Crack‐induced velocity anisotropy in the White Mountains, New Hampshire

Abstract

The compressional wave velocity of rock in situ measured at four sites in the White Mountains of New Hampshire varies as a function of azimuth and is anisotropic. The anisotropy [(Vmax − Vmin)/Vmax], which ranged from 18% to 42%, is attributed to two sets of cracks with different azimuths and with total crack density of order of 0.2 at each site. The anisotropy in velocity correlates only approximately with the orientation of mapped joints at each site.

Integrated use of surface-geophysical methods to indicate subsurface fractures at Tibbetts Road, Barrington, New Hampshire

“…Interpretation; The magnitude of anisotropy, as expressed by the anisotropy quotient, is comparable to that obtained in other studies (Bamford and Nunn, 1979;Crampin and others, 1980;Park and Simmons, 1982;Imse and Levine, 1985). The DC-resistivity data are the most complex of the four data sets.…”

“…Velocity and (or) thickness inhomogeneities in the material overlying the reflector can also generate an anisotropic azimuthal velocity function. 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. APPROACH Fractures represent discontinuities in an otherwise continuous and, in the simplest case, homogeneous body of rock.…”

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.…”

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.…”