Effect of offset on the depth and effectiveness of controlled-source audio-frequency magnetotellurics exploration

An appropriate offset is crucial for the successful application of controlled-source audio-frequency magnetotellurics (CSAMT) exploration. To investigate the effects of offset on the depth of investigation and application effectiveness of CSAMT exploration, inversion with 3D nonlinear conjugate gradient (NLCG) optimization is applied to synthetic and field datasets with various offsets. In the inversion, forward modeling using the staggered-grid finite difference method to discretize the secondary electric field equation is solved via the quasiminimal residual (QMR) method. The results demonstrate that as the offset increases, datasets show a progressive transition from the near-zone field to the middle- and far-zone fields, enabling more low-frequency data to acquire frequency sounding capabilities. The data from the far-zone field are significantly enriched in deep resistivity measurements. The optimal offset represents a trade-off between the maximal signal strength (effectiveness) and the objective depth of exploration. In light of these results, we synthesize and propose a structured five-step guideline for optimal offset selection, derived from established field-zone division principles and the skin-depth formula: (1) simple subsurface electrical structure construction, (2) target depth estimation, (3) frequency determination, (4) wavelength calculation, and (5) offset optimization. Field data from the Lushi Basin, China confirm that the offset affects not only the signal strength but also the reliability of the actual measured data. This study provides a reference for survey design in offset selection for CSAMT practitioners.