Sounding mechanism and response characteristics of short-offset frequency-domain controlled-source electromagnetic

ABSTRACT The frequency-domain controlled-source electromagnetic (FD-CSEM) method is a crucial tool for resource exploration, providing a higher signal-to-noise ratio compared to natural-source methods. The effectiveness of land-based FD-CSEM for short-offset sounding remains a topic of debate, particularly concerning its detection mechanisms and theoretical limitations. A comprehensive analysis of the FD-CSEM detection mechanism is presented from theoretical and numerical simulation perspectives. Analysis of the integral characteristics of the electric field shows that, at short offsets, the response consists of two components: a dominant field and a subtle field. The dominant field, defined as the contribution from the effective interval of the kernel integral where spectral components add coherently, is generated mainly by evanescent waves and reflects shallow resistivity structures, resembling geometric sounding. Reliable frequency sounding based on this field requires offsets of about six times the target depth for apparent resistivity, or three times for inversion. In contrast, the subtle field, arising from the remaining spectral regions outside the effective interval and generated by attenuating traveling waves, has low amplitude but stable presence. It provides frequency-dependent sensitivity to deeper structures consistent with long-offset observations. This capability can be exploited if high-precision data are obtained, with smooth low-frequency trends and a differential response that decays approximately as a 1.5 power law. These results demonstrate that the FD-CSEM method at short offsets not only achieves excellent resolution for shallow structures but also maintains potential for deep subsurface exploration. The findings contribute to a deeper understanding of FD-CSEM’s applicability and potential for short-offset and deep-target detection.