Peng et al. Geotherm Energy (2019) 7:14 https://doi.org/10.1186/s40517-019-0130-y RESEARCH Open Access Geophysical survey of geothermal energy potential in the Liaoji Belt, northeastern China Chong Peng1,2,3, Baozhi Pan1, Linfu Xue2* and Haiyan Liu3 *Correspondence: [email protected] Abstract 2 College of Earth Sciences, The Liaodong area which lies in the Liaoji Belt of northeastern China is rich in geother- Jilin University, 2199 Jianshe Street, Changchun 130061, mal resources, but locating the resources is challenging. Here non-seismic geophysical People’s Republic of China data, existing geological data and the physical properties of rocks are examined to Full list of author information locate and characterise potential geothermal resources. High-precision gravity, aero- is available at the end of the article magnetic and magnetotelluric data are analysed to determine the characteristics of the hidden faults in the region and map the spatial distribution of the subsurface rock mass and its properties. The distribution of hidden faults and the basic characteristics of the rock mass were determined via Euler deconvolution of the gravity and aero- magnetic data. The subsurface extension of concealed faults, the spatial distribution of intrusive rocks, and the specifc locations of underground thermal storage structures were determined from magnetotelluric data analysis, thereby enabling geothermal energy targets to be identifed. The results highlight four potential sites of geothermal energy development in the study area, which indicate that non-seismic geophysical data can provide reliable clues to the geothermal energy potential in a given region. Keywords: Liaoji Belt, Gravity, Aeromagnetic, Magnetotelluric, Liaodong area, Geothermal resource Introduction Economic development has accelerated the consumption of oil, natural gas, coal and other traditional energies, leading to the depletion of traditional fossil fuel resources, heavy pollution and degradation of the natural environment (Dincer and Acar 2015; Li and Xue 2015). Terefore, there is a growing need for clean energy (Fridleifsson 2001). Geothermal energy is one of the most important clean energy options that has attracted growing attention worldwide (Barbier 2011). Geothermal resources have the advan- tage over traditional fossil fuels of possessing large reserves that are clean and environ- mentally friendly (Kütahyali et al. 2011; Croteau and Gosselin 2015). An increase in the global use of geothermal resources would reduce the emissions of particulate matter and harmful gases that result from the combustion of fossil fuels (Lior 2010). Researchers have developed and implemented various methods to investigate the potential reserves and exploitation of geothermal resources with promising results. For example, geophysical methods have been employed to predict the geothermal potential of the Baja California Peninsula (Arango-Galván et al. 2015); deep-buried faults in geo- thermal regions have been successfully identifed using microtremor arrays (Xu et al. © The Author(s) 2019. This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creat iveco mmons .org/licen ses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Peng et al. Geotherm Energy (2019) 7:14 Page 2 of 19 2012); and gravity and magnetic data (Mohammadzadeh Moghaddam et al. 2016; Abdel Zaher et al. 2018), as well as magnetotelluric soundings (Zhang et al. 2015; Hersir et al. 2018), have been acquired and analysed to infer geothermal energy potential. Te North China Craton, one of the oldest cratonic blocks in the world (Liu et al. 1992; Song et al. 1996), has been intensively studied in terms of its geothermal energy potential, with the Liaoji Belt attracting considerable attention (Zhang 1984; Peng and Palmer 1995; Li and Zhao 2007; Li and Chen 2014; Yuan et al. 2015). Previous geochemical analyses have suggested that the heat generation rate of the intrusive rocks throughout the belt is relatively high, which could provide the heat source for the formation of geothermal resources (Wang and Huang 1990; Hu et al. 2001; Li and Xue 2015). Terefore, we analysed our non-seismic geophysical data using Euler deconvolution to investigate the geothermal energy potential in the Liaoji Belt. Euler deconvolution of the gravity and magnetic data provided a better characterisation of the highlighted structures and delineated the extent of the geothermal source. Euler deconvolution was selected because it is a fast inversion and interpretation method for gravity and mag- netic potential feld data that can automatically determine the location of the feld source without any a priori