According to Schlumberger OilField Glossary,
One can access Nickzom Calculator+ (Professional Version) – The Calculator Encyclopedia via any of these channels:
On selecting Petroleum, a bunch of links to major topics in Petroleum Engineering is displayed. Scroll down to Well Logging and click on it.
According to Wikipedia,
Well logging, also known as borehole logging is the practice of making a detailed record (a well log) of the geologic formations penetrated by a borehole. The log may be based either on visual inspection of samples brought to the surface (geological logs) or on physical measurements made by instruments lowered into the hole(geophysical logs).