Measuring Soil
Resistivity
Uses of
Resistivity
Measurements
With intelligent and experienced
assessment of the results of
resistivity
surveys, it can be used to detect
unnatural irregularities in the soil; concentration of mineral deposits,
buried masses, caves, etc. Various types of soil have different resistivities
and areas with such differences or displaying a sudden change in resistivity
can be shown up in a survey. For example, where there is a stratum of sand or
rock below a top soil of loam. Also, such techniques have been used
successfully in locating archaeological remains by showing sudden changes in
resistivity in the vicinity of buried objects and thereby indicating their
position. It is possible with experience and knowledge to deduce from results
of a resistivity survey, the nature of the mineral deposit or buried object.
The use of resistivity measurements can lead to a reduction in the number of
boreholes that would otherwise have to be made in order to adequately
determine underground conditions, etc.
Factors Affecting
Resistivity
Various conditions of the soil
causes differing resistivities and the differences found between them are
quite considerable. Dry soil is generally speaking, a good insulator, and only
becomes a conductor when water is added to it. Water can dramatically reduce (with
a few exceptions) the resistivity of the soil. But water too, in its pure,
demineralized state, is also a good insulator and not until salts are
dissolved in it does it become a conductor (conduction in soil is largely
electrolytic in nature). So to a considerable extent the resistivity of the
soil is a measure of the resistivity of the water held in the soil which in
turn is dependent upon the amount of salts dissolved in the water. By salts,
is meant more than just common sodium chloride, it encompasses all the
naturally occuring chemicals in the soil.
The amount of water held in the
soil is governed by the weather conditions, time of the year, nature of the
sub-soil, etc. To a certain degree the temperature of the soil has an effect
upon resistivity, lower temperatures causing higher resistivities, therefore,
climatic changes have a bearing on this subject. Over the course of a year,
resistivity changes can be quite marked especially for layers of the soil near
to the surface. When water is frozen, its resistance increases so it might be
expected that when soil freezes in severe winter weather its resistivity
increases. Below the frost penetration level the effects of temperature on
resistivity are almost negligible.
Fluctuations occur also in very wet
seasons, natural salts may be washed out by heavy rainfall. Likewise, the
rising and falling of the natural water table level will considerably effect
the resistivity at the particular depth where the table normally occurs. Below
the water table level, soil resistivities are generally far more constant that
they are above this level, where the seasonal factors take effect. Because of
the changes in the moisture content of the soil near the surface, it may well
be considered adviseable to sink earth electrodes deeper to gain the advantage
of more stable resistivity conditions.
The resistivity is not a constant
parameter, but varies from area to area; with the weather conditions and with
the type and nature of the soil. It changes for different depths below the
surface and for different strata formations in the ground. It cannot be
assumed that once measured the resistivity will remain constant. The actual
resistivity must be obtained by measurement at any one time because of all
these effects.
The Wenner
Principle of Resistivity Measurement
Several methods of earth resistivity
measurement have been devised, but many of them are variations of the
principle developed by Dr. F. Wenner of the United States Bureau of Standards.
The basis of this principle is given below with the application
where Miner A-200 resistivity instrument is used as measuring instrument.
The method is sometimes known as
the "four electrodes method" because four electrodes (spikes) are inserted
into the ground and connected to the terminals of the instrument.
The four spikes are equally spaced
in a straight line and driven into the ground such that the depth of insertion
is 1/20 of the distance between the spikes. Current is passed through the
earth via the two outer spikes, which are connected to the current terminals
C1 and C2 of the instrument. The voltage appearing between inner spikes
connected to the potential terminals P1 and P2 of the instrument as a result
of the current flowing, is measured.
The Wenner method gives the
resistivity to a depth equal to the spike seperation distance "a". In this way
different soil structures, strata or buried objects and caves could be
identified. Performing depth surveys in this manner over any particular area
enables the contour of an underground stratum to be found.
"a" is the distance between two
adjacent spikes
and approximately the detection depth.
As "a" increases, detection depth
increases. As the detection
depth increases, the sensitivity decreases.
Since the resistivity is obtained
to a depth equal to the seperation distance between the spikes, by the Wenner
principle, then if the seperation were too small, the buried object might not
show up because there would be little change in resistivity. Similarly, if the
seperation distance is too large, small objects might be missed for the same
reason. So, it is necessary to estimate the depth required at the start of the
resistivity survey when looking for buried objects. Usualy a survey is
conducted for a depth 50% greater than the suspected depth of the object in
question.
The Twin Probe
Principle of Resistivity Measurement
Two electrodes mounted on
wooden skeleton are close together and are moved around the area of survey
while other two electrodes, that are called the reference or fixed electrodes,
remain at fixed locations during these measurements. It must always be assumed
that the earth below the reference electrodes is different from that in the
area of survey. The true pattern of the soil contrast is not important; it is
only important that it is different under both electrode pairs.
The resistivity at the
reference electrodes will change during a day’s survey, although it may not be
so large that it causes difficulties. Until its temporal change is known for
certain to be small, it should be measured before the survey starts each day
and after it is finished. When a twin electrode resistance survey is finished,
the reference electrodes are removed. If the measurements are redone at a
later date, it is unlikely that the reference electrodes can be placed in the
same locations as before, and so the resistance readings would not be the
same. If a survey of a measurement grid takes several days, this same effect
can happen on each of those days if the electrodes are removed at the end of
every day.
Twin array can
detect targets to a depth of 1 to 1.5 times the probe separation, depending on
the size of the target. If the surface is stony then the noise generated in
the background response will reduce detection depth.
Practical
Considerations
Surveying takes time. Take
great care when testing so that each individual test is reliable and accurate.
Take care to connect the leads securely. Take care to connect the leads
correctly, errors of 50% can result by wrong connections. For work at shallow
depths, use the wooden skeleton to hold test spikes firm, so that they can be pushed into the
ground at correct spacing and penetration, will be found very useful. When it
is not possible to drive test spikes to the required depth water the area with
a saline solution.
Miner A-200 resistivity instrument specifications:
- Power supply: 8 x 1.5V AA batteries (not
included)
- Testing Current: 5mA
- Main unit with 4 terminals
(C1, P1, P2, C2)
(orange)
- Test leads (red, black) (each
1.5 metres)
- 4 metal
spikes (electrodes)(long)
- Test wires (2
pcs of 20 metres [black] & 2 pcs of 40 metres
[red])
- Aluminum carry
case (dust-damp proof)
- Wooden skeleton (demounted):
Consists of 4 pieces of bars, a mini table, 4 metal pins, 4 short metal
electrodes
- Function
switch, potential terminals (P1 & P2), current terminals (C1 & C2), TEST
(on-off) button, LCD, low battery
indicator,
current
output indicator (green led), open circuit alarm
(when there is connection failure, no loop).
Miner A-200 resistivity instrument packing details:
- Weight (whole unit
in aluminum case): about 4
kgs
- Weight (whole unit
with wooden skeleton): about 6 kgs
- Dimensions (aluminum
case): 50 cm x 40 cm x 18 cm
- Dimensions: (wooden
skeleton (demounted)):
170
cm x 15 cm x 15 cm