The Method: Magnetotellurics (MT) measures the natural low-frequency electromagnetic field of the earth. The field components measured on the seafloor are influenced by the resistivity of the geologic formations beneath the site, and the resistivity of the subsurface as a function of frequency can be computed from the EM field measurements. Resistivity as a function of depth and lateral position is obtained through inversion and modeling of the resistivity/frequency relationship. Geologic structure is then interpreted from an analysis of the resistivity data and its relationship to corresponding seismic data and well logs. As with any geophysical method, there are limitations on the depth of burial, formation thickness and resistivity contrasts that will affect the viability of an interpretation. The primary, difference (other than the location of the receiver site) between marine MT (MMT) and the more familiar land MT is that, in the marine environment, the higher frequencies are attenuated by the conductive sea water above electromagnetic receivers, limiting the high-frequency range and thus the minimum depth of investigation.
Operations: A self-contained MT station consisting of electric and magnetic field sensors and a data logging system is deployed on the seafloor from a suitable survey vessel. The marine MT station includes an anchor to hold it securely to the seafloor, an acoustic release mechanism to release the anchor, and flotation to bring the station to the surface. The receivers are tracked and located acoustically. The receivers are recovered and the data downloaded, processed, and interpreted in terms of, first electrical units, and then geologic formations and structure.
Applications: Resistivity is important because rock types important to hydrocarbon exploration can be differentiated on the basis of resistivity value. While MT cannot be used to detect oil directly, the identification of favorable rock types and the presence of geologic structures capable of trapping hydrocarbons are critical to successful exploration. Recent successful applications of marine MT have emphasized difficult seismic areas, where because of the presence of volcanics, carbonates, or salt, the seismic data are difficult to interpret or ambiguous. MMT data has recently been used in conjunction with gravity and seismic data to create accurate salt geometry, and subsequently, an enhanced velocity model for seismic prestack depth migration. This is known as Multimeasurement Constrained Imaging (MMCI).
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