Audio-Magnetotelluric Surveying
This webpage is a relatively non-technical description of the use of the audio-magnetotelluric (AMT) geophysical method to groundwater exploration. Some technical information is added as parenthetical elements in the text and endnotes referenced in brackets.
Audio-magnetotelluric surveying is a geophysical technique that uses naturally occurring electrical and magnetic fields [1] to approximate the distribution of electrical resistivity in the relatively shallow part of the Earth's subsurface. The "audio" designation is misleading because sound waves are not used. Instead "audio" refers to electromagnetic wave data in the frequency range of about 1 Hertz (Hz) to 20,000 Hz (a Hertz is one cycle per second), which happens to correspond roughly the range of sound waves that humans can hear (about 20 Hz to 20,000 Hz). This frequency range is mostly in the part of the electromagnetic wave spectrum classified as very low frequency (VLF). Low frequency corresponds to long wavelength. The wavelengths used in AMT range from about 15 kilometers to more than 100 kilometers. "Telluric" comes from the Latin word "tellus" which means earth both in the sense of "soil" and in the sense of "the globe". So magnetotelluric refers to magnetic fields in the subsurface of the Earth.
The electrical resistivity of subsurface materials (rocks, sediment, soil) is a measure of how well the material conducts electricity. How well the material conducts electricity is affected by its composition (lithology), porosity, and water saturation and salinity. Consequently, the subsurface distribution of resistivity may be interpreted to estimate the location and distribution of subsurface geologic and hydrologic features, including aquifers.
AMT has been used as a reconnaissance geophysical method in groundwater and geothermal exploration. Its use is encouraged by the following attributes:
Low cost.
Low environmental disturbance: Does not use artificial sources, or heavy equipment.
Portable equipment: Can be used in remote and poorly accessible areas.
Deep investigation: Can probe depths to several hundred feet.
However, it has significant limitations:
Low resolution: Subsurface features are defined only by differences in apparent resistivity based on electric and magnetic field data collected at the ground surface;
Cultural noise: Electrical noise from power lines and other human-made devices may produce VLF data that is difficult to separate from natural signals, but such data cannot be used to estimate subsurface resistivity.
Complexity: Electromagnetic theory applied to audio-magnetotelluric surveying is complex and is not intuitive. Proper collection and interpretation of the data requires adequate knowledge of (1) electrical instruments and their field installation and operation, (2) theory involving advanced mathematics, and (3) computer languages used in the data processing.
Electromagnetic Signals
[This webpage is still being written. If you have suggestions please send them to ddunn@dunnhydrogeo.com.]
Endnotes for Audio-Magnetotelluric Surveying
[1] Electromagnetic Fields
A field is a physical quantity that has a value at every point is space. Electrical and magnetic fields are force fields. Their value at every point is a vector. A vector is a value that has both magnitude and direction. It is often represented graphically as an arrow whose length corresponds to magnitude. The magnetic field is measured in Teslas or gammas). The electrical field is measured in volts per meter, which means the voltage difference between pairs of electrodes is measured. It is good practice to install the electrodes in deep wet pits several hours before they are used. Magnetic field components are usually measured with induction coils (Vossof, 1990).