Trans. geol. Soc. S. Afr., 84 (1981),107-114
A GEOPHYSICAL STUDY OF THE CAPE FLATS AQUIFER
by
R. MEYER and J.H. DE BEER
ABSTRACT
A geophysical study, forming part of a geohydrological investigation, was undertaken in the Cape
Flats to determine the thickness of the porous overburden and the nature of the underlying bedrock.
The seismic refraction and electrical resistivity techniques were employed during the geophysical survey.
The seismic refraction survey was undertaken first and produced travel-time curves of good quality
which were assumed to represent a three-layer model. The second layer of this model was considered to
comprise water-bearing unconsolidated sand with a relative restricted range of velocities (between
1 550 and 1 800 m/s) throughout the study area. A number of boreholes were sunk on the basis of the
seismic results and in most cases the borehole findings confirmed the interpreted overburden thickness.
However, in some areas in situ weathered bedrock was present which has a seismic velocity that differs
so little from that of the water-bearing sand as to produce no expression of its presence on a travel-time
curve. This problem which resulted in erroneous estimates of the amount of groundwater in storage
prompted the use of the resistivity sounding technique to detect those areas where the water-bearing
sand is underlain by weathered bedrock. The weathered bedrock is a conductive unit which makes it
recognizable on a sounding curve. The electrical method could differentiate between the water-saturated sand and the clay, giving a more conservative but also a more realistic estimate of the available
groundwater in storage.
This study is a good example of how two geophysical techniques can complement each other to yield
more accurate results.
CONTENTS
I. INTRODUCTION ... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
II. SEISMIC REFRACTION SURVEY. . . . . . . . . . . . . . . . . . . . . . . . . . . . .
A. Equipment, Field Procedure and Interpretation Methods . . . . . . . . . . . . . . .
B. Results of the Seismic Refraction Survey . . . . . . . . . . . . . . . . . . . . . . . . . .
III. ELECTRICAL RESISTIVITY SURVEY. . . . . . . . . . . . . . . . . . . . . . . . . .
A. Equipment, Survey Methods and Interpretation Procedure. . . . . . . . . . . . . .
B. Field Work. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
C. Results ofthe Resistivity Survey ..... . . . . . . . . . . . . . . . . . . . . . . . . . .
IV. CORRELATION BETWEEN GEOPHYSICAL AND GEOHYDROLOGICAL
PARAMETERS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
V. CONCLUSIONS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
ACKNOWLEDGMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
REFERENCES. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
I. INTRODUCTION
This geophysical study forms part of an extensive investigation into the feasibility of reclamation, storage and
abstraction of purified sewage effluents and surplus runoff water in the Cape Flats (Henzen, 1973). The aim of
the geophysical survey was to determine the thickness of
the porous overburden and the nature of the underlying
bedrock.
The geologically young sedimentary cover in the Cape
Flats area consists of sand deposits locally interbedded
with peat, clay layers and varying amounts of calcareous
material. These unconsolidated sedimentary strata were
deposited partly under fluvio-marine conditions and
partly under aeolian conditions (Gerber, 1977). The impermeable basement is formed by Cape Suite granites and
Malmesbury Group metasediments in various stages of
decomposition. At the time of the field work very little
was known about the distribution of the different lithological units and the extent of weathering of especially the
Malmesbury Group rocks underlying the Tertiary to Recent sand deposits.
The seismic refraction survey was carried out during
1969 and the electrical resistivity investigation during
1971. Originally, a seismic refraction study was decided
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107
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108
109
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113
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on because preliminary investigations indicated that electrical resistivity work would be difficult due to the very
high levels of industrial electric currents in the ground and
the high contact resistance of the surface sand cover. The
results of drilling that followed the seismic survey
prompted a change of decision and the electrical resistivity survey was undertaken.
