Azimuthal Surveys

Azimuthal Surveys

Azimuthal resistivity surveys are performed to determine the direction of joints or fractures in rock or the direction of anisotropy in soils or rock. Often the Wenner array is used. These surveys are conducted using the same array spacing (“a” spacing) and with the center of the spread on the same position. Each successive spread is oriented in a different direction or azimuth until 170 degrees or 165 degrees are covered in increments of 10 or 15 degrees respectively. A full 360 degrees need not be surveyed at the setup for 0 degrees gives the same result as for 180 degrees and 10 degrees the same result as 190 degrees, etc.

The data are plotted in a polar diagram or “rosette”. If there is no anisotropy, the apparent resistivity will plot as a circle. If the apparent resistivity senses anisotropy, it will plot as an ellipse. The radial coordinate of the polar diagram need not start at zero. If it starts at a little less than the minimum apparent resistivity, the diagram will emphasize the irregularity with azimuth.

Rock joints are often filled with decomposed rock, clay or conductive fluid. If the rock joints are more conductive than the unfractured rock, then the resistivity lines parallel to the joints should have less resistivity.

Before conducting an azimuthal survey, a resistivity sounding should be conducted to determine the optimum electrode spacing for the survey. If the spread is too short, it will not sense the fractures.

Appendix Z contains some references that might assist in interpretation of azimuthal surveys and also some containing examples of these surveys.

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