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Exploration and Detection of Subsurface Water Using Broadband Electromagnetic Sensors
Because of water’s conductivity properties, electromagnetic geophysical surveys are often used in the detection and monitoring of subsurface water. The common coil receiver is a simple and inexpensive instrument that measures the derivative of the magnetic field strength. It has a frequency bandwidth dependent on its physical geometry and a frequency-dependent amplitude response. Generally, these instruments have a limited useful frequency response, requiring different sets of coils if information over more than a few discrete frequencies within a narrow bandwidth is desired.
The use of wide bandwidth receivers in electromagnetic geophysical surveys can provide information that limited bandwidth instruments can not. A magnetic field receiver with a small physical size allows array measurements of electromagnetic fields in a manner similar to arrays used for seismic measurements. This makes feasible 2-D and 3-D electromagnetic imaging. The wide bandwidth of the receiver also allows time and frequency domain measurements to be made simultaneously. Although not a topic for this discussion, it has been shown that reactions of different liquid organic compounds with clays have varying transient responses to a given stimulus; a measurement tool with a sufficiently wide bandwidth to measure these variations allows the possibility of subsurface contaminant identification.
A wide bandwidth, vector-field sensitive receiver has been developed and field-tested for functionality in geophysical applications. The frequency domain results of the detection of subsurface water flow are discussed herein. The availability of such a receiver allows the possibility of not only detecting a conductivity anomaly such as subsurface water flow but the possible identification of contaminants within the water as well.
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