II. SEISMIC REFRACTION SURVEY
A. Equipment, Field Procedure and Interpretation
Methods
A 12-channel Dresser SIE RS-4 refraction system recording on dry paper was used in conjunction with 4,5
and 14 Hz vertical component geophones. Geophones
were placed at 15 m intervals and shotpoints at 75 m or
150 m intervals along a profile depending on the amount
of detail required. A spread consisting of 12 geophones
thus covers 165 m (Meyer, 1974). Normally, only four
shots were fired into each spread, but if sufficient refraction coverage of the bedrock was not attained three additional shotpoints were used, one in the centre of the
spread and two at distances of 67,5 m from the end on
either side of the geophone cable. For the four-shotpoint
coverage, shots were detonated midway between the first
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
108
two and last two geophones as well as 150 m on either
side of these. Hand-auger holes 1 to 2 m deep were
loaded with charges of dynamite varying between approximately 0,2 and 1,5 kg depending on the distance
from the geophones and the seismic noise levels.
Most of the profiles were orientated in an approximately east-west direction, thereby enhancing the possibility of detecting north-south-trending undulations in
the bedrock surface at the expense of east-west trends
(Fig. 1). The 12 seismic refraction profiles undertaken
gave a total of 19,5 km of continuous coverage.
34~tr=30==='=--..----....,.,.,.-=__- - - - - : : : : : - - - - - - - r - - - - - - , 3 4 · 0 0 '
CAPE FLATS
GEOPHYSICAL SURVEY
---•
®-0
""
Seismic profile
~~SOU1di1g
Profile
----;----
-------
_------fJ
34'05'
34'05'
FALSE BAY
lB"30'
Figure I
Locality map showing the positions of the seismic profiles and
electrical soundings.
The seismograms were of good quality showing excellent first breaks. In some places where thin layers of
weathered bedrock with seismic velocities intermediate
between that of water-saturated sand and solid bedrock
occurred, secondary arrivals could be picked. Because
continuous refraction coverage of the bedrock was
achieved, the interpretation method as described by
Hagedoorn (1959) and Hawkins (1961) which involves the
determination of the intercept times and the velocity of
the bedrock at different geophone positions was adopted.
Since these parameters are determined solely by the
travel-times of waves refracted from the refractor concerned, this method of interpretation is far superior to
those which assume plane refractors and straight line
200
travel-time curves, e.g. the critical distance method (Van
Zijl and Huyssen, 1971). The field procedure as well as
the interpretation techniques of this method are described
in detail by Meyer (1978).
B. Results of the Seismic Refraction Survey
The profile marked AA' on Fig. 1 will be discussed
and the seismic results will be compared with those of the
geoelectric survey.
From Fig. 2 it is clear that the travel-time curves were
of good quality. In general, they represented a three-layer
model although along some other profiles the presence of
an intermediate layer overlying the bedrock could be detected on the seismograms. The travel-time curves were
interpreted as representing a thin layer of dry alluvium,
with a velocity ranging between 300 and 1 000 mls underlain by a layer of water-saturated alluvium with a velocity
between 1 550 and 1 800 mls and finally the bedrock. The
velocity of this layer varies over a large range. In an unweathered state the bedrock layer has a velocity in excess
of 4 500 m/s. The occasionally seen layer intermediate between the water-saturated sand and the solid bedrock represents weathered bedrock and has a velocity of between
2 400 and 4 000 m/s. Initially, it was assumed that the
granitic bedrock would be seismically different from the
bedrock consisting of metamorphosed Malmesbury rocks
so that the two types of bedrock could be differentiated.
An inspection of the profiles where borehole control was
available shows that such a distinction is not always possible.
The seismic refraction study indicated that the bedrock
topography is rather uneven, especially in the north-western sector of the study area, and that the thickness of the
sand increases towards the east. The area where the thickness of the low-velocity material overlying high-velocity
bedrock exceeds 60 m as deduced from the seismic survey
is shown on Fig. 3. In several areas there is a local thickening of the sand cover but no clear indication of any
north-south-trending buried channels in the bedrock topography could be detected.
A number of boreholes were sunk on the basis of the
seismic results and in most cases the borehole results confirmed the interpreted overburden thickness. However, in
some areas the borehole results showed the presence of a
clay layer representing in situ weathered bedrock which
escaped detection during the seismic survey. The seismic
velocity of this layer is so close to that of the water-saturated alluvium that it could not be seen as first or later arrivals on the seismograms. At a dam site in Australia,
Palmer (1980) noted that saturated unconsolidated sediments and completely weathered bedrock also gave the
same velocity. Along the west coast, however, where geological conditions are similar, weathered bedrock with a
velocity averaging slightly more than 3 000 mls was seen
as first and later arrivals (R. Kleywegt, pers. comm.).
