Supporting the teaching of Earth sciences at all levels
VIRTUAL EXPERIMENTS
EARTH SCIENCE TEACHERS’ ASSOCIATION
Experiment: Mike Tuke              Realised by: Julian & Phillipa Priddle                  Sponsor: PESGB
RESISTIVITY OF ROCKS AND MINERALS Purpose The purpose of this virtual experiment is to discover which rocks, minerals and fluids conduct electricity and which do not.  This information is important when using resistivity for prospecting. Prospecting using resisitivity The resisitivity of the ground is used in prospecting for ore minerals, in hydrological investigations, in site investigation for civil engineering projects and for archaeological investigations. It is useful as one of the borehole measurements in oil prospecting and even used by forensic scientists looking for the bodies of buried murder victims.  Conductance and resistivity A material is a conductor if there are charged particles, ions and electrons that can move easily through the material.   Generally, metals and ionic compounds are conductors.  If there are no free charged particles then the material has electrical resistance. Covalent compounds tend to be resistors.  If the material is porous then the resistivity will be affected by whatever is in the pores. The resistance of geological materials (rocks and minerals) is greatly affected by the presence of pore water. Water underground always contains some solutes and has very low resistance. Porous rocks underground are generally saturated and so their resistance is dependant on their porosity.  The measurements in Animation 2 are made on dry rock and mineral samples.  In Animation 3, the rock samples are the same, but the content of the pores differs. Instructions 1 Trial run Run this animation to show what the resistivity tester does. It has lights and sound so you may want to turn the volume down on your computer. Here is a summary from that animation. Remember that conductance and resistivity are opposites. 2 Resistivity of dry rocks and minerals Print the table for Animation 2 and fill in the resisitivity with the words: Very high, Moderate, Low, or Very low, by running the animation. 3 Resistivity of rocks with fluids in their pores Print the table for Animation 3 and fill in the resisitivity with the words: Very high, Moderate, Low, or Very low, by running the animation. An oil company is drilling bore-holes in their exploration for oil. Oil and water are trapped in permeable layers undergound. Sandstone is permeable and shale is not, so oil and water will be trapped in the sandstone. Remember that oil is less dense than water, so will oil will always be above water. The survey team drilled 3 test holes on the site. The left hand bore-hole is on the crest of a symmetrical anticline, where 1000m below the surface, a sandstone layer 20m deep, is sandwiched between 2 shale layers. This first bore-hole is found to have a high resistivity right through from the shale above the sandstone to the shale layer below. Oil was found in the sandstone in this bore-hole. The right hand bore-hole was drilled, to find the width of the oil deposit, 100m to the right of the original drilling. The top of the sandstone layer was found 37m lower than in the first bore-hole. When they tested the resistivity of this hole, it was low throughout the sandstone layer and no oil was detected. The team drilled a third hole halfway between the first and second holes and the top of the sandstone was found 11m below the level of the first hole and in this case resistivity measurements showed that the top 10m of the sandstone had a high resistivity and the rest had a low resistivity. Oil was detected in this hole. As a member of the survey team you are asked to draw the cross section through this anticline, showing the sandstone layer, the bore-holes, and where the oil and water are. (How do we know that there is water down there? - Think about your resistivity measurements in Animation 3.) Use a scale of 1cm = 10m. Do not try to include the surface on your cross section (remember that it is 1000m above the sandstone).
Table for Animation 2 Table for Animation 3
This diagram is not to scale.

sandstone

shale

left bore-hole

centre bore-hole

right bore-hole

bore-hole

Sketch of the fold structures below the surface
Location of the bore-holes on the surface