Geology of Oshiri and Determination of Depth to Water Table Using Electrical Resistivity Method

Geology of Oshiri and Determination of Depth to Water Table Using Electrical Resistivity Method

This work involved the observation, description of various rock types and their field relationships at Oshiri and its environs. These rocks give information on the general geology of the area which helps in determining and interpreting the environment of deposition and paleoecology of the study area.

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Extensive and qualitative electrical resistivity survey method, precisely the vertical electrical sounding (VES) was carried out and used to assess and determine the depth to water table within the study area.

The study area is located at the southern part of Ebonyi State, Nigeria. It is located geographically between latitudes 6005’N and 6010” and longitudes 7050’ E and 7057” on the map scale of 1:25,000 in Onicha local government Area. The areas were accessed through the network of Ezza South and Ezzama-Oshiri road, Oshiri-Onicha road, Ezelechi-Amankalu road, Oshiri-Ugwulangwu road and Oshiri-Agbabor-Isu road. Some of the areas during the field exercise were accessed through footpaths
(See figure 1).

Figure 1: Location and accessibility map of Oshiri

The specific aim and objectives of these work includes:
a. To identify different rock units outcroping within the study area.
b. To make a detailed study of rocks encountered in their outcroping locations.
c. To map and demarcate the rocks encountered into lithologic units.
d. To make petrographic study of the rocks where necessary.
e. To produce a geologic map of the area.
f. To carry out geophysical survey, specifically vertical electrical sounding (VES) within the study area.
g. To infer geoelectric and geologic layers from the resistivity plots.
h. To recommend depth to water table within Oshiri.

Physical features such as climate, vegetation, relief, drainage pattern, soil and land use are observed in the study area.

Two main seasons characterized the study area. They are rainy and dry seasons. The rainy (wet) season extends from April to October which includes a short rainless seasons known as August break and the dry season lasts from November to March.
The harmattan sub-season occurs in the middle of the dry season features, cold, dry NE trade wind of this area is within the zone of tropical hinter having maximum temperature of over 270C of 180C to 210C during the month of September and October as explained by Ilorje (1978).

According to Ilorje (1978), the study area belongs to the guinea savannah type of vegetation, which could further be classified as a park land Savannah which occur as a result of long period of free devastation by man and fire Igbozuluike (1975), described the vegetation of the area as rain forest type (see figure 2 below).

Figure 2: Vegetation of the Study Area (Modified after Igbozuluike, 1975)

The study area is dominated by low relief (plain surface) to moderate relief between 30 and 72 meters above seal level and sharply rising toward Umumboke and Okoffia hills.

Drainage Pattern
The study area is characterized by dentritic drainage pattern. The Ebonyi River is the major river that drains the area, and its tributaries namely, Ide and Offia rivers (see figure 3). These tributaries are perennial and usually overflow its banks at the peak of the raining season. .

Figure 3: Drainage pattern map of Oshiri


1.3.5 Soil and Land Use
The major soil type in the study area is clay, loamy and silt soil. The soil type supports cultivation of rice and yam in the area. It is characterized by brown and dark grey soil. The clayey and shallow brown soils are derived from weathering of the shale.
1.4.1 Field Mapping Techniques
The mapping exercise started with a complete desk study and literature review of the area. This was followed by a thorough reconnaissance survey of the area to determine the accessible routes and to be familiarized with the area. Compass and transverse method was adopted in the field mapping. Detailed field mapping was undertaken during which data were collected and contacts of different structural features and lithology were identified and delineated. Samples were collected for analysis and photographs of the samples were taken where necessary.
1.4.2 Materials for Field Mapping
The following materials were used during the field exercise or mapping.
a. Topographic map of the study area
b. Global positioning system
c. Magnetic compass
d. Camera
e. Masking tape, marker, field note, pens and pencil, sample bag and shoe.


Geophysical Survey Techniques
The vertical electrical sounding (VES) using the Schlumberger electrode configuration (figure 4) was carried out in some locations within the study area.

The procedure involves driving current into the ground using a pair of electrodes and the resulting distribution of the potential in the ground measured using another pair of electrodes connected to a sensitive voltmeter (ABEM TERRAMETER).
The difference in the potential in ohms is converted to apparent resistivty by using a factor that depends on the electrode configuration. At each VES point, the compass was used to get the direction of the point and GPS was used to record the co-ordinates of the area. The resulting resistance of each VES point were recorded in a survey data sheet (see appendix III A-F) and resulting apparent resistivity were calculated respectively, using the formulae below as given by Lowrie (1997).