CAPE FLATS
SEISMIC REFRACTION SURVEY
WEST
-1&50-
v.Iocity in nvs
SP21
Shotpoint position and number
EAST
100
Figure 2
Travel-time curves and interpretation along profile AA',
,
200
200m
A GEOPHYSICAL STUDY OF THE CAPE FLATS AQUIFER
Dar Zarrouk parameters. This is seldom the case and
usually borehole data are required at a limited number of
resistivity sounding sites in order to carry out a quantitative interpretation. At such sites the sounding curves can
be calibrated with the aid of the Dar Zarrouk parameters
and borehole thicknesses to obtain resistivity parameters
for the different beds. These resistivity values can then be
used to interpret curves measured away from boreholes if
there are no indications of drastic changes in layer resistivities between the calibration site and the sounding site
under investigation.
After the curve-fitting interpretation was completed the
theoretical sounding graph associated with the interpretation model was calculated and compared with the
measured field data. Adjustments were made to the
model till the model curve gave a satisfactory fit to the
field data.
Finally, the geological and geohydrological significance
of each geoelectrical unit was assessed, because a purely
physical interpretation is of no practical use to geologists
and geohydrologists.
CAPE FLATS
GEOPHYSI CAL
SURVEY
Area where sand
t hic kne ss exceeds
SOm
34'0,'
Figure 3
Area where overburden thickness exceeds 60 m as deduced from
the seismic refraction survey.
This problem which resulted in erroneous estimates of
the amount of groundwater stored in the alluvium
prompted the use of the resistivity sounding technique to
detect those areas where the water-bearing sand is underlain by more conductive weathered bedrock.
III. ELECTRICAL RESISTIVITY SURVEY
The primary purpose of the resistivity survey was to
supplement the earlier seismic refraction survey by delineating the areas where the water-bearing sand is underlain by weathered bedrock. A secondary aim was to establish whether there are significant clay deposits in the
unconsolidated overburden.
A. Equipment, Survey Methods and Interpretation
Procedure
The Schlumberger sounding method as described by De
Beer et al. (this volume) was used in this direct-current resistivity survey. A d.c. current source consisting of a d.c.to-d.c. convertor which steps up a 24 V battery supply to
350 V at 500 mA maximum current was the energy
source. The emission current was measured with a commercial multimeter and the potential measuring device
was a centre zero voltmeter with an input impedance of
10 MQ.
In the measuring procedure the problem of polarization
potentials was obviated by using copper potential electrodes giving an acceptably stable potential that was
cancelled by a compensator. The effect of the remaining
time-variable noise potentials was minimized by inverting
the current direction at about lO-second intervals.
The interpretation method applied in the data analysis
of this survey is a curve-fitting technique based on the use
of the so-called Dar Zarro uk parameters (Mailett, 1947;
Kunetz, 1966), which are determined directly from a
sounding graph using methods outlined by, for example,
Kunetz (1966) and Van Zijl (1977). When a layer is sufficiently thick the apparent resistivity will approach the
true resistivity and for such a bed the true resistivity and
thickness can of course be determined without the aid of
109
B. Field Work
As pointed out before, the resistivity survey encountered problems such as the very high level of industrial
electrical earth currents in the area and the high contact
resistance of the dry wind-blown surface sands. The industrial earth currents are mainly due to the electrified
railway network of the Cape Peninsula. Figure 4 shows a
recording of the type of elecrical noise present. The surges associated with the peak-hour commuter trains caused
the most interference. The two problems are of course interrelated in that a high contact resistance at the current
electrodes caused a small emission current and an associated small potential difference between the potential electrodes. This deteriorates an already low signal-to-noise
ratio even further. The only real solution to the problem
involved an increased emission current to obtain an increased signal-to-noise ratio. In the Cape Flats the situation was improved by, firstly, concentrating the soundings in the low-lying areas between the sand dunes where
the dry surface layer is at its thinnest. Furthermore, threemetre-long current electrodes were used and were driven
as deeply as possible into the sand in an effort to reach
the damp sand in the vicinity of the water table, which is
fortunately in most cases less than 3 m from the surface.
