The Electrical Resistivity Method

Instrumentation

As discussed in a previous section a basic resistivity measurement is generally taken using four electrodes. Initially surveys were collected by using four electrode systems. These systems would consist of an instrument with four electrodes attached to the instrument through long wires. For each measurement field staff would have to reposition electrodes manually.

The mid 1980s saw the emergence of the first so called multi electrode systems. These systems still make a four electrode measurement, but they added so called multiplexers to the system. Each multiplexer (which could be daisy chained with other multiplexers) could be connected to a number of electrodes (typically between 24 and 60). The acquisition hardware allowed one to select any combination of electrodes. This allowed users to set up a survey, connect all electrodes to a multiplexer, and collect a series of measurements in a fraction of the time it would have taken to move all the electrodes around manually. Such systems (schematically shown on the left) also allowed monitoring as they can be left in place and collect data autonomously.

Field system

Multi electrode systems were followed by multichannel systems which were introduced commercially around 2008. In multichannel systems users can measure multiple potentials for one current injection. In such a measurement there is still only one pair of current electrodes, but multiple (typical four - ten) pairs of potential electrodes. This allows for a substantial decrease of acquisition length, and thus allows for an increased temporal resolution in monitoring applications. Such instruments are sold by different vendors.

Data acquisition

As mentioned in the previous section in order to get a sufficiently well resolved distribution of subsurface electrical properties many individual resistivity measurements need to be taken (see here for naming conventions). Prior to the start of a survey the measurements which need to be taken are defined in a list, and this list of measurements is collected by the resistivity instrument. Each measurement typically takes several seconds (1-4, depending on a number of settings), and the collection of a resistivity dataset typically takes anywhere between 10 minutes and one day.