Determination of weathering depth by uphole shooting techique in some parts of Owerri, south eastern Nigeria

Abstract

Certain irregularities associated with seismic operation are due to the extent and depth of weathering in the vicinity of the seismic operation. The extend and depth of weathering as characterized by low seismic velocity is very important in static correction, especially where shots are taken in the weathering layer.

Result from 21 uphole positions recorded during a seismic prospecting programme for petroleum exploration in the study area revealed three seismic layers as follows:

(a) A top intermediate silty sand (Highly weathered)

(b) An intern mediate sand layer(subweathered layer)

(c) A third weathering layer consisting of sand and Gravel beds.

The result indicates a varying depth of weathering between 1.8 to 5.0m with an average value of 3.53m and an average velocity 36m/sec. The intermediate layer with of 9.2m to 20.9m has an average velocity of 733m/sec. The weathering and subweathered layers have a total average depth of 21.76m and an average velocity of 547.3m/sec.The result which is consistent with velocity values of unconsolidated sediments is utilized for weathering correction during processing of seismic data. The area around Ihiagwa marked by close contours are likely inversion zones, indicating possible subsurface complexity.


Key words: Weathering layer, Uphole Shooting Static Correction, Average Velocity and Unconsolidated Sediments.


(Global Journal of Pure and Applied Sciences: 2003 9(4): 533-538)