Controlled‐source audiofrequency magnetotelluric responses of three‐dimensional bodies

Abstract

Modeling the controlled‐source audiofrequency magnetotelluric (CSAMT) responses of simple three‐dimensional (3-D) structures due to a grounded electric bipole confirms that the CSAMT technique accurately simulates plane‐wave results in the far‐field zone of the transmitter. However, at receiver sites located above large conductive or resistive bodies, the presence of the inhomogeneity extends or reduces, respectively, the frequency range of the far‐field zone. Measurements made on the surface beyond a large 3-D body display a small but spatially extensive effect due to decay of the artificial primary field. Situating a 3-D inhomogeneity beneath the source permits an evaluation of “source overprint” effects. When such a body is resistive, a slight shift in the near‐field response to higher frequencies occurs. When a body below the transmitter is conductive, it is possible to make far‐field measurements closer to the transmitter or lower in frequency. However, as the size of the conductor and its secondary‐field response increases, large transition‐zone responses distort the data. For both a plane‐wave source and a finite source, current channeling into a 3-D conductor from conductive overburden enhances the response of a target. The modeled response of a dike‐like conductor shows no better results for either the broadside or collinear configuration. The location and extent of such a body are better defined when measuring the electric field perpendicular to the strike of the prism, but resistivity estimates are better when using the electric field parallel to the strike of the prism, irrespective of transmitter orientation. Models designed from data collected at Marionoak, Tasmania, yield results which indicate that the thin, vertical graphitic unit intersected by drilling is detectable by the CSAMT method, but probably is not the sole source of the large anomaly seen in the CSAMT data.