When this was achieved, the contact resistance was considerably reduced and the maximum emission current allowed by the apparatus could be used. Despite this, the
earth current activity still forced the field party on occasions to take as many as 100 readings to accurately determine one point on the resistivity sounding curve.
About 20 per cent of the sounding curves show a surface layer with a resistivity above 10 000 Q.m. At a few
sounding sites the resistivity of this layer was as high as
17h25
17h26
17h27
17h28
+
'l,n
I
J...
"'~.y,'{
---" ~
)
\.
r""
'\
.f'!....
II"
'\
"',
rI
V
I
/\
r.J r\ r
I
30 sec
I
10 mV
I
I I
Figure 4
Electrical noise levels recorded at approximately 17h25.
'"
110
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
40 000 Q.m. During the course of the original field
work a total of 88 soundings were carried out in the
south-western Cape Flats of which 45 were in the area
shown in Fig 5. At a later stage some additional soundings
were added at pump tested boreholes. Most soundings
were expanded to maximum AB lengths of 200 m with
the longest soundings going to maximum AB lengths of
400 m. Ten sounding curves out of the 88 were not originally interpreted because they were either visibly distorted by, for example, underground metal pipes, or due
to obstructions such as buildings, farming activity and
dunes and/or they could not be extended to a sufficiently
large AB distance to penetrate the conductive overburden. A few others were originally thought to be distorted
because of interpretation problems, but with additional
data most of these yielded acceptable results.
TN
20 Electrical sounding
• position and ES number
1
28.
.44
.49
25·
87.
BAY
FigureS
Positions of electrical soundings in the southern part of the study
area.
c. Results of the Resistivity Survey
The sounding curves in the area varied considerably
from location to location, depending especially on the resistivity of the surface layer, the variation in depositional
conditions of the unconsolidated sedimentary cover and
the state of weathering of the basement under the sedimentary sequence. These features are illustrated in Fig. 6
based on the 12 curves measured along profile AA' in
Fig. 5. The sounding curves along this short section vary
from simple three-layer (e.g. soundings ESll and ES28)
to complicated five-layer types (e.g soundings ES36 and
ES37). The variation of total longitudinal conductance
along the profile as indicated by the position of the final
ascending asymptote, is also apparent. The total longitudinal conductance S of the stations along profile AA'
is shown in Fig. 7. There is a general increase in the quantity from west to east, with sounding ES40 an exception.
The S values for the study area (Fig. 5) are shown in the
form of a contour map in Fig. 8.
With a limited variation in overburden resistivity, such
a map gives a qualitative indication of the variation in
thickness above resistive bedrock. In the area to the west
of Strandfontein Road, the sounding curves generally
have total longitudinal conductances of less than 1 siemens, but these values increase to more than 2 S further
east. This means that in the western region the depth to
resistive bedrock is small, or the water-saturated alluvium
in this area has a high resistivity compared with the same
layer further east. It will be shown later in this section
that the latter possibility is not the case, and that the firstmentioned option applies. This also limits the possible
thickness of the weathered bedrock clay layer. In the area
to the east of Strandfontein Road the larger S values indi-
cate a greater thickness of water-saturated alluvium and/
or a thicker clay layer overlying the basement.
The soundings were completed in the sequence indicated by their numbers. Electrical sounding 27 (ES27)
was done at an existing borehole drilled more than 30
years ago. It was hoped that the borehole data could be
used to calibrate the sounding data. Unfortunately, attempts to do such a calibration indicated that the borehole data were either incomplete or inaccurate. As mentioned before, this sounding curve appears to be a threelayer type. An attempt to interpret it as such, using the
58 m reported for the borehole as depth to bedrock,
proved impossible. The curve-fitting interpretation
yielded hI = 3,2 m; PI = 2500 Q.m and S2 = 1,45 S.
