Integrated Application of Radon Measurement and Conventional Electrical Prospecting in Geothermal Exploration: A Case Study of Lantian Section, Ningdu, Jiangxi Province

Abstract

As a pivotal clean energy source with considerable reserves, geothermal water plays an indispensable role in diminishing reliance on fossil fuels and accomplishing carbon neutrality. This study employed conventional electrical prospecting and radon gas surveys in the Lantian area of Ningdu, aimed at curtailing geothermal development costs by precise targeting of resource locations. The investigations successfully delineated fracture structures within the Lantian region. Distinct anomalies were identified in the electrical profiling along Survey Lines 1, 2, and 4, with the most pronounced features observed on Line 4. Accordingly, characteristic peak anomalies were exhibited by the radon gas measurement profiles S1, S2, and S4 corresponding to Lines 1, 2, and 4, respectively. The synergistic interpretation of resistivity and radon survey data recognized two primary fracture zones: the NE-trending zone F1 and the NEE-trending zone F2. This integrated approach not only ascertained the efficacy of the radon gas measurement, but also lays a robust basis for future geothermal water exploration targeting.

1. Introduction

Given that deteriorating environmental issues urgently compel energy transition and carbon neutrality, geothermal energy, as a stable, low-carbon, and efficient renewable resource, is recognized as an essential asset in the energy transition. Applicable to heating, cooling, and power generation, it boasts dramatic environmental and economic benefits that have been evidenced in a number of countries. For example, Iceland exploits geothermal energy for over 81% of residential heating [1]. In 2024, according to data from the International Geothermal Association (IGA), China leads globally in both total installed capacity (MWt) and annual energy production (TJ/y) for geothermal heating and cooling, accounting for 58% and 56% of the world totals, respectively [2].

By now, geothermal energy is booming in both power generation and direct utilization. As a critical clean energy resource, it is distinguished by its enormous potential for carbon neutrality, and it is anticipated to be one of the preferred energy sources in the future. Therefore, more efforts are required for research on geothermal exploration, development, and utilization [3]. Geothermal exploration relies heavily on fracture structures in controlling the location and distribution of geothermal water. This prioritizes the detailed investigation of fracture structures in target areas before initiating geothermal exploration to bolster efficiency and accuracy.

Fault controls on geothermal activity are principally manifested in two key aspects: the intensity of hydrothermal activity in geothermal anomalies depends on fault zone activity, whilst reservoir temperature and yield are contingent on fault properties, scale, recent activity, and depth. Jin Wenzheng et al. [4] recognized fault geometry as a key and reported the following facts: tensile fractures dominate in normal fault footwalls and thrust fault hanging walls; major strike-slips channel deep heat; and late-stage tensional faults govern recharge. Geothermal water concentrates at fault intersections, fracture zones, and tips.

Geothermal water exploration engages a myriad of geophysical and geochemical methods. By means of DC resistivity, magnetic and gravity surveys, seismic exploration, and electromagnetic methods, etc., geophysical exploration characterizes geothermal systems through parameters such as temperature, resistivity, and magnetic susceptibility, and radon gas measurement is also increasingly adopted to augment the accuracy of exploration [5]. Geochemical exploration identifies chemical anomalies in geothermal systems through mercury measurement, soil and rock analysis, and hydrochemical analysis, delivering evidence for resource localization and assessment.

Among geophysical techniques, electrical methods are positioned as a cornerstone. Rock resistivity exhibits prominent variation with temperature and hydrothermal alteration, so that geothermal water distribution can be subject to a predictive assessment through subsurface resistivity mapping. Direct current (DC) methods, specifically electrical profiling and vertical electrical sounding, are so pioneering that they delineate geothermal structures and their properties, embodying an indispensability in intricate geological settings [6].

Complementing these electrical techniques, soil radon surveys harness gas migration properties to detect critical structural features, e.g., bedrock fractures, karst cavities, goaf areas, and active faults. With the established correlation between geothermal water and fault systems, radon measurements afford commendable means for identifying concealed faults. Scholars typically interpret fault locations and activity based on quantitative analysis of radon anomaly profiles [7,8,9].

