Gamma-Ray Logs Database

The ISGS Geologic Mapping Program Gamma-Ray Logs Database uses an interactive map to allow users to view individual logs and standardized borehole location information for 19 counties in Illinois. A few logs collected from county boundaries in Wisconsin are also included in the database. This page must be viewed using the Adobe Flash Player plug-in.  Read instructions on how to use the interactive map or

Gamma-Ray Logs are now located within the ILWATER map interface.

Using the Database

Individual logs can be viewed online using an interactive map. Each borehole is tagged with a field identifier as well as an American Petroleum Institute (API) number provided by the ISGS Geological Records Unit. This unique identification number is used to standardize the identification of the log and helps with digital database searches. Most boreholes were surveyed using a Global Positioning System (GPS), and the information was translated into section, township, and range. Elevation and total depth of the boreholes are also listed. Call the Geological Records Unit at 217-333-5109 for more information about individual records or contact C.J. Stohr.


Map of Illlinois showing the 19 counties with information in the database

The ISGS has performed downhole logging for more than 50 years for groundwater studies, mineral exploration, and geologic mapping in partnerships with private companies and other government organizations. The Survey also has collected thousands of geophysical logs of various types from private companies and other organizations since geophysical logging began more than 70 years ago. For information on geophysical logging projects, contact Tim Young. For geophysical records not provided in this database, call the ISGS Geological Records Unit.

Since 2000, the ISGS has expanded its downhole logging program to incorporate natural gamma-ray logging into its 1:24,000 Quaternary geologic mapping programs (e.g, STATEMAP, Central Great Lakes Geologic Mapping Coalition). ISGS geologists are collecting natural gamma-ray logs in cooperation with geological consultants and private drilling companies to economically obtain subsurface geologic data and to improve the Survey's databases. This database contains more than 350 natural gamma-ray logs that were collected in 19 counties in Illinois (as well as a few logs collected in Wisconsin) as part of the ISGS Quaternary mapping program. More logs will be made available online as new data are acquired.

Principles of Natural Gamma-Ray Logging

Downhole geophysical logging involves lowering a probe down a boring by a cable to acquire readings which represent the physical properties of the adjacent rock or earth materials. Natural gamma-ray logs record the naturally occurring radiation emitted from sediments. A sodium-iodide detector in the probe registers the natural gamma radiation emitted by potassium-40 and by the natural radioactive decay of uranium and thorium. In Quaternary sediments of Illinois, the log mainly reflects clay content as radioactive minerals tend to concentrate in the fine-grained sediments. This log is useful for distinguishing aquifer materials from other sediments.