Resistivity surveys usually employ a Wenner electrode array (four co-linear, equally-spaced electrodes) and an ABEM Terrameter earth resistivity meter. The instrument reading in ohms (applied current divided into the measured voltage difference) is converted to apparent resistivity (in ohm-meters) by means of a geometric factor determined for the electrode array. For soundings, the inter-electrode spacings are varied approximately logarithmically (six readings per decade) over a range determined for the particular site.
has a fixed coil separation with an investigative depth of 20 feet. The EM-34 requires a crew of two and has three coil separations with investigative depths from 25 to 200 feet. Geosphere has rigged a 4x4 truck to acquire continuous data over large areas using the 10 meter configuration; this method has proved valuable in locating and mapping deep karst features, faults, and buried sand channels. The EM-38 measures shallow soil conductivities to depths of 3 to 6 feet. With all three instruments, data are acquired either on analog strip charts or digitally which are downloaded to a field computer. The in-phase component of these EM systems can also provide valuable data concerning the location of buried metal objects and pipes. Other EM systems available include the EM-39 borehole induction logger.
Data is usually acquired with the use of a survey grid constructed onsite and parallel lines "run" across the area of investigation. Factors considered in data collection include the location, orientation and density of the survey lines as well as the digital sampling interval along the lines. The data are then recorded either on analog strip charts or digitally with a datalogger before then being downloaded to a computer in the field. Positions of known metal objects onsite are noted and recorded for placement on resulting maps.
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