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Research | Concepts | MT Exploration

The marine magnetotelluric (MT) method

Copyright Kerry Key 2003

For a marine MT experiment an array of seafloor EM recorders is deployed from a ship onto the seafloor over a geologic target. Sensors on the EM receivers record the electric and magnetic field time variations on the seafloor. The source field for the MT method arises from the complex interaction of the solar wind and Earth's magnetosphere. This system generates a plane-wave magnetic field which propagates through the atmosphere and into Earth at frequencies of around 0.0001 to 10 Hz. As the magnetic field diffuses into Earth it attenuates at a rate that is proportional to the electrical conductivity of the subsurface. The magnetic field attenuation in turn induces an electric field, the strength of which also depends on the subsurface conductivity. In general, lower frequencies penetrate deeper while higher frequencies only diffuse to shallow depths, giving marine MT a depth sensitivity of 10's of meters to several 10's to 100's of kilometers, depending on the conductivity structure.

So by measuring both the electric and magnetic fields, we can calculate the conductivity structure beneath the seafloor. Whereas the seismic reflection method is predominantly sensitive to structural boundaries, the marine MT method is sensitive to what is contained within the layers. Crystalline rocks tend to be very resistive while porous rocks are usually conductive due to the seawater and brines contained within.

An example of an electrical resistivity model calculated from marine MT data is shown below. This model is from a marine MT survey over Gemini Prospect, Gulf of Mexico. The resistive blue structure at 2-4 km depths is a salt intrusion. Click here to see the corresponding seismic reflection image of Gemini.

 
Last updated: Wednesday, 06-May-2009 14:40:11 PDT
email: kkey@ucsd.edu