With h2 = 55 m, P2 will be about 38 Q.m, a value far
too low for a three-layer curve of this shape. Theoretical
models established that 55 Q.m is an absolute minimum resistivity for this layer. When the borehole data are
disregarded and the curve is interpreted as a three-layer
type with the above S2 and minimum P2 it gives a total
depth to bedrock of 77 m, a depth which is also unacceptable in the light of the seismic results and other existing
boreholes in the area. To solve the problem additional
soundings were positioned in the vicinity of ES27. The results of soundings ES38, ES39 and ES40 proved to be
highly informative. These sounding curves are fine examples of a five-layer geoelectrical section with a thin resistive surface layer, a relatively thin conductive second
layer, a prominent resistive third layer, a conductive
fourth layer and finally a resistive fifth layer indicative of
unweathered bedrock. The conductive fourth layer is
highly equivalent (Fig. 6) and was found to represent the
geoelectrical expression of the clay layer between the unconsolidated sedimentary sequence and the unweathered
resistive bedrock. The geoelectrical second and third layers represent the water-saturated alluvium. A comparison
between ES27 and the neighbouring ES39 explains why it
was so difficult to interpret ES27 as a three-layer curve.
The comparison (Fig. 9) shows that due to the relatively
thick surface layer at sounding ES27 (>3 m) the effects
of the various deeper layers in the sounding graph were
masked to such an extent that the presence of layers 2, 3
and 4 was indicated by a single layer. To further illustrate
this ambiguity caused by equivalence, models with different layer and resistivity parameters that fit the field data
·o f ES27 were calculated. Example a (Fig. 10) is a simple
three-layer case with the following geoelectrical parameters
hI = 3,25 m
PI = 2650Q.m
h2 = 77 m
P2 =
55Q.m
P3 = 1800Q.m
Model b is composed of four layers with parameters
hI = 3,2 m
PI = 2700Q.m
h2 = 34 m
P2 =
54 Q. m
h3 = 61 m
P3 =
75 Q.m
P4 = 3000 Q.m
Model c is, in analogy with ES39, a five-layer interpretation with the following thickness and resistivity values:
hI = 3,25 m
PI = 2650Q.m
h2 = 3 m
P2 =
35 Q.m
h3 = 30 m
P3 =
60 Q.m
h4 = 29 m
P4 =
35 Q. m
P5 = 2000Q.m
Although the three models are very different they all fit
the field data. The five-layer model is most probably the
correct one, because the total thickness of layers 1, 2 and
3 is of the right order for the depth to impermeable base- .
ment in that region and the total depth to resistive bedrock agrees with that reported for the depth to unweathered Malmesbury in the borehole. It would seem that the
transition from the sedimentary cover to the in situ weath-
111
A GEOPHYSICAL STUDY OF THE CAPE FLATS AQUIFER
in a report by De Beer (1972) and more recent borehole
data along the ·section. During the initial interpretation ,
11 boreholes in the study area were used for calibration
purposes and in the part of the area treated in this paper
ered Malmesbury , the clay layer, was not noted in the
borehole log.
Figure 7 gives a comparison between the initial interpretation for the depth to impermeable basement as given
ES10
ES28
ES11
1000
100
101~----~----~----~
10
1
ES36
ES22
,
I
~.
10000~
10000
\.
'.__...........
100
.,,--
,,/
;'
10
\
10
\
100
100
\-----/
100
10
1
I
100
ES38
1000
//
I
10
10
1
10
ES41
\
./-
'.--.---'."
ES27
1000
101
100
-
\.
10
1
1000
ES39
100
\
100~
ES40
101
1000
./
\,--,---"
ES37
-
.~
"
100
10
1
1
E
C
\
1000-
1000
-
\
1000
/
\ -..--"---_./"
10
AB/2
100
100
10
1
'-,,-- _',./
..
10
100
(m)
Figure 6
Typical ~Iectrical sounding curves obtained in the Cape Flats. The dots indicate the measured data points and the lines represent the theoretical sounding curve as computed for the interpreted models.
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
112
East
West
Bh
Bh.
16
53
Bh.
Bh.
30
29
~~
_
Bh.
