Geophysical Study of the Weathered and Near Surface Zone in Parts of Oru Area, Imo State Nigeria Using Seismic Refraction Method

Abstract

Seismic refraction method has been used to map the near surface geological features in parts of the Oru Area. Ten seismic refraction profiles were surveyed within the Area. The instruments used were a 10 kg sledge hammer, Mark 6 digital 12-channel seismograph, Reflexw software was used to process the seismic data. A GPS system was used to obtain the co-ordinates of the profiles. Surfer 11 software was used to produce contours of the seismic velocities within the study Area. This research has established that the distribution of p-wave velocities within the subsurface of this area show a general increase of velocity with depth and velocity varying from 400 m/s to 770 m/s for the weathered layer. The thickness of the weathered layer is on the average 20.4 m. Field mapping shows that the Northern part of the study area rests on Ogwashi-Asaba formation. The area has mudstone, claystone, gritty claystone, carboniferous mudstone, massive sandstone facies whereas Benin formation is found to the southwest from Mgbidi. And also clay deposits are found within the boundary between the weathered layer and the consolidated layer. This research shows the presence of shallow and deep seated aquifers within the area. There are traces or channels of less dense materials which may be conduits for fluid movement within the consolidated layer. These channels suggest a defect in engineering capacity of this layer. Hence cutting and filling may be adopted even at great depths in the study area for the purpose of infrastructural development. The clay deposits within the Area can be exploited for industrial purposes. Also discovered in this research is a Batholitic structure along Nempi-Ibiasoegbe which permits the possibility of quarrying for crushed rocks along the section.