Application of Vertical Electrical Sounding in Mapping Lateral and Vertical Changes in the Subsurface Lithologies: A Case Study of Olbanita, Menengai Area, Nakuru, Kenya ()
1. Introduction
The response of the earth to the flow of electrical current is the basis of electrical resistivity [1], under which the Vertical Electrical Sounding (VES) is the commonest resistivity field application for conventional examination of the subsurface. As noted by [2], deployment of VES in collection of data is feasible in large areas given the easiness and economical nature of the method. The VES method correlates vertical apparent resistivity with depth of the subsurface [1] in models achieved by the use of collinear configurations and the obtained results can be interpreted to mean overburden thickness, water table depth, the thicknesses and depths of subsurface layers [3]. Similarly, [4], successfully used resistivity in mapping features not observed at the surface like rock types, structural formations and fractures. The subsurface resistivity changes depending on the rock’s number of open spaces between particles (porosity), the degree of interconnection of the pores, conductivity of the pore water [5] with fresh compact rocks having relatively higher resistivities compared to saturated sand or gravel [6]. Rigorous work goes unto these VES interpretations, where raw apparent resistivities modelled to give closer to reality (true resistivities), where a close match is attained between the calculated and observed resistivity curves thereafter getting geoelectric layer thicknesses [6].
The resistivity differences basis in VES has been used successfully in delineating the subsurface stratigraphy [7], to the extent of establishing sequences of horizontal or slightly inclined layers with clearly defined thickness and true resistivities [4]. Though, [6] notes that translating the resistivity results into geological senseentails being equipped with knowledge of the characteristic resistivity values for the different types of subsurface geological materials which can be modelled to the surveyed area. Further evidence in research in Mayo Kani Area in Cameroon by [8], indicates that apparent resistivities were interpreted to give subsurface layers enumerated as topsoil/clayey sand/water sutured sand horizon/bedrock; topsoil/sand/clayey sand/porous or fractured formation; topsoil, clayed sand/aquifer/bedrock intrusion, topsoil/clayey sand/sand/altered granite. Accordingly, [9] has shown that VES is useful in identifying subsurface profiles at least from their work done in volcanic island of Weh, Aceh.
Stratigraphic orders of certain lithologies are evident like the old land surfaces [10] are clearly recognizable in the geological logs at depths range of; 164 - 166 m for BH 1, 184 - 186 m for BH 2, 118 - 132 m and 182 - 188 m for BH 3, 186 - 190 m for BH 4, 106 - 114 m and 206 - 210 m for BH 5, 70 - 76 m for BH 6, 110 - 114 for BH 7, 174 - 180 m for BH 7A and BH 8 at a depth range of 170 - 180 m. Despite the linking of the VES interpreted results to lithologies in the subsurface within the study area, many tend to only indicate the vertical changes which are in most cases linked to occurrence of groundwater aquifers. [6] certainly shows that there is indeed a good correlation in a reference point within the study area, between its VES data and the lithologies extracted from borehole log. [11] in his worknotes that geological mapping, geophysical surveys covering VES as well as electrical resistivity tomography (ERT) and geo-log assessments have extensively been deployed in hydrogeology valuations in the area of interest. The focus of the aquifer lithologies by [2], indeed shows that using the VES data with borehole log as control data, could help to distinguish the main aquifer, related aquifer parameters from other aquifers. Several boreholes have been drilled in the study area, though not much has been done to compare and contrast the vertical and lateral lithologic changes in the study area.
The VES method has been widely used in the study area, mainly investigating subsurface structural orientation, groundwater aspects of quality and quantity, and as such mainly focusing on the aquifers of the area. This study has purposed to evaluate the lithological changes both laterally and vertically using VES as the main method of investigation. The work done in the area through various analyses ( [2] [5] [6] & [11] ) show possible irregularities in the subsurface of the study area, and thus a program of soundings and the interpretation can help resolve such problems associated with traversing anomalies. This research is based on VES modelled geoelectric layers compared from point to point and using borehole logs as control data to establish inferences of certain lithology in the subsurface.
