Magnetotelluric responses of three-dimensional bodies in layered earths

The electric and magnetic fields scattered by a three-dimensional inhomogeneity in a conducting earth result largely from current-gathering, a boundary polarization charge phenomenon that becomes increasingly important as frequency falls. Boundary charges cause normalized electric field magnitudes, and thus tensor apparent resistivities and magnitudes of vertical admittance elements, to remain anomalous as frequency approaches zero. However, these E-field distortions below certain frequencies are essentially in-phase with the incident electric field. In addition, secondary magnetic field amplitudes over a body ultimately decline in proportion to the layered host impedance. It follows that tipper element magnitudes and all MT function phases become minimally affected at low frequencies by an inhomogeneity. The applicability of 2-D transverse electric (TE) modeling algorithms is very limited, since this 2-D mode involves no boundary charges and hence no current-gathering. Furthermore, 3-D bodies in layered hosts typical in nature, with layer resistivities that increase with depth in the upper 10 or more km, are even less amenable to 2-D TE interpretation than are similar 3-D bodies in uniform half-spaces. However, centrally located profiles across elongate 3-D prisms may be modeled accurately with a 2-D transverse magnetic (TM) algorithm, which implicitly includes current-gathering in its formulation. In defining apparentmore » resistivity and impedance phase for TM modeling of such bodies, we recommend a coordinate system derived using tipper-strike, calculated at the frequency for which tipper magnitude due to the structure of interest is large relative to that due to any nearby geological noise.« less