31
~~~g ~~
0
,./' Alluvium
~
-10
Z
- 20
~
::
o
;'
;'
;'
Weathered
bedrock -
~-50
-60
ES ES
ES
ES ES
ES
ES ES ES
ES ESj~
-J.::t.1r£.28:"--'ir~J[-2~2~9
_ _ _-1JrJJJt-6_;37
40 27 3941
o
I
500m
L'~~~'
1 Siemens
the agreement with later boreholes has proved to be
good. Along section AA' sounding curve ESlO is of the
same type as ES27 and the three-layer interpretation is
most probably also too deep. The final interpretation
along section AA' as given in Fig. 11 shows models that
are compatible with the results of boreholes 14, 16, 29
and 53. Boreholes 16, 29 and 53 were stopped shortly
after passing through the water-saturated alluvium and
could not be used to obtain a resistivity for the clay layer.
Borehole 14 was drilled to a total depth of 79 m after encountering the basement clay layer at a depth of 35,6 m
below collar level. The hole was stopped when the clay
layer started to yield to harder unweathered Malmesbury
shale. This borehole was used to calibrate soundings
ES38, ES39 and ES41. The resultant geoelectrical parameters are given in Table I. The calibration exercise
Figure 7
TABLE I
Geoelectrical Soundings
Initial interpretation of resitivity sounding curves (top) and total
longitudinal conductance (bottom) along profile AA'.
TN
I
1.~5
Stat ion position with
total longitudinal
conductance S
0.5
lkm
FigureS
Contour map of the total longitudinal conductance.
27
......
1000
,,
,
\
\
\
\
\
\
\
\
\
\
E
C
a..,C"O
100
\
\
,
101L--~--L-LL~~L---~~~~~1~O-0---L~~~
AB/2 (m)
Figure 9
Resistivity sounding curves ES27 and ES39 showing the effect of
the thick high resistivity surface layer.
Sounding
ES38
ES39
ES41
hi (m)
PI(Q .m)
h2(m)
plQ·m)
h3(m)
plQ·m)
h4 (m)
p4(Q.m)
0,9
300
1,5
3 100
0,8
460
2,7
35
3
30
4
33
35
71
31
61
30
65
42
43
43
43
40
35
ps(Q.m)
700
3000
3000
showed that the resistivity of the aquifer is not constant,
but varies not only laterally, but also in depth. This is,
however, not surprising when the heterogeneous nature
of the deposits is considered (Gerber, 1977). The aquifer
is in most cases composed of two geoelectrical layers, a
thin top layer with a resistivity between 30 and 45 Q.m
and a lower, much thicker layer with a resistivity that
varies between 50 and 80 Q.m. Soundings ES11, ES28
and ES40 that were interpreted as three-layer curves perhaps also have this upper conductive layer in the aquifer,
but it is either too thin or has too Iowa resistivity contrast
with the underlying layer to be detected. Incorporation of
these features in the interpretations will have a small effect on the total interpreted depth. From Fig. 11 it is also
clear that the resistive basement as determined electrically corresponds to the high-velocity bedrock as determined seismically.
The conductive layer at the top of the aquifer can be
due to various causes. Gerber (1977) notes that in parts of
the study area, clayey layers occur in the vicinity of the
natural water table. He also mentions the occurrence of
impermeable calcrete and peat layers within a few metres
of the surface. At borehole 14 he describes such an impermeable calcrete layer at a depth of 7 m. This means that
the more conductive geoelectrical layer could also be due
to more brackish water that is confined above the impermeable layers. The brackish near-surface water can be
due to, for example, evaporation or the use of agricultural
fertilizers. Whatever the cause of the more conductive
layer, its presence would seem to indicate less permeable
zones near the top of the aquifer. This has a direct bearing on the storage of water in the alluvium, because such
zones will certainly impede the infiltration of water into
the aquifer. The variation in resistivity of the lower layer
in the aquifer can likewise be linked to the variation in salinity of the groundwater and/or the variation in the clay
and calcareous sand content of the alluvium. Figure 12
113
A GEOPHYSICAL STUDY OF THE CAPE FLATS AQUIFER
I
I
b
a
l
c
--=-".
1000
\
-
E
C
Q..,"'
100 -
-
\
\."-
./
."
