Abstract
Soil samples from 31 shallow boreholes were acquired at depths 0m, 1m, 2m, 3m, 4m, 5m, 7m, 10m, 15m, 20m, 25m, 30m, 35m, 40m, 45m, 50m, 55m, and 60m in Pingida (Kolmani Field) in Ako LGA, Gombe State, Nigeria. Using the same boreholes, seismic refraction data was also acquired. The aim of the survey was to delineate the near-surface lithology and velocity layering. The boreholes were drilled using rotary drilling rig and the core samples acquired and described using Wentworth Scale. Seismic refraction data acquired using a single trace Stratavisor NZXP portable digital recorder. The recording spread consisted of a single SM4- 10Hz geophone positioned at depths where the soil samples were taken. A hammer was used as the energy source and placed 3m away from the hole to obtain the first breaks. The refraction data was interpreted using UDISYS Version 1.0.0.0 software. The soil layers in the Kolmani Field have three distinct layers specified as follows, namely, top weathered and sub-consolidated layers made up of intercalation of sandstone, gravel ash clay and muddy coal shale. The lithologic strata do not correlate throughout the field resulting from the highly variable elevation which ranged from 317m and 524m with average of 389.16m. The top weathered layer of laterite intercalated with cobblestones with compressional wave velocity ranging from 342 ms-1 to 517 ms-1 with an average of 405.03 ms-1. Beneath the weathered layer is the sub-consolidated Clay layer intercalated with silt and laterite of compressional wave velocity ranging from 440 ms-1 to 1854 ms-1 of average of 826 ms-1. The underlying consolidated layer is the shale and coal layer having compressional wave velocity ranging from 1518 ms-1 to 4201 ms-1 with an average of 2162.65 ms-1. The dominant lithologic sequences encountered are laterite, clay, silt, sand, gravel, coal and shale. The results of this work can be used for static corrections in seismic reflection processing, planning and assessing risk for engineering structures, and for groundwater exploration. The laterite, clay, silt, sand, gravel, coal and shale can be utilized in agriculture, construction, process industries, and environmental remediation.
Highlights
The lithology of a rock unit is its physical characteristics seen and described in terms of colour, texture, grain size, and composition (US Geological Survey, 2010; Bates and Jackson, 1984; Allaby and Allaby, 1999; American Heritage Dictionary, 2005)
The near-surface soil in the area of study has 3 distinct layers specified as follows: (a) Top weathered layer of laterite intercalated with cobblestones with compressional wave velocity ranging from 342 ms-1 to 517 ms-1 with an average of 405.03 ms-1 (Figure 8); (b) sub-consolidated Clay intercalated with silt and laterite of compressional wave velocity ranging from 440 to 1854 ms-1 of average of 826 ms-1 (Figure 9); (c) underlying consolidated layer is the consolidated clay intercalated with shale and coal layer having compressional wave velocity ranging from 1518 to 4201 with an average of 2162.65 ms-1 (Figure 10)
The rock layers in the Kolmani Field have three distinct layers specified as follows, namely, top weathered and sub-weathered layers made up of intercalation of laterite cobble, pepple, clay, and silts
Summary
The lithology of a rock unit is its physical characteristics seen and described in terms of colour, texture, grain size, and composition (US Geological Survey, 2010; Bates and Jackson, 1984; Allaby and Allaby, 1999; American Heritage Dictionary, 2005). Lithology is the basis of subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas. Grain size is the diameter of the grains and/or clasts which are used to determine which rock naming system to use. In this system, sandstones and conglomerates, a word describing the grain size range is added to the rock name. In rocks in which mineral grains are large enough to be identified using a hand lens, the visible mineralogy is included as part of the description
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