Two‐level magnetovariational measurements for the determination of underground resistivity distributions 1

We investigate, from a theoretical point of view, the possibility of performing marine two‐level magnetovariational measurements. An apparent resistivity function is denned and calculated after solving the differential equation governing the behaviour of the natural magnetic field variations inside a one‐dimensional earth. In order to generalize the problem, a frequency‐dependent resistivity is assumed to characterize the layers and the distortions caused by the polarization effects are carefully analysed. The computation of three‐layer amplitude and phase diagrams for the apparent resistivity function shows that, in the case of an intermediate polarizable layer, sandwiched between a non‐dispersive overburden and substratum, the H‐type sequence results are the most affected by the dispersion phenomenon as it occurs in magnetotellurics. Finally we consider the problem of the sensitivity of the method, since, in practice, it requires top and bottom sensors separated by a vertical finite distance. It is found that in the higher‐frequency range, due to the strong attenuation of the relative components of the field, the depth of the bottom sensor must be small enough to guarantee detectable signals, well above the full‐scale resolution of the acquisition system. Conversely, in the lower‐frequency range such a depth must be large enough to allow the difference between the top and bottom signals to be above the same recording sensitivity threshold.