Geology Reference
In-Depth Information
Resistivity Sounding
Vertical Electrical Sounding is applied whenever a depth section is to be
determined at a particular place and, in this method, increase in the depth of
investigation can be obtained by gradually increasing the distance between
the current electrodes such that current penetrates deeper and deeper into the
ground. This method gives the information about depth and thickness of
various subsurface layers and their resistivities.
The field procedure of a sounding is to use a fixed centre with expanding
electrodes. The Wenner and Schlumberger layouts are particularly suited to
this technique. But Schlumberger array has some advantages. There are
always some naturally developing potentials (self-potential, SP) in the ground,
which have to be eliminated and nullified. The potential difference developed,
due to the experimentally impressed current, should be taken into
consideration. For different spacings, the apparent resistivity of the ground
for particular array can be calculated.
Interpretation of Sounding Data
The apparent resistivity is plotted against half current electrode spacing on
a double logarithmic paper and the curve so obtained is called sounding
curve. To get the layer parametres (resistivity and thickness) of the subsurface,
these sounding curves are to be interpreted and there are mainly two types
of interpretational techniques: Indirect methods and direct methods.
In the indirect methods, the field curve is compared with a set of theoretical
curves, also called as master curves, for different known layered parametres
prepared in advance. Several albums of master curves are available which
include among others Compagne Generale de Geophysique (1963), Flathe
(1963), Orellana and Mooney (1966) and Rijkswaterstaat (1969). These are
computed from the expression for surface potential (Stefanesco et al., 1930).
L
1
1
V
=
2(
#
KJ rd
)
(
)
0
2
!
r
"
0
where
r
= distance of the measuring point from current source,
1
= resistivity
of surface layer,
K
() = Stefanesco kernel function determined by thickness
and resistivity of surface layer,
J
0
(
r
) = Bessel function of zero order and
first kind and
= Integration variable, a real number with dimensions of
inverse length. When a match of the field curve is obtained with the theoretical
curve, one can get the layer parametres in terms of resistivity and thickness
of the subsurface layers.
In direct methods, one can get the layer parametres from the field curve
itself by using the computer code available. Quite often, it is possible that
the field curve may not match with the available master curves. In the
absence of a proper set of master curves that simulates the geological situation,
one has to compute a theoretical sounding curve that best fits the field
