Wellbore surveying with gyro tools | Part 1 of 2
The technology for surveying in mining has gone through some significant improvements in the last decade. It is perhaps time to review the current situation to get the best possible use of the current suite of tools on the market.
This article (split into two parts) is going to explore surveying on a worldwide basis – many of the world’s mines are in the most remote and hostile places – which also correspond to some of the more difficult surveying challenges. We are also taking information from the cross-over with oil and gas and trenchless technologies to help us in the mining survey environment.
Magnetic vs. Gyroscopic surveying
As the less expensive and more robust option, magnetic tools have always been considered the standard for surveying. They definitely have their place in mine surveying.
But as the awareness and appreciation of the benefits of directional survey accuracy has increased; and gyros have become more robust and reliable, so they have entered the mainstream of mine surveying.
Once considered too exotic, expensive and unreliable, they are now the default tool for surveying.
The uncertainties associated with magnetics can stand being restated. Magnetic tools reference the well direction to Magnetic North.
It is obvious that magnetic tools will not give accurate results when operating in a magnetic environments.
In the latest tools you can download the raw magnetometer and accelerometer data and run QC checks on vibration, magnetic field strength and dip to exclude bad surveys.
Since Magnetic North is constantly moving over time, we need to correct our surveys from Magnetic to Geographic North.
Declination is the offset between Magnetic and Geographic North references.
But even in a magnetic interference-free area, do you know how accurate your declination data is? If you input inaccurate data, you will offset the complete survey, leading to an inaccurate bottom hole location.
The standard method for obtaining declination data is to use a predictive model.
None of those models account for all of the variances in a particular location such as the local geology.
When a model costs $20k plus per year, there is also a temptation to continue using an outdated model for a few years, decreasing time related accuracy.
The current information from the NOAA website suggests even in clean, properly modelled conditions, there is a 0.3 deg residual error and often practically, 3 to 4 degrees error in declination values.
This is one of the uncertainties we can estimate.
What do these error margins mean?
In a 600m borehole, a 0.2 deg declination error creates a 2ft systematic bottom hole location error; a 3.0 deg declination error creates a 31ft systematic error! I can leave you to see the likely error in longer boreholes.
None of this takes into account ferromagnetic mineralogy in your mine or other uncertainties peculiar to magnetic tools that we cannot estimate.
One of these other sources of magnetic interference rarely dealt with in surveying situations is the magnetisation of the drillstring. Rotation and stressing of the drill rods can lead to the magnetisation of the drill rod itself.
This can be clearly seen when breaking connections – one can see rust and iron filings attaching themselves to the magnetic pole which is the connection. The solution – degaussing in an alternating current coil.
Gyro tools as we know are not subject to magnetic interference. They reference to Geographic North, the axis of rotation, thus removing the declination and other errors mentioned above.
The quantum leap made by gyros occurred when we moved from mechanical to solid state.
This instantly increased robustness and reliability to the point where they are now on a par with magnetic tools.
We believe that this concept is not yet fully appreciated by those who have experienced mechanical gyros.
Reference vs North seeking gyros
A gyro used to be a rapidly spinning mass that resists twist in it’s main plane. High end, accurate gyros are now either an optical (Ring Laser or Fibre Optic) device or a vibrating (Hemispherically Resonating) device.
Both are highly accurate, and in general suitable for the mining industry, where high temperatures are unusual.
A reference gyro needs a reference direction. This is used to tie in the start of the survey, with the tool updating the change in position as it is run in hole and out again.
A reference gyro is subject to drift. Hence stops are made to correct the previous drift before continuing running in continuous mode.
A North seeking gyro, by nature of its North seeking capability will find North at each gyrocompassing station. Again, it is run in continuous mode between gyrocompassing stations.
All gyros will drift to some extent. Each gyro manufacturer has worked to reduce and correct this drift and they have their own proprietary methods.
The other quantum leap has been in data processing.
One of the undoubted advantages of magnetic tools is clarity of data. Most experienced clients can review the data for themselves.
You can see if the magnetic field strength and dip do not match the expected values and reject the survey (But what can you do to resolve this situation?)
The gyro industry in general has always been less transparent in that the client cannot possibly know all of the black box filtering that goes into removing and generally tidying up any bad data.
This is the “post-processing” that can take a transfer to head office for QC checks before going to the client.
In this age of chain of custody and data transparency requirements it is surprising that clients will accept this situation.
Ideally, the surveyor gives the client an unedited inrun and outrun survey. The difference in location between start and end is a combination of accuracy, calibration, resolution and repeatability.
The client can review the data and immediately see the survey closure value and calculate the error per survey length.
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