Geophysical intercalated in most cases with clay, clayey

 

 Geophysical techniques were deployed to
assess seawater (saline) intrusion into freshwater aquifers in the coastal
environment of Odimja, Lagos,South-Western Nigeria. Electrical resistivity
method utilizing the Schlumberger technique was used to acquire data for thirty
vertical electrical soundings, and six 2-D electrical resistivity imaging using
the Wenner array to investigate the vertical extent of seawater (saline) intrusion.
The field data were acquired using PASI
16-GL resistivity meter, a 12V, 60Ah Battery and Garmin Etrex model Global Positioning System (GPS) handset. The Wenner electrode array, which
utilizes current electrode half spacing from 1m to 620m, was used to acquire
resistivity data in the proximity of the Atlantic Ocean. Curve types such as
the QHK, HK, and HKH were generated for the investigated area also; 6
geoelectric layers were generated to a maximum depth of 100m. The subsurface
lithology comprise of topsoil to sand intercalated in most cases with clay,
clayey sand and sandy clay. The resistivity of the intruded saline water was
found to range between 1.0 to 22.3 ?m at a depth interval of 1.6 to 86.7 m and the thickness of saline layers
was found to be greater in areas close to the coast. This study shows that both
the 2D electrical resistivity imaging and wenner sounding resistivity methods
are efficient tools for investigating the saltwater-freshwater interface in
coastal areas. Figures 4.1a,4.1b,4.1ci and 4.1cii were extracted from traverse
two(2) with five(5) VES(6,7,8,9 and 10). Geophysical correlation of VES and 2-D
image showed that the results of the geoelectric sections signify topsoil with
resistivity values ranging from 3.9 to 1364.2 ?m within the depth range of 0.4
to 0.8 m, while the 2D result indicates topsoil with resistivity value range of
5.4 to 88 ?m within the depth range of 0 to 5 m. Both results show that the top
soil is composed of clay (saline), clay, clayey sand (saline) and sand. The
second layer on all the geoelectric sections indicates clay (saline), clay and sandy
clay having resistivity values ranging from 2.9 to 69.8 ?m and depth range of 2.0
to 8.5 m which corresponds to the 2D results signifying clay (saline), clay,
clayey sand (saline) and clayey sand having resistivity values ranging from 5.4
to 88 ?m to a depth of 10.0 m. The third geoelectric layer has resistivity values
in the range of 1.0 to 113.3 ?m within the depth range of 4.0 to 37.0 m which
also correspond with the 2D result indicating clay (saline), clay and clayey
sand (saline) layer with the depth of 30.0 m. The fourth geoelectric layer has
resistivity values ranging from 2.2 to 194.4 ?m within the depth range of 11.5
to 93.3 m which correspond with the 2D result indicating clay (saline), clayey
sand (saline), clayey sand and sand within the depth of 40.0 m. The fifth
horizon has resistivity values in the range of 3.4 to 239.8 ?m within the depth
range of 43.7 to 123.6 which correspond with the 2D result indicating clay
(saline), clayey sand, clayey sand (saline) and sand within the depth of 50 m. The
sixth subsurface layer has resistivity values ranging from 18.9 to 96.3 ?m which
correspond with the 2D result clay (saline), clayey sand (saline) and clayey sand.
The depth range could not be determined due to current terminated within this
region. This shows that there is a correlation between the geoelectric sections
and the 2D structures.

It
is recommended that the borehole drilling could be carried out at VES 8 and VES
9 points along the traverses, the depth to the aquifer ranges from 46.6 to 50.4
m with resistivity values of 194.4 to 239.9 ?m. Further studies should as well
be carried out, with a wider spread of about 500 m and above, in order to
determine other possible areas where groundwater can also be explored in the
area of investigation.

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