The basic concept of timelapse resistivity
As discussed in the section on electrical resistivity the application of the electrical resistivity method results in a spatial distribution of subsurface electrical properties. If one collects multiple electrical geophysical surveys with the same parameters at the same location at different times, each one of these surveys will result in a spatial distribution of electrical properties associated with the time the survey was collected.
If subsurface processes occur which change the subsurface electrical properties these changes will be visible by examining the sequence of electrical property distributions.
This approach is known as time-lapse or 4D geophysics, and has been demonstrated to work for a number of geophysical methods (including electric, seismic, gravity and electromagnetic) for a wide range of applications including oil and gas production, water resource management and remedial efforts.
A time-lapse geophysical study has as objective to gain insight into processes (and thus is different from traditional geophysical efforts which are focused on the characterization of properties). A time-lapse geophysical study requires what is typically referred to as a time-lapse survey. A time-lapse survey is made up of numerous individual geophysical datasets, each of which can have many individual geophysical measurements. In order to facilitate the interpretation of time-lapse surveys (and driven by the need to avoid spatial aliasing) each individual dataset is typically collected at fixed time intervals. This interval is typically dictated by the need to avoid spatial aliasing of the processes being imaged. In the case of time-lapse datasets which need to be collected manually cost considerations provide a constraint on the number of datasets collected. However, if acquisition is fully automated the acquisition of continuous time lapse geophysical data can be very low cost. It should be noted that while time-lapse geophysics is well established and has been demonstrated numerous times, the process of interpreting changes in geophysical data in terms of changes in subsurface properties is complex. The interpretation of subsurface changes and processes can be assisted by certain constraints: for instance, in the case of anthropogenically caused changes we typically know the "when". Similarly, laboratory and theoretical studies can tell us the "what", and the relationship between rock properties and geophysical properties can help address the "how".