Geology Reference
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situation to get the proper layer parametres. In some codes, one requires
giving an initial guess model and the curve is interpreted by an iterative
process and in another type of direct methods, the field curve is interpreted
without the necessity of an initial model.
The method of automatic iterative interpretation given by Zohdy (1974)
is based on the similarity in shape existing in many cases between apparent
resistivity curves and Dar Zarrouk curves. Zohdy (1989) has developed an
algorithm for automatic iterative interpretation of field sounding curve, where
no initial guess model of the layered earth are required to be given as initial
model. Based on the field curve an initial model is assumed. Then first
thickness parametres are changed according to an algorithm till minimum
root mean square error is obtained. Later, layer resistivities are changed
according to an algorithm till another minimum is obtained in RMS error.
Using equivalence principle the layers can be suppressed finally.
When the layer parametres are obtained in terms of resistivities and
thicknesses, these can be inferred in terms of lithology of the subsurface by
knowing the geology of the area and correlation of resistivities with various
formations from a known place.
The resistivity data collected in Maheswaram watershed was interpreted
using a computer programme based on the inversion algorithm of Jupp and
Vozoff (1975), which uses digital filter theory (Ghosh, 1971a, b). The iterative
method successively improves the initial model given, until the error measure
is small and the parametres are stable with respect to a reasonable change
in the model parametres. Figure 2 is an example of a sounding curve
interpreted by the above method.
Resistivity
(Ohm.m)
Thickness
(m)
Layer No.
( m)
1
315
0.5
Final Layered Model Curve
2
25
1.1
3
50.5
14.4
4
153.6
9.1
5
1483
Smooth Layered Model Curve
Apparent Resistivity data
Spacing (m)
Figure 2. Resistivity sounding interpreted showing the
field and computed curves.
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