2. Geology and Hydrogeology
The geology of the Rift is formed of Lava flow rocks whose origin is related to fissures and faulting [6] associated with the formation and subsequent expansion of rift valley. The local geology is represented by lava sheets associated with the eruption of Menengai volcano, whose weathering results unto superficial sediments and soils. The works by [12] and [13] indicate that the geology of the Olbanita Menengai area comprises a succession of Miocene-Pleistocene volcanic, mainly phonolites, trachytes and basalts, with pyroclastics and intervening lacustrine beds attributed to the Pleistocene period. [11] further breaks down the lithologies beneath the Olobanita, Kabatini and Baharini based on assessment of borehole geo-logs to top brown soils and sediments (≈ 10 m thick) overlying lava flows of rhyolites, basalts, trachytes or phonolitic trachytes, with thickness ranges 10 - 40 m, with alternating consolidated pyroclastic materials such as tuffs, agglomerates, ashes and their derivatives comprising loose tuffaceous sands or pumiceous gravels. Geological formations have been mapped (Figure 1) with some that appear to be on the surface in one point mapped as a subsurface layer in another point of the region occasioned by the tectonic episodes ranging from minor faulting, major faulting, unconformities and warping [6].
Groundwater in the Kenyan Rift Valley occurs in volcanic rocks and at fluctuating depths and several aquifers may exist top of one another [6]. The formations beneath the study area in some instances have a tendency of collapsing during borehole drilling operations [6]. Groundwater occurrences in the area have been innumerably discussed with [2], qualifying that there exist four aquifers beneath the study area which occur in different depths from one borehole to another [11]. The different depths of water strike are probably attributed to the vertical fault displacement due to volcanic eruptive series followed by major faulting in the area [6]. [5] shows that episodic lava and pyroclastic deposits in the area were followed by periods of weathering and erosion. [10] describes these deposits’ occurrence in depths beneath the subsurface in the area as channels of water which are accommodated by old land surfaces with [11] defining them as highly conductive coarse sand aquifer layers.
3. Research Methods
3.1. Vertical Electrical Sounding Data Collection
During VES data collection there was an assumption of having uniform one-dimensional layering coupled with progressively increasing layer thicknesses with depth, with smallest to largest, respectively. The Schlumberger (Figure 2) array was carefully laid where the AB/2 and MN/2 were geometrically increased thus avoiding crossing resistivity contrasts in the near surface or accommodating the anticipated thickness changes with depth.
With the Schlumberger array, for each measurement the current electrodes A and B are moved outward to a greater separation throughout the survey, while the potential electrodes M and N stay in the same position until the observed voltage becomes too small to measure. Otherwise sometimes there was need to increase the voltage in the settings of the equipment to ensure minimization of noise, and this adjustment automatically adjusts the current into the ground. For checking the reliability of the data, the soundings were interpreted in the field by using curve matching procedures.
where A (C1) and B (C2) are current electrodes, and M (P1) and N (P2) are potential electrodes.
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Figure 1. The geological map of the study area.
The process has an assumption that the medium below the sub-surface of the earth is homogeneous and isotropic and considered to have trueresistivity ρ, otherwise normally, the geology is heterogeneous thus its resistivity is presented as apparent resistivity (Equation (1));
(1)
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Figure 2. Schlumberger array showing electrodes used to perform VES measurements. (after AGI, accessed Nov 14, 2022).
Two hundred and nine (209) apparent resistivity values in a 100 by 100 meter grid, covering nineteen lines (19) called VES points in the East west direction and eleven (11) lines, otherwise referred to as profiles, in the North-South direction were collected through VES from the Olbanita area (Figure 3).
