The document discusses a geo-electrical imaging survey conducted in Edo State, Nigeria to characterize the subsurface geology for environmental and engineering studies. Resistivity data was collected along four lines using the Wenner array and inverted to produce 2D resistivity images. The images indicate resistivity increases with depth and identify three main layers - alluvium deposits from 0-20m underlain by laterite to 20m thick, underlain by sandstone and shale. Areas over 3500 ohm-m represent bedrock of gravel and granite, showing the area is suitable for construction. The study found no evidence of contamination or faults, but low conductivity suggests limited aquifer potential for water supply.

1. GEOELECTRICAL IMAGING
SURVEY FOR ENVIRONMENTAL
AND ENGINEERING STUDIES

2.  ABSTRACT
 INTRODUCTION
 AIM AND OBJECTIVES
 LOCATION OF STUDY
 GEOLOGY OF STUDY AREA
 LITERATURE REVIEW
 METHODOLOGY
 RESULTS AND INTERPRETATION

3. A geo-electrical imaging survey was conducted at Uhunmode Local
Government of Edo State Nigeria for site investigation in order to
determine the applicability of 2-D resistivity imaging for Environmental
and Engineering studies The Wenner array was employed. Field data
were obtained for four electrical imaging lines Using PASI Earth
resistivity Meter. The field data was subjected to inversion Using
RES2DINV in order to remove geometrical effects from the pseudosection
and produce an image of true depth and true formation resistivity. Each
of the profiles indicates that resistivity increases with depth. The first 3
layers across the 4 transverse indicates the presence of alluvium deposits
with scattered deposit of clay in small quantities from 0- 20m in depth
underlain by laterite varying about 15m to 20m in thickness, across the
four transverses with resistivity values ranging from 600ohm.m to about
1000ohm.m. The areas of intermediate resistivity zone 1000 to 3500ohm.m
indicate a natural material like sandstone that have been underlain by a
thin layer of shale and The High resistivity zone 3500ohm.m and above
has been interpreted as bedrock which comprises of gravels and granite.
It has been shown that all the areas are competent for the construction and
also suitable for agricultural purpose.

4. The knowledge of the subsurface geology and the characterization of the
spatial distribution of subsurface physical properties are necessary for effective
environmental monitoring, protection and remediation in polluted areas, as
well as for infrastructure development purposes. These will, in addition, assist
policy makers and environmental managers to take quality decisions required
to preserve and sustain a healthy environment for mankind and the ecosystems
in general, and to effectively and safely manage our natural resources. (
Ahzegbobor P.A 2010 ) The applications of geophysical methods are able to
investigate and determine soil properties, bedrock depth, topography of the
bedrock surface below unconsolidated material, rock type, layer boundaries,
water table, weak zones and expansive clays, inhomogeneities of the
subsurface, cavities, ancient relics and generally underground structures or
bodies that have different physical properties from their geological
surroundings (Aubert, 1984; Carrara et al., 2001). Land that is contaminated
contains substances at the surface or in the subsurface that are actually or
potentially hazardous to health or the environment. In Nigeria there are
numerous sites where land has become contaminated by human activities such
as mining, industry, chemical and hydrocarbon spills and unregulated waste
disposal. Contamination can also occur naturally as a result of the natural
geology of the area, or through intensive agricultural use.

5. AIM OF STUDY
The aim of this Research is to determine if the Lithology of the survey area
is suitable for environmental and engineering studies.
OBJECTIVES OF STUDY
1. obtain a 2D model of the subsurface by inverting the data set
2. Infer from the 2D model, the rock formation in the subsurface from
which one can tell if the terrain is suitable for environmental and engineering
studies

6. GEOLOGY OF THE STUDY LOCATION
Geology of the study area is described in the geology of Niger delta.
Niger delta is one of the ten major sedimentary basins of Nigeria. The
others are Abakaliki basin, Anambra Basin, Benue trough, Bida basin,
Bornu-Chad basin, Dahomey basin, Gongola basin, Sokoto basin and
Yola basement complex. These are Western end of the Cameroun
volcanic zone, Northern Nigeria massif and the eastern end of West
African massif. The niger delta is divided into three formations
AKATA FORMATION
It is characterized by a uniform shale development. The formation is a
marine sedimentary sequence laid down in front of an advancing delta.
These prodeltaic shales are medium to dark grey, fairly hard or at
places soft, gumbo-like
and sandy or silty in several places, the shales of this formation were
found to be undercompacted, and therefore mobile, and may contain
lenses of abnormally high-pressured siltstone or fine-grained
sandstone (Allen, 1965; Reyment, 1965; Short & Stauble, 1967 and
Oomkens 1974).

