Seismic Refraction Investigation Of A Limestone Prospect In Ilaro Area, Southwestern Nigeria

Abstract

A geophysical investigation involving the seismic refraction method was carried out on a limestone prospect located near Ilaro, southwestern Nigeria. The Tertiary Ewekoro Formation composed of clay/shale underlain by limestone constitutes the geology of the area. The objectives of the investigation included the delineation of the subsurface layers within the prospect area, the identification of the limestone deposit at depth, and establishment of a synoptic view of the mode of occurrence and relief of the deposit.
An optimal array pattern of 12 m/39 m shot offset and a geophone-geophone spacing of 12 m were adopted for the survey. Thus, 22 refraction profiles with lengths varying from 134 m to 210 m were shot in forward and reverse manners. The dynamite (0.5-1 kg in weight) was buried at depths ranging from 1.5 to 3.0 m. The first arrival times were picked from the seismograms and plotted as time-distance (T-X) graphs. These were then interpreted to obtain the general geometric/structural disposition and velocities in the subsurface.
Two geologic layers comprising a clay/shale overburden with velocity of between 923 m/s and 2207 m/s and a basal limestone layer with velocity range of 2121-3200 m/s were delineated. Depths to the limestone deposit ranged from 1.5 m to 24.9 m. The deposit is shallowest beneath traverses TR1, TR2 and a segment of TR3, and deepest beneath TR5.
The limestone layer generally dips at a low angle of less than 5o. The overburden-limestone interface is by and large gently undulating with not clearly defined folded structure. The limestone deposit plunges southwards. Normal faults were mapped beneath profiles 3A, 4T2, 4A, 4B, 4D, 5A1 and 5F with throws ranging from 3.1 m to 8.8 m.


Key words: seismic refraction, first arrival times, limestone, subsurface layers

Ife Journal of Science Vol. 7(1) 2005: 79-89