Surveying provides data that defines the position of the borehole in 3
dimensional space. This provides answers to questions such as "Exactly
where did this rock (core or chip) sample come from?" or "Is the
hole going to hit the destination target?".
Say, for example, that the hole was being drilled
steeply downward to depths of 1000m (3300 feet) using NQ* equipment in a
temperate climate. The NQ designation immediately conveys the size of the
drilling equipment and the fact that we can lower and retrieve the
instrument by the same system that is used to transport the core-barrel
assembly inside the drill rods.
If the instrument used is a magnetic azimuth type,
then the survey train has to pass through the drill bit to place the
instrument beyond the influence of the magnetic drill rods. In this
application the drill rods would have to be raised from the bottom of the
hole to allow the survey assembly to pass through.
If the instrument chosen used nonmagnetic sensors (see
“Azimuth or Heading Direction Surveys”) the survey could be done inside
the drill rods. In this case the directional reference would be set at the
surface and transferred by the instrument to the survey site.
In this particular application we have a wide choice
of instruments because the size of the hole in the bit is fairly large
(47.5mm or 1.87 inches), the hole is steep downwards (greater than 40° from
horizontal) and the temperature will not be extremely hot nor cold.