Investigating the Time-Frequency Electromagnetic Method for Detecting Deep Karst Geothermal Reservoirs in Tianjin

Abstract Tianjin, renowned for its geothermal wealth, features the Wumishan Formation of the Jixian System as its principal geothermal reservoir, exemplifying a classic carbonate reservoir. Aiming to delineate the stratigraphic structure and uncover concealed faults beneath Dongli Lake’s 5-kilometer depth in Tianjin, while also pinpointing the spatial distribution and structural attributes of the primary deep thermal reservoirs to forecast potential geothermal zones, the authors have innovatively adapted the time-frequency electromagnetic method—traditionally employed in deep oil and gas exploration—for the deep geothermal investigation of Dongli Lake. This approach involved meticulous data acquisition, sophisticated processing, and resistivity inversion techniques. Utilizing seismic and borehole data, the horizon was calibrated to provide a nuanced portrayal of the subsurface structure. The study postulates the existence of seven concealed faults, characterized as NNE and NWW-oriented tensional normal faults. Notably, the Cangdong Fault (F1), serving as a critical conduit for the area’s principal thermal fluids, is implicated in heat conduction, storage, and the modulation of geothermal anomaly zones’ distribution and configuration. The Wumishan Formation’s thermal reservoir predominantly lies to the west of the Cangdong fault, with the top interface buried at depths ranging from −2200 to −2400 meters, exhibiting a progressive increase in resistivity, indicative of continuous A-type electrical properties. The karst fissures within the third and fourth sections, when well-developed, lead to localized low resistivity anomalies and H-type electrical signatures due to the accumulation of hot water, thereby highlighting the prospective geothermal zones at targeted depths. This paper delineates four such favorable areas, offering vital foundational data and support for the future development and exploitation of these geothermal resources.