3.2. Vertical Electrical Sounding Data Processing and Analysis
The Resistivity is automatically measured and calculated giving the heterogeneity of the subsurface lithologies with apparent resistivity (pa) calculated based on I, ΔV, and electrode spacing. The progressive increase of AB/2 results in a generation of resistivity-depth relationship. Data Processing and analysis were achieved in a multipronged approach. The data presented as apparent resistivity by the equipment is achieved by the internal data processing based on the Schlumberger array. This first phase was digitally achieved with the Terrameter’s data handling internal capabilities considering the instructions keyed into the equipment when collecting the data supported by the Shlumberger protocol installed in the Terrameter. The AGI Earth Imager ID inversion automated computer program used in data analysis requires data in the format
thus requiring conversion of the apparent resistivity data before it could be used in determination of the actual resistivities, thickness and depth of the various geoeletrical layers. The
processed data was input into a format used by the AGI Earth Imager 1D inversion software and the datasets were read by the software which in turn gave the log-log plot of the data and the initial model.
The inversion of each VES curve was obtained using an AGI Earth Imager ID inversion automated computer program. The inversion method modifies the initial model until a very close match between the calculated and the measured resistivity curves was obtained. Based on these interpretations, apparent resistivities and thicknesses of geoelectric layers from models thought to be closer to reality, were estimated, and modified by trial and error until a very close match was attained between the calculated and observed resistivity curves. Accordingly, the automatic curve-matching computer program results in a geoelectric model affirmed when the calculated apparent resistivity matches the given field curve almost exactly. Thus, the interpretation as determined by the program is
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Figure 3. Map showing VES data collection points in the study area.
mathematically correct but may not necessarily correspond to lithological reality. The number of layers as determined by the program are geoelectric rather than lithologic, and the resistivities of some of these layers are sometimes unrealistically small or large, while their thicknesses are too small to be detected by the VES method. In other words, the results may, on some occasions, tend to exceed the limitations of the VES method.
4. Results
The data were processed using excel to convert them into the required V/I statistics. The data were analyzed by using the AGI Earth Imager ID inversion software to obtain the true resistivity values through the inversion modelling. The inversion procedure resulted into both modelled curves and corresponding modelled resistivity values which were then analysed further through plotting using surfer software. The use of surfer helped by combining several points in a straight line considering the depth (X-values), the horizontal (Y-values) and the resistivity values with depth (Z-values), whose consequence was a vertical and lateral correlation between the points. The analyzed VES data interpretation using the curve matching technique that is, the comparison between the standard curves and the inversely generated VES curves was carried out. The curves are based on a three-layered earth classified into H, K, A and Q type curves, based on shape, where their correlation with apparent resistivity they are given as: H-type; ρ1 > ρ2 < ρ3, K-type; ρ1 < ρ2 > ρ3, A-type; ρ1 < ρ2 < ρ3, Q-type; ρ1 > ρ2 > ρ3, and subsequently a combination of the above is used to represent several layers in the subsurface.
There are dominantly H-K and K types of curves in the study areas as presented in the analyzed summarized in Table 1, with exceptions of one type Q, one type H, three type A, two of combined type K-H-K and one of combined type HA. Further results are adduced using geologic logs as control data using four different boreholes drilled in the area in comparison with the VES data collected and analyzed from these points (Tables 2-5).
Accordingly these curves (Table 1) are conspicuously aligned to specific profiles (Figures 4(I)-(XI)) running in the north-south direction in the field with each having nineteen VES points. Given the consistency observed in the arrangement of the curves in relation to profiles in the west-east (VES 1 - 19) direction, further analysis reveals several uniformity, but the variations are noted too. The arrangement of the profiles in the field appears in the order 7, 6, 5, 4, 3, 2, 1, 8, 9, 10 and 11 (Figure 3), and thus in the presentations, and further analysis, an order of 1(7), 2(6), 3(5), 4(4), 5(3), 6(2), 7(1), 8, 9, 10, 11 has been adopted (see Table 1 and Figures 4(I)-(XI)). Profile-wise, in profile 1(7) the analysis is given as eight combined type H-K curves starting from the west (VES 1 - 7, 10) of the profile and eleven type K curves at VES points 8, 9, 11 - 19 in the eastern section. Profile 2(6) in the west-east direction has five type K curves (VES 1, 2, 4 - 6), three type A curves (VES 3, 7, 8), one type Q curve (VES 9), one combined type K-H-K (VES 10), with seven combined type H-K curves (11 - 14, 16 - 18), one type H (VES 15) and one combined type H-A (VES 19). In Profile 3(5) there are seven combined type H-K curves (VES 1, 3 - 8), one combined type K-H-K (VES 2) and eleven type K curves (VES 9 - 19). Profiles 4(4) and 5(3) presents one type K curves, in the west-east (VES 1 - 19) direction. There is a clear demarcation shown in profile 6(2) with nine type K curves (VES 1 - 9), eight combined type H-K curves (VES 11, 12, 14 - 19), with exceptions of combined type H-A at VES 10 and type A at VES 13. Profile 7(1) has nine combined type H-K curves (VES 1 - 9) and ten type K curves (VES 10 - 19). The profiles eight to eleven shows uniformity in the west to east direction, with all having type K curves. Surfer 2D plots (Figures 4(I)-(XI)) were generated from VES data, where the profiles are numbered north-south with each having nineteen (19) VES points except profiles 1(7) and 2(6) with twenty two (22) VES points each.
