Determination of the Physical Properties of Near Surface Layers of Omerulu Area, Nigeria, Using Seismic Refraction Method

摘要

Seismic refraction method is a geophysical technique used to study physical properties of the subsurface such as layer thicknesses, travel times and velocities of seismic signals through the layers. The process of turning seismic refraction data to useful information involves use of first arrival times and offsets. In this study, the near surface investigation of the physical properties of the unconsolidated and consolidated layers was carried out in Omerelu, Rivers State of Nigeria. Omerelu lies between latitude 05sup0/sup 08’N and 05sup0/sup 13’N and longitude 06sup0/sup 51’E and 06sup0/sup 58’E. Previous investigations were based on refection method in which the near surface effect caused misalignment in the deeper horizons observed in the final stack of the reflection data. This problem was solved by running a seismic refraction survey over the area. The study involved identification of points at which data would be acquired to give a good overview of the area under consideration. Twelve sampling points were picked with a grid of approximately 4 × 4 km. A 100 m line with two source points at each end was cleared at each data acquisition point after which the coordinates were taken using Leica Total station (TC 1203 survey equipment). Seismic signals were recorded using OYO McSeis 160M coupled with a 12-geophone harness along with a blasting unit. Upsphere processing software was used to plot time - offset graphs to determine the velocities of the unconsolidated and consolidated layers. The intercept times were also graphically obtained and used to determine the thickness of the unconsolidated layer. Results show that the thickness of unconsolidated/weathered layer in the study area varies between 12.25 and 13.60 m, while the velocities of the unconsolidated and the consolidated layers vary between 500 – 550 m/s and 1790 – 1875 m/s respectively. The results obtained when applied to the reflection data, were able to resolve the static problems; thereby increasing and improving the quality of data available on the lithology of the study area.