A transient electromagnetic survey to define hydrogeology. A case history

A transient electromagnetic (TEM) survey was conducted in northern Nevada (USA), as a part of a groundwater exploration project. The study area consists of alluvium-covered volcanic flows with brecciated fault zones. The objective of the survey was twofold: (1) to identify locations of faults and alluvium with large amounts of clay, both of which are likely to affect groundwater flow; and (2) to identify formations that may be good aquifers.

An optimal range of formation resistivity values was estimated using Archie's law, to serve as a preliminary model for identifying good aquifers. The field survey was conducted along profiles with closely spaced central loop sounding, providing almost continuous coverage. Color modulated pseudosection of logarithmic apparent resistivity versus time were plotted for all survey lines. A one-dimensional numerical model was used to determine depths of investigation and formation resistivities in areas where pseudo-sections did not indicate rapid lateral changes in electrical properties.

The pseudo-sections show two low-resistivity features at depth which are consistent with two faults which have been mapped in the volcanic unit south of the study area. Formation resistivity values from the soundings in the vicinity of four wells were compared with hydraulic tests and electrical well logs where available. For three of the wells, formation resistivity values were in the range of formation resistivity values associated with a good aquifer. Production rates and hydraulic conductivity values at these wells are satisfactory. At the fourth well, which was drilled after the TEM survey to intercept a prominent lineament feature, low-resistivity TEM data had suggested the possibility of poor hydraulic characteristics due to the presence of clay-sized fault gouge material. The presence of fault gouge material subsequently was confirmed by drilling and, as predicted by the TEM survey, the hydraulic properties of the fourth well were considerably less desirable than the other three locations.