Weathering Layer and Topographic Irregularities at the Early Stages of Seismic Data Processing Resolved with Statics Correction Method in Southern Niger Delta, Nigeria

SUMMARY The South central part of Niger delta has the terrain of high weathering thicknesses and unique profile of weathering velocities, with random spurious values. The existence of spurious values of weathering thicknesses is consequent to prolific deposition of sediments of variable lithology, densities, thicknesses and velocities. Uneven or unequal thicknesses about the creek/land boundaries, with clayey deposits on one side, and fine sand on the other exist, thus, the heterogeneity of the vadose zone. Spurious overburden of sediments in this part of the Niger delta was evident on seismic records as the introduction of spikes in the profile of weathering thicknesses. Weathering thickness highs tied to depositional inequality of spurious overburden and low range of weathering velocities. Deposition at many land/creek boundaries gave rise to the trend of weathering thickness and weathering velocities of uneven gradient. When the energy source was buried within the weathering depth, it came out weathered shots of low amplitude data, but it came out as strong events and good reflections when the energy source was at the depth below weathering. Rock types varied from clayey sand to sand, to silt, to shale and fine-sand sediments. There were challenging results of erratic topographic irregularities and weathering depth values resolved by statics computation or correction method which considered the heterogeneity of the weathered zone and adopted a system of slicing-off the slab of heterogeneity. An approach was the use of seismic source located at the base of the weathered layer, viz, single deep holes (SDH). That produced sharper seismic signals of good data quality which digitization and analyses yield enhanced results of improved Signal/Noise ratio. Subsequently more reliable weathering velocities (Vw) were generated and similarly more accurate computation of the thickness of the weathered layer or the low- velocity-layer (LVL). The challenge was resolved by use of statics correction which effects automated replacement of the heterogeneous layer with underlying layer attributes / consolidated velocity. It produced weathering thicknesses ranging from 5.1m to 33m and off-mark spurious patches 37m to 41m while the weathering velocities measured 410m/sec to 550m/sec. Off-mark values of weathering velocities ranged from 390m/sec to 400m/sec. Good seismic signals of the refractor or consolidated layer yielded velocity values that ranged from 1667m/sec to 1750m/sec.