USE OF A SQUARE-ARRAY DIRECT-CURRENT RESISTIVITY METHOD TO DETECT FRACTURES IN CRYSTALLINE BEDROCK IN NEW-HAMPSHIRE

Azimuthal square-array direct-current (dc) resistivity soundings were used to detect fractures in bedrock in the Mirror Lake watershed in Gr afton County, New Hampshire. Soundings were conducted at a site where crystalline bedrock underlies approximately 7 m (meters) of glacial dr ift. Measured apparent resistivities changed with the orientation of t he array. Graphical interpretation of the square-array data indicates that a dominant fracture set and (or) foliation in the bedrock is orie nted at 030 degrees (degrees). Interpretation of crossed square-array data indicates an orientation of 027 degrees and an anisotropy factor of 1.31. Assuming that anisotropy is due to fractures, the secondary p orosity is estimated to range from 0.01 to 0.10. Interpretations of az imuthal square-array data are supported by other geophysical data, inc luding azimuthal seismic-refraction surveys and azimuthal Schlumberger de-resistivity soundings at the Camp Osceola well field. Dominant fra cture trends indicated by these geophysical methods are 022 degrees (s eismic-refraction) and 037 degrees (dc-resistivity). Fracture mapping of bedrock outcrops at a site within 250 m indicates that the maximum fracture-strike frequency is oriented at 030 degrees. The square-array dc-resistivity sounding method is more sensitive to a given rock anis otropy than the more commonly used Schlumberger and Wenner arrays. An additional advantage of the square-array method is that it requires ab out 65 percent less surface area than an equivalent survey using a Sch lumberger or Wenner array.