Frequency Domain Electromagnetics
An aerial photo provides the background for mapping the location and extent of a fault zone defined by a "worm plot" of ground conductivity readings. Red, orange, and yellow colors along the worm plot represent zones of high conductivity associated with the fault zone.
A transect of ground conductivity readings perpendicular to the fault zone shown in the above aerial photo is used to define its approximate width.
Electromagnetic methods are used at Southwest Research Institute (SwRI) to measure subsurface electrical conductivity. Scientists can perform electromagnetic surveys using frequency domain electromagnetic instruments or transient electromagnetic instruments.
Frequency Domain Electromagnetics (Ground Conductivity)
Frequency domain electromagnetics, commonly referred to as ground conductivity, measures the electrical conductivity of soil and rock by measuring the magnitude and phase of an induced electromagnetic current. Secondary electromagnetic fields generated by conductors are also detected and can be used to locate ferrous and nonferrous metal objects.
Frequency domain electromagnetics can effectively delineate:
- Lateral variations in the lithology of soils and rocks
- Near-Surface structures (faults)
- Metallic objects (tanks, drums, and pipelines)
A ground conductivity instrument induces currents, generated by a varying electromagnetic field, into the subsurface in such a manner that their amplitude is linearly proportional to the ground conductivity. These instruments provide bulk measurements of apparent conductivity values integrated over a volume of the subsurface.
Factors affecting ground conductivity include the constituents, structure, and moisture content of the soil or rock. Most soil and rock constituents (such as quartz, feldspar, mica, and iron and aluminum oxide coatings) are electrical insulators of very high resistivity.
In general, ground conductivity is electrolytic and takes place through the moisture-filled pores and passages contained within the insulating soil and rock matrix. Therefore, the conductivity of both soils and rocks is a function of:
- Moisture content
- Concentration of dissolved electrolytes in the contained moisture
- Temperature and phase state of the pore water
- Amount and composition of colloids
Electromagnetic conductivity values (expressed in milliSiemens/meter [mS/m]) are not necessarily diagnostic in themselves. Scientists look at the spatial variations in conductivity values to assess sites.
Advantages of Frequency Domain Electromagnetics
- Rapid data collection
- Integration with differential GPS allows for accurate measurement location
- Good lateral resolution
Limitations of Frequency Domain Electromagnetics
- Limited vertical resolution (2 to 150 ft). Maximum depth of measurement depends on coil spacing or vertical dipole generated by the instrument.
- Susceptible to interference from induced noise from power lines and cultural features such as metal pipes, fences, vehicles, and electric utilities.
A contour map of ground conductivity shows the distribution of soils across a proposed building site. Fine-grained sediments, such as clay and silt, which are good at retaining water, are electrically conductive (blue, green, and yellow colors). Porous, coarse-grained sediments, such as sand and gravel, which are poor at retaining water, are electrically resistive (orange, red, and pink colors).
electrical resistivity • electromagnetics • environmental geophysics • geophysics • gravity • ground conductivity • ground-penetrating radar • induced polarization • magnetics • Near-Surface geophysics resistivity • surface-based geophysics • transient electromagnetics