7. AGBADA FORMATION:
This sequence of strata forms the hydrocarbon prospective sequence in
the Niger delta. The formation is characterized by alternating sandstones
and shales of the delta front, distributary channel, and deltaic plain
origin. Weber (1971) showed that the alternating sequence of sandstones
and shales of the Agbada Formation is of cyclic sequences of marine and
fluvial deposits. The sand content ranges from 50 to 75 %. The sandstones
are medium to fine grained, fairly clean locally calcareous, and shelly.
They consist
dominantly of quartz and potash feldspar with subordinate and illite.
BENIN FORMATION:
This is the uppermost unit of the Niger delta complex. The formation can
be easily distinguished based on its high sand percentage (70 –
1000 %). The sand is dominantly massive highly porous and freshwater
bearing with locally interbedded shale beds, which are considered to be of
braided stream origin. The sands are poorly sorted, ranging from fine to
coarse – grained and occasionally pebbly and they contain abundant
wood, fragments, which become lignitic with depth. Composition,
structure and grain size show deposition in a probably upper deltaic
environment.

8. BASIC RESISTIVITY THEORY
Electrical resistivity is a material property which indicates how well a
material retards electrical conduction. Resistivity relates electrical
potential and current to the geometrical dimension of the specified region.
It is the reciprocal of conductivity. Electrical conduction takes place due to
the movement of charges. Charges are displaced from the original
equilibrium condition under the application of electric potential.
However, charge density depends on the applied electric field and
resistivity of the material. Resistivity can be defined by considering
current flow through a cylindrical section. To define resistivity, assuming
a cylindrical section with cross sectional area and length of A and L, if
current flow is I through section resistance R and potential drop across the
section is V, then resistivity can be expressed by the following equation
ρ=RA/L……………………………..(1)
where, ρ=Electrical Resistivity, R= Resistance of the material, V= Potential, I= Current , A= Cross
sectional Area and L= Length. The schematics of cylindrical section and flow of current are
presented
Figure1 : Diagram showing the
flow of current through a
cylindrical section

9. To obtain a good 2-D picture of the subsurface, the coverage of the
measurements must be 2-D as well. The basic resistivity measuring
technique is just similar to the conventional principles. Basically, four
electrodes are required; two for current injection and two electrodes for
potential measurement. Each array has their own advantages and drawbacks
in data acquisition and processing works. But one might has better
sensitivity and resolution power for vertical as well as lateral structural
variations than the other. For instance, Wenner array in multi-electrode
mode has good resolution power for horizontal structure having vertical
variations but weak for horizontally variable geological structures.
Figure 2: Arrangement of
electrodes for a 2D survey

10. METHODOLOGY
In this research work, the Wenner array in electrical resistivity survey was
adopted. The basic field equipment for this study is the PASI Earth resistivity
meter which displays apparent resistivity values digitally as computed from
ohm’s law. It is powered by a 12.5 V DC power source. Other accessories to the
Earth Resistivity meter includes the booster, four metal electrodes, cables for
current and potential electrodes, harmers (3), measuring tapes, walking, phones
for very long spread. In this configuration, the four electrodes are positioned
symmetrically along a straight line, the current electrodes on the outside and
the potential electrodes on the inside. The spacing between adjacent electrodes is
“a”. With Wenner array, the first step is to make all the possible measurement with
electrode spacing of “1a”. Electrode 1 is used as the first current electrode C1, electrode
2 as the first potential electrode P1, electrode 3 as the second potential electrode P2 and
electrode 4 as the second current electrode C2. For the second measurement, electrodes
2, 3, 4 and 5 are used for C1, P1, P2, and C2 respectively. This is repeated down the line
of electrodes until electrodes 17, 18, 19 and 20 are used for the last measurements with
“1a” spacing. The spacing is then doubled without moving on, the first active electrodes
with “2a” electrode spacing being 1, 3, 5 and 7. For the second measurement, electrodes
2, 4, 6 and 8 are used. This process is repeated down the line until electrodes 14, 16, 18
and 20 are used for the last measurement with spacing “2a”. The same process is
repeated for measurements with “3a”, “4a”, “5a” and “6a” spacing.

