2 0 fS .

where ECa is measured in S m-1; Hp and Hs are the intensities of the primary and secondary magnetic fields at the receiver coil (A m-1), respectively; f is the frequency of the current (Hz); | is the magnetic permeability of air (4n10-7 H m-1); and s is the intercoil spacing (m).

The calibration of EMI equipment (e.g., Geonics EM381), which can be difficult and time consuming, is another dissimilarity with ER. However, the DUALEM-2 does not appear to suffer from the same calibration difficulties as the EM38 due to the increased distance between the transmitter and receiver coils. Complexity of the EMI measurement and difficulties in calibration are distinct disadvantages of the EMI approach that have reduced its use in agriculture. These limitations are the most likely reasons that there are no commercially available EMI mobile platforms. This has caused the use of EMI in agriculture, even today, to be principally as a research tool.

Following the early vertical profiling efforts, research with EMI, and concomitantly with ER, drifted away from salinity and concentrated more on observed associations between ER and EMI measurements of ECa and other soil properties. This research trend significantly contributed to the base of knowledge compiled in Table 2.1.

2.2.2 Measurement of Water Content with eca

Several geophysical techniques have been adapted for agriculture to measure 8 within the root zone including TDR, GPR, CP, AM, PM, EMI, neutron thermalization, NMR, gamma ray attenuation, and ER. Aside from ER and EMI, neutron thermalization, CP, TDR, and GPR have received the greatest use for laboratory and field-scale agricultural applications. The history of the agricultural application of CP and neutron thermalization predates all other geophysical-based approaches for measuring 8 except ER. Gamma ray attenuation has been in use in agriculture since the 1950s, but it

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