During certain drilling operations, data about the drilling fluid ("mud") are collected as it is pumped and recycled: its water content, weight, a description of the rock cuttings brought to the surface, the kind and amount of any brine, oil, or gas, and other information. These data are recorded in chart form, the mud log.
After drilling to a particular depth, a device is lowered into the hole. This device is equipped with a variety of sensors that measure electrical, nuclear, or mechanical properties of the rock penetrated. As the tool is withdrawn from the hole, data representing the changes of rock properties with depth are recorded and a chart is made. Each rock and fluid encountered in the hole produces a set of responses that enable the geologist to determine its type, the presence or absence of oil, gas, or water, and other properties.
There are many specialized logs. A video camera may be used to view the rocks and condition of the hole. Logs may be used to evaluate the mechanical integrity of the hole or target zones for production or in need of repair. Not all drilled wells are vertical holes; some logging equipment is used during drilling to direct and steer the drilling tools in intentionally angled wells that may reach horizontal.
Scanned images of the logs for Kentucky wells are available through the KGS oil and gas data search page.