Briefly, holes bored into the earth for exploration,
mining, engineering, scientific, fluid extractive or fluid transportive
purposes need to be defined/located in 3 dimensional earth space. The
beginning (collar) of the hole can be located (surveyed) by traditional
surveys or GPS without problems.
Beyond the collar lies the path of the earthbound
borehole that is never naturally straight without some type of human
intervention or control, and that path is not available for direct
So, a valid question about a borehole is "Where did it go?"
Or if the drilling had produced samples of a particular rock traversed
in the borehole, the question might be "Where exactly was this
sample rock located in the earth?" Yes, the path of the borehole
can deviate sufficiently to make answers to these questions critically
If surveying the borehole as it progresses tells you
the hole isn't going to hit the 'target' you set, you can proceed to
change its direction by using wedging or directional drilling
techniques. To set up the in-hole tools that will effect the change in
direction toward your target requires tool-facing surveys.
The rock samples (called core) obtained by
core-drilling contain features such as bedding or fracture planes. Once
the core breaks away from the rock it was a part of, the rotational
orientation is lost. If the attitude of these planes in the undisturbed
rock is important, then a facing survey must be made of the core before
it is broken free.
The core facing survey and the subsequent orientation on the surface
of the core in itsí original attitude is an application called core