Delineation of weathered layer using uphole and surface seismic refraction methods in parts of Niger Delta, Nigeria Mfoniso Aka, Okechukwu Agbasi, Johnson Ibuot, Chukwuemeka Sampson.

Abstract EN

-Uphole and surface seismic refraction surveys were carried out in parts of the Niger Delta, Nigeria, to delineate weathering thickness and velocity associated with aweathered layer.

A total of twelve uphole and surface seismic refraction surveyswere shot, computed and analyzed.

The velocity of the uphole seismic refraction ranged from 344.8 to 680.3 m/s with a thickness of 5.45 to 13.35 m.

Surface seismic refraction ranged from 326.6 to 670.2 m/s and 4.30 to 12.0 m, respectively.

The average velocity and thickness ranged from 559.6 to 548.0 m/s and 9.43 to 8.63m with differences of 11.6 m/s and 0.83 m respectively.

The VW/VS ratios ranged from 0.955 to 1.059.

This indicates that the uphole velocity is higher than the surface refraction velocity leading to low VW/VS values.

This is a direct experimental proof of a low velocity zone, confirming the weathered nature of the area.

The results of both refraction methods are reliable; the differences in surface refraction values are due to shot point offsets.

Based on these findings, it is recommended that shots for seismic surveys should be located above 15.0 m in the area to delineate the effects associated with weathered layers to ensure that will be competent to withstand engineering structures