GeoMEM Solutions: Borehole Surveying

Background

A borehole is surveyed to obtain a 3D “locational” path of the borehole so that all other information obtained for the borehole (lithology, structure, geophysics, mineralisation, end point) can be accurately located and therefore provide the best information for later critical processes (such as further drill planning, reserve planning/financial planning). It is also essential where knowledge of the borehole location is critical such as in engineering, construction, tunnelling and quarrying (blastholes).

Despite what drillers may say all holes deviate (that is, curve – or spiral – away from a straight line). The exact deviation depends on a number of factors (rock type, drill rig type/power, drill rods used, driller technique are the more important ones), and will not be the same between holes.

A borehole survey removes the uncertainties (or guesswork) of which path the borehole has taken and where it has ended.

At GeoMEM we are of the opinion that: “Any non trivial** borehole is incomplete unless it has an accurate borehole survey.”

** By non-trivial I mean in the surveying sense – and this probably means that the end of the hole is visible from the collar !

Borehole deviation - in practise

Deviating boreholes can have serious consequencies if they are not surveyed and assumed to be straight. These are some possible and known deviations:

  • Shortish boreholes of 10 to 25 metres (as in blast holes for quarries) can deviate by several metres – this can have a severe effect on explosive requirements and knowledge of location and proximity to other blastholes is essential (and a legal requirement in many countries).
  • We have also seen boreholes of 150m with 50m deviations ! This was an extreme case – usually the deviations are around 10 to 20 metres.
  • When you get to boreholes of 1000m plus the deviations can be large (100s of metres). If these are not surveyed your project could be in serious trouble.

Surveying methods.

The instrument and method used to survey a hole is dictated, to an extent, by the type of hole and the lithology through which the hole has been drilled as well as the inclination of the hole.

The following sections describe some different scenarios and the instruments used and available from GeoMEM:

Non-magnetic scenario

If you are surveying in (mostly) non-magnetic rock and in open hole (that is, no casing, apart from near the collar) and not surveying within the drill string then the standard magnetometer / accelerometer Electronic MultiShot (EMS) systems are suitable.

These use Magnetometers. and Accelerometers (on three axes - triaxial) to define the earths magnetic and gravity fields (respectively) and, hence, the instruments' orientation relative to the fields. The Accelerometers provide data to calculate the inclination (dip) and the Magnetometers the direction (like a compass).

There is some more information on the workings of EMS instruments here: EMS survey instruments (pdf 790 Kb).

The magnetometers will only provide accurate results where there is no distortion of the earths magnetic field so cannot be relied on where there are extensive magnetic rocks, within steel rods or casing or near man made magnetic items. Usually the processing software can deal with short sections of magnetic anomaly (such as a narrow band of magnetic rock).

The instruments themselves are made of non-magnetic material (usually brass) and are run with aluminium spacer rods between the instrument and any steel rods or connectors to minimise external magnetic influence. GeoMEM supply two versions of EMS survey tools from Devico:

Magnetic scenario

Where there are magnetic anomalies/magnetic rock (e.g ironstone, massive pyrrhotite, etc.) or the survey instrument must be run inside rods or casing (this may be necessary if the hole walls are liable to collapse) a non-magnetic tool must be used (where the direction is derived from some other method than magnetometers).

There are a number of options to survey in a magnetic environment – and because the accelerometers are not affected by magnetism, these are normally included to provide an independent measurement of the borehole inclination.

There are two main types of instrument:

  1. Differential: Where the tool measures the angular differences between fixed points as it moves down or up the hole. The starting angles are known and the differences are added incrementally at each station to obtain the borehole survey angles. There are various ways of achieving this – some systems use optical methods. The Devico DeviFlex system, which is supplied by GeoMEM, uses miniature strain gauges to determine curvature and direction of curvature. It is intended to be run inside drill rods / casing to ensure the most accurate survey (it cannot be run in open hole).
  2. Gyroscopic: Where the instrument uses inertial navigation to define the borehole path as it moves. At present some MEMS (Miniature Electro-Mechanical System) gyro system are available. However, there are some concerns about the accuracy and operational reliability at this time.
    A gyro can be run in drill rods / casing or open hole.

 

Inclination only required

In some cases only the dip of the borehole may be required. This can be obtained either using the EMS tools listed above or with DEVICOs dedicated dip-only tool, the DeviDip. This also provides gravity roll angle information which allows orientated placing of wedges if needed. Because it uses only accelerometers this tool works in any environment (Magnetic and non-magnetic).


 

See also: Core orientation, Directional drilling


Some useful sources of borehole surveying information on this site

  • Borehole Surveying Resource Centre. This contains a number of links to sources of information (current and historical) for borehole / wellbore surveyors.
  • Old FlexIT SmartTool support. This contains some support information and Powerpoint presentations about the old FlexIT SmartTool systems (including the MeasureIT and DisplayIT software). It also has information about other legacy software systems developed by GeoMEM in the past, including but not limited to: SProcess; GyroMeasureIT and GyroDisplayIT.