APPLICATION OF ELECTRICAL RESISTIVITY METHOD IN QUANTITATIVE ASSESSMENT OF GROUNDWATER RESERVE OF UNCONFINED AQUIFERS

Abstract

Application of Electrical Resistivity Method in Quantitative Assessment of Groundwater Reserve of
Unconfined Aquifers
was carried out at Port Harcourt and Obio/Akpor Local Government Areas,
Rivers State, Nigeria, in order to locate, delineate subsurface water resources and estimate its
reserve. A Wenner electrode configuration was employed. The field survey was conducted along
thirteen profiles providing continuous coverage. Color-modulated sections of resistivity versus
depth were plotted for, giving an approximate image of the subsurface. The field survey was
accompanied by a laboratory work. Resistivity of rock and soil samples taken from the field
were measured and the resistivity formation factors were obtained. The porosities and hydraulic
conductivities of the same samples were calculated. A relationship between the porosity and
the formation factor has been established. The laboratory established relationship was applied
to the data obtained in the field to calculate the porosity values of the formation present within
the investigated area. The porosity values were contoured and plotted. Depth to the bedrock
for each line was obtained. A commercial computer software was used to enable the
computation of groundwater reserve within the investigated area. The results showed that the
layers associated with the aquifers have resistivities between 345
Ω.m and 8286 Ω.m, and are
located at depths varying from 6 to 51 m. The layers have porosity between 18% and 35%. The
results obtained from the electrical resistivity profiles indicate that the aquifers occupy a surface
area of about 15977900 m2 with a mean depth and net volume of 29 m and 204610000 m3
respectively. The average aquifer porosity was revealed to be 30%, suggesting a usable capacity
of about 61,383,000 ± 7,980,440 m3 reserve of groundwater