information, and efectively delineate the location, depth and spatial distribution of fractures in a rock mass. Terefore, we present the frst investigation of the geothermal energy potential of the Liaoji Belt (Fig. 1) via the integration of geological and non-seismic geophysical data to determine the thermal structure of the area and delineate geothermal targets. Geological setting Te study area is located in the Liaodong region, which lies in the northern Jiao-Liao- Ji Belt of the North China Craton, northeastern China (Fig. 1). Te Liaoji palaeo-belt is a relatively complete and preserved Palaeoproterozoic zone that possesses favourable geological conditions for mineralisation. It has undergone a complex tectonic evolution, including uplift, subsidence, arching and rifting, with metamorphism and deformation at diferent scales within the rift, as well as the formation of complex folds and deep- seated faults (Lu et al. 1998; Zhao et al. 2001, 2005; Zhao 2009; Li et al. 2006; Wan et al. 2006; Lu et al. 2006; Tam et al. 2011; Meng et al. 2013). Repeated, intense episodes of magmatism in the Liao Ji rift produced large intrusive rock masses, with intense hydro- thermal-exhalation along deep-seated faults (Zhao et al. 2001, 2005; Zhao 2009; Li et al. 2005, 2006, 2011, 2012; Zhao et al. 2012; Zhao and Zhai 2013; Peng et al. 2016a). Te region is rich in geothermal resources, with measured geothermal water temperatures and fow rates of up to 70 °C and 4 kg/s (Zhong and Xiao 1990; Table 1). Previous heat fow estimates for the North China Craton are in the range of 29.7– 113.9 mW/m2 (He 2015), with the eastern North China Craton possessing a high back- ground heat fow of ~ 63 mW/m2 (Jiang et al. 2019). Te heat fow estimate for the Liaodong area is ~ 56.97 mW/m2, which is lower than the continental average in China, with the granites, faulted areas, and older metamorphic rocks yielding values of 55.7– 90.4, 64.9–72.0, and 32.1–49.0 mW/m2 (Li et al. 2014; Li 2015; Fig. 1), respectively. Furthermore, the geothermal gradient in the Liaodong area is only 20 °C/km (Li et al. 2014; Li and Xue 2015). We, therefore, infer that radiogenic heat from the granites is the heat source (Zhong and Xiao 1990; Liu et al. 2014; Li et al. 2014; Li and Xue 2015), Peng et al. Geotherm Energy (2019) 7:14 Page 3 of 19 Fig. 1 Tectonic setting of the NCC (a), regional map of the Precambrian geology of the Eastern Block in the NCC (b) and Simplifed geological map of the study area (c), modifed after Zhao et al. (2005) and Peng et al. (2016a). 1. Paleozoic. 2. Permian. 3. Jurassic. 4. Archean granitic gneiss. 5. Paleoproterozoic granite. 6. Early Triassic diorite. 7. Middle Triassic granite. 8. Jurassic granite. 9. Cretaceous granite. 10. Locations of MT measurement lines and points. 11. Fault. 12. Hot spring. 13. Heat fow values with the 10-km-deep zone of radiogenic heating generated by these shallow intrusive rocks playing a major role in the formation of the geothermal resources. For example, the rate of radiogenic heat generation in the mantle and the lower crust is low, which has little infuence on the heat fow at the surface. Recent studies have confrmed that the geothermal resources in the Liaodong area are derived primarily from radiogenic heat generation in granite of various ages. Most of the hot springs that have recently been found in the Liaodong area occur above intrusive rocks and in areas that have undergone tensional fracturing (Fig. 1), producing mainly NW–SE-trending normal faults (Liu et al. 2014; Li and Xue 2015). Te intrusive rocks provide the heat that forms the geothermal Peng et al. Geotherm Energy (2019) 7:14 Page 4 of 19 Table 1 List of hot springs in the Liaodong area Number Position Temperature (°C) Flow rates (kg/s) a 123°03′E 040°23′N 60 – b 123°09′E 040°24′N 50 1.39 c 123°25′E 040°18′N 48 0.97 d 123°15′E 040°16′N 47 – e 123°36′E 040°32′N 70 4.00 f 123°45′E 040°18′N 46 0.97 g 123°56′E 40°46′N 32 3.33 h 123°46′E 40°25′N 31 0.42 j 123°11′E 40°61′N 48 1.00 k 123°16′E 40°40′N 40 0.97 resources in the Liaodong area, and the dense distribution of fractures provides migra- tion pathways that enable the development of geothermal resources at the surface. Methods An efective geophysical interpretation of geothermal potential requires a petrophysical analysis of the relevant rocks (Huang et al. 1998; Li and Chen 2013). Terefore, we used data (Table 2) from Peng et al. (2016a) and divided them into three classes (high/strong, intermediate and low/weak; Li and Chen 2013; Peng et al. 2016a; Table 3). We also sum- marised the physical characteristics of the strata and intrusive rocks (Li and Chen 2013; Peng et al.