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TerraDat is a leading geophysical survey company carrying out non-invasive shallow ground investigations. This technical note describes how our novel techniques can be used to provide geoelectrical information for earthing grid design.
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Soil Resistivity Testing for Earthing InstallationsE
AR
TH
ING
TechnicalNoteAs 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.
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.
No allowance for topography which can result in errors in the calculated depth and lateral positioning of the readings.
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
TRUE RESISTIVITY VALUES
FORWARD MODELLING OF DATA
GEOTECHNICAL/GEOLOGICAL
INFORMATION
Resistivity tomography section
20 25 30 35 40 45 50 55 60
Chainage (m)
285
290
295
Ele
vation
(m)
Inc
rea
sin
g r
es
isti
vit
y
Conductivetill
Resistivebedrock
SurfacePeat/Gravel
Resistivity testing setup
Test
Spacing
(a) (m)
Potential
(V) (mV)
Current
(I) (mA)
Resistance
(R) (Ohms)
Resistivityinstrument
(ρ) (Ohm.m)
Resistivity
calculated
(ρ) (Ohm.m)
1
64
2.235
365.03 0.006123 2.46 2.462118
1
32
9.049
462.89 0.019549 3.93 3.930544
1
16
16.624
352.17 0.047204 4.75 4.745511
1
8 66.471
288.64 0.23029 11.58 11.57565
1
4 477.96
293.85 1.626544 40.88 40.87951
1
2 3031.49
269.8 11.23606 141.2 141.1965
1
1 2952.842
74.36 39.71009 249.51 249.5058
2
32
2.864
405.22 0.007068 1.42 1.421059
2
16
16.367
374.91 0.043656 4.39 4.388761
2
8 79.034
292.68 0.270036 13.57 13.57347
2
4 337.919
268.17 1.260092 31.67 31.66958
2
2 2985.367
249.41 11.96972 150.41 150.4159
2
1 2944.653
70.65 41.67945 261.89 261.8797
Resistivity sounding profile (true resistivity values extracted from modelled section)
TerraDat (UK) Ltd - Tel (08707 303050)www.terradat.co.uk
Resistivity (Ohm.m)
“a”
Spaci
ng
00
100 200 300
20
40
60
80
Test 1Test 2
C1/C2 current electrodesP1/P2 potential electrodesa electrode spacing
V
C1 P1 C2P2
a a a
*C
ost E
ffectiv
e
*N
on-in
vasiv
e
*R
apid
Gro
und C
overa
ge
*E
nviro
nm
enta
lly F
riendly
*S
wift M
obilis
atio
n/G
lobal C
overa
ge
Contact Simon Hughes for more info; [email protected]
