To inject electric current and collect the requisite resistance measurements in the field (Figure 3), a series of electrodes are attached to an ER control unit, which consists of a regulated current source (for example, a 12-V deep-cycle battery or a generator), voltage meter, current meter, and multiplexers (units for switching between electrodes) for multi-channel data collection. Despite the advent of multi–node cables (i.e., cables with connection points, called takeouts, for many tens of electrodes, as shown in Figure 3), considerable labor is still required for the initial deployment of cables. However, once the instrumentation deployment is complete, it can be left in place for many hours to years to explore changes in electrical conductivity through time associated with processes, which is a reason these methods show strength for time-lapse monitoring. Commercially available multi-channel electrical imaging systems commonly accept input files listing the selected four-electrode current-potential pairs, called quadripoles. These files are variously referred to as sequence, command, or schedule files. The control unit then drives current on two electrodes and measures potential differences between two or more electrodes based on the input sequence file.
Figure 3 – Electrical imaging instrumentation, including the control unit in the foreground, electrodes, and cables that connect the control unit to the electrodes. The batteries and multiplexers are inside of this control unit. From Binley and Slater (2020).
Commercial systems on the market today generally are constant-voltage systems, i.e., they establish a constant voltage difference over the current electrodes and measure the resulting current. Thus, the amount of current driven is influenced by the conductivity of the Earth, where lower input currents may be needed in resistive settings where potential differences are larger, and higher input current is needed in conductive settings. Typical currents are usually in the mA to A range. Subsequent measured voltages are usually in the μV to V range. Electrodes are commonly composed of stainless steel or graphite. Steel electrodes tend to be more durable, whereas graphite electrodes resist corrosion and thus are preferred in marine deployments. In applications where spontaneous potential (SP) data (based on naturally occurring voltages in the earth) are collected in addition to ER, non-polarizing electrodes (e.g., copper/copper-sulfate) are necessary for making voltage measurements. For a review on various electrode types, see Morris and others (2004) and LaBrecque and Daily (2008). SP is beyond the scope of this book, but it is worth noting that SP data are complementary to ER and IP data, can help to reduce uncertainty in interpretation of hydrogeologic systems, and can be used to interpret flow directions in some applications.
IP data acquisition uses the same hardware as ER, but additional considerations are needed to acquire reliable measurements. Any hydrogeologist considering the use of IP should be aware that it is substantially more challenging than ER surveying alone, and more time consuming to collect in the field. This is because the signal-to-noise ratio of the IP measurements (Ø, Ma) is typically 2.5 to 3 orders of magnitude smaller than the magnitude of the resistance recorded with ER, and accurate transmitter-receiver synchronization is needed. In addition, electromagnetic and capacitive coupling between the different wires used to connect the current injection and voltage recording electrodes is manifest as spurious charge storage effects that may corrupt the response from the earth. Field procedures have been developed to alleviate these concerns, including separating the wires that connect to the voltage-receiving pairs from those that connect to the current-injection pairs (e.g., Dahlin and Leroux, 2012).
Regardless of the type of electrode used or whether ER or ER and IP data are being collected, it is important to record locations of the electrodes accurately in the field, as well as electrode and transect elevations, which will be incorporated into the inversion procedure to get the correct topography for the upper boundary (see Section 3.6). The accuracy required for surveying and georeferencing of electrodes is highly dependent on the survey design, with greater accuracy required for smaller electrode spacing or in areas of more topographic relief.