Relief effects correction on frequency-domain electromagnetic data

Abstract

During a frequency-domain electromagnetic (FDEM) land survey using transmitter-receiver distances of kilometer order, the receiver and transmitter may be at different altitudes. To increase the signal-to-noise ratio, the transmitting coil size must be increased to the order of a hundred meters and its geometry will be determined by the terrain roughness. Therefore, the equivalent magnetic dipole axis may be neither vertical nor normal to the mean plane representing the terrain surface. Considering the perpendicular loop-loop arrangement, these factors modify the expected secondary magnetic field in two ways: (1) A horizontal primary field arises at the receiving coil position as well as (2) the secondary fields induced by the abnormal currents in the subsurface caused by the tilting of the transmitter dipole axis. A correction procedure is proposed to remove these effects on field FDEM data and tested by using simulated FDEM data with two- or three-layered tilted models to represent the earth with a dipping surface and a nonvertically oriented transmitter magnetic dipole representing a large coil laid on rough terrain. The results demonstrate that the proposed correction procedure has a limited effectiveness, but it can be applied to the FDEM data collected on terrain surfaces having small dipping angles. It is observed that maximum values of the transmitter dipole or surficial plane tilt angle should be 2° to ensure error values in the apparent conductivity less than 10%. Even for the said value, in some combinations of geometric and physical parameters, the tilting and dipping angles can be increased to the order of 5°.