The azimuth of a non vertical hole (usually referred to as an angle hole) is the horizontal direction in which it is drilled and is usually expressed as a compass bearing.
The inclination of a hole is the angle that it makes with the horizontal and is measured in the vertical plane. Inclination is a negative number if the hole is drilled below the horizontal, and a positive number if above. Since holes collared at sur face are almost invariably angled downwards, the negative sign is usually omitted. However, the inclusion of a plus or minus prefix is important when dealing with holes collared underground, since such holes are as likely to be angled up as down.
The drill section is the vertical plane in which a hole is drilled. Although all holes are set up to drill in the vertical plane they may deviate out of this plane in the course of drilling.
Deviation is the amount by which a hole swings away from its initial azimuth and inclination during the course of drilling. Drillers use many techniques to control the amount and direction of deviation of a hole. Details of this specialised process– known as hole deflection– can be found in standard drilling manuals such as Cumming and Wicklund (1985), Hartley (1994) or The Australian Drilling Industry manual (1997).
The Core Axis (CA), sometimes called the Long Core Axis (LCA), is the imaginary line running along the centre of the cylindrical drill core.
An Oriented Hole is one in which a Down Hole Survey has been conducted to determine the exact azimuth and inclination of the core axis at a number of points down the hole. This survey determines the amount that the hole has deviated from its starting azimuth and inclination and would normally be carried out on all holes that are more than 50 m deep. For more information on deviation and down hole surveys.
Core recovered from a hole is only partially oriented by the down hole survey. While the attitude of the core axis is determined, core removed from the hole will have rotated by some unknown amount about the core axis. To fully orient a piece of core, it is therefore also necessary to know the original attitude of a point on the recovered core. To do this, a point with a known orientation relative to the core axis has to be recorded by some means before the core is broken free from the ground and drawn to the surface. This type of survey is known as a Core Orientation Survey and, if successful, provides Oriented Core.
