Borehole logging, or well logging, is a process where a probe is dropped into a borehole prior to drilling for oil, gas, water, or minerals. Borehole logging is also performed by environmental organizations, as part of an environmental or geotechnical study. Probes for borehole logging can measure the composition of soils, map the area or provide other relevant information. Borehole logging produces an extremely detailed description of the area.
A well log is a log that records all of the results of the borehole probe. There are two ways to calculate the results. A geological log refers to the process of bringing samples to the surface of the soil for study. A geophysical log is a log created by a special probe lowered into the borehole that gathers the information passively.
Companies that are extracting minerals from underground often use the bore log to determine if the area contains the target minerals. The log measures both fluid and rock underground, which the company can interpret to determine the amount of oil or gas in the area.
Well logging tools are very sensitive, and can measure for radioactivity, electrical currents, electromagnetic waves, and acoustical activity. Bore logging typically takes place from the bottom of the borehole up. The probe is lowered into the borehole, and as the probe is pulled out, it takes measurements to create a geophysical log. Geophysical logging is considered more precise, because the geological measurements are taken on site.
The information recorded on the well log never makes it to the top of the hole; instead, it travels digitally to the company's office. The company will use the data compiled in the bore log to determine if an area has oil or gas reserves, and if it is worthwhile for drilling. The mineral extraction industry is not the only industry that uses borehole drilling. Environmental and geotechnical companies use borehole logging to measure rock formations and characteristics, the thickness of various formations, dips in rocks underground, the location of any fractures, depth of water, types and density of sediment, and temperature.
Some mineral extraction and environmental companies perform their own well logging, others contract the project out. Many companies specialize in well logging, and travel from site to site ahead of the main work crew performing borehole logging. The equipment and expertise required makes it difficult for a small landowner or interested individual to conduct borehole logging. These people typically hire a consultant if they want borehole logs.