AKAOLISA, CASMIR C. ZANDERS PhD
FEDERAL UNIVERSITY OF TECHNOLOGY OWERRI
Determination of the depth to bedrock and its subsurface geometry are often difficult tasks. Potential field methods generally lack the necessary depth resolution, while seismic reflection data are often contaminated by source-generated noise in the time range of interest (< 100 ms). Massive engineering structures like dams, etc., are always at risk if their foundations are not sited on stable and highly consolidated formations.
Seismic tomography can be used to image the subsurface by inverting first arrival travel times for velocity. In this work, the subsurface imaging at a dam site at Ahmadu Bello University Zaria, Nigeria, using seismic refraction tomography was carried out. Zaria area lies within the northern Nigeria basement complex. The instrumentation consisted of a 24-channel computerised seismograph. The source of energy is obtained from a hammer, which is dropped on a solid plastic plate. This source of energy is considered adequate because the depth to basement in the area of investigation hardly exceeds 50 m.
To process and interpret the comprehensive traveltime data set, a tomographic refraction scheme based on a fast finite-difference eikonal solver and an inversion method that incorporates both damping and smoothness constraints was used. The method of data analysis employed is a high resolution, non-linear travel-time ray tracing for forward modelling and simultaneous iterative reconstruction technique (SIRT) for inversion, which are commonly used in cross-hole travel-time tomography. The result of the analysis shows that the depth to basement in the area is between 20 and 30 m. Multiple interval velocities were obtained from the surface to the basement. This shows that there are several strata within the regolith, which overlay the basement in the area. Low velocity zones (LVZ) were delineated. From the imaging, it was inferred that although the LVZ is attributed to possible area of faulting, the dam is not yet at high risk given that the LVZ is sandwiched by high velocity materials with no traces of cracks or fractures. Seismic tomography inverts first arrival travel times to generate subsurface velocity distributions and has been used to successfully image the Ahmadu Bello University dam.
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