Seismo-electrical survey for ground water in the

Marico Area

Introduction

With the Marico dams drying up in

the winter of 2016 and water for

irrigation becoming scarce,

KitoWater was asked to search for

possible new sources of strong

underground water. A large sand

filled paleo-channel 3km to the west

of the Marico river is a well-known

source of underground water but is now nearing exhaustion due to over-utilisation.

Farmers have drilled numerous boreholes within the contact metamorphic aureole of the Bushveld

Complex, ofter targeting dolerite dykes. These dykes generally appear to be sheets rather than dykes

and certainly in the area that KitoWater investigated appeared to be northward dipping at a shallow

angle. KitoWater noticed several occurrences of Norite in the drill cuttings of these boreholes. The

productive boreholes near these sheets all intersected open fissures with limited recharge and generally

the boreholes do not live up to the early supply promise.

A large fault was interpreted by historical mapping to run approximately where the Marico river now is

and numerous failed attempts were made to detect and drill into this supposed fault. Most of these

boreholes were drilled as close as possible to the banks of the river and although the fault was never

intersected, some of the boreholes did give a

consistent and relatively high flow. What they all

have in common is intersecting rounded gravel at

depths of up to 50m, indicating that there could

possibly be deep paleo-channels along the river

course.

Seismo-electric sounding

KitoWater attempted to delineate paleo-channels

using a simple seismo-electric sounder using a

hammer as seismic source. Seismo-electric sounding

makes use of the flow potential principle where a

small electromagnetic potential is generated when

water moves relative to a permeable matrix. The

same principle is used in Spontaneous Potential

surveys to detect Geothermal-upflow and Dam leaks.

In the case of Seismo-electric sounding the

Figure 1

Deon J Scheepers

deon@kitoresources.com

movement of the water is caused by a p-wave passing through the permeable water filled zone and is

measured over two sets of electrodes either side of the shot-point. Adding signals from the two

opposite-polarity electrode pairs together, eliminate regional cultural noise components. The

instrument was set to record 200milliseconds of data and to stack six shots per reading.

Surveys were done along all three river courses and signs of channels were found in all three cases. Two

of these could be clearly defined. Figure 2 below shows the result on the instrument screen of a

sounding made 70m east of the Klein Marico river over one such a channel. The first peak represents the

surface wave of the shot and

the second probably the

vadose zone. Both these

peaks are in essence ignored

during interpretation. The

third broad peak represents

the location of the paleo-

channel. The zone of

increasing potential

represents the p-wave front

moving through the permeable water filled zone. The attack angle is supposed to be an indication of

permeability but remember that this is also a function of the velocity of the material the p-wave travels

through. In this case the velocity is probably in the order of 2500m/s at the channel expression. The

amplitude of the signal is probably an indication of the amount of water in the channel but not of

borehole yield.

Results

It was found that a simple quantification of the signal amplitude allowed a delineation of the extent of a

paleo-channel in plan as can be seen from Figure 3. In this case I started out searching for a crushed rock

aquifer along a line of trees and stumbled across this narrow channel running parallel to the riverbed, 50

meters to the north of this location. Some channel sediments were exposed further to the east along

this line.

Figure 2

Figure 3

Transects at right angles to the Marico river

uncovered several possibly anastomosing

paleo-channels, the most significant of

which is in line with one of the best

boreholes in the district. It follows a near

straight line 30-50m west of the river. In

this case the channel is in the order of 15-

20m wide. The borehole along this channel

is said to sustainably yield 20000 gallons

per hour. The seismo-electric signal was

repeatedly tested along the line nearest

this borehole and over the course of two

weeks and different times of day the signal

remained remarkably repeatable. After two

days of heavy rains the signal however

dropped significantly. Apparently it is well

known amongst other users of the

equipment that a wet clayey surface can

cause attenuation of the signal. After a

week the signal was again at the same level as before.

A similar channel was found

further to the south along the

Marico river as shown in Figure

5. In this case the channel was

not as clearly defined and I

found greater variability over

short distances. It is assumed

that the channel in the area

contains more clay and the

highly permeable zones are

smaller than in the north. The

largest amplitude reading of the

entire survey was found on this

part of the property but no

extension of this could be

found.

Conclusion

The study showed that with this simple technique sedimentary aquifers can be mapped out both in plan

and in depth. The technique however does not detect narrow open water filled fissures such as

encountered in the metamorphic aureole of the Bushveld Complex in this district.

Figure 5

Figure 4