Application of Integrated Geophysical Methods for the Landslide Studies of the Bonga Town
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Date
2000-03
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Addis Ababa Universty
Abstract
Integrated geophysical methods, particularly refraction seismic, resistivity sounding and
profiling and magnetic were conducted on the landslide affected part of the Bonga town.
Based on the data obtained in the survey refractor velocity and depth maps, total field
magnetic and derivative maps, resistivity maps at three depth levels (AB/2=10, 45, and 100
m) and resistivity and seismic sections on selected profiles are prepared.
Tertiary volcanic, namely basalt and tuff, are the main rocks covering the Bonga town
landslide area. These rocks were affected by high degree of weathering that is responsible
for the formation of different grade, from weak to relatively strong basalt and fine to
coarse-grained tuff. The quatematy colluvial and residual soils are found in the area
covering the volcanic on the gentle to moderately sloping topography, whereas the alluvial
soil is occupying the flood plains. The results of these surveys give that the saturation of
unconsolidated colluvial and alluvial soil and the highly weathered volcanic formations of
basaltic origin from recharge of groundwater system and the high intensity and long term
rainfall in the area are determined to have a major contribution to landslide processes of the
Bonga town. This high intensity and long duration rainfall by wetting and increasing the
volume of the loose colluvial formation to introduce imbalance between the shearing and
resisting forces that generate instability. Geophysicaly inferred structural features were
delineated that have strong correlation with the occurrences and orientation of the
geologically mapped active landslide scarps in the area. The landslide area is classified into five zones based on the measured physical responses,
such as resistivity, P-wave velocity and total field magnetic intensity, together with the
geological observations of which Zone five (Z-5) is determined to be the most unstable one
that favour landslide occurrences. Finally, some preliminary remarks and recommendations
are forwarded which would help to better understand the overall landslide process in the
area. The recommendations are thought to lead towards taking mitigating and controlling
measures by minimizing the effect of landslide occurrences on the wellbeing of the
dwellers in particular and infrastructure development of the area in general.
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Keywords
Bonga Town