Soil salinization has been considered one of the most challenging global threats, affecting large cultivated and irrigated areas all over the world. Its detrimental impacts on environmental quality, agricultural productivity, and socioeconomic stability are about to become even more pronounced in the coming years due to climate change [1]. The extent of soil salinity along with the frequency of floods and duration of droughts, as a consequence of climate change, are expected to be more intense and exacerbated in the arid and semi-arid regions where the sustainability of natural resources is imperative [2]. Considering the increasing demands for global food supplies and arable land, these effects necessitate urgent control and mitigation. For this purpose, regular and accurate monitoring of soil salinity distribution and its spatial variations across multiple scales is crucial for preventing soil salinization hazards and preserving the long-term sustainability of agricultural and environmental systems.

Successful monitoring of soil salinization requires rigorous modeling techniques and advanced tools to reliably assess the soil salinity levels and interpret its severity in different areas of the world. Recently, satellite remote sensing technology has been widely applied as an effective tool for identifying and mapping the soil salinity of large-scale areas [3]. However, the sensors are incapable of detecting the subsurface distribution of the soluble salts and the highly spatial heterogeneities in the soil profile [4]. Thus, their implementation is usually combined with other sensors or data for more accurate results [5].

On the other hand, proximal sensing (PSS) technology with ground-based, electromagnetic induction (EMI) sensors can quantify and characterize the spatial patterns of soil salinity within the soil profile by measuring the soil’s apparent electrical conductivity (ECa). Besides the popular EM38 sensor, a variety of commercial EMI devices have also been developed, enabling the investigation of the solute’s variability at different soil layers and allowing soil salinity mapping, particularly on the field scale [6].

Despite the accessibility of improved proximal sensing devices such as the EM38MK2 (MK2), an upgraded type of EM38, and the range of studies that evaluate various EMI data conversion techniques for the soil salinity assessment ^{[7][8][9][10]}[7,8,9,10], the choice of the most suitable approach for each survey is still a challenging task. The uncertainties that emerge with their application depend on a number of site-specific environmental factors, including the complex interactions of soil properties that affect the ECa and distinct data processing requirements, which can significantly impact the credibility of the results. Furthermore, while the technical guidelines and considerations regarding the employment of EM38 in soil salinization surveying have been well documented by existing reviews and scientific publications ^[11][12][13][11,12,13], a concise compilation of the currently available approaches that convert ECa measurements by the EM38- and the MK2-type sensors into soil salinity as expressed in ECe has not yet been attempted.

