Geotechnical site investigation using seismic refraction and resistivity techniques
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Date
2015-07-11
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Abstract
2D siesmic refraction and electrical resistivity surveys have been conducted at the KNSUT
Business School extension site to find out how suitable the subsurface is for building
construction purposes. The survey focused on using resistivity and p-wave velocity values
obtained from the subsurface at the study area to predict clay zones, voids, fissures and faults,
depth to the bedrock and also examine the campaction of the weathered zone. Fourteen
traverses each 240 m long oriented in the north to south with an inter-profile separations
of 10 m were covered. Resistivity data were collected using the ABEM Terrameter SAS
4000 with the Wenner array and an electrode separation of 4 m. To cover the whole profile
length, the roll-along technique was employed. Seismic data were collected using 10 Hz
electromagnetic geophones with the ABEM Terraloc Mk.6, a 24 channel recording system.
A full version of the RES2DINV software was used to obtain 2D inverse models of the
resistivity data. The 2D resistivity data were collated and into 3D model of the subsurface.
Seismic data was also processed with the ReflexW software into 2D velocity depth models
unveiling three acoustic layers. P-wave velocity ranges of 699-870 m/s, 1189.1-1450.5 m/s
and 3759-5321 m/s were recorded for the first, second and third layers respectively. The use
of the 2D velocity depth models and the geoelectrical inverse models together with geologic
information from the study area helped in the interpretations. Weak zones, distribution of
the subsurface resistivity and possible voids were mapped with the resistivity method. The
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seismic method delineated the bedrock at depths > 20 m at the north which becomes close to
the land surface at about 13 m at the south with p-wave velocity > 3759 m/s. The north and
east display a highly compact weathered layer that can hold the weight of buildings but the
south and part of the west cannot support the weight of buildings due to a highly weathered
subsurface with low degree of compaction.
Description
A thesis submitted to the Department of Physics, Kwame Nkrumah University of Science and Technology in partial fulfillment of the requirements for the degree of MASTER OF PHILOSOPHY.