Possibility of deep crustal probing in orogenic regions by studying variations of the surface impedance parameter of natural electromagnetic fields

摘要

Crustal structure in orogenic regions such as the Himalayan one consists of a tilted sedimentary layer on a granitic basement which is covered by metamorphic rocks. The complex crustal problem is modeled by an almost representative three-layer model of an anisotropic dipping layer with a substratum of a transition layer overlaying a homogeneous half space. The surface impedance of natural electromagnetic fields has been evaluated for varying anisotropy and inclinations of the sedimentary surface and thickness of the metamorphic rock regions. The following interesting conclusions can be drawn from the curves. (i) Generally, crustal electrical mappings are limited in depth by increasing anisotropy of horizontal sedimentary layers (higher longitudinal conductivity as compared to the transverse one). Fortunately, here the plane wave impedance has a better response for higher dip angles and increasing anisotropy coefficients. (ii) The thickness of inhomogeneous metamorphic regions modifies the phase of the impedance parameter quite characteristically, particularly for high anisotropy factors. The study of deeper processes in areas of intense deformation and movements may find ready application of the numerical results of the realistic model, showing promise for deeper diagnostic probing by natural electromagnetic fields.