Given the ample geothermal energy in Ningdu County in Jiangxi Province, China, a plurality of studies were conducted on Ningdu, northern Ningdu, Luopo, Longguishan, and Poxia geothermal areas [10]. Yuan Jing [11] et al. defined 79 favorable geothermal zones by a remote-sensing geothermal GIS prediction method, among which the study area is located in an A-class geothermal anomaly zone. Liu Feng [12] et al. delved into the traits of the geothermal heat flow and the geothermal formation model in the northern part of Ningdu, and through holistic analysis, they concluded that the formation mechanism of the geothermal resources in the northern part of Ningdu is “high heat flow and heat generation from high-yield granite bodies + multi-level secondary faults controlling heat conduction and water transport.” Chen Qiuling [13] et al., combining data, found that the hot spring in Luopo is of a convection-type geothermal system due to uplifting faults. Li Jianhong [14] integrated drilling, faults, minerals, and field investigations to carry out an in-depth analysis of the geothermal in Longguishan, revealing the secondary faults’ control over geothermal energy. Liu Wei [15] et al. discovered through drilling that the heat-controlling and heat-conducting faults in Poxia are northeast-trending faults. In summary, previous research hinged on an array of methods to probe into the type, formation mechanism, and controlling factors of the geothermal water in Ningdu, but the location exploration of Lantian, Ningdu, geothermal water remains inadequate.

These studies demonstrated that the Yingtan–Anyuan deep fault zone passing through Ningdu is the predominant cause accounting for the widespread geothermal water in Ningdu. There are also undiscovered secondary fault structures in Lantian, Ningdu. With the purpose of further exploring the geothermal geological conditions in this area, soil radon measurement and ordinary electrical method surveys should be applied to scrutinize hidden structures. These results are expected to set a basis for geothermal water exploration in the future.

2. Overview of the Research Area

Ningdu is situated in the north of Ganzhou City and the southeast of Jiangxi Province, China. Under the jurisdiction of Zhantian Township, the area involved in this study is located approximately 15 km northeast of Ningdu County which is on the western flank of the Wuyishan uplift zone in terms of the regional structure. The secondary Wuyi Mountain, Yushan uplift zone and Ningdu sedimentation zone run through the entire area, while with the geological structure of the county, the tectonic lattice forms are diverse, among which the dominating tectonic traces are fold and fracture structures. The traffic location map, hydrogeological map of the research area, and geological sketch map of Ningdu County are illustrated in Figure 1.

2.1. Geological Structure

The multi-stage tectonic movement of the South China plate controls the geological tectonic evolution of the Ningdu area [16]. The geothermal activity in the study area is chiefly manipulated by the Yingtan–Anyuan fault zone. During the Caledonian orogeny, the Yingtan–Anyuan Fault Zone arose as a fundamental tectonic boundary, demarcating contrasting geological domains in the central-southern region of Jiangxi. It is manifested by the metasomatic gneiss batholiths to the east and the granodiorite intrusions to the west, accompanied by regional metamorphism intensifying eastward from greenschist to amphibolite facies. Through successive tectonic regimes, the Yingtan–Anyuan fault zone evolved as a persistent structural partition—controlling Late Paleozoic–Triassic sedimentary patterns, channeling Yanshanian (Jurassic–Cretaceous) moderate-acid magmatism along NE-trending reactivations, and ultimately governing Cenozoic basin development by differentiating the Ganjiang Fault Depression from the Wuyi Uplift Belt under Himalayan extension. This multi-stage evolution witnesses South China’s transition from compressional folding to extensional rifting, with the Yingtan–Anyuan fault zone acting as a keystone structure throughout the Phanerozoic [17].

The tectonic distribution of Ningdu County presents typical sub-zone characteristics, manifesting as the central, western, and southern parts of the Ningdu fault-trapping monoclinic basin, the Qingtang complicated oblique basin, and the small westward oblique structure of the near east of the Duifang. The fracture system analysis reveals the dominance of the NNE-oriented fault zone as a control structure in the area. This group of fractures are characterized by the largest cutting depth, the longest extension distance, and the most significant structural scale, spawning an advantageous migration channel for geothermal fluids. Their structural trend is generally NEE, and the cross-sectional dip angle is mostly greater than 50°. These fractures are a part of the tectonic system of the Yingtan–Anyuan deep fault zone. The mechanical properties of the fault zone are basically compressive–compressive torsion, and this is often accompanied by tectonic fracture zones and silicification alteration zones with a bandwidth spanning from a few meters to tens of meters. The fracture system is not only a vital boundary condition for the evolution of the Ningdu Red Layer basin, but also exerted a dramatic control effect on the magma intrusion activities of the Yanshanian and Caledonian periods.