-'
",,/
1000
,./
-
",,'
I
100
I
10
100
\.
\
./
r---\_.-.....-""'",../."
1o1L----~~--~10~0~---~
AB/2 (m)
Figure 10
Different interpretations for sounding curve ES27 to illustrate the principle of equivalence. The dots indicate the measured values and the
lines through them the theoretical values pertaining to the models indicated in each diagram.
Bh.
Bh.
53
16
T?~~
ES
10
ESES
229
2§
Bh.
Bh.
Bh.
29
30
31
Elevation of impermeable
floor in metres
-20
Ele~t.rical sounding
•
ES
poSition
36
:l2
+ Borehole
4662
"E- 20
~ -30
.~
-40
~ -50
W
SCOrn
...........~~.............I
98
GeoelectriC layer resistlvltv In n m
Figure II
Final interpretaton of resistivity sounding curves along profile
AA'. The resistivities of the various units and the seismically
determined depth to unweathered bedrock are indicated.
BAY
Figure 13
Contour map showing elevation of the impermeable floor as derived from the resistivity survey and drilling results.
TN
1
0.5
lkm
51
•
Station position with
resistivity of lower part
of the aquifer in
n.m
At the time of the survey the water table near the coast
was still several metres above mean sea level and sea
water incursion posed no problem. Figure 14 gives an
aquifer isopach map for the study area.
TN
1
0.5
lkm
'-'-.................................~I
2~
Electrical sounding
position with alluvial
thickness in metres
Figure 12
Contour map showing the resistivity of the lower layer of the
aquifer.
gives a contour map of the resistivity of this layer. Although the coverage is uneven and there are inevitably
spurious values among the data, the lower part of the
aquifer would seem to be more conductive in the central.
area, with a general increase in resistivity eastward. The
high resistivity values along the coast clearly show that at
the time of the survey there were no incursions of sea
water into the aquifer;
The elevation of the impermeable tloor that underlies
the alluvial aquifer is mapped in Fig. 13. This contour
map is derived from geoelectrical sounding data and borehole data. In this area the bedrock elevation varies between 11 and 37 m below mean sea level, which means
that sea water incursion could pose a problem if the fresh
water head is drastically lowered by excessive pumping.
Figure 14
Aquifer isopach map as determined from the resistivity survey.
IV. CORRELATION BETWEEN GEOPHYSICAL
AND GEOHYDROLOGICAL PARAMETERS
One of the objects of the geophysical surveys was to determine whether any meaningful correlation between seismic and/or geoelectrical properties and geohydrological
parameters, obtained during pumping tests, could be
found for the Cape Flats aquifer.
114
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
If we assume the aquifer to consist of a granular medium with no clay, which of course is not strictly true due
to the presence of peat and clay layers and occasional
limestone occurrences, the porosity can be estimated
from the seismic velocity, if the water level is known, by
using an equation given by Levshin (1961). Curves showing the relationship between water-table depth, seismic
velocity and porosity computed from Levshin's formulae
are given in Meyer (1974). By using an average velocity of
1 670 mls for the water-bearing sands of the Cape Flats
area and a water level of 3 m below surface, a total porosity of 40,7 per cent is obtained. Gerber (1977) reports
specific yields, which may be considered to be equal to
the effective porosity in unconfined aquifers, of up to 35
per cent with an average of 29 per cent obtained from extensive pumping tests conducted in the same area.
An attempt to obtain a relationship between the geoelectrical and geohydrological parameters, similar to the
techniques described by Van Zijl et ai. (this volume),
failed to produce meaningful results. In certain parts of
the Cape Flats the aquifer is only unconfined whereas in
others it is unconfined as well as semi-confined. Pumping
tests described by Gerber (1977) were, in areas where
semi-confined conditions exist, not designed to tap the
unconfined and semi-confined portions of the aquifer separately, with the result that the quality of the pumped
water is not representative of the semi-confined nor the
unconfined part of the aquifer. Due to the presence of
several clay and peat layers, the aquifer is inhomogeneous, giving rise to large variations in permeability for
different observation boreholes at a certain production
borehole. The fact that the aquifer consists of two geoelectric layers also makes a correlation between the geoelectrical and geohydrological parameters difficult.
v. CONCLUSION
The seismic refraction survey was successful in those
parts of the Cape Flats where low-velocity unconsolidated
alluvium directly overlies higher velocity unweathered
bedrock of the Cape Granite Suite or the Malmesbury
Group. In areas where in situ weathered bedrock is sandwiched" between the alluvium and unweathered floor
rocks, the method was less successful because the upper
two layers have almost identical seismic velocities. This,
together with the perhaps over optimistic total porosity
derived from the seismic velocity, led to erroneous estimates of the amount of groundwater in storage.
