Long-period (10 < T < 20,000 s) Magnetotelluric Studies in Northern Vietnam

摘要

We consider results of magnetotelluric and magnetovariational soundings in the period range 10 < T 20 000 s in the North Vietnam area. The simple structure of magnetovariational responses is shown, which generally reflects the electrically conductive quasi-two-dimensional structure of the Earth's crust. Impedance responses form as a superposition of responses of local complex subsurface and quasi-two-dimensional regional deep-seated structures. Separation of local and regional effects made it possible to construct a geoelectrical depth model of the regional tectonosphere, whose main elements are electrically conductive subvertical trans-lithospheric faults and high-resistivity disturbances of the conductive asthenosphere. These elements, favoring the flow of telluric currents induced in deep-seated electrically conductive systems into the sedimentary cover, form zones of abnormal apparent-resistivity curves. The apparent resistivity monotonously increases with increasing the period up to 20,000 s. We called the combination of such elements of the geoelectrical model a ultradeep fluid-fault system (UDFFS). The modeling has established the location of three orthogonally intersecting UDFFS of NE and SE strikes in the North Vietnam area. It shows that conductive (fluid-saturated) translithospheric faults extending to the base of the sedimentary sequence control the location of petroleum fields and ore deposits. A method for separating local and regional magnetotelluric effects is proposed. It permits one to determine reliably the main strikes of a regional two-dimensional structure and the configuration of the apparent-resistivity curves along them.