Time Lapse Resistivity
The most advanced usage of the resistivity method is conducted through time lapse resistivity to monitor changes of the subsurface. Mostly, these changes are associated with hydrogeological phenomena and resistivity monitoring can help understand highly complex and dynamic systems such as subsurface injections, water flooding, pumping drawdown, contaminant plume movement, leaking underground pipes or tanks, and dam seepage. Currently, HGI has a number of ongoing resistivity monitoring studies underway that are used to examine both slow (e.g., contaminant plumes moving under natural gradients) and fast moving (e.g., subsurface injection) systems.
HGI has a number of ongoing time lapse resistivity monitoring studies that are used to examine both slow and fast moving hydrogeological phenomena.
One advantage of conducting time lapse resistivity is the versatility of the acquisition to monitor changes measured through 1D VES soundings, 2D resistivity profiles, and 3D resistivity grids. Additionally, the same resolution and scaling applied to spatial mapping can be translated to temporal mapping by acquiring snapshots at a frequency that fits the specific problem at hand. Our monitoring philosophy is outlined in a recent publication in the Journal of Applied Geophysics. In general, we can apply time lapse resistivity as means to capture the end state condition, as a continuous resistivity monitoring campaign providing feedback in near real time, or anywhere in between.
An example below shows a set of time lapse resistivity profiles used to monitor changes associated with moisture redistribution after uniform surface irrigation. The data are presented as a percent difference in resistivity compared to a baseline just after irrigation was shut off. Most of the section shows the subsurface becoming drier as the water moved downward out of the investigation area. However, in the center of the profile, a bulbous mass of water has collected and is prevented from complete drainage by a compacted layer below it. These data were used to help qualitatively understand the efficiency of surface irrigation, migration, and redistribution for a mine that leaches metal from low grade ore.
Time lapse resistivity for monitoring surface irrigation at a mine.
At the other end of the resistivity monitoring spectrum is the creation of 4D imaging (3D space + time) with time lapse resistivity data. 4D resistivity imaging may be particularly useful for sensitive projects that rely on safe operations (e.g., subsurface injection or fracking) or systems where time sensitive information is needed for decision making (e.g., remediation). HGI has designed and manufactured a resistivity monitoring system that is capable of performing rapid data acquisition on 180 simultaneous electrodes to acquire a snapshot with 25 minutes. The video below shows an example of hourly resistivity monitoring of an injection on six simultaneous wells. The injection lasted 1 week with approximately 7 million gallons of sulfuric acid injected into the wells. Each color-rendered body in the 4D imaging represents a level of change in conductivity relative to the baseline acquired prior to injection. The results show how the acid propagates outward and follows the topographical gradient of the underliner.