The stratigraphic structure of the study area exhibits typical basal–caprock dual structural traits. More specifically, the northern bedrock area is exposed with Sinian shallow metamorphic rocks, and the primary part is vastly developed with Caledonian tectonic cyclic magmatic rocks. In the central modern riverbed and alluvial geomorphological unit, there exist alluvial strata of the Holocene alluvium with a thickness of 1–3 m. The lithological sequence is bottom-to-top manifested as the interlayer of gravel and silty clay, possessing typical binary structural characteristics. It is worth noting that the main body of magmatic rocks in this area is an intrusive rock body of the Eastern Gary period, of which medium-coarse-grained spotted biotite Two-Long granite is the most developed, producing the core component of the regional basal rock system. The structural lines in the study area strike NE, and the fault structures are well developed [18]. The fault represents a Yanshanian regional fault, dipping to SE at an angle of approximately 60°. The rock in the fault zone is broken and breccia is developed. The analysis validates it as a tensional fault, extending more than 5 km, which imposes prominent control effect on the generation of geothermal resources in the working area.

In view of the foregoing data, granites are broadly distributed in the study area, and the radioactive characteristics of rocks are distinct. Huang Qiwei [19] systematically studied the radioactive traits of the Jiangxi geothermal system rock mass, suggesting the significant geochemical anomalies in southern Jiangxi. More precisely, the average radioactive heat generation rate of rocks in the whole region reaches 3.08 μW/m³, which is distinctly higher than the typical value of craton crust. Among them, the anomaly of the Lantian geothermal field in Ningdu is particularly strong, with an average heat generation rate of rock mass reaching 5.32 μW/m³, which is roughly 73% higher than the regional background value (

Table 1. U, Th, and K contents and radiogenic heat production rate of rock samples from Ningdu, Jiangxi (data cited to Qin Qiuju et al. [20]).

Sampling Location	Number of Samples	U	Th	K (10%)	Radioactive Heat Generation Rate (μW/m3)	Mean
Ningdu	NDZK1-1	7.5	24.48	3.38	3.87	5.32
	NDZK1-2	7.71	15.38	4.21	3.35
	NDZK1-3	8.19	32.68	3.02	4.58
	NDZK1-4	14.6	31.21	4.32	6.2
	NDZK1-5	18.49	40.13	4.25	7.73
	NDZK1-6	10.22	37.54	2.72	5.44
	NDZK1-7	10.56	35.12	4.04	5.45
	NDZK1-8	8.88	46.98	4.39	5.94

).

2.2. Hydrogeology

In this area, groundwater exists in three forms [13], i.e., pore water in unconsolidated rocks, networked fracture water in the weathering zone, and fissured water in the foundation rock structure. The first form is stored in the pores of Quaternary loose deposits of sand and sand gravel, with hydraulic properties primarily as phreatic water, and a relatively weak water yield. The second is largely distributed in Cretaceous strata, with groundwater stored in weathered fissures, tectonic fissures, and dissolution holes in calcium-bearing sand gravel and conglomerate, with moderate water yield, and the last form of groundwater is basically stored in fissures of metamorphic rocks, general clastic rocks, and igneous bedrock.

The groundwater dominating the working area of electrical exploration and soil radon measurement is the unconsolidated pore water which exists in the pores of sand and gravel loosely accumulated in the Quaternary. The depth of the water table is generally small, typically less than 2 m below ground surface, apart from medium-to-poor water content. Hydraulic properties are mainly diving, and water richness is weak.

Located mostly on both sides of rivers in low-lying flat terrain, the groundwater receives atmospheric precipitation or farmland irrigation water infiltration recharge. In hillside areas, it additionally incorporates lateral recharge of bedrock fissure water. Runoff speed is fast with short paths, exhibiting seasonal dynamic changes. Discharge occurs through piece-like flow or springs into the nearest rivers or surface outlets.

There are two drilled wells, ZKI and ZK2, in the study area. The final depth of ZK1 is gauged as 408.1 m, with a water temperature of 81 °C; the final depth for ZK2 is 643.58 m, and the water temperature is 83 °C. The temperature at the mouth of the Lantian geothermal water spring in Ningdu is 85–86 °C; the pH value is 8.03–8.7, implying alkaline; its hydrochemical characteristics are classified as SO₄•HCO₃-Na type, belonging to low mineralization and alkaline hot water. Isotope geochemical studies corroborate that the geothermal water in Lantian is principally derived from atmospheric precipitation recharge [20].

