Deep drill holes or boreholes are applied in prospecting for minerals and where these are gaseous or liquid in character for bringing them to the surface. The high demands ordered upon this branch of engineering have caused the traditional percussion drilling techniques, with their various drawbacks, to be superseded by continuous rotary drilling. Depths of more than 8000 m (26000ft) have been reached by this technique.

The drilling rod is suspended from a pulley block within a lattice steel tower called as drilling derrick which may be used as much as 60m (200 ft.) high and designed for loads of up to 600 tons. For drilling operations on a more limited scale, a jack knife-typed collapsible mast on a mobile chassis may more conveniently be used.

A square rod, which fits with a socket in a power-driven turntable, transmits the rotary motion to the drill rod and thus to the drill bit attached to the rod. For drilling in hard rock a roller bit is used, which consists of three toothed conical steel elements with wedded on hard metal (tungsten carbide) tips. The drill rod is hollow during drilling a flushing liquid is pumped down through the rod and then rises to the surface through the annular space between the rod and the wall of the drill hole.

This liquid, which consists of water to which certain substances which are held in suspension have been added to increase its specific gravity (1.2 to 1.6) and is referred to as drilling mud, is kept in circulation by pumping. It serves to cool the drill bit and to keep the drill hole clean and free of obstructing matter; it washes away the debris produced by the drill and carries it to the surface. On emerging at the surface, the liquid serve to consolidate the wall of the drill hole, preventing its collapse.

Another function of the liquid is to counteract, by its high specific gravity, any gas or oil pressure that may build up in the hole. With rotary drilling it is possible to obtain rock specimens (cores) for examination. For this purpose a core barrel provided with an annular bit is used instead of an ordinary bit. A cylindrical specimen of rock is thus cut from the bottom of the hole and can be brought to the surface.

The drill rod is assembled from units up to 32 m (105ft.) in length. On the working platform in the tower is the control panel for operating the machinery. Below and beside the platform are the electrical motors (up to 2500 hp) for driving the winch, turntable, pumps, etc.

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