Naturally occurring sand and gravel
constitute aggregate resources. These resources have had such a great impact on
our lives to an extent that our current quality of life would be impossible to
maintain without them. This is because sand and gravel deposits are often: (1)
economically significant groundwater and hydrocarbon reservoirs; (2) sources of
economic placer deposits (e.g gold, tin, diamond; and (3) sources of
construction aggregate i.e materials 
used for large scale sand fill reclamation of marginal lands, and also
as a stockpile for the construction of major civil engineering works such as
airports, highways, roads, bridges, commercial and residential buildings,
factories and power generating facilities, water supply and waste treatment
facilities. The drive towards rapid industrialization has placed an unrelenting
demand for aggregates in most parts of the world including Nigeria.

      
      In
anticipation of increase in population in the near future and the need to
provide suitable land for infrastructural development in Amabolou and Norgbene
in Bayelsa State, Shell
Petroleum Development Company of Nigeria (SPDC)  under
the new community
interface model, based on global memorandum of understanding (GMoU)
accepted to fund the sand filling of these communities. Before now,
natural aggregates had to be hauled more than 50 km to build and maintain roads
in these communities. To source the sand for this project, a program of
sand search was commissioned. In the Niger Delta, sand and gravel
are usually explored along river channels using drilling methods carried out on
floating barges. But in this instance,
the riverbed of the creek passing through these communities (Fig. 1) is deeper
than 10 m from the surface of the water. It was considered that most dredgers
in the neighbourhood would consider such depth unviable because of their short
dredging arms. Thus, the search was carried out on land. The
search was to determine the availability of sand, estimate the strippable
overburden and estimate the volume of exploitable sand. This information could
of course be obtained by drilling a network of test holes, an
approach that was thought to be invasive, extremely time consuming and
expensive. Besides, lateral facies changes and stratigraphy cannot be
delineated without extensive drilling programme. It was considered that surface
geophysical investigation using the electrical resistivity method would furnish
the information with minimum test holes for control. The electrical resistivity
technique is a well established method, which had been used to investigate a
wide range of hydrogeological, geological, engineering and environmental
problems 1.  The electrical resistivity
method had been used for hydrogeological investigations in the Niger Delta
2,3. In these studies, the electrical resistivity data displays good
resistivity contrasts between the overburden, sand and gravel, and bedrock; a useful
albeit qualitative indicator for the applicability of the
electrical method. The electrical resistivity method is sensitive to
compositional variations in the subsurface, and can therefore be used to
distinguish between different lithologies, e.g clean sand and gravel (high
resistivity) and clay (low resistivity). If the fluid conductivity does not
vary, the resistivity of unconsolidated sedimentary deposits generally
increases with grain size 4. Thus the electrical resistivity method has the
capability of providing a qualitative estimate of the grain size of a sand and
gravel deposit 5. It was considered therefore that the electrical resistivity
technique will be effective in mapping the lithology and geometry of the sand
and gravel deposit.

       
Since the 1940s, electrical resistivity method is being used as a
viable tool for locating and mapping sand and gravel deposits 6-8. For
instance, 5 used surface geophysical methods to characterise aquifers in the
northeastern United States and reported that the bulk resistivity of an aquifer
was generally representative of the aquifer’s grain size characteristics.  9 used contour maps of apparent and true
resistivity values obtained from electrical resistivity measurements (sounding
and profiling) to estimate the volume of aggregates. Generally, the
applicability of the electrical method to sand and gravel prospecting is based
on the high resistivity contrast between coarse grained materials and the
surrounding soil, clay or silt.  The
study of 9 also confirmed the ability of the electrical method to determine
the physical characteristics of sand and gravel important for evaluating the
quality of the deposit. Furthermore, the
authors reported that differing degrees of weathering could be seen in
electrical profiles due to the effect of weathering on porosity.

      In this
study,
in order to reduce the uncertainty and thus have robust subsurface models, we
have integrated geoelectrical sounding and 2D resistivity imaging to map sand
and gravel deposits. In areas with promising potential for sand
and gravel, the volume of exploitable sand and gravel was
determined using the areal extent of the deposit determined from the 2D
resistivity cross-sections.

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