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.