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The movement of a small plume of fresh water through the vadose zone was monitored using surface resistivity tomography and pole‐pole potential measurements. Sets of potential measurements on a square grid at several times throughout the experiment show gradual, progressive, and systematic development of low‐resistivity zones that are inferred to be loci of fluid concentration. A procedure for inverting percentage changes in potentials is developed here and used to map maximum potential changes of 13% into resistivity decreases of up to 40% through 3-D resistivity inversion. The resulting patterns are complex, with both resistivity increases and decreases needed to match the observed data. The resistivity reductions show a spatial connection to the plume’s source and are suggestive of the fluid migration. Resistivity increases generally appear to form rims surrounding the decreases and may be artifacts of the inversion process. However, some isolated increases in the surface layer may be caused by evaporation of fluid from previous precipitation events. Interpretation of the complex patterns may limit the usefulness of this method for monitoring fluid migration. Nonetheless, the resulting pattern of resistivity reductions may show details of fluid migration that are unavailable with more conventional monitoring techniques. In this experiment, comparison of the volume with reduced resistivity with the volume of injected water predicts only a 0.5% increase in saturated porosity, demonstrating that fluid flow in the vadose zone was most likely controlled by the distribution of fine‐grained clays and silts and occurred by capillary action.

The San Jacinto Basin is a northwest‐trending, pull‐apart basin in the San Jacinto fault zone of the San Andreas fault system in southern California. About 24 km long and 2 to 4 km wide, the basin sits on a graben bounded by two strands of the San Jacinto fault zone: the Claremont Fault on the northeast and the Casa Loma Fault on the southwest. We present a case study of shallow structure (less than 1 km) in the central basin. A 2.75-km refraction line running from the northeast to southwest across the regional structural trend reveals a groundwater barrier (Offset I). Another line, bent southward and continued for 1.65-km, shows a crystalline basement offset (Offset III) near an inferred trace of the Casa Loma Fault. Although a basement refractor was not observed along the 2.75-km line, a mismatch between the estimate of its minimum depth and the basement depth determined for the 1.65-km line suggests that an offset in the basement (greater than 260 m) exists around the junction of the two refraction lines (Offset II). By revealing more faults and subtle sedimentary structures, the reflection stack sections confirm the two refraction offsets as faults. Offsets I and III each separate sediments of contrasting structures and, in addition, Offset III disrupts an unconformity. However, the sense and amount of the offset across Offset III contradict what may be expected across the Casa Loma Fault, which has its basinward basement down‐thrown to about 2.5 km in the better defined southeastern part of the graben. The Casa Loma Fault trace has been mislinked in the existing geological maps and the trace should be remapped to Offset II where the reflector disruptions spread over a 400-m wide zone. Our Offset III is an unnamed, concealed fault.

Two common methods for detecting leaks from storage ponds are detection of contaminants in monitoring wells and use of two asphaltic liners with a seep collection system between them. However, significant quantities of the impounded fluid may have leaked by the time contaminants are detected in the wells. Double liners have the advantage that leaks are detected before the fluid escapes into the environment, but they are expensive. An alternative approach is to use electrical resistivity methods to detect leaks. Several studies have mapped conductive zones with electrical methods and have inferred the presence of contaminated groundwater (e.g., Greenhouse and Harris, 1983). A fundamental ambiguity in any such survey is that the subsurface may contain conductive sediments (such as clay or silt) that may be mistaken for contaminated water, however. Parra (1988) proposed an alternative method that detects holes in a liner by mapping the electrical response due to a short circuit across an insulating liner which results from conductive fluid in a hole. An important result is that the electrical response of the leak is detectable only within a few meters with this method.

We have developed a practical algorithm for inverting gridded resistivity data for three‐dimensional structure and applied it to data from an experiment designed to detect leaks from ponds. This method yields relatively accurate reconstructions of structure when applied to synthetic data, but lateral contrasts in resistivity are mapped much more accurately than are vertical contrasts. The best results are obtained when transmitting electrodes are located directly above the suspected leak. Application to real data yields results which are consistent with well data and an adjacent Schlumberger sounding.

Significant errors may result when applying one‐dimensional (1-D) or two‐dimensional (2-D) interpretation methods to magnetotelluric (MT) data collected in three‐dimensional (3-D) environments. Both depths and resistivities can be grossly incorrect if 1-D or 2-D methods are applied in a 3-D setting. We present examples of MT sounding curves generated using a 3-D modeling program (Madden and Park, 1982) and illustrate some interpretation pitfalls if 3-D effects are not considered. The 3-D effects discussed herein are attributed to a surface layer heterogeneity, and can be readily identified in MT data from a well‐designed MT survey. The MT survey must include stations chosen to yield information about regional structure. Alternatively, carefull examination of a geologic map will help the intepreter estimate the regional effects present in the data.