Measuring Soil Resistivity; Geologic Surveys - Gossen MetraWatt GEOHM C Manual De Instrucciones

5.6

Measuring Soil Resistivity

The dissemination resistance value of a given earth electrode is depen-
dent upon soil resistivity. Soil resistivity must be known in order to allow for
advance calculation of dissemination resistance for the design of earthing
systems.
Soil resistivity ρ
(see also chapter 3 on page 7) can be measured with the
E
®
GEOHM C earth tester in accordance with the Wenner method.
Four earth spikes of greatest possible length are driven into the ground in
a straight line separated from one another by distance a, and are con-
nected to the test instrument (see Figure 7).
The earth spikes are usually 30 to 50 cm long. Longer earth spikes can be
used in poorly conducting soils (sandy soil etc.). The depth to which the earth
spikes are driven into the ground may not exceed 1/20 of distance a.

Note
Erroneous measurement results may occur if pipes, cables or
other metallic underground lines run parallel to the measuring
circuit.
Measurement is performed as described in chapter 5.4 on page 16. Enter
distance a, and soil resistivity is displayed.
22
Figure 7
Soil resistivity is calculated in accordance with the following formula:
ρ = 2π ⋅ a ⋅ R
E
where:
π = 3.1416
a = distance between earth spikes in meters
R = calculated resistance value in Ω (this value corresponds to R
by means of the 4-wire measurement setup)

5.6.1 Geologic Surveys

Except in extreme cases, soil is measured to a depth which is
approximately equivalent to distance a between the earth spikes.
It is thus possible to draw conclusions concerning various layers within the
ground by varying the distance between the earth spikes. Highly
conductive layers (water table) within which earth electrodes are best
positioned can thus be located within otherwise poorly conducting
surroundings.
Soil resistivity is subject to great fluctuation which may result from various
conditions such as porosity, moistness, concentration of salts dissolved in
ground water and climatic fluctuations.
Soil resistivity ρ
START
ature and negative temperature coefficient of the ground) closely resemble
a sine wave.
E ES S H
a a a
Measuring Soil Resistivity
characteristics as influenced by season (ground temper-
E
as determined
E
GMC-I Messtechnik GmbH