LABORATORY AND FIELD INVESTIGATIONS OF A VARIANT OF THE TWO-ELECTRODE D.C. RESISTIVITY ARR
LABORATORY AND FIELD INVESTIGATIONS OF A VARIANT OF THE TWO-ELECTRODE D.C. RESISTIVITY ARR
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Date
1996-01
Authors
MAKINDE, VICTOR
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Abstract
The present work has considered the application of a
variant of the two-electrode array to the detection of, and
prospecting for horizontal layerings within the earth. The
mode of this two-electrode array makes use of only the two
current electrodes without the potential electrodes. In its
use, the electrode separation is varied by moving the two
current electrodes equidistantly away from the sounding
point. The result of this is that the current I, delivered
by the power source through the earth also changes. The
driving potential as indicated by the source voltage
indicator was taken as the voltage developed across the
section of the earth between the two current electrodes. In
investigating the capability of this technique, called the
C-C mode, in delineating horizontally-layered media, a
laboratory experiment was set up. Different materials such
as water - an electrolyte, lateritic clay, river sand and
mixed-soil (a mixture of lateritic clay and river sand) were experimented for the one-layer (overburden) case. The experiment was conducted at various frequencies within the
operating frequency range of the tank. All the frequencies
used, including 0 Hz and 50 Hz, gave good alignment of the
resisitivity point for all the materials and combinations
used. The lateritic clay, river sand and mixed-soil were
again variously combined to form two-layer and three-layer
cases, all underlain by the non-conductive perspex as
"basement". Graphs were plotted to show the variation of the
"bulk resistivity" with the electrode separation (or its
reciprocal). The most direct and easily interpretable
results was given by the graph in which the "bulk
resistivity" (b was plotted against the electrode separation
AB/2 on a log-linear scale. The accurate delineation of the
layers in the laboratory investigation encouraged the
initiation of the field study. The Kubanni River Basin was
chosen as the field study area. The basin occupies the
centre of the South-Eastern sector of the Zaria sheet (No.
102 S.W.) of the 1:50,000 sheet ordinances series of the
Nigeria ordinances survey map, with an average area of about
150 km2. It is underlain by Precambrian rocks of the
Nigerian Basement Complex comprising mostly of older
granites and biotite granite-gneiss overlain mostly by
superficial deposits recognised by their brown cellular
aspects. In carrying out the field investigations, thirtyone
soundings were made in twenty-three stations using
conventional Schlumberger VES method and the expanding C-C
technique. Depth horizontal profilings were also carried out
in one of the twenty-three stations and one other
independent station using the Wenner array and the constant
spacing C-C mode of the two-electrode array. Typical graphs
as done for the laboratory model studies were plotted and
deductions made. Results from the interpretation of the
collected data using the C-C mode show that the depth to
basement within the basin varied from about 7.9 m to about
36 m on the average. The result at each sounding location
was compared with the interpreted result of the collected
data using either the Schlumberger array or the Wenner array
as the case may be. The layer delineation by the C-C mode
agreed well with that delineated by the Schlumberger array.
The C-C technique has delineated layers and has also given a
good estimation of the layer thickness and an uncomplicated
way of determining the surface layer resistivity.
Description
A THESIS SUBMITTED TO THE DEPARTMENT OF PHYSICS, AHMADU
BELLO UNIVERSITY, ZARIA IN PARTIAL FULFILMENT OF THE
REQUIREMENT FOR THE AWARD OF THE DEGREE OF DOCTOR OF
PHILOSOPHY IN GEOPHYSICS
JANUARY, 1996
Keywords
LABORATORY,, FIELD,, INVESTIGATIONS,, VARIANT,, TWO-ELECTRODE,, D.C.,, RESISTIVITY,, ARRAY