Engineering geology

Logging of drillhole samples and Drilling contracts

Information from drillhole comes from the following main sources: rock, core, or chips; down-the-hole geophysical equipment; instruments inside the drillhole; and performance of the drilling machinery. In this section, we are only concerned with geological logging, but the geologist on-site at a drill location must be familiar with all sources of information.

Geological logging

Effective core recovery is essential – that is, the length or volume (weight) of sample recovered divided by the length or volume (weight) drilled expressed as a percentage. If recovery is less than 85–90% the value of the core is doubtful as mineralized and altered rock zones are frequently most friable and the first to be ground away and lost during drilling. The core is not then representative of the rock drilled, it is not a true sample, and it is probably misleading (Fig. 1).

drillhole

Core drilling

Often, initial, rapid core logging is done at the drill site. This information is used to decide whether the drillhole is to be either continued or abandoned. A wetted core is more easily examined, using either a hand lens or a binocular microscope. Most organizations have a standard procedure for core logging and a standard terminology to describe geological features.

Field data loggers are now used to gather company standardized digital data, which are down loaded to the central database upon return to the field base or office. Onions and Tweedie (1992) discuss the time and costs saved using this integrated approach to data gathering, storage, and processing.
Once the initial logging at a drill site is complete, the core is moved to a field base, where a more detailed examination of the core takes place at a later date.

drillhole

Nevertheless, the main structural features should be recorded (fracture spacing and orientation) and a lithological description (colour, texture, mineralogy, rock alteration, and rock name) with other details such as core recovery and the location of excessive core loss (when say >5%). The description should be systematic and as quantitative as possible; qualitative descriptions should be avoided.

These data are plotted on graphical core logs and used as an aid in interpreting the geology of the current and next holes to be drilled.
A core is stored in slotted wooden, plastic or metal boxes short enough to allow two persons to lift and stack them easily (Fig. 2; see Fig. 3.).

drillhole

The core is collected for a variety of purposes other than geological description, e.g. metallurgical testing and assaying. For these latter purposes the core is measured into appropriate lengths (remembering the principle of stratified sampling) and divided or split into two equal halves either by a diamond saw or a mechanical splitter.

Half the core is sent for assay or other investigations whilst the other half is returned to the core box for record purposes. Obviously structural features have to be recorded before splitting and good practice is to photograph wet core, box by box, before logging it, to produce a permanent photographic record (Fig. 4).

drillhole

When core from coal seams is to be sampled or the samples are to be collected for geotechnical analysis, the core should be sealed as soon as the core leaves the core barrel. This is to prevent loss of moisture, which can adversely affect the measurement of relative density of the core (Preston & Sander 1993) or alter the nature of the material.

Rock chips and dust (“sludge”) can be collected during core drilling; they represent the rock cut away by the diamond drill bit. Drilling with air circulation in relatively shallow holes (as in most percussion drilling) delivers cuttings to the surface within a minute or so.

However, with core drilling, water circulation, and longer holes, there is an appreciable time lag before cuttings reach the surface. It is estimated that from a depth of 1000 m cuttings can take 20–30 minutes to reach the surface, with the inherent danger of differential settlement in the column of rising water due to differences in mineral and rock-specific gravities and shape.

Consequently, rock sludge is rarely examined during core drilling.
Obtaining core is expensive so it is sensible to retain it for future examination. However, adequate and long-term storage involves time, space, and expense, but the value of the contained information is important particularly as during mining some drill locations may
be permanently lost.

Noncore drilling

In noncore drilling the chips and dust are usually collected at 1- to 2-m intervals, dried and separately bagged at the drill site. After washing they are relatively easy to examine with the use of a hand lens and binocular microscope.

Samples can be panned so as to recover a heavy mineral concentrate. It is a good practice to sprinkle, and glue, a sample of rock chips and panned concentrates from each sample interval on to a board so that a continuous visual representation of the drillhole can be made. Again, descriptions must be systematic and quantitative.

Drilling contracts

Drilling can be carried out with either in-house (company) equipment or it can be contracted out to specialist drilling companies. In the latter case, the conditions of drilling, the amount of work required, and the cost will be specified in a written contract. The purpose of drilling is to safely obtain a representative sample of the target mineralisation in a cost-effective manner.

Once the mining company has been assured, in writing, of the drilling company’s safe working practices, the choice of drilling equipment is crucial and much depends upon the experience of the project manager. Unless the drilling conditions are well-known test work should, if possible, be carried out to compare different drilling methods before any large-scale program begins.
The main items of cost in a contract are as follows:

  1. Mobilization and transport of equipment to the drilling site. This can vary from movement along a major road by truck to transport by helicopter.
  2. Setting up at each site and movement between successive drillhole locations. Again costs can vary greatly depending on distance and terrain.
  3. A basic cost per meter of a hole drilled.
  4. Optional items costed individually, e.g. cementing holes, casing holes, surveying.
  5. Demobilization and return of equipment to driller’s depot.

All items of cost should be detailed in the contract. There may be a difference of interest between the driller and the company representative on site, normally a geologist, who is there to see that drilling proceeds according to the company’s plan and the contract. The driller may be paid by distance drilled during each shift whilst the geologist in charge is more concerned with adequate core recovery and that the drillhole is proceeding to its desired target.

The company representative usually signs documents at the completion of each shift where progress and problems are described and it is on the basis of these documents that the final cash payment is made. Consequently, it is important for this representative to be thoroughly familiar with the contract and the problems that can arise at a drilling site, particularly if in a remote location.

The client (i.e. the company) is at liberty to fix specifications such as, say, a plus 90% core recovery, a vertical drillhole with less than 5 degrees deviation. It is then at the discretion of the drilling company as to whether or not these requirements are accepted, but if they are and the conditions have not met the failure is remedied at the expense of the contractor.

Adapted from C. J. Moon, Michael K.G. Whateley
& A. M. Evans

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