Geophysical Surveys in Khwisero, Kenya
By Scott Patterson
March 2012
What is a geophysical survey?
Geophysical surveys are utilized to determine characteristics of subsurface materials (e.g. rocks,
water and oil) from the surface of the Earth. The physical properties of the focus material (e.g.
density, velocity, induction, etc.) determine which type of geophysical method and parameters of
that method to perform.
Why are geophysical surveys preformed?
Cost and convenience are the drivers for implementing geophysical surveys. It is generally
cheaper and easier to perform a geophysical study or a suit of surveys than to drill many wells
or to excavate a large hole to access the subsurface rocks and fluids. Most geophysical
surveys can also be carried out over a large area that lends themselves to regional studies.
There are many types of geophysical surveys such as seismic, magnetic, gravity and electrical.
Electrical resistivity is the method that is used in Khwisero, Kenya.
Why are geophysical surveys used in Khwisero, Kenya?
The geophysical survey is a cost effective procedure that assesses the potential of groundwater
at a specific location (e.g. a school). Specifically, the geophysical survey is trying to distinguish
between water bearing fractured rock and solid host rock. This can be done because the host
rock does not conduct electricity whereas the groundwater does quite well. The cost of this kind
of geophysical survey is significantly less than the cost of drilling a borehole.
How are geophysical surveys preformed in Khwisero, Kenya?
The methodology utilized in Khwisero is the Electrical Resistivity. Vertical electrical soundings
are performed in Khwisero because there is little horizontal variation but great vertical variation
in electrical (and lithologic) properties. The aquifers in Khwisero are highly fractured rock in the
weathering zone of igneous and metamorphic rocks.
The Electrical Resistivity method relies on the application of Ohm’s Law to the Earth. Ohm’s
Law explains the relationship between Voltage, Current and Resistivity (Figure 1). Voltage (V)
is Electric Potential or the difference in “electric pressure” between two points in a circuit and is
measured in volts. Current (I) is the rate of flow of electrons and is measured in amps.
Resistivity (R) is impedance of flow of electrons and is measured in ohms. By setting up a
circuit with the Earth and inducing a know current at an established voltage the resistivity can be
calculated using Ohm’s Law. This is done at predetermined depths to compile a set of
resistivity measurements at various depths (Figure 2, 3, 4 and 5).
Figure 1. Explaining Ohm’s Law, where V = voltage, I = current and R = resistivity.
Figure 2. Cross section demonstrating how Ohm’s Law is applied to the Earth to measure
characteristics of rock that are buried deep in the subsurface. GL = ground level and the
orange and blue shapes represent a building.
Figure 3. Through changing the local of electrodes at the surface different depths of the Earth
can be measured.
Figure 4. Further measure of deep rock resistively.
Figure 5. After taking resistively measures from several depths one can start to have a handle
on electric stratigraphy of the subsurface at this particular location and interpretations of the
rock and fluid type can be provided.
To focus the circuit at different depths is where the concepts become more complicated. The
geometry of the electrodes (sources of voltage (a battery) and current) determine the shape of
the circuit. The Schlumberger array (which is a special way to orient the electrodes) is a linear
configuration where the voltage electrodes are located close together and fixed to the center of
the array whereas the current electrodes move outwards. Increasing the linear distance
between the voltage and current electrodes increases the depth of penetration under
investigation. The survey is conducted by moving the voltage electrodes to predetermined
distances, inducing a current, measuring the resistivity, then moving the voltage electrodes and
repeating.
The resulting data are plotted on log-log graph with current electrode distance plotted on the X
axis (which is related to depth in the subsurface) and resistivity on the Y axis. The measured
resistivity values are compared to know rock samples (Figure 6) and integrated with an
understanding of the local geology to interpret the subsurface rock and fluid characteristics.
From the table in Figure 6 you will notice that the water bearing rock units generally have lower
resistivities, which is consistent with the concept of resistant to electricity. Electricity has a
difficult time traveling through high resistant (large ohm values) materials and the inverse is true.
Figure 6. In this table are typical resistivity values for different rocks with and without water.
From Water Resources Management Authority, Geophysical Survey Report, Munyanza Primary
School, 2006.
References:
Kirsch, Reinhard, Groundwater Geophysics: A tool for hydrogeology, 2nd edition, 2009, Springer.
Weight, Willis D., Hydrologeology Field Manual, 2nd edition, 2008, McGraw Hill.
Khan Academy http://www.khanacademy.org/science/physics/v/circuits--part-1
Water Resources Management Authority, Geophysical Survey Report, Munyanza Primary
School, 2006.