One of the more challenging situations in geophysics is the need to identify and characterize aquifers in highly saline environments.
Saline environments are inherently conductive, which means conventional electrical resistivity techniques are less effective because the signals don’t penetrate very far into highly conductive ground and the contrasts between water and surrounding substrates are much more subtle. This can make it difficult, for example, to distinguish between permeable and impermeable aquifer materials, because both permeable and impermeable formations typically measure as highly electrically conductive regardless of the material composition.
As a result, many geophysics professionals have been turning to nuclear magnetic resonance (NMR) tools for more effective hydrogeologic characterization in highly saline situations. Vista Clara, the global leader in magnetic resonance for groundwater investigation, has done extensive research on the differences between electrical resistivity logging and NMR. This brief post presents the highlights.
Magnetic resonance can provide direct and reliable estimates of key aquifer properties, including how much of the water at a given location is tied up in fine grained material (bound water) versus how much is mobile (unbound water). NMR logging tools have been successfully used in highly conductive brine and petroleum reservoirs for decades in the oil and gas industry. This capability is now being extended for use in groundwater surveying.
When an NMR logging tool is immersed in an electrically conductive environment, the transmitted and received RF fields can be affected by electromagnetic skin effects, which will somewhat reduce the field intensity. If unaccounted for, this field reduction can lead to underestimation of water content and porosity. As shown below, by calibrating the NMR data against fresh water, the impacts on detected water can be made negligible. Note: NMR relaxation times, used to determine pore size and permeability, are generally unaffected by increased salinity.
In actual field situations, the impact of conductive water is generally lower than shown in the table because it represents only a fraction of the volume between the tool and the NMR sensitive zone of investigation. Note: for very conductive formations, NMR measurement accuracy can be further improved if necessary by calibrating the tool in a brine tank.
An example of using NMR to characterize a saline aquifer is shown below, based on data from a site at Leque Island in Western Washington, USA. Situated at an agricultural site that was reclaimed from marshland adjacent to Puget Sound, this zone is impacted by the nearby saltwater body. The upper 20m of the subsurface consists of interbedded, unconsolidated sediments, including sand, gravel, silt and clay.
The data shows a comparison of direct push electrical resistivity vs direct push NMR measurements. As can be seen, the resistivity data log is dominated by the conductivity of the groundwater at the site and therefore provides little indication of the existence of three high permeability zones. In contrast, the direct push NMR log clearly indicates the presence and extent of three distinct high permeability zones. In addition, NMR provides quantified estimates of bound and mobile porosity and hydraulic conductivity.
In summary, it is well known that an electrically conductive earth affects electrically based geophysical techniques, including NMR. This is especially true when the site under investigation has a high level of conductivity from high saline content. However, as shown above, the effect of the electrical conductivity on the magnetic field patterns can be accurately modeled and accurate inversion of water content and other aquifer properties can be achieved.