Geophysical Assessment of Two Collapsed and Concealed Septic Tanks at a Residential Site in Zaria Area, North-Western Nigeria

Abstract

A geophysical study was carried out in the vicinity of two collapsed and concealed septic tanks to delineate the structures responsible for their failure. Two-dimensional resistivity and seismic refraction tomography data were collected along a profile laid across the septic tanks. The data were collected using ABEM Terrameter SAS 4000, aided with LUND imaging equipment and seismograph, Terralloc MK6 respectively. The data were processed using REFLEX-W and RES2DINV software respectively. Pseudosections obtained show that the vicinity of the septic tanks is characterized by resistivity of 10–5200 Ωm and p-wave velocity of 320–2400 ms^-1. Results show that these ranges encompass those of clay and sandy clay at shallow depths, and granite and gneiss in a shallow and undulating basement. The zones where the tanks were apparently sited show very low p-wave velocity of < 600 ms^-1 and on the contrary, very high electrical resistivity of >5000 Ωm which suggest the presence of trapped air column. Based on the interpreted results, the tanks are estimated to be with lateral extent of about 6.5 m and depth of about 2.9 m. The delineated clayey soils at shallow depths suggest that there could be seasonal soil swelling and shrinkages due to seasonal variation in moisture content of the clay. These most likely led to annual ground movements, cumulative soil creep and the subsequent collapse of the septic tanks.