Working with downhole survey data
This topic explains input requirements for the downhole survey calculations in Micromine Geobank, with particular reference to the north adjustment angles used for determining azimuth.
Purpose
Downhole survey measurements provide a way of determining the path of the drillhole in 3D space. Essentially, 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 (north = 0°), while the inclination measures the deviation from the horizontal (where -90° is "vertically down").
Using these measurements, it is possible to determine the grid coordinates (northing, easting and height) at a given distance along the path of the hole. This is necessary in order to accurately determine the coordinates of seam intersections, sample midpoints, etc. for the purposes of modelling the deposit.
The functionality for calculating downhole coordinates is available in Micromine Geobank; however, the use of this functionality is entirely optional, because the desurvey calculations are usually done in the resource modelling package (e.g. Micromine). In Micromine Geobank, the use of downhole coordinates is generally limited to reporting.
What is "North"?
In terms of the downhole survey, "North" is whatever direction was recorded as 0° by the measuring instrument at the time of the survey. However, the term "north" can have several meanings, viz:
| True North | Determined by the position of the Earth's north pole |
| Grid North | North according to the map grid you are using (e.g. UTM or Local Grid) |
| Magnetic North | North as indicated by a compass |
This distinction is important when dealing with downhole survey measurements.
Calculating True North from Magnetic North
The angle between a compass bearing and true north is known as the magnetic declination. This is the value that needs to be added to a compass bearing in order to calculate a true north bearing. For example, if the magnetic declination at a given point is 7°, then a magnetic (compass) bearing of 10° at that point would convert to a true north of 10 + 7 = 17°.
In addition to the effect of local variations in the magnetic field, magnetic declination is affected by the apparent movement of the Earth's magnetic north pole over time. This shift amounts to several kilometres per year and therefore the effects can be quite significant over long periods of time.
If the downhole surveys were measured with an instrument that used magnetic north as a reference, then the magnetic declination for that point (at the time that the measurement was made) would have to be known.
Calculating UTM Grid North from True North
To adjust a true north bearing to a bearing relative to the map grid, we have to allow for the angle between true north and grid north.
In the case of a UTM grid, the difference between grid north and true north is generally assumed to be zero. However, keep in mind that the UTM projection uses straight lines to represent curved meridians. Along the central meridian of the UTM zone, true north and grid north are identical; however, as we move farther away to the east or west of this line, grid north departs steadily from true north due to the convergence of the meridians.
The angle between a meridian and the corresponding north-south line of the UTM grid is known as the grid convergence. The magnitude of this angle increases as we move farther away from the central meridian of the UTM zone (or farther away from the equator).
In the southern hemisphere, grid convergence is considered to be positive to the east of the central meridian and negative to the west. In other words, it specifies the value that must be added to a true bearing in order to calculate the grid bearing. For example:
| True North | Grid Convergence | Grid North |
|---|---|---|
| 17° | -0.5° | 17 + (-0.5) = 16.5° |
Calculating UTM Grid North from Magnetic North
To calculate a grid bearing from a magnetic bearing, we have to combine the two steps discussed before, viz:
| Mag Bearing | Mag Declination | True North | Grid Convergence | Grid North |
|---|---|---|---|---|
| 10° | 7° | 10 + 7 = 17° | -0.5° | 17 + (-0.5) = 16.5° |
Using a Local Grid
A Local Grid (in the Micromine Geobank sense of the word) is based on a UTM grid. The local grid may be rotated relative to the UTM grid, introducing an additional angle that has to be taken into account when working with a north bearing. For example, if the local grid were rotated 45° relative to UTM, then a bearing of 10° for the local grid would correspond to a bearing of 55° for the UTM grid.
In some cases, downhole survey data may be based directly on a local grid orientation. For example, an inclined drillhole that is oriented due "north" (0°) on this local grid will in fact be oriented at 45° east of north when measured relative to the underlying UTM map grid.