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Where (Geometric factor)
= (Resistance)

The apparent resistivity data were plotted against the electrode spacing (AB/2) in order to generate the relevant geoelectric curves. The processing of the data was enhanced with the use of interpex 1X1D software, which enabled the generation of the pseudo-section.
Some precautions were taken during the field surveys in order to generate an accurate or near accurate data. For example, the electrodes were hammered firmly into the ground to at least a depth of 35cm in order to avoid error reading and achieve good electrical contacts with the ground. Water was poured at the point of electrode contact with the ground to possibly enhance ionic flow of current. The survey was carried out in the dry season. Also, positions of the electrodes were kept far away from power lines to avoid altering the value of the potential difference.
The Materials for Geophysical Survey
The materials used for the geophysical survey includes.
a. The ABEM terrameter
b. Recording/survey data sheet
c. Global positioning system (GPS)
d. Magnetic compass, hammer and masking tape
e. Car battery and four electrodes ( two current and two potential electrodes)

Figure 4: Geophysical survey field layout equipments

Static Water Level Measurements
The static water level of some drilled wells and hand dug wells were measured using a water level meter called Heron instrument (Figure 5). The co-ordinates of the locations of the wells and their elevations were taken and properly recorded (see Appendix IV). The hydraulic heads were obtained by subtracting the depth to the water table of each well location from the surface elevation (i.e the difference between static water level and the elevation). The processing of the data was done by the use of sulpher 8 software, which enabled the generation of the contour maps (Elevation and depth to water table maps) (see figures 35 and 36).

Figure 5: Heron water level meter

Sieve Analysis
Sieve analysis is a method of mass measurement used in determining the size frequency distribution of sandstones particles especially grain size ranges. The equipment for sieve analysis includes a set of standard sieve, an automatic sieve shaker, sensitive weighing balance and mechanical sample splitter. 50 gram of sandstone was measured from the sample collected at the field from a particular location and poured into the automatic sieve shaker for about 15minutes to shake particles and then weighed to determine the weight of each particle grain size.

Thin Section Analysis
Procedure for thin section analysis, preparation of rock sample using Hillquist thin section machine instruments used.
Hillquist thin section machine,
Model 1005 and 1010 U.S.A.
1. Hot plate with thermostat
2. Epoxy A, B, C, and D, (various types of adhesives)
3. Epoxy syringe
4. Cover slip
5. Carborundum (600 grit and 800grit).
6. Aluminum foil.

b. Initial cutting of specimen
The rock was cut with saw arm of Hillquist machine into a slice along a marked orientation that is not less than 8mm thick and the specimen trim mined to dimension that are less than the length and width of the glass slide. Initial’s grinding of specimen.
With the aid of a 45 micron full coat diamond lap on the Hillquist thin section grinder, the surface of the specimen (rock) was grinded and cemented until all disturbed structures and saw marks were removed and the surface is absolutely flat. The flatness was checked by reflecting light off the prepared surface.


c. Frosting of Glass Slide
Frosting of the glass slides was done to brush off the oily surface and to make the thickness of the glass slide uniform. This was done with fine mesh silicon carbide (800grit) on a glass plate or on a totaling stainless steel lap.
d. Preparation of hot plate and heating
After frosting, the hot plate was set at temperature of 1750F and the specimen placed on it with preferred side up. The specimen was heated until its warm and water is out of it. The frosted glass slide was also heated. The surface of the heated slide was covered with aluminum foil before the specimen is placed.
e. Measurement and mixing of epoxy
The epoxy syring was used to measure 7 parts of epoxy A and 3 parts of epoxy B. This was mixed thoroughly in a small Petri-dish container.
f. Cementing of specimen and slide
The mixed epoxy in (e) above was spread generously on the prepared surface. Then, the specimen was cemented on the glass slide. All bubbles were removed by pressing the specimen down firmly. This was covered for 30 minutes with heat at 1750F.
g. The cored specimen was allowed to cool naturally.
Excess epoxy was removed with methylated spirit.
h. Final cutting of specimen
The saw arm of the Hillquist machine was used to cut the cemented and cored specimen to about 1.5mm thickness.

