Application of integrated geophysical exploration methods in target area selection for geothermal field in Xianxian County

Xianxian County is recognized as "the largest geothermal enrichment area in North China," boasting abundant medium- and low-temperature geothermal resources. To address the insufficient understanding of deep geothermal resource distribution and heat-controlling structure development in the southeastern block of Xianxian County, and to support the large-scale development of bedrock fracture-type thermal reservoirs, this study employed an integrated geophysical exploration approach combining the Magnetotelluric (MT) method and microtremor survey. It conducted exploration and interpretation of stratigraphic, fault, and thermal reservoir characteristics in the study area, with the reliability of results verified using drilling data.The study area is situated in the northern part of the Xianxian Uplift, Cangxian Uplift, Bohai Bay Basin. Within the depth of 4000 m, the strata from top to bottom include the Cenozoic, Mesozoic, Paleozoic, and Middle-Upper Proterozoic Erathems. For fieldwork, 5 MT profiles (total length: 28.67 km; measurement points: 60) were deployed. A V8 electrical instrument recorded orthogonal electromagnetic field components, and electrical structures were derived via multi-method inversion (1D Bostick, 2D RRI, 2D Occam). For microtremor surveys, 1 profile (length: 8.54 km; measurement points: 19) was laid out. An EPS-D10 broadband seismograph extracted Rayleigh wave dispersion curves to invert the underground shear wave velocity structure, and the two methods constrained each other to reduce geophysical non-uniqueness.Results show: (1) 6 secondary faults and fractured zones were identified, mainly distributed in the northwest of the study area. Well-developed fractures around fault zones provide channels for geothermal fluid migration, serving as key drilling targets. (2) Burial depths of the top/bottom boundaries of major strata were clarified: Quaternary System (Cenozoic) bottom boundary: 415−471 m; Neogene System bottom boundary: 980−1420 m; Wumishan Formation (Jixian System, Middle-Upper Proterozoic) top boundary: 980−1430 m. Strata exhibit a "deeper burial in the northwest, shallower in the southeast" pattern. (3) Geothermal gradient laws were revealed: the highest heating rate occurs in the Cenozoic and Mesozoic (Cenozoic geothermal gradient: 3.5−4.0 ℃/100 m); after entering the Middle-Upper Proterozoic basement, the heating rate slows down (bedrock geothermal gradient: 1−1.5 ℃/100 m); the temperature in the middle of the Wumishan Formation thermal reservoir is 70−80 ℃, decreasing gradually from northwest to southeast. (4) Suitable and relatively suitable geothermal mining areas were delineated. Karst-fracture thermal reservoirs of the Jixian System (Middle-Upper Proterozoic) are distributed throughout the area, with thermal reservoir thickness: 500−600 m, water inflow: 57−140.19 m³/h, and specific water inflow: 0.266−8.67 m³/(h·m).Verification via Exploration-Production Well 1 in southeastern Xianxian County (well depth: 2508 m; water temperature: 70 ℃; water inflow: 100 m³/h) shows high consistency between geophysically interpreted stratigraphic sequences/burial depths and actual drilling data. This study confirms that the combined application of MT and microtremor methods effectively characterizes geothermal geological structures, providing technical support for the scientific development of Xianxian's geothermal resources and the achievement of regional "dual carbon" goals.