TechnicalNote Soil Resistivity Testing for Earthing Installations
Traditionally, these tests are carried out using a 4-pin survey technique, which provides a vertical electrical profile at a central location point. This approach has some inherent limitations that include: An assumption of lateral homogeneity of the ground within the test area which may be up to 150m away from the central test location point for deeper readings. The results are usually reported in terms of apparent resistivity values i.e. average values from surface.
Given the limitations of the traditional 4-pin methodology, TerraDat resistivity tests are carried out using a multi-electrode resistivity tomography approach. Not only does this provide true resistivity values at the selected depths, the modelled true resistivity section provides detailed cross-sectional information that will allow the identification of, and compensate for, lateral changes such as geological boundaries and hydrological variations within the subsurface. This can make a significant difference through optimal placement of an earthing system.
RESISTIVITY SOUNDING V a
a P1
C1
a
TRUE RESISTIVITY VALUES
C2
P2
FORWARD MODELLING OF DATA C1/C2 current electrodes P1/P2 potential electrodes a electrode spacing
GEOTECHNICAL/GEOLOGICAL INFORMATION
Resistivity testing setup
Chainage (m)
Elevation (m)
Resistive bedrock
20
25
30
35
40
45
50
55
60
Surface Peat/Gravel
295
290
285
Increasing resistivity
Conductive till
*Cost Effective *Non-invasive *Rapid Ground Coverage *Environmentally Friendly *Swift Mobilisation/Global Coverage
No allowance for topography which can result in errors in the calculated depth and lateral positioning of the readings.
EARTHING
As part of our services aimed at the Energy and Infrastructure sectors, TerraDat regularly carries out soil resistivity measurements for earthing installation design and corrosion engineering. These resistivity measurements are made by passing a DC electrical current through the ground using a pair of electrodes and measuring the resulting potential gradient within the subsurface using a second electrode pair. There are a number of different electrode configurations and by gradually increasing the spacing between the current/potential electrodes, the depth of investigation is increased.
Resistivity tomography section 0
Resistivity (Ohm.m) 100 200
300
0
Test
“a” Spacing
20
40
60 Test 1 Test 2
1 1 1 1 1 1 1 2 2 2 2 2 2
Spacing (a) (m) 64 32 16 8 4 2 1 32 16 8 4 2 1
Potential (V) (mV) 2.235 9.049 16.624 66.471 477.96 3031.49 2952.842 2.864 16.367 79.034 337.919 2985.367 2944.653
Current (I) (mA) 365.03 462.89 352.17 288.64 293.85 269.8 74.36 405.22 374.91 292.68 268.17 249.41 70.65
Resistance (R) (Ohms)
Resistivity instrument (ρ) (Ohm.m)
0.006123 0.019549 0.047204 0.23029 1.626544 11.23606 39.71009 0.007068 0.043656 0.270036 1.260092 11.96972 41.67945
2.46 3.93 4.75 11.58 40.88 141.2 249.51 1.42 4.39 13.57 31.67 150.41 261.89
Resistivity calculated (ρ) (Ohm.m) 2.462118 3.930544 4.745511 11.57565 40.87951 141.1965 249.5058 1.421059 4.388761 13.57347 31.66958 150.4159 261.8797
80
Resistivity sounding profile (true resistivity values extracted from modelled section) Contact Simon Hughes for more info; simon@terradat.co.uk
TerraDat (UK) Ltd - Tel (08707 303050) www.terradat.co.uk