## 1 Introduction
The project site was selected where high-temperature granites were located at relatively shallower depths, as estimated by a temperature distribution map and a seismic reflection as well as the gravity survey results. An electromagnetic survey was conducted to evaluate the underground resistivity structure. A number of injections and production wells were drilled, tapping the geothermal reservoir. Water circulation through the reservoir was successfully conducted, and produced electricity of 1 MW in 2013. [PERSON] et al. [21] investigated geothermal HDRs in the Gonghe Basin, China, using gravity data to delineate a shallow underground fault and basement structure using a gravity analysis. Based on the interpretation of the gravity section inversion and Euler deconvolution, they concluded that the study area was in a high-heat-flow spot, and fitted well with the regional geological structure. A more recent study was performed by [PERSON] et al. [22] in the Gonghe Basin. They applied the 3D inversion method to satellite gravity data of the Gonghe Basin to analyze the underground structure and the mechanism of the geothermal system. They identified an internal heat source and a heat transfer channel, and concluded that crustal high-temperature partial molten mass was likely to be the main heat source for the hot dry rock geothermal resources in the Gonghe Basin. In Saudi Arabia, EGSs are mainly encountered in the northwestern parts of the Midyan Terrane. A massive belt of high-heat granitic rocks that attains anomalously high-radioactive elements (U, Th, and K) are recognized [23]. It mainly consists of intrusive alkali, peralkaline granitic, and granoidorite rocks, along with a few extrusive rhyolites and dykes with high-radioactive magmatic sources. The heat-generation capacity of these granites varies from 2 to 134 \(\mu\)Wm\({}^{-3}\). Merely 1 cubic meter of such granites can generate 10 MWe for 30 years. Assuming at least 2% of energy is extracted from such granites, on a conservative amount, 120 \(\times\) 10\({}^{6}\) TWh electricity can be generated [24]. Generally, there is a big gap in the knowledge and information concerning the potential of geothermal resources in Saudi Arabia. During the last decade, a few studies have been made regarding the geothermal resources of Saudi Arabia; among them, [23; 24; 25; 26; 27; 28; 29; 30] are the most important. Good work has been achieved by [PERSON] et al. [29], who built a database of the available geothermal resources in the kingdom and produced the first geothermal favorability map for the kingdom. However, more detailed and much more comprehensive exploration studies are required to evaluate the geothermal resources in Saudi Arabia, with a special emphasis on EGSs. In this regard, audio magnetotelluric (AMT) and gravity geophysical measurements were obtained for a geothermal exploration of the Midyan area. The aims of this study were to investigate the subsurface structural pattern affecting the EGSs, describe the regional deep and shallow fault systems, characterize the thicknesses of the sedimentary cover, and detect the subsurface geometry/extension of the high-production granitic rocks; this will help to enhance detailed volumetric estimations, which are necessary for further geothermal reserve studies.