Electromagnetic Methods
For shallow profiling (up to 20 feet in depth), a Geonics, Inc. EM-31 Terrain Conductivity meter is used. An EM-34 is used for depths of investigation between 30 and 180 feet. These tools are extremely sensitive and accurate, capable of detecting variations in conductivity of as little as 3%. Data are automatically stored in an electronic data logger for later transfer to a computer.
The EM-61 instrument is a high resolution, time-domain device for detecting buried conductive objects. It consists of a powerful transmitter that generates a pulsed primary magnetic field when its coils are energized, which induces eddy currents in nearby conductive objects. The decay of the eddy currents, following the input pulse, is measured by the coils, which, in turn, serve as receiver coils. The decay rate is measured for two coils, mounted concentrically, one above the other. By making the measurements at a relatively long time interval (measured in milliseconds) after termination of the primary pulse, the response is nearly independent of the electrical conductivity of the ground.