Determination of Weathering Depth Within the Upper Cretaceous and Tertiary Sedimentary Succession in the Upper Benue Trough, Ne-Nigeria Using Uphole Seismic Refraction Technique.

Abstract

A total of 31 up-hole seismic refraction data was acquired and analyzed from 31 
borehole sections in Kashere area, Upper Benue Trough, NE Nigeria, to determine 
the depth to bedrock by obtaining the thickness as well as the velocities of the 
weathered and consolidated layer(s). The essence of the study is to determine the 
suitability or otherwise of these layers for civil engineering structures such as Oil 
well drilling rig platforms, Dams and also for delineation of area most suitable for 
underground dumpsite. The study is equally useful in the determination of depth of 
burial of explosives when acquiring seismic reflection data using dynamite as well 
as static correction during processing of 2D/3D seismic reflection data. Analyses of 
the data shows that the near surface geology in the study area is relatively 
heterogeneous with moderate velocity contrast; weathered depth within the study 
area varies enormously ranging between 4.5m to 20.9m, with average depth of 
about 10m. Thus, the near-surface weathered layer can be thought of as having a 
flat top but a highly irregular internal structure. Elevations in study area vary 
between 317 m and 524 m with average of 389 m; implying that the topography of 
the area is highly variable and undulated. The velocity of the weathered layer ranges 
from around 342 m/s to 698 m/s, averaging 551 m/s. The average velocity of the 
underlying consolidate layer in the area is 2097m/s, this shows a general trend of 
increase in the velocity with amount of consolidation of the bedrock and is sufficient 
enough to support engineering structures. The depth of weathering and lower 
velocity can be observed to be more prominent along the NE-SW direction 
suggestive of higher weathering rate along the structural trend of the Benue 
Trough.