the Homestake Formation located at a depth of 1478 m in the Sanford Underground Research Facility (SURF) in the Black Hills of South Dakota, USA. We deployed a 24-element linear array of three-component geophones in an area in the Homestake Mine called 19-ledge. An airless jackhammer source was used to shoot two profiles: (1) a walkaway survey to appraise any distance dependence and (2) a fan shot profile to measure variations with azimuth. Slowness estimates from the fan shot profile show a statistically significant deviation with azimuth with the expected 180° variation with azimuth. We measured P-wave particle motion deviations from data rotated to ray coordinates using three methods: (1) a conventional principal component method, (2) a novel grid search method that maximized longitudinal motion over a range of search angles and (3) the multiwavelet method. The multiwavelet results were computed in two frequency bands of 200–600 and 100–300 Hz. Results were binned by azimuth and averaged with a robust estimation method with error bars estimated by a bootstrap method. The particle motion results show large, statistically significant variations with azimuth with a 180° cyclicity. We modelled the azimuthal variations in compressional wave speed and angular deviation from purely longitudinal particle motion of P-waves using an elastic tensor method to appraise the relative importance of crystalline fabric relative to fracturing parallel to foliation. The model used bulk averages of crystal fabric measured for an analogous schist sample from southeast Vermont rotated to the Homestake Formation foliation directions supplied by SURF from old mine records. We found with average crustal crack densities crack induced anisotropy had only a small effect on the observables. We found strong agreement in the traveltime data. The observed amplitudes of deviations of P particle motion showed significantly larger variation than the model predictions and a 20° phase shift in azimuth. We attribute the inadequacies of the model fit to the particle motion data to inadequacies in the analogue rock and/or near receiver distortions from smaller scale heterogeneity. We discuss the surprising variability of signals recorded in this experimental data. We show clear examples of unexplained resonances and unexpected variations on a scale much smaller than a wavelength that has broad implications for wave propagation in real rocks.

Item Type:Article
Related URLs:
URLURL TypeDescription
Brownlee, Sarah J.0000-0003-4461-0161
Pavlis, Gary L.0000-0003-2128-9897
Additional Information:© The Author(s) 2020. Published by Oxford University Press on behalf of The Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model ( Accepted 2020 August 14. Received 2020 July 22; in original form 2019 November 17. This paper would not have been possible without the amazing support the staff at SURF provided to collect this experimental data. We especially thank Tom Regan and Neil Engle for keeping us safe underground and being great helpers. Jaret Heise was invaluable in coordinating the experimental work with SURF personnel and getting us access to geologic data from Homestake. Sincere thanks to the DUGL team who fielded the broad-band experiment and to which this experiment was an added component. We also thank Terry Stigall of Indiana University who was a key player in keeping instrumentation working and coming up with her usual creative solutions to problems. Sincere thanks to Jeffrey Park and an anonymous reviewer for constructive comments that helped improve this paper significantly. This paper emerged from the B.S. thesis of JA. The text was extended and revised by all authors led by GLP. The multiwavelet code is the legacy of a body of work by GLP spanning two decades. SJB did the model calculations. The seismic instruments were provided by the Incorporated Research Institutions for Seismology (IRIS) through the PASSCAL Instrument Center at New Mexico Tech. Data used are available through the IRIS Data Management Center. The facilities of the IRIS Consortium are supported by the National Science Foundation under Cooperative Agreement EAR-1261681 and the DOE National Nuclear Security Administration. This research was supported by the National Science Foundation under EAR-1526894.
Funding AgencyGrant Number
Department of Energy (DOE) National Nuclear Security AdministrationUNSPECIFIED
Subject Keywords:North America, Wavelet transform, Seismic anisotropy, Wave propagation, Wave scattering and diffraction
Issue or Number:1
Record Number:CaltechAUTHORS:20201217-124922238
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Official Citation:James Atterholt, Sarah J Brownlee, Gary L Pavlis, Direct P-wave anisotropy measurements at Homestake Mine: implications for wave propagation in continental crust, Geophysical Journal International, Volume 224, Issue 1, January 2021, Pages 121–137,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:107159
Deposited By: George Porter
Deposited On:17 Dec 2020 21:42
Last Modified:16 Nov 2021 19:00

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