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Use of a Square‐Array Direct‐Current Resistivity Method to Detect Fractures in Crystalline Bedrock in New Hampshire
Author(s) -
Lane J. W.,
Haeni F. P.,
Watson W. M.
Publication year - 1995
Publication title -
groundwater
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.84
H-Index - 94
eISSN - 1745-6584
pISSN - 0017-467X
DOI - 10.1111/j.1745-6584.1995.tb00304.x
Subject(s) - bedrock , geology , anisotropy , depth sounding , fracture (geology) , azimuth , geophysics , electrical resistivity and conductivity , outcrop , square (algebra) , mineralogy , seismology , geometry , geomorphology , geotechnical engineering , optics , oceanography , physics , mathematics , electrical engineering , engineering
Azimuthal square‐array direct‐current (dc) resistivity soundings were used to detect fractures in bedrock in the Mirror Lake watershed in Grafton County, New Hampshire. Soundings were conducted at a site where crystalline bedrock underlies approximately 7 m (meters) of glacial drift. Measured apparent resistivities changed with the orientation of the array. Graphical interpretation of the square‐array data indicates that a dominant fracture set and (or) foliation in the bedrock is oriented at 030° (degrees). Interpretation of crossed square‐array data indicates an orientation of 027° and an anisotropy factor of 1.31. Assuming that anisotropy is due to fractures, the secondary porosity is estimated to range from 0.01 to 0.10. Interpretations of azimuthal square‐array data are supported by other geophysical data, including azimuthal seismic‐refraction surveys and azimuthal Schlumberger dc‐resistivity soundings at the Camp Osceola well field. Dominant fracture trends indicated by these geophysical methods are 022° (seismic‐refraction) and 037° (dc‐resistivity). Fracture mapping of bedrock outcrops at a site within 250 m indicates that the maximum fracture‐strike frequency is oriented at 030°. The square‐array dc‐resistivity sounding method is more sensitive to a given rock anisotropy than the more commonly used Schlumberger and Wenner arrays. An additional advantage of the square‐array method is that it requires about 65 percent less surface area than an equivalent survey using a Schlumberger or Wenner array.