Groundwater Geophysics Phần 9 pps

438 M.A. Mabrouk, N.M.H. Abu Ashour, T.A. Abdallatif, A.A. Abdel Rahman

at VES 11. This resistivity range represents, according to geologic evidence,

the upper part of chalky limestone layer related to the Senonian (Upper

Cretaceous). The thickness of the layer is nearly uniform throughout the

area where it ranges from 46 m to 53.6 m. At VES 14 the layer decreases

in thickness to reach 23.8 m.

(6) The sixth layer is characterized by low resistivity that changes

within narrow range (0.3- 1.6 Ωm). At VES 14, the layer showed higher

resistivity value (17 Ωm). According to the geologic information, this

resistivity represents a layer that is mainly composed of argillaceous

limestone. The anomalous resistivity value at VES 14 is due, mainly, to the

decrease of the argillaceous material within this layer at the location of this

sounding. The layer exhibits a uniform thickness ranging from 12 m to

16.5 m.

(7) The seventh layer attains resistivity values that vary from 32 Ωm to

175 Ωm. Nevertheless, at most of the soundings the resistivity of this layer

lies within the range from 40 to 80 Ωm. This range is greatly similar to that

of the fifth layer. According to the data of the wells, this layer corresponds

to the lower part of the chalky limestone layer, which has the same

composition of the fifth layer. The base of this layer has not been reached

at many of the sounding stations. However, the detected thickness at the

other soundings has been found to be uniform (23- 24 m).

(8) The eighth layer has been recorded at six soundings with relatively

low resistivity (5.5 - 21.5 Ωm). Disregarding the extreme resistivity values,

the resistivity of this layer in most cases lies within the range from 12 to

16 Ωm. According to the geological data, this layer corresponds to soft

chalky limestone related to the Lower Senonian (Upper Cretaceous). The

thickness of the layer, where it is detected, ranges from 37.4 to 45.4 m.

(9) The ninth layer attains resistivity variations from 32.6 to 95 Ωm.

However, at most of the soundings the layer is characterized by a

resistivity within the range from 50 to 60 Ωm. As recorded in the nearby

wells, this layer corresponds to the upper part of the Turonian dolomitic

limestone. This zone is not recorded as water bearing in Nekhl well or

Hamth well, which has been specially drilled and designed to utilize the

Nubian sandstone aquifer. However, the relatively low resistivities indicate

possibility of groundwater occurrence. The base of this layer has not been

reached.

The vertical and horizontal extensions of the detected layers along with

the structural elements that affected the succession are illustrated through

three electrical cross sections (Fig. 14.28).

14 Aquifer structures – pore aquifers 439

Fig. 14.28. Geoelectrical cross sections

As shown on the location map (Fig. 14.26), the cross section AA’ ex￾tends from sounding 1 (at Nekhl well) across Wadi El Hamth to sounding

8 at the northern part of the investigated site. The cross section BB’ ex￾tends parallel to AA’ from sounding 9 to sounding 13. The third one CC’

runs also in south-north direction from sounding 15 to sounding 20. The

cross sections indicate that the first, second and third layers show general

decrease in their thicknesses towards north. The layers show regular regional

440 M.A. Mabrouk, N.M.H. Abu Ashour, T.A. Abdallatif, A.A. Abdel Rahman

dip towards the south. Most of the layers extend along the cross sections

with nearly uniform thickness except for the fourth layer due to structural

and erosional processes. Four normal faults have been found to affect the

succession. The faults F1, F3 and F4 throw down towards north, whereas

F2 throws down towards the south and forms with F3 a horst structure.

The fault F1 affected the whole succession, except for the surface alluvial

deposits, whereas the faults F2, F3 and F4 affected the succession, which

is older than the upper three layers. This means that the fault F1 is younger

than the other faults and the upper three layers were successively deposited

on the erosion surface of the underlying faulted layer.

In order to provide better insights into the structural configuration in the

investigated site, the lower surface of the fourth layer (corresponding to

the Esna shale) is selected to draw a structural contour map (Fig. 14.29

left). From this map, it is obvious that the fault F1 strikes in ENE-WSW

direction, whereas the faults F2 and F3 strike nearly in E-W direction and

the fault F4 strikes in WNW-ESE direction. According to the relative

displacements of the identified faults different structural highs and lows

Fig. 14.29. Left: Structural contour map for the lower surface of the fourth layer,

right: groundwater potentiality in layer 9

14 Aquifer structures – pore aquifers 441

and F4 throw down nearly towards

north, whereas the fault F2 throws down to the south. A graben is

developed between F1 and F2 followed by a horst between F2 and F3.

14.4.4 Groundwater occurrence

The geophysical results have been used to map the spatial extension of the

layers which were known as water bearing from borehole data. The

groundwater potentiality of these layers throughout the investigated sites

has been evaluated in view of the distribution of the resistivity exhibited

by the concerned layers. This has been achieved through isoresistivity

maps for the water bearing layers in each of the investigated sites.

Furthermore, the identified structural elements have been illustrated

together with the isoresistivity maps in order to indicate their impact on the

groundwater occurrences.

As a rule of thumb, values around the recorded resistivity of the water

bearing layer at the location of a well are here considered to represent high

groundwater potentiality. On the other side, the groundwater potentiality

decreases at zones of low resistivity values in clay rich layers, where the

clay content is the resistivity controlling factor or at zones of high

resistivity values in clay free layers where the fracture density and degree

of water saturation are the resistivity controlling factors.

The geophysical results indicate that groundwater possibly occurs in the

upper part of the Turonian dolomitic limestone (the ninth layer). This layer

is characterized, at most of the soundings, by resistivity values ranging

between 50 Ωm and 60 Ωm with anomalous high and low resistivity

values. The groundwater potentiality within this layer has been determined

by making use of the isoresistivity contour map (Fig. 14.29, right).

The groundwater potentiality within the concerned layer increases by

decreasing the resistivity values due to increasing the density of the water

saturated fractures. Facies change to materials of low resistivity such as

clay is excluded according to the lithologic description of the concerned

layer (dolomitic limestone) in the wells. Based on the same concept, high

resistivity values are considered to represent zones of less fracture density.

From Fig. 14.29 (right) it is obvious that the groundwater occurrence is

restricted to a small area at the horst structure, which is bounded by the

faults F2 and F3. Elsewhere, groundwater almost does not exist. Generally,

the layer shows poor groundwater potentiality. It is also evident that the

highly fractured zone initiated under the effect of the two adjacent faults

contributes to the groundwater occurrence.

had been developed. The faults F1, F3

442

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M.A. Mabrouk, N.M.H. Abu Ashour, T.A. Abdallatif, A.A. Abdel Rahman