Conductivity - Frequency-Domain ElectromagneticsSubsurface ion concentrations (i.e. salts) greatly affect soil conductivity. As such, distributions of conductive soils can be delineated with frequency-domain electromagnetic induction methods, also called EM surveying. In oil and gas applications, ions tend to migrate further than associated hydrocarbons. Therefore, maximum impact extents can usually be determined with an EM survey.
A transmitter coil in the instrument emits an alternating electromagnetic field that induces eddy currents in the ground. Subsurface eddy currents produce a secondary electromagnetic field coupled to the primary field. Secondary field magnitudes are measured by a receiver coil that relate to bulk subsurface conductivity and indirectly correspond to ion concentrations.
The instruments measure two components of the secondary response. The first, and most frequently used, is the quadrature-phase that relates to apparent conductivity. Apparent conductivity implies that measurements do not directly represent the actual subsurface conductivity at any specific layer but yield a bulk representation of various layers dependent on several electromagnetic properties described below.
The second measurement is the in-phase component. Reactive electrical properties in the soil, such as capacitance and inductance, shift the phase of the secondary field. In-phase magnitudes correlate to this shift, which represents magnetic susceptibility (a property of metals). The instruments are not designed to detect metals; however, in-phase measurements can help differentiate between ion concentrations and buried metal responses. This feature is useful in some applications such as landfill assessments. When metal detection is the goal of the investigation, however, time-domain, methods should be employed.
Below is some information on the instrumentation we use. In our experience, Geonics manufactures the most rugged conductivity meters on the market. We have yet to encounter weather conditions which prevent us from obtaining results.
The instruments have an approximately linear response below one hundred millisiemens per meter. Responses over these levels are usually caused by artificial sources, except occasionally in areas of naturally saline soils. Discriminating anthropogenic (i.e. human) impacts from natural trends requires some interpretation based on shape, pattern, and magnitude. Response magnitudes are relative and should be compared to background (i.e. naturally occurring) values. Background levels vary widely depending on local geology. Fine, silty soils, and clays are more conductive than course, dry, sandy soils, and gravel or rocky terrain. Survey limits must extend wide enough to determine background values to identify naturally occurring trends.
The effective depth of penetration, or skindepth, is defined in electromagnetics as the distance where current density, due to an alternating field, drops below 1/e (about one third) the intensity at surface. Because field extent is theoretically infinite, skindepth approximates the range where a majority of the response will be located. It is largely determined by the operating frequency and the distance between the transmitter and receiver coils. In conjunction, coil configuration and instrument frequency are designed to target various depths. However, skindepth is greatly affected by overall soil conductivity. (Higher conductivities decrease field penetration.) The instruments, therefore, detect the depth onset of conductive layers rather than the depth extent. Lateral EM surveys yield very limited depth information.
Furthermore, a principle called equivalence suggests that conductivity distributions at different depths can produce equivalent responses. Shallow small conductors, for example, imitate larger conductors at depth. Targeting different skindepths with multiple instruments and comparing the results aids in generalized depth interpretation. However, vertical conductivity profiling is the best method for delineating distribution depths. Together with electromagnetic surveying, vertical profiling offers high precision, three-dimensional distribution, and volumetric information.
There is a well-known trade-off between depth of investigation and image resolution, a property of electromagnetic fields. Although tools targeting depths down to sixty meters are available, such large footprints are not usually practical nor are the depths necessary for typical environmental applications.