A borehole is the generalized term for any narrow shaft drilled in the ground, either vertically or horizontally. A borehole may be constructed for many different purposes, including the extraction of water or other liquid (such as petroleum) or gases (such as natural gas), as part of a geotechnical investigation, environmental site assessment, mineral exploration, temperature measurement or as a pilot hole for installing piers or underground utilities. Also boreholes can be made for geothermal installations. As well as pumping petroleum from an underground well through a borehole, liquid or gas can be pumped into it, for that process, or for underground storage of unwanted substances., e.g. in Carbon capture and storage.
Engineers and environmental consultants use the term to collectively describe all of the various types of holes drilled as part of a geotechnical investigation or environmental site assessment. This includes holes advanced to collect soil samples, water samples or rock cores, to advance in situ sampling equipment, or to install monitoring wells or piezometers. Samples collected from boreholes are often tested in a laboratory to determine their physical properties, or to assess levels of various chemical constituents or contaminants.
Typically, a water borehole is completed by installing a vertical pipe (casing) and well screen to keep the borehole from caving. This also helps prevent surface contaminants from entering the borehole and protects any installed pump from drawing in sand and sediment.