A MAGNETOTELLURIC STUDY OF RESISTIVITY ANISOTROPY

The apparent resistivity curves obtained from the magnetotelluric field measurements at the University of Alberta Geophysical Observatory, near Leduc, Alberta, reveal a strong anisotropy. The significance of the direction of the incident magnetic field in interpreting the structure from the apparent resistivity curves is examined in detail for the case of a linearly polarized incident field. The tensor impedance analysis of the data indicates that the directions of the principal axes of anisotropy are aligned parallel to the north-south and east-west measuring axis. The high coherency between the orthogonal horizontal components of the incident magnetic field is in complete agreement with this interpretation. The two apparent resistivity curves are interpreted and an anisotropic layered model is derived. The anisotropy is found to lie in the Precambrian basement down to a depth of 14 km. The isotropic upper layer has a resistivity of 8 ohm-m which is characteristic of the sedimentary section. The anisotropic resistivity structure proposed for the Precambrian basement may be attributed either to local lithological changes in the basement or to major but more distant structure. © 1972 Society of Exploration Geophysicists.