Transient electromagnetic responses to deep low resistivity targets beneath thick resistive ice sheet

Thick, high-resistivity ice sheets extensively cover bedrock and sedimentary layers, posing significant challenges for the identification of subglacial hydrological systems and associated geological structures. Subglacial water systems not only play a crucial role in regulating ice-sheet dynamics and material transport, but also serve as important indicators of deep geological environments, fluid activity, and potential mineralization conditions.However, high-resistivity ice sheets significantly enhance electromagnetic energy attenuation during field propagation, resulting in insufficient recoverable low-frequency signals and thereby limiting the detectability of deep low-resistivity anomalies. This challenge is widespread in polar environments and exhibits strong similarities to those encountered in other high-resistivity-covered mineral exploration settings.

In this background, this study applies the loop-source transient electromagnetic (TEM) method to to systematically analyze the spatial distribution characteristics of transient attenuation curves and electric field components by constructing various underground models (e.g., subglacial water systems and fluid-rich anomalies). Results indicate:

(1) In models containing high-conductivity anomalies (such as saturated sedimentary layer), the presence of conductive bodies significantly slows electromagnetic field diffusion. As a result, response signals maintain relatively high amplitudes during late-time sampling, resulting in attenuation curves exhibiting a characteristic S-shaped bulge. This indicates that transient electromagnetic methods possess high discrimination capability for identifying water-bearing low-resistivity anomalies.

(2) As the ice thickness increases from 50 m to 500 m, the transient electromagnetic response curve exhibits an overall rightward and downward shift. The rightward shift reflects the elongated propagation paths and delayed response times of electromagnetic fields within thick resistive cover, whereas the downward shift indicates enhanced attenuation of electromagnetic signals by the overburden, thereby reducing sensitivity to deep subsurface structures. In addition, increasing cover thickness amplifies response differences among distinct subsurface targets, leading to reduced resolution in inverted models.

(3) Under conditions of thin ice cover, differences in transient responses induced by varying transmitter loop sizes are relatively minor. However, as ice thickness increases, the required transmitter magnetic moment rises substantially. Large transmitter loops (e.g., 300 m and 500 m) generate stronger transient electromagnetic fields owing to their higher magnetic moments. Their late-time responses exhibit higher amplitudes and longer persistence, indicating enhanced sensitivity to deep low-resistivity anomalies. This improvement contributes to better imaging performance and more reliable identification of deep subsurface targets.

Overall, the loop-source transient electromagnetic method demonstrates strong applicability for detecting subglacial hydrological systems in polar regions. It exhibits significant detection potential for identifying low-resistivity anomalies associated with fluid activity and potential mineralization within thickly covered environments.These findings provide valuable technical references for subglacial hydrological investigations, deep geological structure studies, and deep mineral exploration in polar regions and other areas characterized by thick resistive cover.