On the Use of Different Sources for Controlled Source Radiomagnetotellurics

In the controlled source radiomagnetotelluric (CSRMT) sounding method, different types of sources are in common use. However, no systematic examination of their advantages and disadvantages exists. In this paper, we analyze the electromagnetic fields of different CSRMT sources: horizontal electric dipole, HED, horizontal magnetic dipole, HMD, and vertical magnetic dipole, VMD, using both numerical modelling and field data. Positions of the boundary between the far-field and transition zones have been determined. Using 1D and 2D modelling and results of field experiments, we have shown that the HMD source has the smallest transition zone, while the VMD source has the largest one. In general, the HMD and HED sources are preferred for soundings in the far-field zone, due to the versatility of the transmitter’s geometry, and to the possibility of tensor measurements and use of 2D-3D magnetotelluric codes for data interpretation. In the case of the homogeneous half-space, for all sources the boundary between the transition and the far-field zone is farther away from a source for the impedance phase than for the apparent resistivity. Comparison of the signal magnitudes’ decay indicates that the field from the VMD source shows the slowest decrease with distance in the transition zone, while the field from the HMD source shows the fastest decrease, confirming the shorter range of measurements using the latter source. Using field experiments, we have compared the magnitudes of HED-, VMD-, and HMD-signals at odd subharmonics relative to the signal magnitude at the main frequency. We find that use of a HED source has definite advantage over loop sources for broadband frequency measurements with the square waveform from transmitter.