We demonstrate the use of resistivity/induced polarization (IP) monitoring of salt transport under natural hydraulic loads. Electrical monitoring of saline tracer transport during forced injection has been demonstrated previously. Detection of tracer transport under natural hydraulic loading is difficult because neither the hydraulic load nor the tracer resistivity can be controlled. In one study, we identify the electrical response to salt transport in a dynamic beach environment. Resistivity/IP imaging resolved the structure of the saltwater‐freshwater interface and evidence for tide‐induced groundwater transport. Resistivity increases in the near surface and at depth, upbeach of the high‐tide mark, accompanied by tidal transgression. We attribute this to desaturation and decreasing salinity in the near surface and to decreasing salinity at depth, despite tidal transgression. Monitoring of groundwater levels indicates a phase lag between the tide level and groundwater level, supporting the electrical data. IP was insensitive to groundwater salinity variation. In a second study, we identify the electrical response to recharge‐induced salt transport from a road‐salt storage facility. Conductivity and IP models for monitoring lines, located on the basis of an EM31 survey, resolved the subsurface salt distribution. IP modeling resolved the sediment‐bedrock interface. Modeling of monthly conductivity differences revealed conductivity increases and decreases at the locations of salt contamination, which correlate with the recharge pattern. We attribute near‐surface conductivity increases after heavy rainfall to increasing saturation and ion dissolution. Corresponding conductivity decreases at depth are attributed to flushing of the bedrock with freshwater. Essentially, the opposite response was observed during a quiet monitoring period following heavy recharge. Near‐surface IP changes are consistent with this interpretation. Salt transport occurring under natural hydraulic conditions was monitored with resistivity imaging. IP improved characterization of the hydrogeologic framework but was of limited value in monitoring salt transport in these environments.
Theoretical and field tests indicate that the controlled‐source audiomagnetotelluric (CSAMT) method provides an efficient means of delineating the shallow resistivity pattern above a hydrothermal system. Utilizing a transmitter overcomes the main limitation of conventional audiomagnetotellurics—variable and unreliable natural source fields. Reliable CSAMT measurements can be made with a simple scalar receiver. Our calculations for a half‐space show that the plane‐wave assumption is valid when the transmitter is more than 3 skin depths away in the broadside configuration and more than 5 skin depths away in the collinear configuration. Three‐dimensional (3-D) numerical modeling results for a bipole source 5 skin depths away compare well with those for a plane‐wave source, showing that the method is valid. A CSAMT survey at the Roosevelt Hot Springs geothermal area in Utah produced apparent resistivity contour maps at four frequencies: 32, 98, 977, and 5208 Hz. These maps show the same features as those of a dipole‐dipole resistivity map. We also collected detailed CSAMT data at 10 frequencies on two profiles. Two‐dimensional (2-D) plane‐wave modeling (transverse magnetic mode) of the resulting pseudo‐sections yields models similar to those derived by modeling the dipole‐dipole resistivity data. However, CSAMT resolved details not shown by the resistivity modeling. Thus, high resolution along with an efficient field procedure make CSAMT an attractive tool for geothermal exploration.
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