Assessment of Bedrock Depth Utilizing Vertical Electrical Sounding (DDBR)

Abstract

This study is aimed at evaluating the depth to bed rock around Liji area of Gombe Northeastern Nigeria. The area covers 18𝒌𝒎𝟐and lies between Latitudes 11°11ʹ20.4ʺ, and 11°14′ 37.4″Eand Longitudes 10°16′ 20.8″, and 10°19′ 16.0″N, (Gombe N.W Sheet 152). The study area consists of Basement rocks (Coarse grained Biotite Granite,), Aptian-Albian indurated Bima Sandstone, Gombe Formation and Mudrock. Ten (10) Vertical Electrical Sounding (VES) were profiled around Liji and environs, employing Schlumberger array with a maximum electrode separation of AB/2 = 100m to determine the depth to bedrock. The data obtained were processed and interpreted using WinResist program. The results showed that seven (7) VES points (VES 1, 2, 3, 5, 6, 7 and 10) displayed three layers while the remaining (VES 4, 7 and 9) had four layers. The Geo-electric sections along profiles A-A’ and B-B’ revealed that the area is underlain dominantly bythree layers, (top soil, weathered sandstone and basement). Thefirst layer has a resistivity value ranging from 2.3 to 193.5Ωm witha thickness that ranges from 0.4 to 7.1m. The second layer isweathered Sandstone with a resistivity range of 2.8 to 4607.6Ωmand thickness of 0.8 to 7.3m while the third layer is Basement withresistivity value of 93.6 to 37356.9Ωm with an infinite thickness.VES 4, 7 and 9 had four layers and the third layer represent thefresh Sandstone with resistivity ranges of 290.2Ωm to 4607.6Ωmand a thickness of 3.3m to 17.3m while the forth layer in therespective three VES points is basement, having resistivity rangesof 3.1Ωm to 15717.4Ωm with an infinite thickness. At the end ofthe study, the results revealed that Basement rocks are present atVES 1, 2, 4, 5, 6, 7 and 10 at the depth of 10m, 9.3m, 19.7m, 6.5m,5.7m, 14.7m and 24.0m respectively with an infinite thickness,therefore these serves as the depths to be removed to reached thetop of the basement for the construction of a sound foundation.