Azimuth or Heading Direction Surveys

   The orientation of a borehole at a specific depth can be absolutely defined by measuring its azimuth from the earth's magnetic field and the inclination* of the hole from the earth's gravity field. These measurements can be made at any survey station in the hole and provide the necessary information independent of any other survey station. This fact is the strength of compass based azimuth or magnetic azimuth surveys.

   All other methods require that a reference direction be established for the instrument at the collar before lowering and all surveys in the borehole will provide a heading relative to that reference direction.  Gyro, accelerometer and light beam based instruments are of this type.  The strength of the relative heading surveys is that these can be run in magnetite bearing rocks, inside drill rods, casing or pipe and adjacent to steel objects that locally alter the direction of the earth's magnetic field.

    The compass based instruments are the work-horses in borehole surveying because they do not have to maintain the collar reference direction and are therefore simpler and less expensive to use. In fact, some these benefits are great enough so that very expensive nonmagnetic drill pipe (collars) are frequently used in oil and gas well drilling to allow the use of compass based surveys inside the otherwise magnetic drill pipe. On the other hand, nonmagnetic drill rods are seldom used in the diamond drilling industry because of the expense and a traditional concern about costs in the small diameter drilling market.
    It has long been known that the top 1-3 m (3-10 feet) of many drill holes are not surveyable by compass because of smeared steel on the walls as the drill hole establishes a stable direction. There is an extension of this problem down hole in many holes where the walls of the hole are smeared with steel at a tight bend in the hole (called a dogleg) or caked with a paste consisting of rock powder, ground particles from the steel rods and drill rod grease. This magnetic contamination is highly variable and is dependent on:

• the straightness of the hole
• the abrasiveness of the rock drilled
• drilling factors such as bit loading, the amount of rod grease used, and
• the magnetic and wear qualities of the drill rods.

    Irrespective of the complex variables that control this effect, the bottom of the drill hole has no smeared steel or magnetic cake over the depth of the last run and insignificant contamination over the last few runs. Where this contamination has occurred, it increases up-hole from the bottom (but not regularly) which roughly correlates with the length of exposure to drill rod rotation.
    The presence of this contamination can be detected by resurveying  a former station at a shallower depth in the drill hole. A difference between the original reading and the resurveyed reading indicates magnetic contamination. With over 35 years of observations on this subject , we find the effect in 80% of holes drilled in Precambrian greenstone belts and have yet to detect it in shales.
    The contamination effect is of no concern if the compass based surveys are being done at the bottom as the hole progresses. But, if check surveys are required, these should be done as soon as possible before the hole has progressed more than two runs past the station. It also means that multishot magnetic surveys of the completed hole are not going to correlate with the magnetic surveys done during progressive drilling if magnetic contamination has occurred.
    The greatest amount of deviation in resurveyed stations we have recorded to date has been 8º, with 2 to 4º being common. If the borehole water is oxygenated, the contamination effect will decrease with time as the steel oxidizes.