GROUNDMAGNETIC INVESTIGATION INTO THE CAUSE OF THE SUBSIDENCE IN THE ABANDONED LOCAL GOVERNMENT SECRETARIAT, OGBOMOSO, NIGERIA
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Groundmagnetic was used to investigate the cause of the subsidence at an Abandoned Local Government
Secretariat, Ogbomoso, Southwestern Nigeria with a view to determining whether the sinking and cracks in the
plasterworks experienced at the Eastern side of the study area is due to subsurface features or insufficient use of building
materials. The study took place at an Abandoned Local Government Secretariat Ogbomoso, Southwestern Nigeria which
falls within latitude 080 09′ 50.6″ - 080 09′ 55.9″ and longitude 0040 15′ 36.1″ - 0040 15′ 42.3″. Proton Precision
Magnetometer model GSM-19T was used to carry out the geophysical survey along four traverses in East-West and North-
South directions. The traverses length ranges from 70m to 150m with inter station spacing of 10m. The results were
presented as groundmagnetic profiles of varying magnetic intensities, groundmagnetic contoured map and groundmagnetic
surface map. The basement topography was obtained from the profiles using half-width of the amplitude method. Areas
with high magnetic values are the competent zones for construction while areas with low magnetic values are the weak
zones. The quantitative interpretation gave the overburden thickness to the top of the magnetic basement rock as varied
between 7.0m to 13.0m. Interpretation of groundmagnetic data revealed that the weak zones present at the Southeastern,
Eastern and Northeastern part of the study area are the cause for the subsidence that occurred towards the Eastern side of
the building. Contrary to the opinion of people that the building contractor used inferior building materials for the
construction, we found out that half of the building is constructed on either collapse tunnel, fault or fracture which resulted
into the sinking and new cracks in plasterworks experienced at the Eastern part of the abandoned Local Government
secretariat.
Keywords
Q Science (General), QC Physics