3. Measurement Methods and Data Acquisition

3.1. Conventional Electrical Prospecting

As an indispensable geophysical exploration method [21], the apparent resistivity combined profiling technique harnesses resistivity contrasts in subsurface targets. By measuring apparent resistivity variations across lateral positions and depths, this approach characterizes geological structures and anomalies, critically constricting the location, scale, strike, and dip of tectonic fracture zones [22]. Its merits lie in operational simplicity, distinct anomalies, and strong resolving power [23].

While sharing fundamental principles with combined profiling, the vertical electrical sounding method specifically targets vertical structural information. By progressively expanding electrode spacings at each sounding point, it systematically probes subsurface resistivity variations from shallow to deep horizons [24]. This controlled depth penetration is impetus to reconstructing vertical stratigraphic sequences.

Two integrated three-electrode arrays are engaged in the apparent resistivity combined profiling method. With electrode C positioned at infinite distance, measurements are recorded at the midpoint of electrodes MN, i.e., the O point, as depicted in Figure 2. At each survey point, simultaneous operation of both three-electrode arrays plots dual apparent resistivity curves along the profile, designated as ${\rho }_S^A$ and ${\rho }_S^B$.

Instrument and Line Arrangement

In light of the regional geological data and on-site measurements, the electrical properties of the rock and soil mass of the site are listed in

Table 2. Resistivity of each rock mass.

	Quaternary Remnant Slope Deposited Silty Clay	Granite	Fault Fracture Zone
Resistivity (Ω·M)	20–200	500–2000	300–500

:

Given this analysis, electrical differences can be noticed between the rock and soil masses and the surrounding rocks in the study area. There is a certain geophysical premise to find the fractured area of underground faults by electrical exploration.

Field investigations leveraged the DZD-2 electrical prospecting system with auxiliary equipment, all pre-calibrated to fulfill technical specifications. Survey points were positioned via RTK-GPS, and marked with red cloth markers, apart from a 20 m station interval for combined apparent resistivity profiling and maximum AB/2 spacing of 420 m for vertical electrical sounding. This instrument features a DC voltage measurement range of ±4000 mV with an accuracy of ±1% of reading ±1 digit and a resolution of 0.01 mV. Current measurement achieves ±1% accuracy ±1 digit with 0.01 mA resolution. During data acquisition, quality assurance protocols executed duplicate measurements at 20% of stations (mean square relative error < 10.0%) and verification measurements at 10% of stations (mean square relative error < 5.0%). As demonstrated by the resultant dataset, the reliability for geological interpretation is sufficient, which fully complies with industry specifications.

The layout of the measurement lines was based on the principles of vertical heat control and the conductive fault zone. A total of four ordinary electrical method measurement lines were laid out, with varying inter-profile distances and a measurement point spacing of 20 m. The total length was 2020 m, alongside 144 effective physical points and 4 resistivity sounding points, as visualized in Figure 3.

3.2. Radon Measurement

Radon is a radioactive noble gas stemming from the disintegration of uranium and thorium, which exists in almost all rock and soil types. There are three natural isotopes of radon: ²²²Rn,²²⁰Rn, and ²¹⁹Rn, with half-lives of 3.82 days, 55.6 s, and 3.96 s; these are derived from the ²³⁸U, ²³²Th, and ²³⁵U decay series, respectively [25,26,27,28]. On account of the short half-lives of ²²⁰Rn and ²¹⁹Rn, the radon measured on faults is ²²²Rn. The highly mobile Rn can migrate from underground to the soil surface with a unique migration pathway.

Soil radon measurement is often involved in locating the abnormal position of a deep fault structure, which lays the foundation for later geothermal exploration, boasting its strengths in economy, simplicity, flexibility, and easy repetition of observation.

