Near Surface Characterization of Ebem Community, Ohafia Abia State, Using Seismic Refraction Tomography

Seismic refraction tomography was carried out in Ebem community, Ohafia, eastern Nigeria, which recently experienced erosional landslide. The aim was to determine the near surface characteristics in the area, offer possible geologic explanation to the frequent landslides which occur in the area and provide information on potential slide zones to mitigate future disasters. A total of three seismic refraction profiles were carried out using a 12-channel ABEM Terrace MK III digital seismograph with inter-channel spacing of 10 m. Shots were generated by means of a 6 kg sledge hammer and metal plates at 23.5 m before the first and after the last geophone group respectively, and at every geophone station, giving a total length of 157 m for each profile. Velocity structure from the study shows that the area is made up of three different layers on account of velocity distribution, which includes a top layer of unconsolidated, low velocity dry sands assumed to be weathering layer with velocity ranging from about 412 m/s to 1,300 m/s. This layer varies in thickness from about 9 m at the flanks to about 35 m at the center, creating a thick zone of low velocity at the center. This is underlain by a very thin, apparently more compacted sub-weathering layer containing a mixture of sands and clay in which velocity and thickness vary from about 1,300 m/s to 3,000 m/s, and 2,31 m to 3.01 m respectively. Compressional velocity in the area varies from about 412 m/s to about 4.090 m/s, and velocities in the bedrock generally exceed 3,000 m/s; the velocity generally increasing with depth from the surface. The results suggest that the frequent landslides in the area are likely the result of the thick, unconsolidated and permeable dry sands underneath the surface which presumably acts as a sinkhole for surface water run-offs orchestrated by the frequent rainfall which is common in the area. Areas with the highest thickness of weathering are the most vulnerable to sliding in the area