A 2-D profile of apparent resistivity generated from multiple 1-D TEM measurements using a 100-m loop was used along with magnetic readings to interpret the location of the fault illustrated in the profile.
Electromagnetic methods are used at Southwest Research Institute (SwRI) to measure subsurface electrical conductivity. SwRI scientists can perform electromagnetic surveys using frequency domain electromagnetic instruments or transient electromagnetic instruments.
Transient electromagnetic (TEM) instruments allow greater depth of penetration of an induced current by using large transmitter loops and lower frequency signals.
TEM methods, also known as time-domain electromagnetics, measure the electrical conductivity of soil and rock by inducing pulsating currents in the ground with a transmitter coil and monitoring the decay of the induced current over time with a separate receiver coil.
Multiple one-dimensional soundings are used to generate two- and three-dimensional images of the subsurface. TEM measurements can be applied to:
- Delineation of the depth and thickness of geologic and hydrologic units
- Detection of the water table and water quality (salinity)
Advantages of Transient Electromagnetics
- Good lateral and vertical resolution
- Depth range of measurement approximately 20 to 3,000 ft
Limitations of Transient Electromagnetics
- Deep measurements require a large transmitter coil for which space may not be available
- Susceptible to interference from induced noise from
power lines and nearby metal objects such as:
electrical resistivity • electromagnetics • environmental geophysics • geophysics • gravity • ground conductivity • ground-penetrating radar • induced polarization • magnetics • Near-Surface geophysics resistivity • surface-based geophysics • transient electromagnetics