Well logs: The rock record on paper (GC-11)
Layers of rocks beneath the earth's surface have different chemical content, electrical properties, radioactivity, and density. Specialized instruments that are lowered into drill holes from equipment mounted on a truck record these differences on long, graph-like strips called "logs." Geologists use these electric, radioactive, and other types of geophysical logs in their search for oil and gas, water, coal and other mineral resources. Certain rock layers show characteristic log patterns. These patterns enable the geologist to determine depths and thicknesses of particular rock layers, to trace these layers from one well to another, and to trace some of the over very wide areas. From these logs the attitude (the dip, for example) of rock layers can be determined and changed in thickness and composition of the rock can be seen. All of this information is important in the search for oil and gas and other minerals.

As the hole is drilled, a continuous record (a written log) is kept of the depth and type of rock brought up in the well cuttings. This information can also be plotted to form a graphic log.

Thousands of these logs are available for public use at the Petroleum Section of the Indiana Geological Survey in Bloomington.

Our Hoosier State Beneath Us: Geochemistry/Geophysics