## 3 Methods
### Time-Domain Electromagnetic  
The operating principle of the time-domain electromagnetic (TDEM) method is to circulate an electric current through a transmitter coil (usually square in shape) for short time intervals. When the current flow is abruptly interrupted, a magnetic field is produced that induces, according to [PERSON]'s law, a variable electric current in the subsurface, which in turn generates a transient secondary magnetic field. These currents flow in closed paths and migrate at depth, decreasing in intensity over time.  
Changes in the secondary magnetic field over time induce a transient voltage in the receiving coil. The shape of the decay of this voltage provides information on the conductivity distribution of the subsoil, which can be used to characterize it [21; 33; 34; 35]. TDEM has been typically applied to finding mineral deposits [36; 37; 38], to investigate groundwater bodies [35; 39], and to analyze sedimentary basins [40; 41]. In recent years, this method has also been used in environmental studies [42; 43] and to characterize marine intrusion [44; 45].  
The maximum prospecting depth depends on the decay time of the signal from the current cut, the current intensity, the loop size, the signal frequency, and the conductivity of the subsoil [33; 34; 35]. Although there are various measurement configurations, the most common one is to place the (smaller) receiving coil in the center of the (much larger) transmitting coil (central-loop configuration). Another option is to use the same coil to carry out both functions (single-loop configuration).  
We used a TEM measuring system developed and produced by ELGEO Research & Production Company (AIE-2 model). It is a device with a maximum output power of 200 W and a current intensity of up to 10 A. The TDEM receiver is based on a 16-bit analogue-to-digital converter and a signal processor that provides an analogue-to-digital conversion immune to input voltage noise and real-time signal pre-processing. It offers a range of measurements between 5 us and 10 us, with an input voltage of 5-20 V with signal compensation.  
In the field campaign, a total of 10 TDEM measurement stations were acquired along a north-south profile. The points located in Figure 2 refer to the center of the measuring stations for square loops of 200 m on each side. At each of these points, measurements were collected with central-loop and single-loop configurations, and the measurement parameters (intensity, time, voltage) were varied, which allowed us to compare the effectiveness of the different setups. To reach prospecting depths on the order of 400 m, transmission loops of 200 m \(\times\) 200 m are needed. A 6-mm\({}^{2}\) section copper wire was used. This (larger than usual) wire decreased the resistance and thereby allowed it to obtain a higher effective current intensity. When the central-loop configuration was used, the receiving coil had a dimension of 20 m \(\times\) 20 m, using a 5-turn cable and a \(\times\)10 amplifier. For the visualization and editing of the different curves, the TEMBIN software was used. The modeling and inversion processes were performed with ZondTEM1D and ZondTEM2D software [[http://zond-geo.com/english/zond-software/electromagnetic-sounding/zondtem1d/](http://zond-geo.com/english/zond-software/electromagnetic-sounding/zondtem1d/)]([http://zond-geo.com/english/zond-software/electromagnetic-sounding/zondtem1d/](http://zond-geo.com/english/zond-software/electromagnetic-sounding/zondtem1d/)).