Despite the introduction of sophisticated surveying techniques such as satellite guided global positioning, the seemingly simple task of providing the coordinates and elevation of the drill hole collar remains a frequent source of error at all stages of mine development. The errors are so common that it is imperative that basic checks be routinely built into every drilling campaign.
These include checking for differences between the set-out pegs and the as-drilled collar locations, which frequently are quite different, and checking that the datum of the map or computer model used to plan the campaign is identical to that used at the mine site to set out the hole.
Preferably, core drilling should be performed using triple-tube core barrels where the inner tube is split. In the case of very weak and/or degradable rock the split inner tube can be replaced by a PVC sleeve that can be capped on removal and sent directly to the laboratory. In weak ground face discharge bits should be used. These steps are critical to minimise ground disturbance, core loss and core disturbance when the core is removed from the barrel (section 2.4.7). Exceptions may occur in massive competent rock, when standard double-tube systems may suffice.
Drill hole deviation is potentially a significant source of error in the geological and structural models. Reliable downhole surveys are therefore a must in any drilling campaign. The decision on what type of survey method(s) is appropriate for the given drilling program is critical and must be made before drilling commences.
There are two common uses for downhole surveys:
■ surveys to determine the correct geometry (dip/ orientation) of the drill hole trace, typically done after the drill hole is completed;
■ continuous surveys performed while drilling in order to correct any drill hole deviation and reach target areas (also known as directional drilling).
Today’s modern instruments employ two basic techniques – the magnetic compass and the non-magnetic gyroscope.
The accuracy of the magnetic methods depends on the latitude of the drill site, the local variation of the Earth’s magnetic field and the magnetic signature of the rock mass. The most widely used magnetic downhole survey techniques are:
■ single-shot instruments, which are capable of one survey per trip into the drill hole. A single-shot
instrument is preferred for directional drilling when successive surveys enable periodic corrections to the direction of the drill hole;
■ multi-shot instruments, which can perform several readings per trip. Surveys performed with multi-shot instruments tend to be more accurate than those performed with a single-shot instrument. Multi-shot instruments are also efficient where a large number of previously drilled holes have to be surveyed and/or resurveyed.
Where magnetic disturbances are prevalent and in high latitudes the best downhole survey results are obtained using gyroscopic tools. Three types are now commonly available:
■ free-spinning gyroscopes, operating on the basis of a known direction, with changes in azimuth referenced to the starting direction, typically the azimuth of the drill hole collar;
■ rate gyroscopes, which measure the point-to-point change in azimuth while the probe is in motion along the drill hole. Typically, the output of the rate gyroscope is integrated to give a change in azimuth referenced to the drill hole collar;
■ north-seeking gyroscopes, which measure an absolute azimuth referenced to the Earth’s geographic axis. This measurement minimises the systematic error that can be introduced from an inaccurate drill hole collar azimuth or poor calibration.
The most accurate positional survey is a combination of the rate gyroscope for a continuous measurement of azimuth and the north-seeking gyroscope for absolute accuracy, which can now be achieved using a single tool. For drilling programs where a downhole survey is critical for the accurate location of structures or geological contacts, it is recommended to use two systems and compare the results.
A number of downhole core orientation techniques are available. The choice may depend on a number of factors, including the anticipated drilling conditions and the experience of the drilling crew, but is very often guided by equipment cost and ease of operation. Some of today’s most commonly used direct (physical marking) and indirect (digital) marking techniques are outlined below. Table 2.10 contains a summary highlighting the main advantages and disadvantages of each system.
Direct marking techniques
There are four main types of direct marking techniques.
■ Weighted core barrel. As the name suggests, the weighted core barrel technique uses gravity and an impressionable substance to record the geometry of the surface or ‘stub’ left at the bottom of the hole after the core has been broken and returned to the surface. Typically, the core barrel is 50% weighted to help induce a consistent orientation as it free-falls down the hole. Clay, plasticine and spears have been used to form the impression. Limitations exist with drill holes inclined at shallow angles (<30o) as the weighted barrel may not reach the proper equilibrium in air or water.
■ Ballmark® system. Unlike the spear or other weighted core barrel techniques, which return to the bottom of the hole after the core has been recovered, the Ballmark® system is designed to orient the core as and when it is broken from the bottom of the hole. It does this by indent marking a soft disc with a non-magnetic free-moving ball, which gravity dictates lies at the bottom or low side of an angled hole. The indent marking process utilises the action of the inner tube back end during core-breaking, which transfers load to the outer tube via compression of a spring. Difficulties can arise in broken ground, where there is no force required to break the core. In these situations the indent triggering mechanism may fail to activate.
■ Scribe system. Scribe orientation systems commonly provide a core which has been scribed by three tungsten carbide knives. The system’s basic equipment generally consists of a multi-shot directional survey instrument which records on film the inclination, direction and orientation of the entire core, a modified double-tube core barrel, a diamond-impregnated core bit and a scribing sub situated immediately above the core bit that contains three triangular tungsten carbide knives. The recovered core is continuously scribed by the three differentially spaced knives as it enters the barrel. The reference scribe has a fixed known relation to an orienting lug which appears on the compass face of the survey so that the scribed core is continuously referenced to the drill hole azimuth and inclination data. The frequency of survey data can be varied depending upon the competency of the rock.
■ EZY-MarkTM system. The EZY-Mark system is designed to provide core orientation at most drill hole angles (up or downhole) without needing to positively break the core or run a separate tool back into the drill hole. Orientation is achieved by taking the profile shape of the bottom of the drill hole at the beginning of the core run by means of up to three independent methods (pin profile, pencil zero-point and clay impression) while simultaneously taking three independent gravitational and non-magnetic orientations of the bottom side of the drill hole prior to starting each core run. This technique provides a measurement of true bottom dead centre to within 5°. The EZY-MarkTM system is especially applicable in conventional underground drilling operations that require constant making and breaking of long core barrels prior to the orientation data being transferred from the tool to the core. The system can also provide an audit on each orientation using a recording system which provides a record of each orientation.