Reference Systems for Downhole Surveys

Downhole survey measurements provide a way of determining the path of the drillhole in 3D space. This is done by measuring the direction and inclination of the hole at regular depth intervals. The direction is measured as the angle clockwise from the north axis (where "north" represents 0° in the current frame of reference), while the inclination measures the deviation from the horizontal (where -90° is "vertically down").

In the context of the downhole survey, the term "north" refers to whatever direction was recorded as zero degrees by the measuring instrument at the time of the survey. However, the term can have several meanings, depending on the reference system used.

True North	The direction of a line that connects the observation point with the North Pole
Magnetic North	North as indicated by a magnetic compass
Grid North	The direction along a line that is parallel to the central meridian used for a map projection (e.g. a north-south grid line on a map based on the UTM projection). This type of reference system is not relevant to this chapter, as it is already catered for by standard projected coordinate systems.

The angle between a magnetic compass bearing and true north is known as the magnetic declination. If the magnetic declination at a given observation point is 5°, then a compass bearing of 0° at that point is in fact equal to a “true north” reading of 5°.  In general terms, the angle relative to true north can therefore be calculated from the magnetic bearing as:

True North Bearing = Magnetic Bearing + Magnetic Declination

For example, if the magnetic declination at the point of measurement is 5°, then a reading of 60° on the compass is pointing at 60 + 5 = 65° North.

Magnetic declination depends on local variations in the magnetic field as well as the migration of the magnetic north pole. This shift can be quite significant over a long period of time.

In cases where downhole survey readings were taken with an instrument that used magnetic north as a reference, the magnetic declination at that location (at the time that the measurement was made) would have to be known in order to accurately calculate a true north and grid north azimuth for each reading.

A special type of coordinate reference system is used for cases like these. It is essentially a geographic coordinate reference system that has been extended to cater for the magnetic declination data.

A value for the average magnetic declination can be specified in the reference system editor, as shown above.

The check box determines how this value is used.

• With the option enabled, OCRIS will always use the specified value for all downhole surveys linked to that coordinate reference system.
• With the option disabled, OCRIS will first attempt to determine the appropriate value by way of the magnetic declination reference table stored in the database. If no reference data is available for the survey location, then the specified default value will be used.

The second method is more accurate and is therefore the recommended option. However, this method depends on the availability of a grid of temporal magnetic declination values covering the survey area, as explained later..

When dealing with downhole surveys where the magnetic bearing used for downhole azimuth readings has already been corrected to true north, the magnetic declination is no longer required. This eventuality can be catered for by defining a reference system where the value of the magnetic declination will always be zero.