The electrical resistivity survey showed that the watersaturated alluvium has a higher resistivity than the in situ
weathered clay layer. The result of this survey was a more
accurate determination of the thickness of the alluvial
cover. This, together with porosity estimates and other
geohydrological parameters, determined from extensive
pumping tests, led to more realistic estimates of the
amount of groundwater available for extraction. Unfortunately, it was not possible to obtain any meaningful corre-
lation between the geoelectrical and available geohydrological parameters for the area studied.
From the results obtained during these two surveys it is
evident that different geophysical techniques can complement each other and in larger surveys more than one
technique should be used if at all possible.
ACKNOWLEDGMENTS
The authors would like to express their thanks to Dr
G.G. Cillie, Messrs W.R. Ross, A. Gerber and A.J du
Toit of the National Institute for Water Research and to
Dr M.R. Henzen, present chairman of the Water Research Commission, for their part in these geophysical
surveys.
REFERENCES
De Beer, J.H. (1972). An electrical resistivity survey of a portion
of the Cape Flats in the vicinity of Zeekoe Vlei. NPRL
Contr. Rep. (unpubl.), 9 pp.
- - - -, Joubert, S.J., and Van Zijl, J.S.V. (1981). Resistivity
studies of an alluvial aquifer in the Omaruru delta, South
West Africa/Namibia. Trans. geol. Soc. S. Afr., 84, 115-122.
(This volume)
Gerber, A. (1977). 'n Ondersoek na die hidrouliese eienskappe
van die ondergrondse waterbron in die Kaapse Vlakte. M.Sc.
thesis (unpubl.), Univ. Orange Free State, Bloemfontein.
Hagedoorn, J.G. (1959). The plus-minus method of interpreting
seismic refraction sections. Geophys. Prosp., 7, 158--182.
Hawkins, L. V. (1961). The reciprocal method of routine shallow
seismic refraction investigations. Geophysics, 26,806-819.
Henzen, M.R. (1973). Die herwinning, opberging en onttrekking
van gesuiwerde rioolwater in die Kaapse Skiereiland. Na-
tional Institute for Water Research, CSIR. Pretoria.
Kunetz, G. (1966). Principles of direct currrent resistivity prospecting. Gebruder Borntraeger, Berlin, 106 pp.
Levshin, A.L. (1961). Determination of groundwater level by
the seismic methods. Bull. Acad. Sci., 9, 867-870. (Geophys.
Ser. English translation.)
Maillet, R. (1947). The fundamental equations of electrical prospecting. Geophysics, 12, 529-556.
Meyer, R. (1974). jnterpretation methods for shallow seismic refraction prospecting and their applications. M.Sc. thesis (unpubl.), Univ. Stellenbosch, 123 pp.
- - - - (1978). The continuous seismic" refraction method. Bull.
Ass. Eng. Geol., 15,37-49.
Palmer, D. (1980). The generalized reciprocal method of seismic
refraction interpretation. K.B.S. Burke, Ed. Soc. Expl. Geophys., Tulsa, Oklahoma, 104 pp.
Van Zijl, J.S.V. (1977). A practical manual on the resistivity
method. Rep. FIS 142, CSIR, Pretoria, 132 pp.
- - - -, and Huyssen, R.M.J. (1971). Some aspects of seismic refraction investigations for water in arid zones of southern
Africa. Trans. geol. Soc. S. Afr., 74,33-43.
Geophysics Division,
National Physcial Research Laboratory,
P.O. Box 395,
0001 Pretoria.
Accepted for publication by the Society on 8.10.1981.