APPLICATION OF SEISMIC REFRACTION TOMOGRAPHY FOR DETERMINATION OF ROCK ELASTIC PROPERTIES FOR SUB-SOIL EVALUATION IN AN ENGINEERING SITE

In the design of engineering structures, joint efforts must be ensured to determine in-situ subsurface soil conditions and depth to competent layers across any proposed site so as to ensure the stability of civil engineering structures, to avoid the devastating effects of structural failure and collapse rampant of late. Geophysical investigations play an important role in the geotechnical detection of subsoil properties. The study presents one of the geophysical methods, seismic refraction tomography, which allows for an accurate and spatially instant recognition of the subsurface geology and the phenomena occurring in it. Five seismic shots were sounded along a 92m length of profile in a site located inside the PTI-Osubi campus. Compressional wave velocity (Vp) and shear wave velocity (Vs) were extracted from the seismic dataset for estimation of Poisson ratio (a geomechanical parameter). Shear waves velocity obtained from each shot was gathered together to produce a shear wave-velocity profile section which delineates subsurface rock layers. The shear wave section shows a highly heterogeneous subsurface structure with velocity increasing downwards with respect to depth. At depth between 0 – 6m a low velocity structure (233 m/s) was observed. At depth of about 7m and beyond, a high velocity structure (318 m/s) was observed. At depth of 27m, a very high velocity structure (382 m/s) was observed intruding into the low velocity layer overlying it. This typifies the heterogeneous nature of the subsurface. Contour maps of Poisson ratio computed at depths of 3.125m, 4.58m, 6.25m and 8.125m within the subsurface ranges from 0.45 to 0.49 and are indicative of presence of sand and clay formations, which typifies the subsurface sequence in the Niger Delta region. Saturated clay is not a good engineering material, as clay is a weak and unstable material. Thus, it was inferred that Poisson ratio values estimated within depths 3.125m, 4.58m, 6.25m and 8.125m indicates that the geological materials within these depths are weak and unstable, hence not competent enough for design of engineering structures.