Determination of weathering trend using seismic reflection data of south-west Niger delta basin

Abstract

A procedure for delineating the weathering trend along a seismic line from reflection data of a study area OML-39 of South-West Niger Delta Basin is presented. Data obtained from SP 422 – 500, Line 105 of the OML 39 of the Okuibome prospect, Delta State, South West Niger Delta Area are presented as a case study. A plot of first break time at a geophone station versus the geophone take-off distance from a seismic shot is made for total of 40 shots, using first breaks time for 14 active traces per shot point. From these plots, the elevation velocity (consolidated layer velocity) for the different shot points were obtained, which ranges from 1642ms^-1 at the western part of the field to 1862ms-1at the eastern part with an average value of 1724 ms^-1 across the line profile. A datum surface was assumed, and a plot of both the shots and geophones elevation above the datum surface along the line gives the surface topography of the area. Subsequently, the weathering thickness along the line was obtained using the elevation velocity obtained, shot depth, weathering velocity and calculated modified intercept time Tmod of the first break and take off distance plots for each of the shot points. With a weathering velocity of 500ms^-1 (supplied from uphole survey of the area), the weathering thickness obtained for the area along the line ranges from 35.46m in the low side to 50.87m in the high side, with an average thickness of 42.66m for the study area along the line, the variation of the weathering thickness along the line is significant. By plotting the weathering thickness, shot and geophone elevation at the stations, the weathering trend of the study area was delineated. The determination of the weathering  thickness at a shot point requires consideration of the shot and geophone  configurations which have to be modified in the computation to suit the actual field situations. From the results obtained, the thickness is highly influenced by the topography along the area, thus weathering thickness along the line of the study area is topography dependent.

Key words: Weathering trend, Weathering thickness, Weathering layer, First break time, Take-off distance, Intercept time, Reflection data and Low-velocity layers.