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: