St. Petersburg Coastal and Marine Science Center
Direct Current Resistivity Mapping
Streaming resistivity mapping can be used to identify active sites of submarine groundwater discharge, variations in freshened subsurface water masses, the role of tidal-forcing, and to pinpoint discrete sites for additional monitoring.
Direct current resistivity mapping is accomplished by using an Advanced Geosciences Inc. (AGI) Marine SuperSting R8 multi-channel system connected either to a (1) streaming cable consisting of two current producing electrodes and 8 potential electrodes acquiring data with dipole-dipole geometries, or (2) an external switching box controlling the flow of current along a 56 electrode, stationary cable. In stationary mode, current potentials are measured in a distributed array, with array geometry set by the user. For every resistivity measurement (~1 per sec), the SuperSting R8 injects an optimized current, reverses the polarity and then re-injects the current again to cancel spontaneous voltages that may occur down-cable. This process is repeated, and if the error is less than a pre-determined threshold value (i.e., 7%), then the next reading advances. Replicate measurements provide a means to assess wave-induced artifacts.
To process the resistivity measurements an inverse modeling routine (AGI EarthImager) is utilized that accommodates both water-column salinity and depth observations. The resolution is optimized by using a starting model with a resistivity pseudo-section. A best-fit layered model is developed using an iterative least squares smooth model inversion method.
For more information on Streaming Resistivity Profiles, see http://coastal.er.usgs.gov/sgd/html/resist_electrical.html