Is short-offset frequency-domain controlled-source electromagnetic survey using an equatorial configuration a good choice on land?

Abstract

The frequency-domain controlled-source electromagnetic (FD-CSEM) method can be used to collect data in both the large-offset plane-wave zones and the short-offset zone using an axial configuration within conductive marine environments. In contrast, the equatorial configuration is more commonly adopted for field surveys on land. However, selecting a suitable offset using an equatorial configuration on land remains a challenge. Therefore, the primary objective of this study was to choose a reasonable offset using an equatorial configuration on land. To this end, we compared the distribution and decay characteristics of the horizontal electric field. Additionally, we analyzed the detectability of the horizontal electric field to targets based on the root mean square (RMS) misfit and three-dimensional (3D) simulation at different offsets. The 3D simulation results show that the maximum normalized response at different offsets is similar, indicating that electric fields at different offsets share similar resolution limits. However, the horizontal electric field at small offsets (less than 3 times the target depth) only have the detectability for the target resistivity and are unable to make more precise distinctions on the target thickness. The detectability for the target thickness at very small offsets (about one time the target depth) is limited, as frequency sounding cannot be effectively established. A larger offset distance (larger than 4 to 6 times the target depth) is required when observing the vertical magnetic field compared to the horizontal electric field. Finally, we observed the electric field data at different offsets in the Kalatongke nickel–copper deposit in Xinjiang, China, which verifies our theoretical analysis.