2. Fundamental Principles and Considerations of the EM38 and MK2 Sensors

2.1. Basic Operational Features of EM38 and MK2 Sensors

The deployment of electromagnetic induction (EMI) instruments has been consolidated in agricultural science and soil surveying since the 1970s ^[14][15][16][14,15,16]. Owing to their low cost and their capability to detect the spatial variations in edaphic properties and heterogeneities within the field and at larger scales, in real time, and non-destructively ^[17][18][17,18], they have been studied and used for numerous environmental and geophysical applications [12]. Unlike other geophysical methods such as TDR and GPR, the quantification of soil salinity’s spatial variations by electrical resistivity (ER) and EMI devices has become a vital component of precision management implementations ^[11][19][11,19]. This is mainly amplified by the measurements of apparent electrical conductivity (ECa), which have been found to be correlated with soil salinity estimates and can be used as an indirect indicator for many soil properties [20]. Frequency domain reflectometry (FDR) technology, including WET sensors, has also been successful in the appraisal of soil salinity in the laboratory ^[21][22][23][21,22,23] and in situ ^[19][24][19,24] using ECa measurements. Nevertheless, the single utilization of these probes might be exacting and locally restricted since they have a substantially smaller measurement volume and are invasive, as their operation is based on contact with the soil and its sublayers [24]. Over the years, commercial EMI sensors, such as the DUALEM (Dualem Inc., Milton, ON, Canada), EM31, EM34 (Geonics Ltd., Mississauga, ON, Canada), and Profiler EMP-400 (Geophysical Survey Systems, Inc., Salem, NH, USA) have been investigated for the assessment of various soil properties, including soil salinity, and their performance has been documented ^[25][26][27][25,26,27]. The ground conductivity meter EM38 (Geonics Ltd., Mississauga, ON, Canada), introduced in 1980, was revolutionary in soil salinization surveys due to its light weight, portability, and the large volume of ECa measurements taken in various types of soils and fields, e.g., stony, which until then were difficult to acquire with electrode-based devices ^[28][29][28,29]. Thus, it quickly gained the attention of the agricultural community and became the most frequently applied tool for monitoring and mapping soil salinity [20]. The adaptation of EM38 can also be attributed to the fact that it was intentionally designed to support the assessment of near-surface variations in soil properties and specifically of soluble salts that affect crops within the rooting zone ^[30][31][30,31]. The EM8 sensor is constructed with two coils, one transmitter and one receiver coil, which are installed at the opposite ends of the instrument with a fixed spacing of 1 m, and it operates at a 14.6 kHz frequency. The orientation of the coils determines the cumulative depth response of the instrument associated with the ECa measurements. When located in the horizontal configuration (EMh), the device’s signal corresponds to a depth range of roughly up to 0.75 m, whereas in the vertical mode (EMv) the penetration depth is approximately up to 1.5 m. The depth range of the instrument sufficiently covers the root volume of most plants [32]. These depth-weighted responses of ECa, however, are theoretical measuring depths that rely on a non-linear function in homogeneous soils. As a result, the absolute depth values cannot be easily defined [33]. Since its release, the EM38 has undergone several modifications, updates, and technical improvements, including the addition of a GPS receiver, which allows for accurate georeferencing of the data, and the development of user-friendly software for data analysis and visualization. The dual-dipole EM38 DD sensor is an example of these modifications. This version consists of two EM38 units attached together and placed horizontally and vertically for recording simultaneous EMh and EMv measurements ^[34][35][34,35]. In 2008, the EM38MK2 (Geonics Ltd., Mississauga, ON, Canada) was launched as an updated version of the original EM38 instrument [30]. The MK2 type encompasses new attributes and enhancements regarding the depth range response, stability, and facilitations concerning the field survey and data acquisition. Compared to its predecessor, the MK2 has hardware and software that offer more automation in operation, easier processing, and better interpretation of the raw data. In particular, it can simultaneously measure both soil conductivity (Q/P) and magnetic susceptibility (I/P) within two discrete depth ranges. It entails temperature compensation circuitry, which reduces the occurring temperature drifts during the survey, and it supports automatic calibration without laborious adjustments. This can be achieved through a wireless Bluetooth data logger, which enables the collection of the data and the communication with the instrument conveniently from a relative distance. Alternatively, data recording can be performed through a serial port. The duration of field operations has also been enhanced with the addition of a power connector, which allows for the use of an external rechargeable battery [36]. Besides the technical advancements, the main fundamental difference between the two sensor types lies in the second receiver coil of the MK2, which corresponds to an additional depth range of measurements. The MK2 consists of one transmitter coil and two receiver coils that are positioned at two fixed distances of 1 m and 0.5 m from the transmitter coil, respectively. Hence, in the MK2, the effective depth range is determined by both the coil separation and the dipole modes of horizontal and vertical orientation. Consequently, with the new coil (0.5 m coil separation), measurements of ECa can be additionally taken at two distinct depths: at 0.375 m depth when the device is placed in horizontal mode (EMh) and at 0.75 m in the vertical mode (EMv). This version allows users to detect and investigate variations in shallower layers, which may be optimal for precise agriculture practices ^[26][33][26,33]. Moreover, along with the rest of the depth ranges of the sensor, the profile of soil salinity distribution up to a 1.5 m depth can be promptly acquired and evaluated. The relative differences, advantages, and applicability of the EM38 and MK2 sensors over various geophysical instruments in mapping of soil properties have been discussed in a few studies ^{[12][26][37][38]}[12,26,39,40]. Gebbers et al. [26], by comparing a variety of EMI and other technology devices (ARP03, CM-138, EM38, EM38-DD, MK2, OhmMapper, Veris 3100), concluded that the main disadvantage of EM38, EM38-DD, and CM138 sensors is their sensitivity to deeper soil layers, which is irrelevant to the crop’s rootzone. On the other hand, MK2, ARP03, OhmMapper, and Veris3100 were found to be more effective in detecting shallower soil variations that are important for precision agriculture. Likewise, the EM31 and EM34 sensors, which have exploration depths of up to 6 and 60 m, respectively, may also be considered inappropriate for detecting the variability in shallower soil layers [12]. In a study conducted by Dooltitle et al. ^[37][39], the use of EM38 and the multifrequency device GEM 300 was investigated, revealing that both sensors provide reasonable estimates of soil salinity. Moreover, Urdanoz et al. ^[38][40], by comparing the EM38 and the DUALEM sensor, indicated that although EM38 tends to produce slightly higher horizontal ECa readings than the DUALEM, both sensors can be used interchangeably. Generally, the EMI sensors exhibit close similarities in their collected data, with the main differences attributed to the different operational modes and sensing depths [30].

