This technique is used to measure soil resistivity (or it's reciprocal conductivity) using electrodes in physical contact with the soil. Typically four or more electrodes are employed with two used to inject a current into the soil and the remaining electrodes measuring the voltage potential accross set displacements. The electrode displacement can be used to target a depth volume.
Induced Polarisation (IP) measurements use contact electrodes as above however a pulsed or alternating current is injected into the soil enabling the measurement of both the charge loss (conduction) and charge storage (polarisation).
Measurement of ECa.
ECa mapping using rotating disc electrodes.
Soil hydraulic properties [1][2].
Similar to EMI methods however requires physical electrode contact with the soil.
Commercial
A good overview of ECa measurement using electrode, EMI and TDR methods and applications [3].
1. Börner FD, Schopper JR, Weller A. 1996. Evaluation of transport and storage properties in the soil and groundwater zone from induced polarization measurements. Geophysical Prospecting 44: 583–602.
2. Binley A, Slater L, Fukes M, Cassiani G. 2005. The relationship between frequency dependent electrical conductivity and hydraulic properties of saturated and unsaturated sandstone. Water Resource Research 41: 13
3. Corwin DL, Lesch SM. 2003. Application of soil electrical conductivity to precision agriculture: theory, principle and guidelines. Agronomy Journal 95: 455–471.