## 2 Materials and Methods
### EM38-MK2 Description and Data Acquisition  
EM38-MK2 provides the measurements of in-phase (magnetic susceptibility) and quad-phase (conductivity) components within two different depth ranges without any requirement to contact with or penetration of the instrument into the soil. The maximum practical depth of exploration is 1.5 m. Therefore, the EM38-MK2 applications in general soil and agriculture sciences are typical. The EM38-MK2 contains two receiver coils, divided to 1 m and 0.5 m from the transmitter, supplying the data from the effective depth ranges of 1.5 m and 0.75 m, respectively when placed in the vertical dipole orientation. On the other hand, it supplies the data from the effective depth ranges of 0.75 m and 0.375 m, respectively at horizontal dipole orientation. Once the EM38-MK2 is positioned within the collapsible calibration stand, the instrument can be calibrated automatically within seconds, without any necessity for iterative adjustments. In addition, the EM38-MK2 is supported by temperature equilibrium circuitry to improve temperature-related drift aspects. The data can be collected by any appropriate field computer or the DAS70-AR Data Acquisition System, which can be connected by either Bluetooth wireless technology or RS-232 serial cable. The DAS70-AR provides an improved level of support for data collection, and it includes a rugged field computer with 8 GB storage memory. For more details about this instrument, refer to Geonics Ltd., Mississauga, ON, Canada.  
The diagram of the EM38 and its related electromagnetic fields is described in Figure 6. The primary magnetic field (Hp) is generated from the transmitting coil (Tx) at one end of the sensor, this field produces current loops in the ground below, and in turn, the current loops generate their own magnetic field (Hi). The generated secondary field is superimposed on the primary field, and both Hp and Hi are measured in a receiving coil (Rx) at the other end of the sensor [36]. The meter is intended to be placed or held in either the horizontal or vertical orientation; this orientation position controls the signal penetration depth and depth weighting response pattern. In a homogeneous soil profile, the vertical (EMV) signal penetrates to a depth of 1.5-2 m, while the horizontal (EMH) signal primarily reflects the shallower soil zone (0.75-1.0 m); Manual of EM38-MK2, (Geonics Limited).  
Therefore, the instrument works by emitting an alternating current through a transmitter coil, which induces a primary magnetic field to pass perpendicular to the coil orientation and into the adjacent media. Eddy currents are generated as these currents pass through the coil. This strength is a function of the (a) amount of negative charge on the clay particle; (b) clay content; (c) concentrations of salts in the soil solution; and (d) soil moisture content. The more conductive the soil, the greater the secondary magnetic field produced. The ratio of the primary and secondary magnetic fields determines the apparent electrical conductivity (ECa).  
Electromagnetic induction sensors, such as EM38-MK2, are becoming more widely used for mapping relative differences in soil characteristics in agricultural fields, including soil salinity; see [15]. In this study, EM38-MK2 was used in vertical mode to measure the electrical conductivity (EC) at 75 and 150 cm depths at about 2240 sites along the study area (see Figure 4). The acquired data using the EM38-MK2 conductivity meter were processed using the DAT38 MK2 Ver. 1.05 package (Geonics Limited) and exported to XYZ format for the twenty EC measured profiles.