2.2. Principles of the EM38 and MK2 Operation

The operation of the EM38 and MK2 instruments is based on the principles of EMI and has been established by McNeil ^[39][40][41,42]. Once the sensors are turned on and properly calibrated for recording ECa measurements, the transmitter coil sends, at a frequency of 14.6 kHz, an alternating electrical current to the soil, generating a primary magnetic field (H_p). When the primary field interacts with the subsurface, it induces electrical currents (eddy currents) that, in turn, produce secondary magnetic fields (H_s). These secondary fields interact with the receiver coils by inducing alternating currents in the coils. The sum of the amplitude and phase of the induced voltages from the primary and secondary fields is amplified in an output voltage, which is read by the user. Accordingly, under low induction number (LIN) conditions, where Nb << 1 and assuming homogeneity in the depth profile, the apparent electrical conductivity, ECa, is sensed and expressed as the ratio of the primary (H_p) and the secondary magnetic fields (H_s) (Equation (1)), where f is the operating frequency (Hz), μ_ο, the magnetic permeability of free space (4π10⁻⁷ H m⁻¹), s is the intercoil spacing (m), and ω = 2πf ^[39][41]. Besides other factors, ECa readings by the EM38 and MK2 sensors for an investigated depth range are influenced by the orientation and coil spacing of the instruments. The relative ranging depths for the horizontal and vertical modes have been determined by McNeil ^[39][41] in homogeneous soils as non-linear functions that describe the relative contribution to the secondary magnetic fields in respect to normalized depth z. Consequently, the depth-weighted response, which indicates the cumulative depth response R(z) of the sensors, is a non-linear function that represents the relative contributions of all soil electrical conductivities from a soil volume below a normalized depth z. The R(z) equations, based on the horizontal and vertical orientation and expressed as a percentage (%) of the measured signal, have been defined for 1 m (Equations (2) and (3)) and 0.5 m (Equations (4) and (5)) coil separation ^[33][39][33,41]: where z (m) is the depth and R_H(z) and R_v(z) are the cumulative relative ECa for horizontal and vertical mode, respectively. From the derived cumulative functions, the depth of investigation (DOI), which refers to the depth from which more than 70% of the signal response derives, can be determined for each sensor. Heil et al. [33] compared the two instruments and examined the effective depth responses for each orientation and coil distance. The coil spacing of 0.5 and the horizontal mode are generally influenced by near-surface variability, making them more suitable for shallower depths. Instead, the 1 m spacing coil and the vertical mode seem to have an increased sensitivity along with the depth. It is noteworthy that the EM38’s vertical response decreases drastically at depths above 90 cm, in contrast to MK2. Practically, the DOI of the sensors may vary under natural soil conditions due to existing heterogeneities and the interrelations of ECa with subsurface soil features that affect the signal. In addition to the effective depth ranges when placed on the ground surface, both devices can be lifted at different heights above the soil surface to investigate interval depth variations and model the distribution of salt content in the soil layers ^[41][42][43][43,44,45]. Also, they are designed for handheld measurements or can be mounted on non-metallic sleds and attached to vehicles for mobile measurements. The mobile and real-time collection of ECa data by the EM38 and MK2 sensors is a simple process owing to their software and the direct connection to the GPS. Thus, they can be an ideal option for monitoring and mapping soil salinization at field scales ^[44][45][46][46,47,48].

2.3. Considerations in the EM38 and EMK2 Applications

One of the key factors in the employment of the EM38 and MK2 sensors is that on all occasions of soil salinity surveying, either at field or larger scales, site-specific calibration is required. Therefore, soil sampling for ground-truth data cannot be omitted [30]. Another important consideration when using these probes for the collection of ECa measurements is their susceptibility to metal and electrical interference, like fences and power lines. In comparison to other technologies, such as capacitance sensors, the presence of metallic objects in the study area can affect the signal, especially in the horizontal configuration [36]. Although the detection of metals may be beneficial for archaeological prospecting ^[47][49], for efficient soil salinity estimation and mapping, uniform, metal-free soils are a prerequisite. Furthermore, as the manufacturer recommends, in the automatic mode of ECa recording, more frequent calibrations might be needed to minimize any potential effects from the drifts on the accuracy ^[48][50]. The drifts by temperature are stronger in the original EM38 ^[20][33][20,33], while for the MK2, they are considered insignificant due to the internal enhancements. An exception might arise in the case of near-surface measurements with the 0.5 m spacing coil, where the effects from the drifts need to be managed ^[48][50]. Finally, one of the most concerning and constraining aspects of EM38’s utility for determining solute distribution within the soil is its application under dry moisture conditions or in fields where there is insufficient moisture through the penetration depths. Conducting ECa surveys in fields where soil water content levels are less than those of field capacity and reportedly under 50% ^[49][51] can lead to unreliable and biased results. Likewise, ECa measurements in shallow and moderately deep soils above bedrock should be avoided ^[30][50][30,52].