The analysis further deployed the use of the surfer software capabilities, combined the VES data (VES 1 - 19) for every profile and thus accounting for eleven
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Table 1. Showing field orientation of surfer profiles with their VES points assigned the types of curves match they represent.
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Table 2. Profile 8 VES 17 Analyzed data compared with borehole 1 geologic log (VES done near BH1).
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Table 3. Profile 8 VES 13 Analyzed data compared with borehole 2 geologic log (VES done near BH2).
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Table 4. Profile 11 VES10 Analyzed data compared with borehole 3 geologic log (VES done near BH3).
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Table 5. Profile 1(7) VES 16 Analyzed data compared with borehole 5 geologic log (VES done near BH5).
profiles (1(7) to 11). The points where VES data were collected are shown in Figure 3 showing the lines (VES points) in the west-east direction and the profiles in the north-south direction. The generated profiles have a length of 1800 meters (at 100 meter spacing) covering nineteen VES points, except profiles 1(7) and 2(6) with length of 2100 meters (with 22 VES points each) as shown in Figures 4(I)-(XI). All the VES points have an equal depth consideration of 305.60 meters determined from the maximum spacing AB/2 of 320 meters and MN/2 of 25 meters , and considering a depth factor of 2.5 in the AGI Earth Imager ID inversion software. This depth factor was deployed and was precise in determining the exact geoelectric depths which perfectly translated unto specific depths determined by borehole log data.
Further extracts have been made on the curve types to try and show the differences existing between the shapes of same curve types given in Profile 4(4) as shown in Figure5 and Figure6, which shows transition in the west-east direction at VES 8 and VES 9 along the profile. Further correlation is made in appendix FigureA2 and FigureA3.
5. Discussion
The VES curves (Table 1) achieved in the study area varies from the 3-layer (A, H, K and Q types), to 4-layer (H-A and H-K) types and 5-layer (K-H-K) types. The dominance in occurrence can be determined by percentages of the 3-layer A type at 1.44 percent, H type at 0.48 percent, K type at 76.55 percent and type Q at 0.48 percent. The 4-layer curve represented by HK has18.66percent representation and 0.96 for HA type, and 5-layer curve of KHK with 0.48 percent. This analysis in itself indicates that in the area of 1.8 square kilometers there exists curves portraying several subsurface layers. A closer check at the curve types in
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Figure 5. Profile 4(4) VES 8 showing measured (blue dots) and modeled (red line and dots) VES data.
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Figure 6. Profile 4(4) VES 9 showing measured (blue dots) and modeled (red line and dots) VES data.
their appearance in the profiles (Table 1), there too exists a sequence showing a shifting of the order of arrangement. For example, the profiles 1(7), 2(6), 3(5), 6(2) and 7(1) evidently show shifting at about VES points 8, 9 and 10, from either 3-layer to 4-layer or vice versa. The lateral change is noted from the curves as shown by the number of layers, and basically happens at a point diving VES 8 and VES 9 (Table 1, and Figure5 and Figure6). The shape of the K type curves of VES points 1 - 8 are different (Figure5; appendix FigureA2) from the VES point curves 9 - 14 (Figure6; appendix FigureA3), indicating that though profile 4(4) is represented by K type curve, the resistivities’ values vary where the VES points 1 - 8 having their curves starting at above 100 Ωm, while for VES points 9 - 19 starting below 100 Ωm.
