Credibility
Of Using Electric Resistivity and Seismic Refraction in Detecting
Quaternary Sedimentary Variations, Nile Delta, Egypt
ElKhedr H Ibrahim
Geology Dept., Fac. Sciences, Mansoura Univ., Egypte |
Thirteen vertical electric soundings (VES's)
and 3 seismic refraction profiles, using 12-channel seismograph, have been done at
or close to drilled water wells in the Nile Delta. The geophysical data are discussed and
compared with the lithic and hydrogeologic data of the Quaternary sediments.
The lithostratigraphic and hydrological characteristics put some limitations to the use of
the electric resistivity technique in detecting the lateral and vertical facies variations
of the Quaternary sequence in the Delta. It is found that a lithic layer (sand or clay)
may be subdivided into two or more geoclectric resistivity layers depending on differences
in water content and salinity. Also, the very rapid alternation of lithofacies in
the sedimentary sequence gives an electric resistivity value representing the whole lithic
unit and not differentiating thinner layers. In addition, in areas of saline underground
water, the electric resistivity is problematic and fails to distinguish between the sand
and clay layers.
On the other hand, the sand and clay layers within the sequence are difficult to be
differentiated, using the seismic refraction technique. This may be due to the absence of
continuous increase in the seismic velocity with depth in such sequence, or due to an
insufficient velocity contrast between these deltaic sediments. However, the boundary
between the aeration zone and saturated zone on the topmost layer of the sequence was
defined by an increase in the seismic velocity with depth in this layer due to
increase in its water content.
From the attained results, the quantitative interpretation of shallow electric resistivity
and seismic refraction data in the Nile Delta Quaternary sequence is not so simple and
needs a great consideration to all physical and geological factors before reaching
conclusions.
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