Magnetotelluric exploration for hydrocarbons

The magnetotelluric (MT) method utilizes naturally occurring low-frequency electromagnetic energy to determine the electrical resistivity of the Earth´s subsurface. The resistivity distribution of the subsurface is then interpreted in terms of rock type and geologic structure. MT-derived resistivity data are usually integrated with other geophysical results, and with surface and subsurface geologic data if available, to arrive at an interpretation, The basic relationships that govern the application of MT to hydrocarbon exploration are reviewed along with the electrical properties of rocks, MT energy sources, and briefly, MT theory. Data acquisition and processing and field procedures are covered. It is shown how by measuring the complete vector electromagnetic field an impedance tensor may be calculated, which, when evaluated as a function of frequency, samples the electrical characteristics of the Earth as a function of depth through the skin-depth relationship. A discussion of current usage is included, concluding with representative case studies which illustrate the application of MT to hydrocarbon-exploration problems