There are differences equally picked in the surfer plotted profiles, where heterogeneity is observed in the mapped subsurface. The different resistivity values’ merging is achieved between the points lengthwise in each profile, carrying out gridding method with minimum curvature using a banding of 100 Ωm. Each of the VES point has a depth of 305.6 meters, giving a two dimensional profile along nineteen (19) points in 100 meter separation (1800 by 305.6 meters) and this subsequently shows the changes both vertically and laterally. A keen check in Figures 4(I)-(XI), there exists a feature Y mapped between lengths 1600 and 1700 meters in all profiles except blockage by orientation of feature X witnessed at profiles 4(4), 5(3) and 6(2). In the North-South profiles of 16 NS and 17 NS shown in appendix, FigureA1, the feature appears as a tunnel well mapped with resistivities of below 100 Ωm from the depth of approximately 80 meters to beyond the mapped depth of 305.60 meters. These 16 NS and 17 NS profiles match with lengths 1600 meters and 1700 meters in profiles running west to east in Figures 4(I)-(XI). This is evident that VES has a capability of mapping the vertical and lateral changes in the subsurface.
There is noted shift of the subsurface formations at least from the curve types (Table 1) where profiles 1(7), 2(6), 3(5), 6(2) and 7(1) have their curves in the VES points for one to about eight, nine or ten being different from those from nine, ten or eleven up to nineteen. Profile 1(7) records H-K type curve at VES points 1 - 7, and 10, and the rest being curve type K at 8, 9 and 11 - 18, with the drift observed at VES 10. This is an indication that on the same line traversing an area measuring 1800 meters , there occurs a change at 600 meters , in the number of curve type determined layers from 4-Layer to 3-Layer. A similar change is evident in the surfer profile 1(7) (Figures 4(I)-(XI)) where the constriction of the modeled resistivity values’ lines up to 80 meters deep changes at 1000 meters in the west-east direction.
The analysis of Profile 2(6) shows a change transiting from VES 7 - 10 from curves K to HK through A, Q and KHK curves (Table 1) and this is a distance of about 300 meters shift from 600 to 900 meters along the line in the west-east direction. This is from 3-Layer through a 5-Layer to 4-Layer curve. The intensity of the involved changes are outstanding in surfer profile 2(6) (Figures 4(I)-(XI)) where the modeled resistivity values’ line constriction is amplified between 600 meters to 800 meters as compared to the other parts of the profile. There is noted lateral change of resistivities from the outer (600 meter point and 800 meter point) ranks to the centre (700 meter point) as shown in the surfer profile 2(6) (Figures 4(I)-(XI)). Though less intense, changes in profile 3(5) move from curves HK to curve K at VES points 8 and 9 (Table 1). As is the case with unnoticeable transitional change in the curves but rather sudden, so is the same with the surfer profile 3(5) (Figures 4(I)-(XI)) from line 700 meter and 800 meter. Though there isn’t change in the curve type in profile 4(4), a closer examination show a change in curves’ shapes as shown in Figure 5 and Figure 6. The change happensat 700 meter point in the west-east direction of the profile, showing the possible north-south shifting of the subsurface. The surfer profile 5(3) shows a transition observed at 900 meter where beyond a depth of 100 meters, the VES modeled resistivities are more than 200 Ωm from VES 1 to VES 9, with VES 10 to VES 19 having less 200 Ωm in similar depths.
The drifting for profile 6(2) happens at VES points 9, 10, 11 from curve K to combined curves H-A and finally to combined curves HK as shown in Table 1 with transition happening at VES point 10 (900 meter point on the profile) shown in surfer profile (Figures 4(I)-(XI)). This transition point exhibit highest resistivity valued feature vertically oriented and obvious in the depths of about 80 meters the considered depth of 305.60 meters. Equally evident for profile 7(1) at VES points 9 and 10 there is a change from combined curve H-K to curve K. Similarities between the drifting in curves and changes in the surfer profile 7(1) are eminent and indicates points of drifting or otherwise called transition.