In the fault zone, radon often appears in positive anomalies, which can be pinpointed by two facts, as illustrated in Figure 4. (1) The radon is produced by groundwater movement. It may be a result of the decay of radium deposited on the groundwater surface and migrates to the surface above the structure through diffusion, convection, suction, gas pressure, pumping, and relay transmission. Generally, the radon escapes in the fracture zone, producing a radon measurement anomaly. It can also be observed that radioactive substances (e.g., radium and radon) originally dissolved in groundwater gather near a fracture zone along with water flow. With the existence of fracture zones, groundwater storage space evolves from closed to open; water pressure drops from high to low; and groundwater flows from high-pressure to low-pressure places, i.e., a fracture zone [29]. (2) ²²²Rn is ultimately produced by the decay of ²³⁸U within rocks; hence, variations in Rn concentrations reflect lithological variations in U concentrations and by extension in heat production.

Radon measurement curve is highly susceptible to the measurement environment. The thickness of loose overburden exerts a pronounced effect on radon content and its abnormal form: in the event of a thin overburden, the abnormal peak and the form are prominent; when the overburden is thick, the abnormal tends to be low while the range is expanded. Generally, according to the relationship between anomaly morphology and fault occurrence, the faults can be classified into three categories (

Table 3. Manifestations of soil radon profile.

Type of Fault	Soil Radon Profile Morphology
Gently dipping fault	Presenting multi-peak anomalies with significant amplitude fluctuations, and reflecting intersecting fracture networks.
Steep dip fault	Displaying asymmetric unimodal anomalies, characterized by gentler gradient transitions on the down-dip flank.
Vertical fault	Producing symmetrical unimodal anomalies with steep gradient boundaries.

).

Radon concentrations are usually higher on the surface of the soil in geothermally active areas than in normal areas. Therefore, geothermal anomaly areas are found by means of measuring the radon content in soil and analyzing its distribution curve. In recent years, radon measurements have become commonly used in locating geothermal targets, ranging from stand-alone soil radon measurements to integrated surveys that merge radon measurements with myriad other geophysical methods [30,31,32,33,34,35,36,37]. Soil radon measurement is especially applicable to the exploration of geothermal fields controlled by fault structures, which can more precisely infer the exact location of the fracture. Coupled with drilling verification, this method is conducive to delineating the water-bearing boundary of a geothermal field and predicting the source and channel of thermal reservoir. Generally, in geothermal exploration, there is a vast application prospect for radon measurement in soil, and its achievements furnish a crucial verification basis for exploration conclusions.

Figure 4. Radon gas migration diagram (modified from Jia Wenyi [38]).

Figure 4. Radon gas migration diagram (modified from Jia Wenyi [38]).

Instrument and Measurement Line Layout

The instrument engaged in this measurement is a DURRIDGE RAD7 radon detector. The RAD7 is equipped with a solid-state detector made of semiconductor material (usually silicon), which directly converts α radiation into a signal. RAD7’s calibration accuracy is approximately 5%, and the system attains a minimum detection limit of 0.1 pCi/L and a linear range of 0.1–20,000 pCi/L.

The measurement results of soil radon gas can be affected by external environmental factors. Two controllable factors, “sampling depth” and “sampling duration,” are selected to optimize the accuracy of soil radon measurement. According to the study of Luo Hao [39] et al. on refining soil quantitative radon measurement methods, each measurement point during actual field soil radon content testing must ensure a sampling depth > 0.6 m and a measurement time > 6 min.

Hence, for this measurement, the collection depth was configured from 0.6 to 1 m, with three sets of measurements taken at each sampling point, each set lasting 3 min. This resulted in a total collection time of 9 min, and the last set of data was selected as the final data point. The layout of the measurement line was arranged to basically coincide with the electrical exploration. A total of five measurement lines were set up, with varying inter-profile distances, and the distance between sampling points ranged from 10 to 30 m, alongside 93 sampling points, as illustrated in Figure 3.

4. Data Processing and Analysis

4.1. Characteristic Analysis of Apparent Resistivity Combined Profiling and Vertical Electrical Sounding Method

• Line 1

The joint profile curve of apparent resistivity in the Zone Line 1 (AO = BO = 110 m, MN = point spacing = 20 m) is depicted in Figure 5a. It can be noticed that at point 46.5, ${\rho }_S^A$ and ${\rho }_S^B$ intersect, and the analysis confirms this as a special intersection point. The background value of resistivity in this section is 650 Ω·Μ. To the left of point 46.5, ${\rho }_S^A$ > ${\rho }_S^B$, and it is the opposite situation to the right. Point 46.5 is the orthogonal anomaly point reflected by the joint profile curve, with a low-resistivity anomaly body below.

