Hydrogeologic investigations to determine the flow direction and velocity of<br>groundwater movement are crucial to the development and assessment of groundwater<br>resources. Information from monitoring wells, tracer tests, and flow meters are often used<br>to determine the direction and rate of groundwater flow. Surface and borehole-to-surface<br>electrical measurements can also be used to determine this information using conductive<br>plumes within the aquifer. The direction of the detected plume with respect to the source<br>indicates the direction of groundwater flow and the change in length with respect to time<br>provides information about the flow velocity.<br>Initial methods of determining groundwater flow paramters were through the<br>application of surface geophysical techniques. Glaccum et al. (1982) and White (1988)<br>discuss experiments in which a conductive plume was introduced into an aquifer and<br>mapped by surface electrical methods. The results of these tests were used to demonstrate<br>the methodology in determining flow directions and estimates of groundwater velocity.<br>However, one problem associated with using surface geophysical techniques is that if the<br>conductive plume is located too deep below the surface, surface-only methods become<br>relatively insensitive to variations in potential (Wilt et al., 1983). Another factor is the<br>inhomogeneity of the near-surface material or presence of cultural features may produce<br>strong variations on the measured potentials such that the effects may mask electrical<br>signals from deeper conductive anomalies of interest such as an induced plume.<br>By placing one or more electrodes beneath the near-surface layer or in contact<br>with a conductive region improves the detection sensitivity of the electrical survey. Thus<br>the borehole-to-surface electrical technique has an advantage over surface-only methods<br>in that it is possible to resolve conductors which are deep in the subsurface or whose<br>contrast is so small that they cannot be detected by a surface survey. Wilt and Tsang<br>(1985) further demonstrated that a subsurface current electrode in contact with a conductive<br>medium can improve measurement sensitivity by an order of magnitude over surface<br>measurements alone.


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