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

Cyril N Nwankwo ,D O Ogagarue

doi:10.12691/jgg-7-2-3

Abstract

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.

Highlights

Many communities in Ohafia, Abia State have in recent times been ravaged with slope failures and landslides (Figure 1) resulting to loss of farm lands, roads, economic trees and other infrastructures
In the several attempts by local authorities to seek for ways to mitigate the risk and reduce the disaster associated with the landslides, less emphasis has so far been placed on the type and strength of subsurface geologic materials in the area which could be a pathway to erosions as well as affect foundations of large civil engineering structures in the area
We employed seismic refraction tomography to determine the velocity structure and overburden thickness in Ebem, a community in Ohafia Local Government Area of Abia State, Nigeria, that had suffered numerous erosional landslides with the aim of providing information on active or likely slide zones, to mitigate the risks associated with the landslides frequently occurring in the community and reduce future disasters, in addition to offering geological explanation to the frequent occurrences

Summary

Introduction

Many communities in Ohafia, Abia State have in recent times been ravaged with slope failures and landslides (Figure 1) resulting to loss of farm lands, roads, economic trees and other infrastructures. The conventional refraction methods have limitations in applicability in areas of complex velocity structures and steeply dipping and discontinuous refractors such as are associated with landslides, they have been applied in the past in landslide studies in the determination of depths to failure surfaces and lateral extent of landslides [10,11,12,13,14]. We employed seismic refraction tomography to determine the velocity structure and overburden thickness in Ebem, a community in Ohafia Local Government Area of Abia State, Nigeria, that had suffered numerous erosional landslides with the aim of providing information on active or likely slide zones, to mitigate the risks associated with the landslides frequently occurring in the community and reduce future disasters, in addition to offering geological explanation to the frequent occurrences. The tomography method utilized in this work does not require an assumption of initial velocity structure or layering, but uses only the first arrival travel time and the data acquisition geometry as input to determine the near surface characteristics of the area

Location and Geological Setting of the Area

Data Acquisition

Data Processing, Results and Discussion

Conclusion