Analysis of Magnetotelluric Response Functions at the Cam Thuy Site, Northern Vietnam

Abstract

During magnetotelluric studies in North Vietnam, an effect where the impedance phases Zxy and Zyx simultaneously left their quadrants was detected at one of the sounding sites, CTH, in the Cam Thuy District, Thanh Hoa Province. Investigation of this phenomenon has shown that it is accompanied by negative (bottom-up directed) components of the Umov–Poynting vector of electromagnetic energy flux density in the entire range of the recorded periods of magnetotelluric variations. This direction of the energy flux remains unchanged when the initial coordinate system is rotated by angles of 45° and 60°. Another characteristic feature of this phenomenon is an unusual behavior of the apparent resistivity curves: the apparent resistivity values increase starting from a period of 200 s up to a period of 104 s, and this increase occurs practically along the asymptote. The analysis of the polarization of the magnetotelluric field and the asymmetry parameter (skew angle) β of the phase tensor has shown that the section exhibits significant three-dimensional distortions as manifested by β values ranging from 20° to 90°.