A Study of the Weathering Layer using Uphole Method of Seismic Refraction Survey in Obayantor and Environs, Edo State, Nigeria
1Omoruyi, S. O.; 2Ebahili, E. O. & 3Selong, U, E.
1,2Department of Geology, University of Benin, Benin City, Nigeria
3Department of Geology, University of Calabar, Calabar, Nigeria
Email: ebahili789@yahoo.com
Corresponding Author: Ebahili, E. O.
ABSTRACT
This study is aimed at using uphole method of seismic refraction survey to investigate the thickness of the weathering layer as well as the velocity regimes of the weathering, unconsolidated and consolidated layers in meter/second (m/s). This was accomplished by using the information on the thickness of the weathering layer obtained from the uphole survey to plan depth parameter for high frequency seismic waves (shots) as it applies to 3-D seismic reflection survey and also carryout static corrections, geometry error (linear move-out) correction and migration as it applies to 3-D seismic processing. The study was carried out in Obayantor between Oredo and Ikpoba Okha Local Government Areas, Edo State. The field equipment set-up in uphole method of seismic refraction survey consisted of a calibrated hydrophone harness with marine rope in a well bore filled with water, a recording equipment called OYO Mc Seis version 3.0, a Master-Slave controlled blaster, a printer and a field battery to power the equipment was used. While, the processing and interpretation software was UDYSIS. Fifteen uphole locations were acquired in this study with identity, A,B,C,D,E,F,G,H,I,J,K,L,M,N and O at different locations throughout the study area. Rotary drilling method was used to drill to depths of 63m to 66m and logging was at 60m, 50m, 4om, 30m, 25m, 20m, 15m, 10m, 5m, 3m, 1m and 0m. The lithologies of the formations showed mainly sand and clay. From this study, the weathering layer velocity obtained ranged between 318m/s to 606m/s with an average value of 449m/s. This shows low velocity with which seismic waves are propagated. The thickness obtained for the weathering layer ranged between 4.2m to 10.9m with average value of 7m. The velocity of the unconsolidated layer obtained ranged between 151m/s to 1220m/s with an average value of 1017m/s. The thickness of the unconsolidated layer ranged between 4.2m to 22.2m with an average value of 16m. The value of the velocity obtained for the consolidated layer ranged between 1528m/s to 1950m/s with an average value of 1714m/s.
Keywords: Uphole, Weathered Layer, Hydrophones, Seismic Refraction, consolidated layer
