Geophysical Characterization of Subsurface Layers and Soil Competency of Naraguta Campus, University of Jos-Nigeria

The utilization of geophysical methods can increase the effectiveness of civil engineering works since it can provide the information which the conventional civil engineering method was hard to determine due to the concern of money, time and quality. One of such geophysical methods is the Electrical resistivity using the Schlumberger array which was employed in this study. The study identified three lithological units of top lateritic soil, weathered / fractured basement and the fresh basement. The thickness of the top lateritic soil ranges from 0-4m, the weathered/ fractured basement has an average thickness in the range of 3.0-25 m while the fresh basement thickness ranges from 25 m to infinity. The resistivity values of the sub-surface layers range from 11.40 Ωm to 5896.7 Ωm with most of the lithological units rated competent to highly competent. This study serves as basis for further engineering investigation for the design and construction of proposed buildings and has shown the relevancy of the application of geophysics to civil engineering works.

The study area falls in the basement terrain underlain by the Precambrian crystalline rocks (600 ± 150 Ma) typical of the Nigeria Basement Complex located on the Naraguta Sheet 168 SW. These rocks exhibit structures like joints, veins, faults and dykes. The rock types found in the study area include; the Jos biotite granite, Aplopegmatitic granite gneiss, the Neil's valley granite porphyry, and Naraguta quartz-pyroxene-fayalite porphyry with the dominant rock type being the Aplo-pegmatitic granite gneiss (MacLeod, 1971).
This research study was necessitated by the recent massive infrastructural development works being carried out within the Naraguta Campus of the University of Jos, which falls majorly within area of study. The work will complement directly or indirectly the geotechnical Standard Penetration Test drilling which may not be cost effective.

Method of Study
Electrical resistivity method of geophysical prospecting was employed using the Schlumberger electrode configuration with the aid of Omega SAS 3000 Resistivity Meter. This method measures the conductivity of the subsurface materials by passing electric current through the ground. Twenty-four (24) VES points covering four (4) profiles A-A' and B-B', C-C' and D-D' respectively ( Figure 1) were occupied and the maximum current electrode separation (AB/2) utilized for the Schlumberger sounding was 125 m. In groundwater and engineering studies for instance, the relevance of the method is based on the usually significant resistivity contrast between the weathered zone and or fractured  (Olorunfemi and Fasuyi, 1993). The purpose of the vertical electrical sounding method is to investigate the change of the formation resistivity with depth. To achieve this purpose, it is necessary to arrange measurements in such a way that at different measurements, the value of the measured potential difference is affected by the formation resistivity at different depth ranges. This may be accomplished by changing the distance between the current electrodes so that the depth ranges to which the current penetrates is changed. However, the distance between the potential measuring electrodes and their position with respect to the current electrodes also affect the depth range from which information on the formation resistivity is obtained. In the configuration employed in this study (Schlumberger) the four electrodes are positioned on a straight line, with the two current electrodes on the outside of the potential measuring electrodes. The interpretation of the VES data is based on the assumption that the subsurface consists of a sequence of distinct layers of finite thickness and each of these layers is assured to be electrically homogeneous and isotropic (Koefoed,1979), and the boundary planes between subsequent layers are assumed to be horizontal.
The VES is based on the principle that a fraction of the electric current injected into the ground, penetrating below a given depth, increases with the separation of the current electrodes. Thus as the current electrode separation is increased, the depth of penetration will increase.
The field data was processed for easy interpretation by calculating apparent resistivity (ρa) value for each layer depth, which was done by multiplying the resistance by their respective geometric constants K, (i.e. ρa=R × K).The processed data were interpreted using the computer Software called IPI2WIN where the field curves were compared with the computer-modelled curves to deduce the layer thickness (m), true resistivity values (Ὠm) for Journal of Environment and Earth Science www.iiste.org ISSN 2224-3216 (Paper) ISSN 2225-0948 (Online) Vol.11, No.10, 2021 31 each layer and the percentage fitting error of each plot model.

Results and Discussion 3.1: Sub-surface Lithology
Information on the existing subsurface conditions on site is a critical requirement because it is useful in the planning and design stage of the building foundation and other underground works.
Three lithological geo-electric layers were delineated from the study area consisting of the top soil which is mostly lateritic, weathered/ fractured basement and the fresh basement as seen in Figures 2, 3, 4 and 5 respectively. The top lateritic soil thickness varies from 0.1-4.0 m; the weathered/ fractured basement has thickness that varies from 3.0 -25.0 m while that of the fresh basement ranges from 25.0 m -infinity. The resistivity values of these layers vary from 101 Ωm-821 Ωm for the top soil, 11.40 Ωm -99.98 Ωm for the weathered / fractured basement while that of the fresh basement ranges from 442 Ωm-5896.70 Ωm (Table 1) below. The VES points 13, 15, 19 and 24 have the thickest layers of weathered/ fractured basement while VES 4, 5, 6, 7, 8, 14 and 16 have shallower fresh basement thicknesses which implies that these areas are capable of anchoring heavy implied loads because of the high bearing capacity associated with fresh basement rocks.

3.2: Soil Competency
The aim of any engineering site investigation whether through direct or indirect means is to determine the nature (competence) of the sub-surface strata that are capable of carrying the load that will be applied to it. Electrical resistivity values have been used by authors such as Sheriff (1991), Idornigie et al. (2006) and Bayowa and Olayiwola (2015) to classify earth materials into incompetent, moderately competent, competent and highly competent as shown in Table 2. Clay is characterized by low resistivity usually less than 100 Ωm are regarded as incompetent material as they tend to flow under stress whereas, sands and crystalline rocks are regarded as competent due to the fact that they can withstand stress because of the presence of sands mixed with clay. The sands in this zone would enhance the engineering capacity unlike the purely clayey horizon (Mosuro et al., 2012). Comparing the resistivity values in Table 1 with the standard as proposed by Bayowa and Olayiwola (2015)

Conclusion
Electrical resistivity method of geophysical prospecting has been used in geotechnical and engineering geology, groundwater exploration, contamination studies etc. The cost effectiveness of this method coupled with its suitability in determining the depth to bedrocks, types and thickness of sub-surface materials has made very convenient for use by Civil Engineers and Geotechnical/ Engineering Geologists to characterize the sub-surface before any meaningful construction can commence. Other advantages of the electrical resistivity method are that it saves time and it is environmentally-friendly because of its non-invasive nature. The data obtained in the course of the study identified three lithological units comprising of the top soil which are mostly lateritic, the weathered/ fractured basement and the fresh basement. The sub-surface layers range from competent to highly competent in view of the resistivity values measured. However, this can be complimented with borehole data to arrive at a more convincing conclusion.