Analysis of Subtle Field From Frequency-Domain Electromagnetic Response With Short-Offset Grounded-Wire Sources

Short-offset frequency-domain controlled-source electromagnetic (FD-CSEM) with grounded-wire sources often face limitations due to near-source and shallow structural effects, which diminish sensitivity to deeper targets. This study introduces a method that approximates and subtracts the lowest frequency response as a quasi-direct current field. This technique enables the extraction of a subtle field that maintains frequency-sounding capabilities while effectively reflecting deeper geological structures. Through theoretical and numerical analyses, we demonstrate that this subtle field is sensitive to deep anomalies and is minimally influenced by offset, surpassing the performance of conventional total-field signals. In addition, inversion techniques applied to the normalized subtle field improve detection depth and resolve both shallow and deep resistivity features. Field experiments validate these findings, illustrating the effective identification of subsurface anomalies. This work advances FD-CSEM by enhancing short-offset detection capability and offers promising avenue pathway for deep terrestrial electromagnetic exploration.