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The development of geothermal energy has received extensive attention because of global energy scarcity and environmental pollution. The Tangquan area is in the north of the Yangtze fold belt, and the lack of magmatic rock development has resulted in a low-temperature type of geothermal reservoir. There are sporadic hot springs and cold springs exposed in the study area, but the uncertain deep geological structure, unknown geothermal reservoir formation mechanism, and lack of detailed exploration data seriously restrict the exploration and development of geothermal resources in the area. Filling underground faults with water can significantly reduce electrical resistivity, while traps filled with warm water can cause a decrease in S-wave velocity. Thus, a new integrated geophysical method, including the controlled source audio-frequency magnetotelluric (CSAMT) and microtremor methods, is applied for geothermal exploration in the region. The combination of CSAMT and microtremor methods can determine thermal-controlled and water-conducting structures more effectively and locate geothermal storage more accurately. The source, channel, storage, and cover of regional geothermal reservoir formation are analyzed using geophysical and geological data. That is, the regional NE- and NW-trending faults are explained as thermal-controlled and water-conducted structures, respectively. The deep Sinian Dengying Formation supplies a heatand water-bearing space. The tectonic intersection area, especially the tensional fault zone, is found to be closely related to the existence of geothermal reservoirs, which is well verified by later drilling results. Finally, a geothermal reservoir model is established to comprehensively understand the distribution of geothermal energy in the region. [ABSTRACT FROM AUTHOR]

Controlled source audio‐frequency magnetotelluric is frequently used in association with other geophysical methods, especially in complex geological areas to highlight geological structures such as water reservoir rock. Although it gives satisfactory results, combining several methods requires time and expense. In addition, despite this combination, several drilling locations proposed after geophysical investigations were inaccurate, resulting in many unsuccessful drillings. The latter occurs due to the difficulty to emphasize the fracture zones properly. To work around this problem, we proposed a novel approach called pseudostratigraphic to reduce the repercussion of unsuccessful drilled boreholes and to demarcate the water reservoir rock. The technique consists to discretize the resistivity of the inverted OCCAM2D model based on the true layer resistivities collected from the borehole log data (observed layers). The discrete resistivity model is known as the new resistivity model. It is used to generate the pseudostratigraphic log at each station by pseudo‐demarcating the thicknesses of the observed layers with a low margin of error. Moreover, the combination of multiple new resistivity models from different survey lines creates a three‐dimensional pseudostratigraphic map useful to emphasize the water reservoir rock. The pseudostratigraphic implementation is carried out in the Xingning area as a real‐world case study. The results show that the intersection of the main fault (F1) and the conductive zones (≤100 Ω m) indicates the potential water reservoir rock. Its thickness is estimated around 150–600 with an error equal to ± 7°m. Based on the three‐dimensional pseudostratigraphic map, the water found in the fracture zone located under the reservoir rock is considered much hotter due to the intense geothermal activity along F1, thereby making it a better place for hot water exploitation. Finally, the pseudostratigraphic technique could be an innovative and cheap strategy to find groundwater reservoirs in complex geological areas. [ABSTRACT FROM AUTHOR]

On the selected cases, Gotalovec in the area of Pannonian basin and Baška in the Dinaridic karst area, that are representing a common hydrogeological model in both regions of Croatia, CSAMT data together with data of other geophysical methods (electrical resistivity tomography, electrical sounding and seismic reflection) enabled the definition of a reliable prognostic geological model. The model consists of carbonate aquifer which underlies an impermeable thick package of clastic deposits. There are great variations of the dolomitic aquifer depths in the Gotalovec area due to strong tectonic activity, while in the Baška area depth changes are caused by the layer folding. The CSAMT method provides the most complete data on lithological and structural relationships in cases of hydrogeological targets deeper than 100 m. Based on the presented models we can conclude that the CSAMT method can provide greater exploration depth than electrical resistivity tomography (ERT) and can be considered as a fundamental geophysical method for exploration of buried carbonate aquifers, deeper than 100 m. But, the CSAMT research may demonstrate its advantages only in the case of very dense layout of CSAMT stations (25–50 m), due to the greater sensitivity to noise in relation to resistivity methods. Interpretation of CSAMT data is more complex in relation to resistivity methods, and a forward modelling method sometimes gives better results than an inversion due to possibility of the use of additional data acquired by other geophysical methods (ERT, electrical sounding and seismic reflection). At greater depths, the resolution of all electrical methods including the CSAMT method is significantly reduced, and seismic reflection can be very useful to resolve deeper lithological interfaces. [ABSTRACT FROM AUTHOR]

