A resistivity log is a data gathering system used to judge the electrical potential in rocks. Since different types of rocks have different electrical conductivity, resistivity logs can give information about the various compositions of rock formations buried beneath the surface. These logs are often used by the oil industry to find natural oil reservoirs by a process called formation evaluation.
Resistivity is expressed in Ohms or Ohms/meter. It is frequently expressed in a logarithmic scale because each area studied usually has a range of resistivity. Also, the amount of current registered is dependent on the area that is being studied. Larger areas will have less dense currents, so to determine the ohms/meter measurement, the density of the recorded current is divided by the area being tested. Since electrical current cannot penetrate far into the ground, boreholes are drilled to aid in the study of formations farther below the surface.
Two different types of equipment used to obtain resistivity logs are laterologs and induction coils. Laterologs rely on information obtained by electrodes, which send electric current into the rock, placed in various positions along the borehole. Induction coils rely on magnetic fields produced by the coils and are less frequently used in formation evaluation.
Several factors contribute to the electrical conductivity of rocks. Porousness and salinity increase the electrical conductivity. Conversely, nonporous rock and areas made up of mostly clay decreases it. Oil and gas have high resistivity and may read 50-500 ohms on the resistivity log, whereas water will only have a reading of 1-10 ohms. The logs themselves look much like seismograph or lie detector readings — a series of wiggly lines on a length of paper.
In oil drilling, a petroleum geologist studies the resistivity and other logs to see whether it is likely that the amount of oil found near new wells is worth further drilling. Once a borehole is drilled, a petroleum geologists will be called to analyze the amount of oil present nearby. These geologists look at more than just the resistivity log, however, since nonporous rocks and salt water both have a high resistivity but no oil. Other logs, such as gamma ray and porosity logs, are also examined during the process.
Resistivity is also used in scientific studies to discover more about the Earth's geology. In lithostratigraphy, the study of the strata of the Earth, scientists use resistivity to gain information about the Earth's layers. By studying the make up of rocks created over millennia, scientist can reach conclusions about Earth's history and evolution.