In Situ Seismic Measurements of Crack Anisotropy in the Carboniferous Limestone of Northwest England*

Abstract

Seismic velocity anisotropy is predicted for cracked rocks containing either a naturally‐occurring or an artificially‐induced preferred orientation of cracks. Methods developed for the study of velocity anisotropy in the uppermost mantle have been applied to the in situ measurement of P‐wave velocity anisotropy in fractured Carboniferous Limestone in Northwest England. At three different sites, first‐arrival travel‐time data has been obtained using conventional refraction equipment and a weight‐drop source. Velocity anisotropy of 15–29% is identified with directions of maximum and minimum velocities that can be broadly related to previously mapped orientations of joints.

Integrated use of surface-geophysical methods to indicate subsurface fractures at Milford, New Hampshire

“…Interpretation: The magnitude of anisotropy, also expressed as the anisotropy quotient, is comparable to that obtained in studies of other areas (Bamford and Nunn, 1979;Crampin and others, 1980;Park and Simmons, 1982;Imse and Levine, 1985).…”

“…P-wave refraction experiments that have successfully detected fractures include: Bamford and Nunn (1979) and Crampin and others (1980) in carbonate rocks; Park and Simmons (1982) in granites, quartz syenites, and volcanic rocks with glacial overburden; and Imse and Levine (1985) in carbonate rocks with glacial overburden.…”

Structural Synthesis: Correlation of Structural Fabrics, Velocity Anisotropy, and Magnetic Susceptibility Data

“…In contrast, a well-developed velocity anisotropy does not appear to be present in the sediments below the décollement. Curves are best-fitting function of the different data sets, following Bamford and Nunn (1979;see also Byrne et al, this volume). …”

Use of surface and borehole geophysical surveys to determine fracture orientation and other site characteristics in crystalline bedrock terrain, Millville and Uxbridge, Massachusetts

“…Some of the limitations of this method include (1) multiple fracture sets that may result in a complex azimuthal distribution of seismic velocity, (2) velocity and (or) thickness inhomogeneities in the material overlying a reflector that also can cause anisotropic azimuthal velocity determinations, and (3) the effect of single fractures or small, isolated fracture zones that are unlikely to be detected because the velocity changes caused by these zones generally are too small to be detected. Despite these limitations, refraction surveys where fractures have been detected have been conducted by Bamford and Nunn (1979) in carbonate rock, Park and Simmons (1982) and others (1992a, 1992b) in crystalline rock with glacial overburden, and Imse and Levine (1985) in carbonate rock with glacial overburden.…”