A borehole is a shaft that is drilled into the ground vertically or horizontally. Although these shafts are generally narrow, they may be very deep. Boreholes are created for a number of reasons, including to access water, to explore for natural resources, or to conduct environmental assessments. Methods of doing this include rotary drilling, percussion drilling, and vibracoring.
There are no exact specifications for a shaft to be considered a borehole. The diameter and the length of the shaft will generally depend on the purpose for which it is being used. It is pointless, for example, to make a shaft to obtain resources if the hole does not extend deep enough to access those resources. In addition to size and depth, it is generally important to consider factors such as soil softness and varying geological formations with different degrees of hardness. Other considerations include the limitations of the drill and the skill of the driller.
One of the reasons for drilling a borehole is to gain access to a natural underground water supply. This is commonly done in third-world countries where clean water is limited or not available at all. First, the borehole is drilled, which is a task that requires specialized skills. If it is not done properly, there are risks that an underground collapse will seal the shaft or contaminate the water. Once the shaft is drilled, a pump is lowered into the ground, and generally a mechanism is added above ground to facilitate extraction and to control flow.
Boreholes do not have to be drilled on dry earth. These shafts can be, and commonly are, drilled off-shore. This may be done to explore for resources such as oil and gas or to extract these materials. Off-shore boreholes are generally either drilled from ships or from rigs that are connected to platforms.
It is also common for geologists to use boreholes. There is a substantial amount of information that can be gathered by drilling into the ground. For example, soil quality may be assessed, or the contents from the shaft can be tested for contamination.
Vibracoring is a technique that may be used to make boreholes for geological purposes. This technique creates a borehole after a vibrating pipe is pushed into the ground and an undisturbed core sample is trapped inside and removed. Though the techniques likely differed, the act of creating boreholes is not a modern practice. The British Geological Survey (BGS) claims to have records proving that boreholes were used as early as the 18th century.