Determination of Weathered Layers Characteristics of Field-K, Western Niger Delta, Nigeria Using Uphole Refraction (Borehole) Survey Technique

Sofolabo A. O., Ebeniro J. O.

Abstract: An up-hole survey was carried out on Field-K in Western Niger delta basin to determine some major characteristic of the weathered layers, namely the velocity, thickness, depth to interface of the weathered layers in the study area. A total of 37 up-hole refraction shots were taken and from the data recorded, a time versus depth curves for each of the up-hole location was obtained which was used to calculate the velocities of the weathered layers. Similarly, up-hole survey time versus depth curves were derived which was used in the determination of the thickness of the weathered layer. From the obtained results, the computed weathered (unconsolidated) layer seismic velocity of the study area ranges from as low as 210m/s at UPH-4 location of the study area to as high as 593m/s in UPH-29. The average weathering (unconsolidated) layer velocity over the entire area was found to be 360m/s. The thickness of the weathered layer ranged from as low as 3.1m around UPH-21 and UPH-28 locations of the study area to as thick as 6.8m around UPH-12, while the average thickness obtained for the weathered layers was 4.7m. For the consolidated layers, the seismic velocities obtained ranged from 1131m/s at UPH-32 to as high as 1987m/s at UPH-3 and the overall average consolidated layer velocity was 1707m/s. It was observed that these consolidated layers across the study area were sufficiently competent for civil engineering applications judging from their recorded high seismic velocity values. The depth to the interface (Z) ranges from 0.45m around UPH-28 of the study area to 1.50m around UPH-33. The result obtained from the study gave an overview of the lateral variations in the velocity and thickness of subsurface materials around the area of study. The results shows that the velocities of the weathering layers can be easily and accurately determined using the uphole survey compared to the downhole survey with some level of uncertainties.

Keywords: Weathering Layer, Uphole Survey, Refraction, Consolidated, Unconsolidated, Velocity, Thickness, Elevation, Travel Time