 Cathodic Protection Training Course

Module 8

Resistivity measurements

There are several conventional methods to measure the resistivity of the ground for the purposes of cathodic protection. The most commonly used is the Wenner four pin method.

This is based on the principle of passing an alternating current through the ground between two pins and measuring the volts drop between two other pins placed in between. This shows a section through the ground and the hemospheres of resistance being measured. In fact the ground is layered with a variety of resistivities in each layer. This results in the charges distributing themselves through the areas of least resistance shown hypothetically above.

The Wenner Four Pin method.

It uses a 4-pole digital ground resistance meter, such as the Megger 5/4 or the AEMC 4500 meters, probes, and conductors. It requires inserting four probes into the test area.

The probes are installed in a straight line and equally spaced. The probes establish an electrical contact with the earth. The four pole test meter injects a constant current through the ground via the tester and the outer two probes. The current flowing through the earth (a resistive material) develops a voltage / potential difference. This voltage drop resulting from the current flow is then measured between the two inner probes. The meter detects the amount of current that is flowing through the earth and the voltage drop across the two center probes. With this information the meter uses ohms law (R=E/I) to calculate and display the resistance in ohms

This displayed resistance value is in ohms and must be converted to ohms-meter, which are the units of measurement for soil resistivity.

Ohms-meter is the resistance of a volume of earth that is one meter by one meter by one meter, or one cubic meter.

To convert from the displayed ohms to ohms-meter, the meter reading is multiplied by 1.915 and the result multiplied times the probe spacing.

The following shows the calculation in a formula. ? (ohms-m) = 1.915 x R x A? = soil resistivity in ohm-m (?-m ). 1.915 constant R = digital readout in ohms (?). A = distance between electrodes in ft.

Readings are usually taken at probe spacings of 5, 10, 15, 20, 30, and 40, 60, 80 and 100 feet.

The calculated soil resistivity is the average of the soil resistivity from the surface to a depth equivalent to the probe spacing.

For example, a probe spacing of 20 feet between each probe will provide the average soil resistivity between the surface and a depth of 20 feet. However, this theory cannot be definitive, even if the electrolyte is completely homogenous. We know the ground is layered or stratified and this is studied by archaeologists, geologists and petroleum engineers. We know that charges will pass from a higher level to a lower level of charge and the example of water flow using the energy in gravity is frequently used to illustrate this activity. I personally liken electrical pressures to gas pressures equalising to fill all available space. In fact electrical potential is like neither water nor gas and is difficult to visualise at first. Science has some formulae to explain this and it is agreed that energy diffuses everywhere, something like radiation. It is a three dimensionl thing. Different types of ground have different conductance and therefore each layer of ground diffuses the charges accordingly.

The diffusion of charges from each pin in turn (AC) is towards 'remote earth' where there is no resistance at all because there is an infinite number of resistors in parallel. The ingress of charges into each pin, in turn, meets fewer resistances in parallel but less charges to balance the pressure. It can be likened to water running down a slightly blocked plug hole.

There is no such thing as zero charges. Mr Gibbs of scientific fame will not allow it. We have to set our own zero or let our meters set it for us.

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