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i. Final grinding of specimen
the cored specimen was cut, this was grinded using the lap arm and dial indicator by sweeping the specimen back and forth across the wheel cup between + 0.06 and 0.07mm. This was to leave the specimen between 40 to 50 microns thick.
j. Finishing of specimen to 30 microns
15 microns was installed on diameter lap on the Hillquist thin section grinder. The light finger pressure and moderate amount of coolant was used to remove scrates and bring the section to proper thickness of 30 microns (0.03mm).
k. Installation of cover slip
Cover slip was installed on thin section for permanent protection. To achieve this, 4 parts of epoxy C and 1 part of epoxy D were measured to mixed thoroughly. The thin section was warmed on the hot plate at 1730F with the prepared surface and installed cover slip. This was pressed firmly and bubbles checked.
i. Microscopic viewing of thin section
The thin sections were critically viewed under the microscope to ascertain the modal composition of minerals in the rocks. The slides were viewed under plane polarized lens and cross polarized lens to enable the observation of minerals such as plagioclase, pyroxene and biotite etc. Moreover extinction on grain size of the minerals were also taken into consideration.
m. Taking the photomicrograph of thin section
With the help of digital camera, photo micrograph of each of the thin section was done; this requires extreme carefulness to ensure the brightness of the photomicrograph as this will help in the description of the rock samples.

Geophysical methods are used to obtain more accurate information about subsurface condition such as type and depth of materials variation to bedrock, depth of weathered or fractured zone and also depth to groundwater (Bouwer, 1978). The application of geophysical methods especially electrical method have been the most effective methods in providing information on the geological structure, soil properties and hydrological conditions of the subsurface (Nnokwe et al., 2014). The electrical methods have been applied effectively for geoelectric assessment (Zohdy et al., 1974). The method applied is electrical resistivity method precisely vertical electrical sounding (VES). This method is based on the fact that certain physical properties of rock change considerable depending on their water content, thereby creating a physical boundary between the host rock and the water bearing strata. The superiority of this technique over others to predict or assess the geoelectric layer was confirmed by the work of (Selemo, et al., 1995; Olanyinka and Mbachi, 1992). The basis of any electrical resistivity method is measuring a contrast between physical properties of the target, so efficacy of this technique lies in its ability to sense and resolve the hidden subsurface heterogeneities or variation. An electrical resistivity method is one of the simplest methods to be used. It is done by putting two electrodes into the ground and introduction of an electric current, a potential field is created. Two additional electrodes are used to measure the potential. Increasingly deeper measurements are achieved by using a bigger separation between the current electrodes. The schlumberger electrode configuration is used where the potential electrode is fixed and only the current electrode is moved.

The Benue Trough extends from the south where it merges with the Niger Delta to the North where its sediments are part of the Chad Basin. The trough burficates near its northeastern end and the Northern branch continues beneath the Chad formation as an elongate depression that extends well beyond Lake Chad (figure 6).
The Benue Trough is a failed arm of a rift system of the Gulf of Guinea, South Atlantic and Benue Mesozoic triple junction whose centre is occupied by the Niger-Delta (Grant, 1971). After the evolution of the Benue Trough, sediments were into the trough with Asu River Group being the oldest sediment followed by Ezeaku Group, and Awgu Group respectively (Nwajide, 1990).

Figure 6: Divisions of the Benue Trough (Modified after Obaje et al., 1999)

Santonian age marked the stage when the basin experienced another phase of tectonic event that involved deformation, folding faulting and uplift of the Pre-Santonian sediments leading to the formation of Anambra Basin which evolved as a depression to the west of the uplift (Benkhelil, 1987) (see figure 7). Anambra Basin is a Cretaceous depo-centre that received Campanian to Tertiary sediments (Nwajide, 1990 and Obi, 2000). Murat (1972), propose major tectonic phases which took place in Albian-Santonian and late Eocene or early Oligocene time. These major tectonic phases resulted in the formation and subsequent remodeling of the Benue Trough, these three phases were (a) the Abakaliki-Afikpo Benue phase (Aptian-Santonian) ((b) the Anambra Basin phase (Campanian-mid Eocene) and (c) the Niger Delta phase (late Eocene Pliocene). The Santonian tectonic phase resulted in the series of folding and fracturing of these rocks giving rise to chains of anticlines known as the Abakaliki anticlinorium” around the Abakaliki area and synclines known as “Afikpo synclines” around the Afikpo area (Reyment, 1965).