Exploration and use of groundwater in hard rocks is a difficult but yet a significant issue, especially when the local surface water source is polluted. In this study, integrated geophysical and nuclear methods are used to delineate groundwater-bearing zones in a granite terrain in the Boshan region, Shandong Province, China. Structural analysis showed that fractures extend in a NW-SE direction, which is the same as the direction of river valley. A controlled source audio-frequency magnetotelluric survey conducted along the river valley indicated several different low resistivity anomalies or potential water-bearing fracture zones. They are seated at depth ranges down to 400 m from the surface along the river valley. The locations of deep water-bearing zones were further confirmed by higher gamma radiation measurements found near the river valley. A fault with a NW-SE strike near the river was deduced as an anomalous water-bearing zone. Two tube wells were drilled targeting at the water-bearing zone that has achieved single-well water yield greater than 360 m/day. The well water was sampled for water chemistry and stable isotope analyses. Shallow groundwater samples show similar water chemistry and isotopic feature to river water, indicating connection between the two. Deep groundwater from the water-bearing fracture zones is not hydraulically connected with shallow groundwater, ensuring safety of the deep groundwater in the granite from being affected by the polluted shallow groundwater or river water. Integrated geophysical and gamma exploration methods helped to identify two disconnected groundwater systems, offering an alternative freshwater source for the region if shallow groundwater or surface water is polluted. [ABSTRACT FROM AUTHOR]

Controlled source audio-frequency magnetotelluric (CSAMT) is one of the frequently used geophysical methods in various exploration problems such as fault mapping and groundwater exploration around 1 km depth. CSAMT is commonly used in combination with other geophysical methods to emphasize underground structures. Although it seems to be a good alternative, such a survey is expensive and requires more logistics which is not profitable for companies with low operating budgets. In addition, despite the combination of the CSAMT method with other geophysical methods, several boreholes drilled for groundwater exploration have failed due to the inaccurate locations to emphasize the fractures zones. Therefore, we developed the Python toolbox for CSAMT to solve this problem. The toolbox encompasses all the standard data processing as well as the modeling and implements a novel approach called pseudostratigraphic to forecast the strata log at each station. The advantage of this technique is twofold. First, it predicts the strata log at each station by demarcating the fracture zone, and secondly, it estimates the layer thicknesses with less margin error useful before the drilling operations. The approach was tested in the Xingning area, Hunan province, China. As a result, the model predicted with the proposed approach is similar to the mechanical borehole drilled after the geophysical survey with thickness errors of less than six meters. Furthermore, the technique would reduce the rate of unsuccessful drilling and be beneficial for geophysical and drilling companies to save money. • pyCSAMT is a Python toolbox for analyzing CSAMT, geological, and borehole data. • The toolbox forecasts the stratigraphy log. • It reduces the rate of unsuccessful drilling. [ABSTRACT FROM AUTHOR]

Controlled source audio-magnetotelluric (CSAMT) is an indirect and effective geophysical prospecting method in subsurface prospecting. Although CSAMT investigation tools are equipped with new techniques, it must be noted that the models obtained still do not automatically decipher the reality of groundwater in bedrock with very high resistivity contrasts. Our study proposed a new CSAMT data processing scheme, which combined the 2D modeling and 3D visualization for groundwater exploration in Tongkeng area, Hunan Province, China. To directly highlight exploitable groundwater, we used the open-source and the OCCAM2D inversion software, coupled with the geological information of the area to define the 3D imaging with emphasizing on the network of producing fractures. We deployed three CSAMT profiles with transverse magnetic (TM) mode recorded on 21 frequencies (8192 Hz to 8 Hz). The data were filtered, pre-processed and carefully analyzed from station to station. Analysis of the apparent resistivity and phase maps show the conductive zones (<150 Ωm) and resistive zones (> 500 Ωm) separated by transition zones (150 to 500 Ωm). The 2D models and stacked sections show discontinuities of fractures marked by two major faults and a main fracture in the zone interconnected by minor fractures. A 3D geological map was obtained from the result of the best models of the different lines. The ITER2DAT function was used to export the data from OCCAM2D inversion and then display it to array and depth channels data for each station. Geological and borehole ZK01 data were used to build pseudo stratigraphic drillholes based on the range of very low (<10 Ωm) to high values resistivity structures (>1500 Ωm). The 3D view shows stratigraphic discontinuity that is between carbonaceous formation and the metamorphic granites. The intruded Devonian limestone layer, which is the potential water tank, and the carbonate vein layer are constantly heated by the geothermal gradient of the area. • We combined the 2D modeling and 3D visualization for groundwater exploration. • The inversion results agree with geological data and the borehole result. • Several measures can improve the processing and increase the interpretation. [ABSTRACT FROM AUTHOR]