Line Traverse; Calculation Of Resistivity - Megger DET2/2 Guia Del Usuario

Line Traverse

The most common method of measuring soil resistivity
is often referred to as the line traverse. Four test spikes
are inserted into the ground in a straight line at equal
distances 'a' and to a depth of not more than 1/20 of 'a'.
The instrument is connected to the test spikes as
shown.
a
P1
C1
Soil resistivity measurement.
The instrument is operated and the measurement
made in the normal way. The resistivity may be
calculated from the formula given opposite or from the
nomogram overleaf. This is the average soil resistivity
to a depth 'a'.
The four test spikes are then re-positioned for further
tests along a different line. If both the spacing 'a' and
the depth a /20 are maintained, a directly comparable
a a
C2
P2
reading will be obtained each time, and thus regions of
lowest resistivity can be located over a given area (at
the constant depth 'a').
Re-spacing the test spikes at separations 'b', 'c', 'd', etc
will yield results from which a profile of the resistivity at
new depths b /20, c /20, d /20,etc.can be obtained.
If the same line for the test spikes is maintained, but the
separation of them is progressively widened, resistivity
values at various depths can be obtained. By this
means depth surveys may be made.
More details can be found in the Megger Limited
publications. See 'Accessories'.
a

Calculation of resistivity

20
Assuming that the tests were carried out in
homogeneous soil the resistivity is given by the
formula:-
where 'R' is the resistance measured in ohms, 'a' is the
test spike spacing in metres and 'ρ' is the resistivity in
ohm-metres.
For non-homogeneous soils the formula will give an
apparent resistivity which is very approximately the
average value to a depth equal to the test spike spacing
'a'.
ρ = 2πaR
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