The apparent resistivity curve of the joint section method in the Zone Line 1 (AO = BO = 170 m, MN = point spacing = 20 m) is plotted in Figure 5b which presents the consistent fluctuations of ${\rho }_S^A$ and ${\rho }_S^B$, without any intersection. The background resistivity value of this section is 650 Ω·Μ, and no obvious anomalies are spotted in the joint section curve.

2.

Line 2

The apparent resistivity curve of the joint section method for the Zone Line 2 (AO = BO = 110 m, MN = point spacing = 20 m) is illustrated in Figure 5c. It can be found that between points 41 and 45, ${\rho }_S^A$ and ${\rho }_S^B$ exhibit a certain length of low apparent resistivity. Analysis demonstrates that the background resistivity value of this section is 550 Ω·Μ. Between points 41 to 45 of Line 2, the joint section curve reflects low-resistivity anomaly points, with a resistivity of about 300 Ω·Μ, and a low-resistivity anomaly body exists below.

3.

Line 3

Figure 5d unveils the apparent resistivity curve of the joint section method in the Zone Line 3 (AO = BO = 90 m, MN = point spacing = 20 m). In light of the analysis, the background value of resistivity in this section is 550 Ω·Μ, while the abnormal points reflected by the joint section curve are not evident.

4.

Line 4

The apparent resistivity curve of the joint section method in the Zone Line 4 (AO = BO = 110 m, MN = point spacing = 20 m) is shown in Figure 5e. Between points 21 and 39, the ${\rho }_S^A$ and ${\rho }_S^B$ curves display obviously close values. As revealed by the analysis, the background resistivity value of this section is 550 Ω·Μ. Between points 21 and 39 of Line 4, the low-resistivity anomaly points reflected by the joint section curve possess a resistivity of about 300 Ω·Μ, and the fissure structure is relatively developed. Compared to other lines, the anomaly points of the Line 4 joint section measurement line are more pronounced.

Taking into account the characteristics of apparent resistivity joint section, there are abnormal points for Line 1, Line 2, and Line 4, so the distribution and fracture degree of rocks below abnormal points can be detected by resistivity sounding. Given the traits of abnormal points, 37 points of Line 1, 27 points of Line 2 and 37 points of Line 4 are involved for resistivity symmetric quadrupole sounding (Figure 6). The results are listed in

Table 4. Characteristics of vertical electrical sounding points.

Depth Measurement Point	Detection Depth	Lower Bedrock	Strong Regolith Depth	Bedrock Fracture
37/1 line	280 m	granite	5–12 m	35–115 m and 280 m
27/2 line	210 m	granite	5–15 m	60–125 m
37/4 line	210 m	granite	10–40 m	60–90 m and 140 m

.

4.2. Radon Data Processing and Profile Line Characteristics

In existing studies, most scientists employ traditional statistical methods when processing radon gas data. These methods generally entail data to guarantee the consistency with a normal distribution [40]. Fortunately, when the data follow a lognormal distribution instead of a normal one, these methods remain applicable [41]. In our study, 93 sets of soil radon gas data were collected, yet they failed to follow a normal distribution. Therefore, a natural logarithmic transformation was applied, and the transformed data successfully passed the normality test (p > 0.05), affirming that they were lognormally distributed.

In conventional statistical approaches, when data conform to a normal or lognormal distribution, the mean is typically used as the background value, and the mean plus n times the standard deviation is harnessed as the lower limit for identifying anomalies [42]. In this study, the radon measurement data, having undergone natural logarithmic transformation, conform to a normal distribution, and the geological background of the study area is relatively homogeneous. This ascertains the applicability of traditional statistical methods. Hence, the mean is adopted as the background value, and the mean plus one standard deviation serves as the lower limit for identifying anomalies [31].

The mean of total data is regarded as background value, and the mean plus 1–2 times variance acts as the abnormal lower limit in this measurement. Then the background value of radon concentration is 25,806 Bq/m³, while the mean plus 1 variance is 51,695 Bq/m³.

The calculation results of the five cross-sectional lines are summarized in

Table 5. Profile background values and standard deviation (Bq/m³).