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Figure 7: Tectonic map of south-eastern Nigeria (Adeigbe and Salufu, 2009).

The origin of the Anambra Basin and Afikpo Syncline is intimately related to the development of the Benue Rift (Murat, 1972).
The Benue Trough was subjected to four main depositional cycles, each of which was associated with transgression and regression of the sea.
The first sedimentary cycle lasted from the Mid-Albian – Late-Albian, and is thought to have been initiated by the opening of the South – South Atlantic Ocean. This is associated with the deposition of the Asu River group (figure 8), a lateral equivalent of the Bima sandstone in the Upper Benue Trough and the Awe Formation in the Middle Benue Trough.
The second sedimentary phase occurred between the Upper Cretaceous and the Middle Turonian, and was associated with the deposition of the Eze-aku shale. Its lateral equivalents are the Amasiri and Makurdi sandstones in the Afikpo basin and Middle Benue Trough respectively; while the Gongola, Jessu and Dukul Formations are its lateral equivalents in the northern portion of the Benue Trough – the Upper Benue Trough.
The third sedimentary cycle ranged from the Upper Turonian to the Lower Santonian. It is associated with the deposition of the Awgu shale and Agbani sandstone which are lateral equivalents of the Fika shale in the Upper Benue Trough. The Turonian transgression which marked the start of the cycle is believed to have commenced from the Gulf of Guinea through the Anambra basin to the Benue Trough. Most deposits of this cycle have been eroded as a result of the Late Cretaceous tectonic activity.
The Fourth sedimentary cycle was marked by the deposition of the Nkporo shales, the Owelli sandstones, Afikpo sandstones and Enugu shales during the Campano-Maastrichtian transgressive phase. This cycle also marked the deposition of the coal measures including the Mamu Formations, the Ajali sandstones and the Nsukka Formations. Its lateral equivalents are the Numanha shales and Gombe sandstones in the Upper Benue Trough.

Figure 8: Map showing the stratigraphy of South-Eastern Nigeria (Chukwu et al. 2014)

The Lower Benue Trough
The Lower Benue Trough as shown in Fig 7 is a linear, intra-cratonic, graben basin trending NE-SW, associated with the separation of the African and South American plates in the early Cretaceous. It is characterised by an uplifted basement block which is flanked by deep basin containing sediments of different ages, about 6,000m thick endowed with various mineral deposits. It is the southern section of the Benue Trough. The geology of the Lower Benue Trough is associated with the tectonic activities that were recorded during the Cenomanian period (Fatoye et al, 2013).

Table 1: The Stratigraphic Succession of the Lower Benue Trough from (Reyment, 1965).

Sedimentation in the Lower Benue Trough commenced with the deposition of the marine Neocomian – Mid-Albian Asu River group, although there are reports of igneous activities of Aptian – Early-Albian age; Ojoh, 1992. The sediments of this group comprises predominantly of shales with localised sandstones, siltstones and limestones and some igneous materials of the Abakaliki shales in the Abakaliki area and the Mfamosing limestone in the calabar flank; Petters, (1982). The Asu River group has been further described as consisting of arkosic sandstones, volcanoclastics, marine shales, siltstones and limestones overlying the Precambrian to Lower Paleozoic crystalline basement rocks by Akande et al, 2011. Sediments of this group have been interpreted as sediments of the first transgressive cycle into the Lower Benue Trough (see Table 1).
Sedimentation continued with the deposition of the marine Cenomanian – Turonian Nkalagu formation (Eze-aku) as well as the inter-fingering regressive sandstones of the Agala and Agbani formations of the Cross River group which rests on the Asu River group (see Table 1), although, sandstone, limestones and shales with calcareous sandstones of the Odukpani formation were deposited unconformably on the basement rocks in the Calabar flank during the Late-Albian.
The major tectonic events recorded in the Lower Benue Trough occurred in the Santonian, a period of non-deposition and major folding and faulting; and consequent uplift and erosion of the sediments. It was the intense Mid-Santonian deformation and magamatization in the Benue Trough that displaced the major depositional axis westward that led to the formation of the Anambra basin (Fatoye, 2013).

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