The use of a total-field magnetometer in the magnetotelluric method of vertical resistivity profiling

Recent work in this country and abroad has indicated that orthogonal E and H components of the magnetotelluric-micropulsation field may be used in the investigation of the vertical resistivity structure of the earth. Most of the techniques used are based on the assumption that the micropulsation field results from vertically incident electromagnetic waves. In their simpler form, the techniques are also based on the assumption that the earth model to be studied is layered in a horizontally homogeneous manner. Under these conditions, the vertical H component is zero and a total-field magnetometer will measure the north-south H component of the geomagnetic micropulsation multipled by a constant. This measurement may be used in the usual way with an east-west earth current measurement for the determination of resistivity. Thus, it is possible, in regions of known horizontal homogeneity, to exploit the high sensitivity, portability, and freedom from vibration effects of an optical pumping magnetometer. An experiment is described in which the ratios of ∣Hy∣/∣Ex∣ for steady-state magnetotelluric waves of the locally predominant periods were computed from E-W earth current and total-field magnetometer records at a test site near Dallas, Texas. The test site was located in a sedimentary basin in which 14,000 feet of horizontally lying Cretaceous and Paleozoic sediments of low resistivity rest on a highly resistive, Precambrian basement. The experimentally determined ∣Hy∣/∣Ex∣ ratios were in good agreement with theoretical values obtained by the method of Berdichevsky and Brunelli who used resistivity and structural data obtained from a nearby well.