Seasonal Impact on Seismic Data Quality: A Case Study of Zaria Basement Complex, Nigeria

The impact on depths and P-wave velocities deduced from seismic data, due to subsurface water variation, resulting from the variation in the amount of annual rainfall, was investigated in this study in order to establish the best season for reliable seismic surveys in Zaria Area, Nigeria. A Terraloc MK6 Seismograph was used to collect the seismic refraction tomography data across a borehole of standard log with the centre of the spread situated at the borehole site. Using the same spread and parameters this procedure was repeated severally in a year for four years and the choice for each survey time depended on when there was significant change in rainfall data. The interpreted 2-D results suggested that the average P-wave velocities of the subsurface in the area were generally higher when the ground was wet than when it was dry. The calculated average seasonal changes in P-wave velocity is about 1.8 ms-1 for every millimeter change in the amount of rainfall between March and May when the subsurface was fairly saturated and about 1.3 ms-1 in August when it was most saturated. The velocities generated in this study for the interpreted rock types were compared with a standard velocity scale and the computed average error showed a little disparity of 3.8%, 3.0%, 8.7% and 7.3% between the standard velocities and the velocities for the tomograms generated from the seismic data collected when the average amount of rainfall was 0 mm, 44.8 mm, 322.1 and 62.8 mm respectively. The results also suggested that the overburden of about 9.0 m in thickness, the weathered basement of about 14.0 m in thickness and the fractured basement at a depth of about 23.0 m best fitted the borehole log. This best fit was consistently obtained in the months between March and May when the average total rainfall was about 44.8 mm in the area. The overall results of this study prompted a conclusion that for more detailed and reliable seismic studies in Zaria Area and its environs, with similar climatic condition, the surveys are best conducted between March and May.