In the experiment we use an ohmmeter to measure electrical resistance of dry soil and compare it to the resistance of soil soaked in salt water and then with oil soaked soil. The dry soil will probably have a very high resistance, perhaps so high that it will not register on the ohmmeter at all. With salt water there will most likely be a measurable resistance. With oil instead of water the resistance should be very high.
We have been using two closely related terms: resistance and resistivity. Both are measurements of the extent to which the flow of electricity is hindered when passing through a substance. The difference is that resistivity is a property of a kind of material while resistance is measured for a particular object. In the diagram below on the left, the squiggly line represents a wire. The ohmmeter measures its resistance. On the right we have a shorter wire made of the same material. It has less resistance. Electricity goes through it more easily because it is shorter.
The resistivity of the material the wire is made of is the same in both cases. Metals have fairly low resistivity and allow electricity to flow easily. That's why electrical wire is made of metal, usually copper. Rubber, glass, air, and most kinds of rock have high resistivity. Materials with low resistivity are called conductors. Those with high resistivity are insulators.
More formally, resistivity is defined as the resistance of a 1 meter path through a cubic meter of a substance.
The resistivity of a rock formation is of interest to people who prospect for oil. As our experiment shows, oil soaked ground has a higher resistivity than water soaked ground. An underground region of high resistivity may contain oil. Or it may not. An area of high resistivity could be dry rather than full of oil.
The real situation is quite complicated, so a variety of measurements are taken and looked at together to get the fullest possible picture of underground geological formations. The articles about The Making of Oil, The JOIDES Resolution, and CyberGeologist describe some of these techniques and methods. Our experiment focuses on electrical logging, which is only one of the repertoire of techniques that are used.
The Absorbency of Rock experiment is closely related to this one. Before soaking a rock in water, use an ohmmeter to measure the resistance from one side to the other. If you are using a piece of chalk, you could wrap some aluminum foil around the ends and touch the ohmmeter probes to the foil. With dry rock you should get very high resistance, probably too high to register any movement of the ohmmeter pointer. After soaking in water, the rock will probably have a lower, measurable resistance.
Salt water has a lower resistivity than fresh water. In fact, pure water has a very high resistivity. But water is almost never found in pure form, particularly underground where minerals are dissolved in it.
Send us your results. We'll periodically post new information here along with the reports from other people who have done these experiments.
Note: A short form of the Resistivity experiment is available as a PDF file which you can download, print and duplicate for your students.
