Resistivity soundings

The apparent resistivity is used to explore the floodplain groundwater and to understand the groundwater levels along the floodplains. Underground water may be characterised using parameters obtained by methods such as electrical resistivity, seismic, magnetic and gravity

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methods. Resistivity survey in particular has the potential for tracing the groundwater levels in an area. For the purposes of this research work, electrical resistivity survey method using vertical electrical sounding was applied. Vertical electrical sounding is a geoelectrical method commonly used to measure vertical alterations of electrical resistivity. This method has been recognised to be more suitable for hydrogeological survey of sedimentary basins than the other resistivity methods (Kelly and Stanislav, 1993; Coker, 2012; Chambers et al., 2013;

Orlando, 2013; De Carlo et al., 2013). Among the types of geophysical methods available, the Schlumberger electrical method is commonly used in the region (Ariyo and Adeyemi, 2009).

Application of the vertical electrical sounding method with a Schlumberger array is popular because of its ease of operation, low-cost and its capability to distinguish between saturated and unsaturated layers (Nejad, 2009; Okolie et al., 2010; Asfahani, 2013).

Earth resistivity is related to important geoelectric parameters which include type of rocks, soil or sediment, porosity and degree of saturation (Ndlovu et al., 2010; Mogren et al., 2011;

De Carlo et al., 2013). This method is regularly used to assess a wide variety of groundwater problems, for example, estimation of groundwater level in an unconfined aquifers formations (Song et al., 2012), estimation of aquifer porosity and hydraulic conductivity (Loke, 2010;

Niwas and Celik, 2012), assessment of contaminants from unsaturated and saturated zones (Sainato et al., 2012), characterising the origin of the water losses through dams (Al-Fares, 2011; Moore et al., 2011), assessment of aquifer vulnerability (Gemail, 2011; Osazuwa and Chii, 2010), determination of depth, thickness and boundary of aquifer (Bello and Makinde, 2007), groundwater potentials (Coker, 2012), determination of aquifer characteristics (Perttu et al., 2011; Igboekwe et al., 2012), assessment of near-surface alluvial deposits (Orlando and Pelliccioni, 2010), determination of boundary between saline and fresh water zones (Khalil, 2006), determination of aquifer depth for indicating water-bearing strata (Burazer et al.,

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2010), determination of groundwater quality (Arshad et al., 2007), estimation of aquifer transmissivity (Tizro et al., 2010) and estimation of aquifer specific yield (Onu, 2003).

Factors affecting the value of resistivity

Resistivity is one of the variable physical properties. It is therefore affected by some factors, which include the presence of water, quality, salinity, temperature and geological factors, etc.

1. Water Saturation: - The basic mechanism affecting resistivity in moist sediments and water bearing rocks occurs as a result of the movement of ions and the ability to transmit ions is governed by the conductivity which is a basic property of all materials (Abu-Hassanein et al., 1996). The presence of water in a formation results in increased conductance of electric current. In general the more water presents in a formation the lower the apparent resistivity (ρ_a).

2. Salinity of the water: - the more saline of the water, the lower its resistivity and the higher the conductivity.

3. Temperature: - Electrical conductivity of electrolytes increases with increase in temperature. For temperature up to 150 to 200 ˚C the resistivity of pore fluid decreases with increasing temperature. The dominant factor is increasing mobility of ions caused by lower viscosity of the water. Dakhnov (1962) described the relation as:

where ρ is the resistivity value; is resistivity of the fluid at temperature T; α is temperature coefficient of resistivity; α is 0.023 for T=23 ˚C and 0.025 for T=0 ˚C.

4. Water quality: - When the ionic contents of dissolve minerals increases the apparent resistivity reading increases.

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5. Geological factors: - they include the amount and arrangement of pore spaces, the matrix conductivity, the porosity, sorting, shape and size of the particles, etc. (Abu-Hassanein et al., 1996). Generally, there is an increase in resistivity with decrease in porosity. That is why the basement rocks are characterised by high resistivity because of low intergranular porosity.

Table 2.6 show variation in resistivity range for different types of alluvial sediments along the floodplain.

Table 2.6: Variation in resistivity with some common materials (Source: Jackson, 1975 cited in Fikri and Azahar, 2011)

Typical surface water 5 to 50

Shale 10 to 80

Limestones 80 to 1000

Sandstones 50 to 8000

Coal 500 to 5000

Electronic configuration array

The principle of electrical resistivity prospecting and the technique of electrical prospecting are of different types, namely the vertical electrical sounding (VES) and horizontal resistance profiling (HRP). The various electrodes used in VES are Schlumberger array, Wenner array and dipole-dipole array (Wightman et al., 2003). In fieldwork, the various types of the surface electrode configurations are used for the current and potential electrodes in resistivity. While a large number of electrode types have been used in a resistivity survey, two have gained recognition: Schlumberger array and Wenner array.

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For the purpose of this research project, Schlumberger array was used, because the Schlumberger method is easier to use in the field than the Wenner method (Wightman et al., 2003). Another justification for selecting Schlumberger electrode array is because the positions of the potential electrodes are changed only after changes in the current electrodes are noticed (Sirieix et al., 2013). This will reduce the working hours and keeps the operator errors small, because only current electrodes are moved at a time.

Schlumberger array configuration

For the Schlumberger array configuration (Figure 2.25), the current electrodes (AB) are placed much further apart than the potential electrodes (MN). Separation is continuously increased as the survey progresses while the potential difference is kept fixed until such a time when the resistance becomes too low to measure. The operational principle involves introducing current into the ground through pair of current electrode and with the aid of pair of potential electrode resistivity measurements is obtained. The pair of current electrodes is moved while a pair of potential electrodes is kept fixed. The potential electrodes are moved only when measurement become too low to measure (Wightman et al., 2003; Eke and Igboekwe, 2011; Sirieix et al., 2013). The relationship between the potential difference electrode spacing and the current electrode of AB > 6MN has to be achieved.

Figure 2.25: Schlumberger configuration array arrangement. AB – current electrode separation, MN – potential electrode separation, a – distance between the potential electrodes, S – midpoint-distance between current electrodes and station.

65 Wenner array Configuration

In the Wenner array Configuration (Figure 2.26), four electrodes arrays are used at the surface, one pair of electrode introducing current into earth, the other pair of the electrode for the measurement of the potential electrode with the current. In field operation with Wenner array all the four electrodes area moved between the successive observations (Wightman et al., 2003). Each potential electrode is separated from the adjacent current electrode by distance, “a” which is one-third the separation for the current electrode.

Figure 2.26: Wenner Configuration array arrangement. AB – current electrode separation, MN – potential electrode separation, a – distance between the electrodes.