THE TELLURIC‐MAGNETOTELLURIC METHOD

Abstract

The telluric‐magnetotelluric method uses magnetotelluric measurements at the base site, but only telluric measurements at remote sites. It thus combines the economy, simplicity, and speed of the traditional telluric method with the quantitative advantages of the traditional magnetotelluric method. The dominant features of the combined method are the following: First, the time required to set up a telluric site is less by a factor of at least 5 than the time for a complete magnetotelluric site. Second, one does not need to record magnetic field data at the base site simultaneously with the electric field recorded at each remote site. One needs only enough magnetic data to adequately determine the base tensor. A telluric transfer tensor coupling electric field measurements at the base site and each remote site can be used to transfer the base impedance tensor to an impedance tensor at each remote site. By being much more selective of the magnetic data used in the analysis, one can significantly improve the signal‐to‐noise ratio. Third, the data are analyzed to determine each element of the complex impedance tensor so that important phase information as well as amplitude information is available for interpretations which are more sophisticated than those currently attempted in conventional telluric surveys. Finally, in making the ultimate interpretation in terms of the impedance tensor rather than the telluric tensor used in conventional telluric surveys, one essentially refers the interpretation of remote electric field observations to the magnetic field at the base site rather than to the electric field. Both experience and model studies demonstrate that the magnetic field is much more homogeneous than the electricfield in the vicinity of lateral heterogeneities; thus the selection of a proper base site is not as critical in the combined method as it is in the conventional telluric method.