## 6 Results from Time-Domain Electromagnetic Surveys  
An electromagnetic profile was carried out in a N-S direction, in the direction of the presumed dominant geological structure in the area, starting at the Triassic facies and ending at the Cretaceous terms. The length of the profile was 3200 m, and it was produced from the execution of a total of five TDEM surveys. To place the TDEM's 100 \(\times\) 100 loops accurately, a compass Silva SSM360 LA Professional Optical Compass was used. This is a high-precision and high-sensitivity compass with sapphire bearings and quick position adjustment.  
The TDEM surveys were carried out using the Terra-Tem equipment (Figure 8) designed by Monex GeoScope, with a \"coincident loop\" configuration and with square loops (transmitter and receiver) of 100 m on each side, using a minimum stacking of 1000 measurement repetitions per channel and testing various gains to minimise the effect of any environmental electromagnetic noise. The results of the different decay curves obtained in the field are shown using the Terratem equipment software \"TEM Plotting Program v 2.0.0\" from Monex GeoScope (Figure 9).  
Figure 6: Electromotive force vs. time [23].  
Figure 7: Secondary magnetic field vs. time [23].  
After the measurements, the resistivity/time curves were obtained and processed using the IX1D software from Interpex to obtain the geoelectric resistivity/depth column for each TDEM survey carried out (Figure 10). These data were used, also through the IX1D software, to obtain a geoelectric profile, through which a geological and hydrogeological interpretation of the studied area could be made.  
Figure 8: Terra-Tem equipment in the process of measurement.  
Figure 9: Decay curves obtained in the field.  
We chose Occam inversion [24] on this occasion for its greater simplicity and softness when comparing the geological profile. The profile obtained (Figure 11) identified the presence of a total of three geoelectric units:  
Figure 10: TDEM location map with borehole and control piezometer and pseudo-2D profile (**A**), resistivity-time curves and their respective resistivity/depth profiles processed with Occam inversion (green) and quantitative layered inversion (red) (**B**). Pseudo-2D profiles build on the data from the processed with Occam inversion (**C**) and quantitative layered inversion (**D**).  
* Geoelectric Unit 1: present at the SW end of the profile, is of a conductive nature, with resistivities below 30 Ohm.m, corresponding to Triassic materials (shales, gypsum and sandstones). The low resistivities of this unit may be influenced by the low hydrochemical quality.
* Geoelectric Unit 2: corresponding to the Finitiraissic and Jurassic carbonate resistive materials (limestones and dolomites). This is a unit located on top of the previous one, appearing between metres 400 and 2600 of the profile, with more than 140 m of thickness. The high resistivities of this unit (100-500 Ohm.m) indicate a high proportion of permeable materials (limestones and dolomites) compared to impermeable materials (marls and marl-limestones). In addition, high resistivities are usually indicative of the good chemical quality of the groundwater.
* Geoelectric Unit 3: appears around metre 2600 of the profile and extends to the end of the profile. The unit shows a resistivity that is intermediate to the previous ones and corresponds to Cretaceous materials (sands, marls, marlstones and limestones).  
The general structure observed, according to the geophysical data, is concordant with the geologically predefined one, in which the Mesozoic sequence appears normal in character, dipping slightly, about 15 degrees to the northeast, with all the terms of the series progressively outcropping in that direction.