SPECIAL PUBLICATION NO. 57


 iv. A shielded antenna should always be used to avoid interference from
 overhead and metallic objects. GPR instrumentation conditions, including
 the frequency of the antenna used, time settings, and other relevant
 parameters, should be included in the report.

g. Electrical resistivity methods can be especially useful where there is shallow
 groundwater, hardpan soils, or shallow clays (but the method is not limited to
 such situations, in fact it can be applied to deep groundwater and clay-free
 subsurface environments).

 i. Electrical resistivity soundings (ER)

 1. It is critical that the locations chosen for ER investigation be
 selected with cognizance of electrical interference, such as fences,
 utilities, and the structure itself. All possible interference sources
 should be noted in the report and program design must account for
 such.

 2. If ER is utilized, it is recommended that the ER investigation be
 coupled with GPR data or a second independent data set (such as
 borehole data).

 3. All conditions of the testing, including electrode configurationss,
 data reduction methods, and number of iterations required to
 produce the final interpretations, will be discussed in the report.

 ii. Two dimensional electrical resistivity (2DER or MER)

 1. All conditions of the testing, including electrode configurationss,
 data conditioning (removal of data points, etc.), data reduction
 methods, and number of iterations to produce the final depth
 section or other interpretative results, must be discussed in the
 report.

 2. It is critical that the locations chosen for these investigations be
 selected with cognizance of electrical interference, such as fences,
 utilities, and the structure itself. Locations of potential
 interference must be discussed in the report and accounted for in
 program design.

 iii. Note that ASTM Standards D-6429, D6431-99, and G57-95a address
 various ER procedures and data acquisition procedures. These should be
 followed as appropriate or deviations noted and explained.

h. Capcitively-coupled resistivity methods (CCR).

 i. Generally limited use in areas with shallow water table. However,
 instrument can penetrate to greater depths if the near-surface materials are