Field and analog tank model studies were done to examine the response of various<br>electrode arrays and to try to develop an optimum field technique. Field studies were on<br>fractured tills and on glacial outwash fan deposits. In the latter, anisotropy in electrical<br>resistivity (and by inference, the hydraulic conductivity) is a result of primary depositional<br>structures, principally the stacked braided stream channels. Schlumberger, Wenner, Pole-<br>Dipole, Pole-Pole and other modified arrays were tested. Various ways of normalizing and<br>plotting the results were developed. All arrays showed clear anisotropy effects and there is no<br>single best array or field technique which is most sensitive to anisotropy. Choice of technique<br>depends on the size of the field crew, access or ease of installing remote electrodes, and size<br>of the field or open space. Clearly, a Vertical Electrical Sounding (VES) must first be done<br>near a control well in the area to establish the appropriate electrode spacings for investigating<br>the depth range of interest. Field studies at one outwash fan site concentrated on vertical<br>variations, where. 12 azimuthal plots were made for a full range of current electrode halfspacings<br>of 1 to 100 m about one central point. A second phase of the work at this site studied<br>lateral variations, k the center point was moved 180 m laterally to obtain a measure of the<br>local geologic “noise” or variability. The lateral variability was less for larger electrode<br>spacings, as these sampled and averaged over a much larger volume.<br>The tank modeling was done to study the effects of the distance laterally and vertically<br>from the top of the fracture(s) to the array center point. The azimuthal apparent resistivity<br>diagram changed significantly with lateral position relative to a single “fracture”. For the<br>application of azimuthal resistivity techniques to fracture and joint investigations, a number of<br>azimuthal surveys at closely spaced points must be done to remove ambiguities caused by<br>unknown fracture spacing, and by unknown lateral distance to the nearest fracture. For multisheet<br>models, increasing the thickness of a homogeneous overburden eliminated the “Paradox<br>of Anisotropy” effect and rotated the ellipse 90 degrees.<br>Azimuthal surveys should prove to be useful at sites where detailed ground water flow<br>models are required, as resistivity azimuthal effects of 10 - 40% are common and presumably<br>the directional hydraulic conductivity variations may have similar magnitudes. The<br>disadvantage of the technique is that it is cumbersome and requires a large circular open space<br>free of interfering conductors; thus it is not applicable at many sites. The information<br>provided, however, is unattainable by any other means within reasonable economic limits.


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