ERT (Electrical Resistivity Tomography) for Earthmat Design

In general Wenner four electrode method is used for assessment of ground resistivity as per IS Code 3043- 1987 . This method gives apparent resistivity values and results are acceptable in soils where the ground is homogeneous. However in case of rocky area ground is not homogeneous and large resistivity changes are met in both horizontal and vertical direction. Simple geophysical resistivity measuring techniques can be utilized for resistivity measurements and details of true ground resistivity in vertical direction can be determined. However finer details cannot be measured.

ERT in comparison to conventional resistivity survey can provide finer details of ground resistivity to design a safe, economic and effective earth mat. This approach has been used at some Hydro Power and PSP projects.

Codes for Earth Mat Design and Resistivity Measurement

Following codes are under use for Earth mat design and for procedure of resistivity measurement at plant site.

Is code 3043-1987 (amendments issued in Jan 2010), code of practice for earthing: This code recommends detailed resistivity measurements in eight directions utilizing wenner electrode configuration.

IEEE Std. 80 2000 , IEEE Guide for Safety in AC Substation Grounding: This standard has detailed deliberations on resistivity measurement and recommends for adoption of highly refined techniques for assessment of finer details of resistivity for Earth mat design. Procedure for determining true resistivity using two-three layer earth model based on geophysical survey is suggested for a layered earth. Large numbers of available theoretical master curves with known parameters (resistivity and thickness) are compared with the measured apparent resistivity field data. After curve matching true resistivity and thickness of different layers can be determined.

IEEE Standard 81- 1983, IEEE guide for measuring earth resistivity, ground impedance and Earth surface potentials of grounding system: This standard recommends that detailed resistivity measurements are necessary if there are variations of resistivity with depth. The number of such readings should be greater where the variations are large, especially if some readings are so high as to suggest a possible safety problem.