11. To get the best results, the measurements in a field survey should be
carried out in a systematic manner so that, as far as possible, all the
possible measurements are made. This will affect the quality of the
interpretation model obtained from the inversion of the apparent
resistivity measurements (Dahlin and Loke, 1998).
As the electrode spacing increases, the number of measurements
decreases. The number of measurements that can be obtained for each
electrode spacing, for a given number of electrodes along the survey line,
depends on the type of array used. The Wenner array gives the smallest
number of possible measurements compared to the other common arrays
that are used in 2-D surveys.
METHODOLOGY CONT’D
EQUIPMENT USED FOR THE FIELD WORK
INCLUDE
Measuring Tape, Electrodes (Current And Potential Electrode), Cables,
Terrameter, Hammer , Battery, Cutlass, Gps (Global Positioning System

12. RESULTS
Figure 3: Inversion result from Location 1 Ekiugbo Community

13. Figure 4 Inversion result from Location 2 Ekiugbo Community

14. Figure 5: Inversion result from Location 3 Ekiugbo Community

15. Figure 6: Inversion result from Location 4 Ekiugbo Community

16. DISCUSSION OF RESULTS
The obtained electrical resistivity images of the subsurface of the four transverses are
presented as models in figures 4.1, 4.2, 4.3 and 4.4. The root mean square errors
obtained in the inverted models were between a minimum of 4.6 to a maximum of
11.7% after 3 iterations for each of the transverse. The maximum depth of penetration
for transverse 1 is 53.6m while that of transverse 2, 3 and 4 are 54.3m, 53.6 and 54.3
respectively. There is a very good correlation between the subsurface images of the
four transverses which shows that the area of investigation has very well compacted
soil with even deposition across the area. Each of the profiles indicates that resistivity
increases with depth. The first 3 layers across the 4 transverse indicates the presence of
alluvium deposits with scattered deposit of clay in small quantities from 0- 20m in
depth underlain by laterite varying about 15m to 20m in thickness, across the four
transverses with resistivity values ranging from 600ohm.m to about 1000ohm.m. The
areas of intermediate resistivity zone 1000 to 3500ohm.m indicate a natural material
like sandstone that have been underlain by a thin layer of shale and The High
resistivity zone 3500ohm.m and above has been interpreted as bedrock which
comprises of gravels and granite. From the interpretation it can be shown that the
investigated area is competent for engineering purpose as revealed by the geoelectric
imaging of all the transverses. It has been shown that all the areas are competent for
the construction of factories that make use of heavy machines However; the area might
be difficult to access aquifer because of its low conductivity. The four study areas also
shows no form of contamination.

17. CONCLUSION
The electrical resistivity imaging has been successfully
used to determine the subsurface in the study area from
which it was shown that the study area has compacted
soil and an underlying layer of bedrock which comprises
of gravels and granite the area also shows that there are
no faults which could lead to collapse that are suitable
for engineering purpose. There is also no sign of
contamination which means the soil is not polluted and
can be used from agricultural purposes. However low
conductivity of the study area reveals that it is not
aquiferous and thus might not be suitable for water
supply purposes.

18. REFERENCES
Ahzegbobor P.A 2010 Acquisition geometry and inversion of 3d geoelectrical
resistivity imaging data for environmental and engineering investigations
Allen, J.R.L., 1965. Late Quaternary Niger Delta and adjacent areas: sedimentary
environments and lithofacies. Am. Assoc. Petroleum Geol. Bulletin, Vol. 49, pp.
547-600.
Aubert, M. (1984). Resistivity and magnetic surveys in ground-water prospecting
in volcanic areas. Case history Maar of Beanit, Puy de Dome, France Geophys.
Prospect. 32: 63–554.
Carrara, E. (2001). Resistivity and radar surverys at the archaeological site of
Ercolano. J. Environ. Eng. Geophys. 6: 32–123.
Short, K.L. and stauble A.J. 1967. Outline geology of Niger delta A.A.
PG-V.5. I P. 761 -799.
Oomkens, E., 1974. Lithofacies relations in Late Quaternary Niger delta complex:
- Sedimentology. Vol. 21, pp. 195-222.

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