Estimating crack parameters from observations of P‐wave velocity anisotropy

Abstract

Cracks are a very common feature of crustal rocks, and their behavior and properties under various conditions are of growing importance to a variety of applications, including investigations of oil and water resources in cracked reservoirs, extraction of geothermal heat, and earthquake prediction by the seismic effects of dilatancy. This paper discusses mapping cracks by measurements of velocity‐anisotropy in underground refractors. The technique is demonstrated by interpreting the velocity measurements of Bamford and Nunn (1979) in shallow fractured limestone. The estimated crack distributions are found to be consistent with rose‐diagrams at neighboring outcrops, and the degree of saturation of the cracks has a major effect on velocity variations. Elastic constants can be derived for an equivalent purely elastic solid which has the same velocity variations as the cracked solid. This allows synthetic seismograms to be calculated for propagation through cracked structures, which demonstrates that use of polarization anomalies in three‐component recordings is a sensitive technique for investigating cracked solids.

Azimuthal Seismic Anisotropy of 70‐Ma Pacific‐Plate Upper Mantle

“…The observed gradients can be explained by "dry" oriented microcracks filled with damaged material and/or hydrous alteration products, which are predicted to produce a 2θ velocity signal (Hudson, 1981;Thomsen, 1995). Assuming that the velocity at 7-km depth is representative of the uncracked solid, the 10.1029/2018JB016451 Journal of Geophysical Research: Solid Earth velocity at the Moho in the fast direction can be accounted for with a dimensionless crack density of~0.05 (e.g., Crampin et al, 1980;Garbin & Knopoff, 1973, 1975a, 1975b where the crack density is defined as ε = Na 3 /v for N cracks of radius a in a volume v. Since cracks filled with hydrous minerals would not be expected to close, the crack density for filled cracks would need to decrease with depth. Under the assumption that the entire volume of the cracks is serpentinized, a crack density of 0.05 corresponds tõ 0.5 wt% water in the uppermost few kilometers of the mantle (Carlson & Miller, 2003).…”

Experimental Relationship Between Compressional Wave Attenuation and Surface Strains in Brittle Rock

“…Extensive work has been performed using LUT as a tool for diagnosing the state of damage in various applications (Acosta-Colon et al, 2009;Cai & Zhao, 2000;Ghazvinian, 2015;Gheibi & Hedayat, 2018;Hedayat et al, 2014a;Hedayat et al, 2018;Hildyard et al, 2005;Huang et al, 2014;Jones, 1952;Modiriasari et al, 2017;Pyrak-Nolte, 1988;Sayers et al, 1990;Scott et al, 1993;Suaris & Fernando, 1987;Yang et al, 2018;Zhao et al, 2006). Significant efforts have been made to analytically model the effects of cracks and fractures on the ultrasonic wave propagation (Blum et al, 2011;Chaix et al, 2006;Crampin et al, 1980;De Basabe et al, 2016;Gross & Zhang, 1992;O'Connell & Budiansky, 1974;Hudson, 1981;Piau, 1979;Pyrak-Nolte et al, 1990a, 1990bWaterman & Truell, 1961;Yang & Turner, 2003). These analytical models can be broadly categorized as the displacement discontinuity model (Mindlin, 1960;Pyrak-Nolte, 1988;Schoenberg, 1980) and the equivalent medium model (Crampin, 1981;Hudson, 1981;Waterman & Truell, 1961) based on the fracture size and spacing in relation to the ultrasonic wavelength (Perino et al, 2010).…”

Crevasse-induced Rayleigh-wave azimuthal anisotropy on Glacier de la Plaine Morte, Switzerland

“…Bradford and others (2013) investigated basal crevasses by means of anisotropy in electromagnetic and seismic wave velocities caused by the preferential alignment of the crevasses. Vertical fractures in an otherwise homogeneous ice body lead to a transversely isotropic medium with a horizontal axis of symmetry (or horizontal transversely isotropic) which is subject to azimuthal anisotropy in seismic wave propagation (Crampin and others, 1980; Bakulin and others, 2000). Apart from azimuthal anisotropy, the presence of crevasses may lead to band gaps in elastic wave propagation (Freed-Brown and others, 2012).…”