Magnetotellurics (MT)

Magnetotelluric (MT) methods measure the earth's impedance to naturally occurring electromagnetic waves in order to glean information about variances in conductivity (or, resistivity) of the earth's subsurface. MT data is invaluable in mineral, petroleum, geothermal and groundwater exploration, characterization, and monitoring.

The MT method is based on frequencies in the order of 10-3 Hz to 1 Hz; however, controlled source methods supplement the earth's natural fields with transmitted frequencies from 1 to 100 kHz allowing for shallower exploration. This is most useful in groundwater and some mineral exploration.

The cross section below illustrates interpreted MT data with the known geology superimposed. To simplify, areas which are less resistive (i.e. more conductive; the red to yellow range) are clearly shown here and suggest the presence of gold.

Similarly, MT and CSAMT methods can delineate areas of fresh and salt water in aquifer assessment and can be used to study tectonic activity deep within the earth's crust. Contact EMI for detailed Case Histories.

The same principles of MT apply to ocean floor exploration. Marine EM has great potential for the discovery of oil and gas reserves below salt structures on the ocean floor, where traditional seismic methods sometimes fall short. Simply put, salt structures reflect soundwaves, but, due to their high resistivity (in comparison with surrounding geology), are ideal for MT imaging. In fact, the lack of cultural noise (like trains, radio signal, automobiles) makes the ocean floor environment almost perfect for MT exploration. Moreover, Marine EM surveys can be completed at a fraction of the cost of seismic methods. At millions of dollars per well drilled, the value of preliminary MT reconnaissance cannot be overstated.

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