The VES method has successfully been used in mapping subsurface changes as evidently shown both laterally and vertically using the types of curves (Table 1) and the differences in profiles (Figures 4(I)-(XI)). The vertical variations are attributed to changing lithologies with depth in a particular VES point, as can be shown in the control points done using boreholelogs (Tables 2-5). Notably the boreholes 1 and 2 happen to fall within the same profile 8 separated by about 400 meters, in VES points 17 and 13 respectively. Both the VES modeled curves are 3-Layer type K, shown in same uniformity in subsurface surfer image and are located to the east of the inferred line of transition in the generally noted trends of VES curve types and surfer profiles. In terms of borehole log lithology, there are noted differences in depth between the two boreholes but equally the resistivity modeling has assigned different values to different lithologies. At a depth of about 80 meters in the surfer profile, a similarity between the BH1 and BH2 is picked in terms of modeled resistivity layers, whose further examination of the specific lithologies (Table 2 and Table 3), same similarities are picked where BH1 has dust underlain by dark grey pumiceous rock and BH2 has brownish red silty clay overlying the greyish weathered trachytes. The logs of the two boreholes (BH1 and BH2) show similarities and minimal differences in the lithologies with depth and equally to their point comparison in the modeled resistivity surfer profile and of course with agreeing VES curve type.
The borehole BH3 has its lithologic log given in Table 4 and it is located at the transition boundary at Profile 11 VES 10 and the correlated VES curve is type K. The lithologies are dominated by either compacted rocks or soils, insinuating a scenario of compression experienced in this part of the subsurface. This is augmented by the transition noted in Table 1 and modeled resistivity at VES point along the profile (Figures 4(I)-(XI)) which is the starting point of increasing resistivities from the relatively low values in the Eastern part of the deciphered transition boundary and the increased values in the western part. The expose by borehole BH5 shows a good correlation between the modeled resistivity values and the specific lithologies within the interrelated depths. There is specific distinction between the subsurface modeled resistivity values associated with boreholes BH1 and BH2 found in the eastern part of the demarcated area of transition and the modeled resistivity values of boreholes BH3 and BH4 in the western fragment. A correlation of the VES curve types and their change from one point to another in the study area (Table 1) supported by the surfer generated profiles from the modeled VES data (Figures 4(I)-(XI)) shows that there exists and inferred fault line running in the north-south covering about 300 meters at least. The fault line exists in the subsurface and inferred by VES mapping, it is magnificently outlined by the geological map, shown encircled in a blue vertical rectangle (Figure 1). A further evidence is given in the surfer profiles (Appendix, Figures A1(1)-(19)) where those in the east (profiles 12 NS to 19 NS) of the inferred fault line are displaying a relatively top to bottom layering and those in the western (Profiles 1 NS to 11 NS) part of the fault line showing a side to side layering.
6. Conclusion
There is exuded evidence from this study that the application of VES is able to help map the lateral and the vertical changes in the subsurface of any area but the evidence of the specific lithologies has to be supported by availability of borehole log control data.VES data collected was modeled using the AGI Earth Imager ID inversion automated computer program and VES curve types were generated. The evaluation of the curves portrayed the existence of 3-layer, 4-Layer and 5-Layer, but also exposed a transition point where the curves were changing. Further analysis using surfer profiling confirmed the departure/transition area which was established to be a fault line through the further analysis of the borehole logs and the area geological map.
Acknowledgements
Special appreciation is given to Water Resources Authority, Kenya, for facilitating me with requested data and the technicians who were crucial in field data collection. Equally acknowledgement goes to the Ministry of Petroleum and Mining, Kenya from whom geological and topographic maps were obtained, which were useful in production the final geological map of the area. I also wish to recognize the prayers, efforts, encouragement, humour and cooperation received from my wife Lydia Gesare and our Children (Amygrace Kerubo Tsitsi, Merrybell Meita, and Madiba Mandela) in the entire period of this research. They allowed me use the meagre family resources for the research and am dearly indebted.
Appendix