Geophysical Investigation of the Origin of Abrupt Changes in Subsurface Properties in Zaria Area, Nigeria.

The causes of some arbitrary and sharp changes, within a short lateral distance, of some physical properties of the subsurface in Zaria area were investigated in this work. The research area is located within the Kubanni Valley which is characterized by a high concentration of valley networks. The study was carried out in the vicinity of a valley in the study area. 2D seismic refraction tomography data were collected along four profiles approximately perpendicular to the strike of the valley and two profiles, each on the flanks of the valley, using ABEM Terraloc MK6 Seismograph with 24 channels. The data collected were tomographically inverted. The results suggest that the basement in the area is characterized by fracture zones which are identified on the tomograms as low p-wave velocity zones flanked by high p-wave velocity zones. The results also suggest that the p-wave velocity range of about 5120 ms -1 - 9093 ms -1 of part of the basement rock on one side of the valley differs significantly from the range of about 2550 ms -1 - 5831 ms -1 on the other side. This sharp difference in the p-wave velocity ranges suggests sharp differences in the physical properties of part of the basement rocks on either side of the valley. It also suggests contacts of different rock blocks in the area. This result prompted further investigation using resistivity tomography. The average basement resistivity of about 2500 Ωm in the western part of the valley and about 12500 Ωm in the eastern part further suggests contacts of different rock blocks in the area. It was concluded that contacts between different rock blocks and high concentration of fracture and fault zones are most probably responsible for the arbitrary and sharp changes of some physical properties of the subsurface in the study area.