Section Number	Concentration Value Range	Mean	Variance	Background Value	Abnormal Lower Limit	Number of Outliers Exceeded
S1	1890–101,000	14,405.42	12,511.58	25,806	51,695	4
S2	8140–68,600	31,725	19,370.43			4
S3	8800–55,800	29,900	20,390.4			3
S4	1090–53,300	14,405.42	12,511.58			1
S5	2291–132,000	36,913.40	41,475.68			3
Total	1090–132,000	25,806.32	25,889.03			15

.

The classification of radon anomalies is heavily contingent on the fluctuation of radon concentration on the profile, and the existence and size of faults are judged as per the expression of peak shape [43]. There are five profile lines included in the field radon measurement, as shown in Figure 7.

The azimuth for the S1 profile measuring line is 259°, and 30 detection points were completed. Regardless of the narrow gap between the background value of the S1 profile and the overall background value, the standard deviation of the profile is larger than the background value, accompanied by a considerable degree of data dispersion. The main peak value of the anomaly reached 101,000 Bq/m³, which is four times the background value. A high-value anomaly occurring at the measuring point 7–14 was manifested as a multi-stage peak anomaly, and the secondary peak was relatively symmetrical. A small peak appeared at the measuring point 19–22. Based on the background value and the morphology of the section, the appearance of faults is speculated in this section.

The S2 profile measuring line possesses an azimuth of 271°, and 16 detection points were completed. Compared with other side-line profiles, the overall radon concentration rises, displaying a wide and gentle peak. Reservoirs and swamps are present at the origin of the on-site survey transect. The low value in the middle of the profile may be related to the wetter environment.

The azimuth of the S3 profile line is 143°, and eight detection points were completed. Because the dense vegetation coverage in extending northwest would introduce errors in the measurement results, it was not continued further. The profile line shows significant undulations, and based on the trend, it probably exhibits a bimodal anomaly. The background value of S3 is relatively higher than the overall background value, and both peaks of the bimodal anomaly are roughly 50,000 Bq/m³.

The S4 cross-section line possesses an azimuth of 287°, and 24 detection points were completed. In contrast to other cross-sections, the overall radon concentration is relatively low, but the anomaly points are prominent. The cross-section displays an asymmetric single-peak anomaly, apart from a peak value of 53,300 Bq/m³. At points 4–9, the cross-section shows a single-value peak anomaly. It can be inferred from the cross-section morphology that a fault may have passed through.

The azimuth of the S5 profile line is 53°, and 15 detection points were completed. The radon background value of the S5 profile line is 36913.40 Bq/m³, marking the highest among all profile lines in terms of overall background value, with a high degree of data dispersion. The highest radon value reached 132,000 Bq/m³, which is approximately five-times higher than the overall data background value. High-value anomalies were observed at detection points 7–13, reflecting a single-peak anomaly in the profile. A fault is suspected based on the background value and profile morphology.

5. Discussion

Electrical prospecting is based on the electrical property difference between rock and ore (expressed by resistivity parameters), and fault structure is identified by analyzing the abnormal characteristics of apparent resistivity. The principle of radioactive decay chain underlies radon gas measurement. Radon geochemical anomalies are formed when radon gas generated in deep underground migrates vertically to the surface along permeable dominant channels such as a fault fracture zone. Electrical prospecting and radon gas measurement can favorably dissect the spatial distribution of faults by detecting the discontinuity of the electrical structure and the gas migration path of the fault zone, so that multi-data coupling analysis can be realized.

The results of combination profile and radon gas measurement are roughly consistent on the same distance profile line, which can judge whether the abnormal area is in the same position, so as to obtain the position of the fault zone.

In electrical exploration, 190–230 m in Line 4 exhibit a certain continuity and directional low-resistance abnormal zone. Unimodal anomalies also occur in the radon gas measurement profile Line S4 from 110–310 m (Figure 8). Based on the combined apparent resistance profile, resistivity bathymetry, and radon gas measurement results, the presence of a zone of fracture structure can be deduced. Line 4 reveals that at a depth of 80 m, the width of the affected area of fracture and crushing is about 180 m. Analysis of the Line 4 vertical electrical sounding curve affirms the better water-rich conditions of the fracture structure.

