Water Divining Geophysical Siting Of Boreholes

There are mainly two ways of doing the water site survey; one can choose to make use of a traditional water diviner or more scientific methods such as the resistivity method, the electromagnetic method or electro-seismic method, to mention a few. The traditional water diviner typically uses rods to show where the best potential for a water bearing borehole lies. The rods will bend or come together when indicating the potential drill point. A Geophysical survey is used to interpret the sub-surface breaks and the presence of groundwater.

Water conductivity is the most important cause of electrical conductivity in rock formations so this method is will indicate the groundwater occurrences. When this information is applied and interpreted it can indicate the depths at which there exists a potential water bearing breaks, fissures or faults. The equipment does not only pick up water but also indicates the change in rock formations and the subsequent breaks, fissures and faults that occur at these points. Where the rock is weathered and broken down the resistivity will be lower. The more porous (weathered) the rock is, the lower the resistivity as indicated in the table below:

 

Formation	Porosity %
Soils	50
Clays	45
Sands & Gravels	30 to 40
Completely decomposed rock	30 to 40
Highly decomposed rock	16 to 24
Moderately decomposed rock	4 to 16
Slightly weathered rock	1.3 to 3
Fresh rock	0.05 to 0.3

 

 

3. Geophysical: The Geophysical survey is conducted to locate the most viable of these drilling sites with the use of the most                  advanced proven technology utilizing the principle of Ohm’s Law and Electrical Resistivity. The resistivity technique is the    longest established geophysical method used to site boreholes in Africa. It has been used successfully for more than 50 years. The    most common resistivity survey method used in Africa is vertical electrical depth sounding (VES for short). Ground resistivity is      measured by passing an electrical current through the ground and measuring the potential difference between two points. Ohms l    law is then used to calculate the resistance. Electrodes are expanded on either side of a single midpoint and then moved to a set of  distances. When the electrode spacing is very wide, the electric currents pass deeper into the ground.

The objective of the geophysical survey is to locate weathered zones in the rock formation. A weathered zone can be described as a zone where rock has degraded into smaller particles such as clay, sand, gravel and stone. Weathered zones are usually formed through the breakdown of the rock either through water erosion or chemical breakdown. The compact hard rock will have a very high resistivity. But decomposed/weathered rock saturated with fresh water will have much less resistivity. Resistivity decreases with increasing saturation of water in the rock. We do not only want to find those areas of decreased resistivity (high conductivity), we also want to find the thickest weathered zones.

The resistivity of a medium is dependent upon its minerals and degree of water saturation. Most minerals are having very high resistivity. The compact hard rock has also very high resistivity. But decomposed/weathered rock saturated with fresh water will have much less resistivity. Resistivity decreases with increasing saturation of water in the rock. Based on this aspect the common range of resistivity of rocks and water is given in table:

 

Rock	Range of Resistivity of Rock Types (Ohm-m)
Clay	1 – 10
Sand	30 – 60 fresh water, 1 to20 saline water
Weathered Granite Rock	20 – 40 fresh and 5 to 15 saline water
Fractured granite rock	40 to 150 fresh and 500-1500 and more for Dry rock
Massive Granite	1000 to > 10,000 Ohm-m

It is clear that the degree of resistivity values varies depending on the degree of the extent of weathering and saturation of fresh or saline water. The objective of the resistivity surveys are as follows:

• Depth & extent of aquifer in a stratified form
• Depth, thickness, extent of weathered/ fractured zones & the extent of massive rock.
• Structural and stratigraphic conditions controlling the occurrence movement & distribution of ground water.
• Potential of aquifers depending upon the effective porosity,
• Salinity distribution, pollution and contamination.
• Movement of ground water
• Depth of water table

 

 

This method is based on the phenomenon of electro-seismic signals that are generated through the relative movement of the water against the substrata rock matrix. This movement of ground water is established through a seismic wave. The presence of water is determined by the processing and interpretation of the electrokinetic signal. The estimated depth and probable geology can also be determined. The water is responsible for the generation of the electro-seismic signals.

Through the ages man has always strived to locate usable ground water but up until recent times the only way to determine the presence of water was by drilling. This unique method of locating groundwater brings a new dimension to geophysics. It improves the time and cost of groundwater development and investigations.

 

The electromagnetic method

This method measures the difference in the electromagnetic conducting abilities of the rock formations. The change in the normal magnetic field of the earth is perceived which can be then used for picking up concealed geological contacts such as dolerite dykes. Knowledge of electromagnetic theory is desirable for a successful interpretation as an experienced knowledge of the interpreting the data will result in a successful borehole in the right geological area.

The limitations of this method is the problem of “cultural noise”. Man-made structures that are constructed using ferrous material, such as steel, can interfere with the quality of the data. Features to be avoided include steel structures, power lines, metal fences, steel reinforced concrete, surface metal, pipelines and underground utilities. The inability of the interpretation methods to differentiate between various steel objects is another disadvantage. For example, it is not possible to determine if an anomaly is the result of a stack of steel plates or a zinc roof.