A Comparative Study of Down - Deep Hydrophone Seismic Refraction Survey and Drill Logs for the Determination of Near Surface Layer Velocity and Thickness Over ‘Galos’ Field, Niger Delta, Nigeria

Abstract

Uphole acquisition was conducted to determine the weathering thickness and velocity of strata layer, in essence obtain values for static correction to be used in the processing of reflection data. Drill logs from each Uphole point was used in conjunction with the theoretical interpretations as a physical set of data to aid in making inferences for boundary distinction. Fifty two (52) Uphole locations were drilled and logged. Each location was drilled to 66m and logged to 60m. Litho logy from the bore samples reveals dominantly sand and clay units. The sand sizes ranges from fine to coarse.The records / playbacks have good signal quality with distinct first break times. The UDISYS interpretation software guided by surface (shot depth) correction and shot offset correction displayed a two-layered case. Analysis on the weathering thickness and velocities within the prospect area reveals that uphole locations have weathering thickness ranging from 2.7m to 11.0m. The highest velocity within the weathering layer is 924m/s (254m/s to 924) while the velocity of the consolidated layer ranges from 1528ms to 1927ms. Appreciably, this correlates with the formation samples collected at various uphole lithology logs. More so, it is of interest to note that low velocity layer / weathering thickness within the prospect area synchronized with previous study of the Niger Delta weathering velocity layer.

Keywords: Drill-hole, down-deep, Critical angle, Near surface, weathering layer.