The Line 2 section from 410 to 450 m is a low-resistance anomaly area, corresponding to the S2 profile line from 315 to 510 m showing multiple peak anomalies (Figure 9). The two verify one another, indicating the presence of a fault. The S2 profile morphology tapers off towards the large-numbered point, confirming the fault dip direction as northwest. Given the results of Lines 4 and S4, as well as Lines 2 and S2, the study area is deduced to have a fault trending NE, dipping NW, with an angle of 65° to 75°, designated as F1. Based on geological inference, the NE-trending fault F1 is primarily located between points 21–25 (190–230 m) on Line 4, with a width of approximately 40 m.

At 460 m on Line 1, an orthogonal anomaly point exists with a broken zone at the bottom. Correspondingly, in the radon measurement S1, anomalies appear between 480 and 500 m, suggesting the presence of a fault (Figure 10). In light of the electrical exploration data, the fault probably extends eastward, provoking the increased measurement of the S5 profile. In the S5 profile, a single-peak anomaly arises, and the analysis of the S1 and S5 profiles implies a NEE-trending fault, numbered F2. A notable multi-level peak anomaly in the S1 profile is worth noting between 200 and 340 m, which may convey the presence of a fault.

The genesis mode of the Ningdu Lantian geothermal system can be summarized as a fracture-controlled deep-circulation convection geothermal system. The geothermal water is chiefly derived from the infiltration of atmospheric precipitation in the hilly and mountainous areas on the east side of the research area and seeps down through the tectonic fracture zone and fracture network of the upper plate of the fractures [20]. Medium-temperature geothermal fluid is formed by heat exchange with the surrounding rock; the heat source stems from the regional earth heat flow and the decay of radioactive heat-producing elements; and geothermal water migrates upward along the F_1-1 fault zone and its associated secondary cracks. The genesis model reveals the fracture system’s dual roles as a water-diversion channel and a heat-transfer medium, which steers the fluid migration path and heat anomaly distribution pattern of the geothermal field. Moreover, in the research area, the enormous content of radon gas in the geothermal water evidences that faults are an indicator for exploring geothermal water in Lantian. By locating underground faults in the research area, the scope of geothermal anomalous areas can be delineated based on the replenishment, diameter, and discharge conditions of geothermal water, so as to lay a basis for the subsequent drilling of geothermal wells.

The comprehensive results of electrical exploration and radon gas measurement underlie the fracture structure development in the NE direction of the research area. Based on these results, we infer the existence of a fault zone F1 (Figure 1) on the basis of the original tectonic line. The orthogonal anomalies of the radon gas measurement profiles S1 and S5 and Line 1 in the electrical survey reveal the existence of fault zone F2. Relying on radon value data to draw a plane contour map, and in light of the results of electrical exploration, the specific locations of F1 and F2 can be inferred from the radon value contour map (Figure 11).

Soil radon gas measurements, albeit useful, are highly uncertain for independent usage and are impeded from accurately determining fault orientations and angles. Merging radon measurements with electrical exploration methods enhances effectiveness for delineating target areas and identifying key structures, especially in the early- and mid-stage of geothermal exploration. However, drilling and additional precise geophysical, geochemical, and geothermal surveys are essential for confirming resource volumes and guiding development decisions.

6. Conclusions

Integrated electrical and radon surveys in Ningdu Lantian reveal key structural characteristics as below:

(1)

Electrical prospecting identifies granite bedrock with thin Quaternary overburden, ascertaining radon method applicability to radioactive terrains with shallow cover.

(2)

Resistivity profiles (Lines 2/4) and radon anomalies (S2/S4) delineate a NE-trending fault zone F1 (65°–75° NW dip; about 40 m width). Complementary data from Line 1 and profiles S1/S5 indicate an NEE-trending fault zone F2.

(3)

Stress concentration at the F1–F2 intersection creates fractured rock masses that are conducive to groundwater accumulation, designating this zone as a preferential geothermal exploration target.

(4)

Strong correlation between electrical and radon methods substantiates their synergistic application for fault detection. Radon anomalies provide distinct, intuitive signatures in fracture-controlled geothermal systems, demonstrating efficacy for structural mapping. Yet this approach warrants expanded implementation in Lantian’s geothermal exploration program.

(5)

Radon gas measurement results are considerably susceptible to environmental factors. Therefore, it is imperative to conduct in-depth studies on how the environment affects radon content and to develop appropriate correction models. Future endeavor may focus on drilling in the fault structure complicated fracture zones of the study area, combined with geochemical analysis and geothermal measurements, in order to holistically interpret the formation mechanisms of geothermal zones.