The purpose of this article is to highlight considerations for dealing with downhole survey azimuths for mineral exploration drilling.
Downhole survey data (dips and azimuths) and drillhole location (east north and elevation) are used to calculate the 3D spatial locations of downhole data including logging, sampling and assays. It is important that survey data are managed and interpreted in a way that maximises the accuracy of the downhole data, which can ultimately affect resource modelling and mine planning. It is not uncommon for azimuths to be misinterpreted and this article aims to provide basic guidelines for understanding and interpreting downhole survey azimuths.
Background
A downhole survey reading consists of 3 key components – depth, dip and azimuth:
- The depth is the distance from the starting position of the drillhole (the collar) to the “survey station”. Historically drillholes were surveyed at 30 metre intervals down the hole, whilst with modern tools, survey stations every 10 metres or less are more common.
- The dip (or inclination) is the angle of the drillhole’s deviation from a horizontal plane. In the mineral exploration industry, a vertically down drillhole is denoted as -90 degrees and a horizontal drillhole is denoted as having a dip of 0 degrees.
- Azimuth is the angle in the horizontal plane between “north” and the drillhole’s direction (or bearing) at the survey station. Azimuths are conventionally measured in a clockwise direction from “north” with 0 degrees corresponding with north, 90 degrees east and 180 degrees south. A consideration for the azimuth is that “north” can refer to 3 reference systems:
- Magnetic North
- True (Geographic) North
- Grid North
Azimuth and North Reference Systems
We know that the azimuth reading is the angle between “north” and the drillhole direction – but which “north” is this referring to? Magnetic North? True North? Or Grid North? And why does it matter?
The key to determining which reference system that the azimuth is referenced to is in knowing which downhole survey instrument was used – these are discussed in the section “Downhole Survey Tools and Implications for Azimuths”.
The reason we need to know which reference system is because a drillhole azimuth must be expressed in reference to Grid North when modelling – and this may require the application of a magnetic declination and/or grid convergence angle. In Australia, the magnetic declination angle varies between -3 degrees in south western Australia to +15 degrees on the south eastern Australia. Thus, correctly calculating, or miscalculating, the grid azimuth of a drillhole can considerably impact the downhole location of the drillhole data, particularly in locations with high degrees of magnetic declination. The section “Magnetic Declination and Grid Convergence” discusses how to determine these angles and when to apply them.
Downhole Survey Tools and Implications for Azimuths
Numerous downhole survey tools have been used over the past 30+ years in the mineral exploration industry and survey tools can be broadly split into 2 key categories – magnetic tools and non-magnetic tools. Knowing the tool type is important for understanding whether the survey data are referenced to magnetic north, true north or grid north.
Magnetic Tools
Magnetic tools determine azimuth relative to earth’s magnetic field and as such can be affected by magnetic interference from magnetic rocks and metallic objects such as drill strings. Subtypes include mechanical tools, such as the Eastman camera, electronic single shot and electronic multi shot tools. Survey data from mechanical tools are typically only encountered in older drill hole records and open file data as these tools have been superseded by the modern electronic tools.
The azimuth readings from magnetic tools are referenced to Magnetic North and require the application of the magnetic declination and grid convergence to calculate the grid azimuth.
Non Magnetic Tools
Non magnetic tools, most commonly gyroscopes (gyros), are unaffected by magnetism and are increasingly use by the minerals exploration industry as affordability improves. Subtypes include Reference gyros, which require the manual input of a reference starting azimuth, and North Seeking gyros which reference azimuths in relation to True North.
Collar Survey Dip and Azimuth
Collar surveys are the dip and azimuth readings at drill hole collar (at zero depth). These readings can be achieved by surveying with a gyrocompass rig aligner or a north seeking gyro, which both reference azimuth in relation to True North, or by a survey pickup of the drill string.
Magnetic Declination and Grid Convergence
Magnetic declination, or magnetic variation, is the angle on the horizontal plane between Magnetic North and True North – this angle varies spatially and temporally, hence the importance of knowing the date surveyed for magnetic azimuth readings in addition to the location. In Australia, the magnetic declination is currently 0.84 degrees in the Kalgoorlie Goldfields and 11 degrees in the Victorian Goldfields.
Grid Convergence is the angle on the horizontal plane between True North and Grid North.
The magnetic declination and grid convergence for a location can be determined using online calculators, such as the Geoscience Australia Geomagnetism Tool (for magnetic declination) and the Geoscience Australia Geodetic Calculator (for grid convergence).
Azimuth Conversions
Convert from Magnetic Azimuth to Grid Azimuth
To convert from magnetic azimuth to true azimuth or a grid (UTM) azimuth, the following rules apply.
- Magnetic North differs from True North by the magnetic declination
- Grid North differs from true north by the grid convergence
grid azimuth = (magnetic azimuth + magnetic declination) + grid convergence
Example 1 demonstrates how to calculate the grid azimuth from the magnetic azimuth reading for the drillhole.
| Location | Drill-hole Azimuth | Grid Reference | Year | Magnetic Declination | Grid Convergence |
| Marble Bar | +135° | Magnetic | 2021 | +1.3° | +1.0° |
grid azimuth = (135° + 1.3) + 1.0°
grid azimuth = 137.3°
Example 2 demonstrates how to calculate the grid azimuth from the magnetic azimuth reading for the drillhole.
| Location | Drill-hole Azimuth | Grid Reference | Year | Magnetic Declination | Grid Convergence |
| Orange | +135° | Magnetic | 2004 | +11.5° | +1.2° |
grid azimuth = (+135° + (11.5)) + (1.2)°
grid azimuth = +147.7°
Convert from True Azimuth to Grid Azimuth
To convert from true azimuth to grid (UTM) azimuth, the following rules apply.
- Grid north differs from true north by the grid convergence
grid azimuth = (true azimuth) + grid convergence
Example 3 demonstrate how to calculate the grid azimuth from a drill-hole azimuth referenced to True North.
| Location | Drill-hole Azimuth | Grid Reference | Year | Magnetic Declination | Grid Convergence |
| Marble Bar | +135° | True North | 2021 | +1.3° (not applicable) | +1.0° |
grid azimuth = (135°) + 1.0°
grid azimuth = 136°
Where modelling is done in a local grid, a rotation angle must be applied to the grid azimuth in order to determine the local grid azimuth.
Conclusion
Knowing the grid reference of a survey azimuth is key to understanding the correct handling of the downhole survey data.