## 2 Geological Settings  
The study area is located at the northern margin of the Ebro Foreland Basin in the central sector of the South Pyrenean fold and thrust belt, which formed during the continental collision between the Iberian and Eurasian plates, from the Late Cretaceous to Miocene [15; 16; 17; 18; 19] (Figure 1). This collision resulted in the growth of an antiformal stack of basement-involved thrust sheets in the axial zone acting as a boundary between the South and North Pyrenean fold and thrust belts [20]. The South Pyrenean fold and thrust belt consists of a system of south-verging thrust sheets and the related Ebro Foreland basin emplaced in a piggy-back sequence from the Late Cretaceous to the Oligocene Pyrenean compression [21; 22; 23; 24]. These tectonic units detached predominantly above Upper Triassic evaporites [25] and above Eocene evaporites deposited in the foreland basin [26; 27]. The Foreland Ebro Basin is the late non-marine stage of the South Pyrenean foreland basin, which developed since the middle Priabolonian at 36 Ma [28]. The system of folds deforming this foreland basin detached above the Cardona salt and gypsum from the Barbastro Formation [27; 29] resulting in the formation of the Barbastro-Balaguer Anticline [30].  
Figure 1: Simplified geological sketch of the Pyrenees and Ebro Basin area. Blue dotted line indicates the location of the survey area reflected in Figure 2.  
Figure 2: Geological map of the study area based on 1:50,000 cartography (sheet 326, Monzón, Instituto Geologico y Minero de Espáña, 1974). Location of the Monzón-1 borehole and TDEM survey points are displayed with geophysical profiles described in Section 5.  
The study area is located in the surroundings of the Monzon-1 well. It is located at the southern flank of the Barbastro-Balaguer anticline [31] which detached in the evaporites of the Barbastro Formation at the front of the Southcentral Pyrenean unit [32; 33]. According to [PERSON] [31], the infra-detachment stratigraphy consists of: (1) Lower Triassic red sandstones and claystones from the Bundanstein Facies; (2) Middle Triassic limestones, dolostones and anhydrite from the Muschelkalk; (3) Upper Triassic red claystones and evaporites from the Keuper; (4) Lower Jurassic limestones, dolostones and carbonic breccias; and (5) Lower Eocene limestones. Folded strata from the Barbastro anticline comprise Cenozoic strata related to the later stages of evolution of the South Pyrenean foreland Basin. The main detachment is the Barbastro Formation, which consists of Pariohonian gypsum and grey luttites deposited in a lacustrine evaporite sedimentary environment [34], with interbedded decameter halite and anhydrite packages [33]. Significant halite deposits are also known in this formation, as evidenced in the Monzon-1 well as in different wells where this material is exploited for the electrochemical industry. Significant halite deposits are also known in this formation, as evidenced in the Monzon-1 and other wells in the area where this material is exploited for the electrochemical industry. The upper part of the Barbastro Formation consists of lacustrine limestones, mudstones, sandstones and minor contents of gypsum acting as a transition to the overlying Peraltilla Formation. The Barbastro Formation outcrops in the core of the Barbastro-Balaguer Anticline [30; 35], a 150 km NW-SE direction structure parallel to the Sierras Marginales front. The thickness of the gypsum zone of the Barbastro Formation is difficult to measure at the surface, due to the great deformation on the anticlinal outcropping area, although it decreases progressively towards the South [36]. The Peraltilla Formation consists of Middle Rupelian to Chattian conglomerates, sandstones and mudstones covered by similar sedimentary rocks from the Chattian to Aquitanian Sarifena Formation [36; 37; 38]. Both detrital siliciclastic formations are interbedded with decimeter-thick lacustrine limestones and gypsum [39; 40].