ELECTRICAL RESISTIVITY TOMOGRAPHY INVESTIGATION OF TERMITE MOUND AS A CONSTRUCTION MATERIAL IN PART OF OBAN MASSIF SOUTHEASTERN NIGERIA

Electrical resistivity tomography (ERT) is a geophysical method increasingly utilized in various geological and environmental studies, including the investigation of termite mounds. This technique involves measuring the electrical resistivity of the subsurface to create detailed images of its structure (Loke & Barker, 1996). Termite mounds, particularly in tropical and subtropical regions like Southeastern Nigeria, present unique geological formations that can significantly impact local hydrology and soil properties (Whitney et al., 2020). In the Oban Massif region of Southeastern Nigeria, understanding these mounds through ERT can provide valuable insights into their structure, composition, and their broader environmental implications (Asomani-Boateng et al., 2021). Electrical resistivity tomography (ERT) has emerged as a powerful geophysical tool for subsurface imaging, offering detailed insights into the distribution and properties of underground materials (Telford, Geldart & Sheriff, 1990). ERT measures the resistance of the ground to electric currents, revealing variations in material composition and structure that can be crucial for various applications, including environmental studies and resource exploration (Aine, 2010). Recent advancements in ERT have expanded its applications to unconventional materials, such as termite mounds, which are of particular interest due to their prevalence and unique properties in tropical regions (Levin & Oreskes, 2009). Termite mounds, constructed by termites using soil and organic matter, exhibit distinctive physical and chemical characteristics that could influence their potential as construction materials (Roisin, 2000). Understanding these characteristics is essential for evaluating their suitability for construction purposes, especially in areas where conventional materials might be scarce or costly (Bignell & Eggleton, 2000). In the Oban Massif of Southeastern Nigeria, the abundance of termite mounds presents a unique opportunity to investigate their properties using ERT. This region, characterized by its diverse geological formations and climatic conditions, provides a relevant context for such studies (Okereke & Aniebue, 2016). Investigating the resistivity of termite mounds in this area could offer valuable insights into their mechanical properties, porosity, and overall potential as alternative construction materials (Ezugwu et al., 2019). This study aims to leverage ERT to assess the feasibility of termite mounds as construction materials in the Oban Massif. 1.2 Theory of electrical method Principle of electrical method Theory of Electrical Resistivity Method The electrical resistivity method is a geophysical technique used to investigate subsurface materials by measuring their electrical resistivity. The fundamental principle behind this method is that the distribution of electrical potential in the ground around current-carrying electrodes depends on the electrical resistivities and distribution of the surrounding soils and rocks. Measurement Process The standard approach involves using a four-electrode configuration, where two electrodes are designated as current electrodes (A and B) and two as potential electrodes (M and N). A direct current (DC) is applied between the current electrodes, creating an electric field in the subsurface. The potential difference (voltage) between the potential electrodes is then measured. This setup allows for calculating what is known as transfer resistance using Ohm’s Law, expressed as: V = IR 1 where R is resistance, V is voltage, and I is current. Theory of Electrical Resistivity From Ohm’s law, the resistance R of a given conductor is given by; R= V/I 2 Where Rn the resistance, V is voltage and I is the current. The resistance is also proportional to the cross-sectional area and the distance between the electrodes and the relationship is given as: Ra L/A 3 Introducing the constant of proportionality called apprarent resistivity we obtain; R = PL/A 4 Where p = the apparent resistivity, A= the cross-sectional area and L = the length or distance between the two electrodes. Combining equations (1) and (2) we get; V/I = PL/A 5 Recalled also that from circle geometry, surface Area A =2πr Data from resistivity surveys are represented by apparent resistivity which takes into account arrangement and spacing of electrodes. From equation (4) above, the potential at any point is given as: V= PL/2πr 6 Where v = the potential in volts, p=the resistivity of the medium and r = the distance between the electrodes. For an electrode pair with current l at electrode A, and I electrode B, in fig. 1, the potential at a point is given as the algebraic sum of the individual contributions: Hence V = VA +VB Pl 1/2πγA+ 1/2πγB 7 pl/(2π ) (1 )/A^r 1/B^r 8 rA and rB are the distance from the point between electrodes A and B. Two pair of electrodes M and N fig.1 carry no current but are used to measure the potential different between the points M and N. The V may be measure as; V=vM - vN Combining equations (7) and (10) in terms of equation (8) we have: pl/2π 1/A_M - 1/B_M + 1/B_N - 1/A_N 9 And VM are potential at M and N. Am is distance between A and M. AN is also distance between A and N. The distances are the actual distance between the respective electrodes, whether they are on a line Replacing pl/(2π ) 1/A_M - 1/B_M + 1/B_N -1/A_N 10 V = pl/K 11 The equation gives resistivity p = pl/K 12 K the geometrical factor, and is a function of the geometry of the electrode arrangement. Hence resistivity can be obtained by measuring values of V, I and K m