Field Work

Based on what is known about the site and the object of the resistivity survey, the type of survey and the electrode array are selected. Under most survey objectives, either Wenner profiling or Wenner soundings are used. For locating fractures or nearly vertical dikes, the azimuthal survey or square array survey technique might be considered. For details on types of surveys, see the following section of this manual, Resistivity Methods.

Some of the field factors to consider are:

· Read the safety warnings in Appendix A.

· Keep the instrument dry. Before working in moist weather, please read the Care of the MiniRes section of this manual.

· Keep the current (transmitter) cables away from the potential (receiver) cables. See Appendix E for more details on cable and electrodes.

· Try to avoid frozen ground. Frozen ground causes electrode installation to be difficult and introduces error into the data. Frozen ground is much more resistant than the same earth that is not frozen. Variations in the depth of the frozen layer and the degree of freezing cause scatter in the field data. Appendix S shows the variation of resistivity with the temperature of earth. If the depth of frozen ground is not too great, it is best to have the electrodes penetrate through the frozen zone into the unfrozen earth. A large heavy pry-bar could be helpful in making a hole through a thin frozen surface. Into this hole the electrode can be inserted into the unfrozen earth.

· Avoid severe terrain if possible. Ridges spread out the equipotential lines and make resistivity seem less than its true value. Gullies concentrate equipotential lines and make resistivity seen greater than its true value. Appendix T gives several references that deal with this problem.

· Try to avoid tight undergrowth that would be time consuming for working with cables. Keeping cables in good condition is important to obtaining reliable field data. See Appendix E for some suggestions on the selection and care of cable.

· Avoid paved areas where installation of electrodes would be very difficult and time consuming. Some surveys have been conducted where concrete pavement exists by drilling a hole through the concrete for each electrode position. Where there is asphalt pavement, surveys have been performed by punching a hole for each electrode position with a pointed heavy pry-bar.

· Try to avoid interfering cultural features such as metal fences, railroad tracts, and electrical grounds. If a tape measure is used to locate the electrode positions, a nonmetallic tape measure is safe. If a metallic tape measure must be used, be sure to real it up before taking resistivity measurements. Be sure to take careful field notes of nearby cultural features that are likely to introduce errors into the interpretation of the survey.

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