by Duncan McLeod, CEO
and Dag Billger, Chairman
at Inertial Sensing
In recent years, we have published several articles in Coring Magazine addressing various aspects of borehole survey instrument performance. In this article, we consolidate those discussions and propose that the mining industry would benefit greatly from an agreed-upon standard for borehole survey instruments. Such a framework would enable more consistent performance evaluation, improve data quality, and ultimately serve the interests of all stakeholders in mining and drilling operations.
Background and evolution of surveying practices
Around twenty years ago, MEMS-based gyroscopic survey instruments began entering the mining market as cost-effective alternatives to the high-end oilfield gyros of the time. In parallel, survey service companies, once responsible for conducting high-quality surveys on behalf of mining operations, began to disappear. Traditionally, these companies supplied both specialized equipment and experienced surveyors who possessed deep practical knowledge of running the equipment under local conditions.
As the industry shifted toward in-house surveying by drillers, responsibility for survey quality often fell to personnel whose primary focus was on drilling productivity rather than survey data accuracy. Frequent staff turnover compounded this issue, resulting in a loss of institutional expertise and surveying best practices.
At the same time, survey tool marketing has evolved. Competition based on technical performance and accuracy has given way to an emphasis on ‘driller-friendly’ features such as ease of use and speed. While usability is important, this trend has often come at the expense of accuracy and, in some cases, the design choices that enhance usability have often degraded measurement performance.
The rise of low-cost, off-the-shelf IMUs (Inertial Measurement Units) has further encouraged the proliferation of inexpensive but low-accuracy gyro survey systems. In the opinion of the authors, the overall accuracy of many borehole surveys has now declined compared with a decade ago.
Understanding accuracy specifications
Most gyro instrument specification sheets include nominal accuracy values for key parameters such as azimuth and dip. However, these specifications are frequently misunderstood or misapplied.
For example, take azimuth accuracy. A single quoted value, say 0.2°, rarely captures the variation in accuracy under different hole geometries and latitudes. A vertical hole, by its very nature, has no azimuth, and uncertainty increases dramatically as the hole approaches vertical. Moreover, for north-finding gyro tools, the quoted accuracy typically refers to 1-sigma at the equator, which represents the best-case scenario and covers only about 67% of survey conditions. At higher latitudes, such as 60°, the error approximately doubles. A more realistic 2-sigma value at 45° latitude for a tool rated at 0.2° accuracy may in fact yield an effective azimuth accuracy closer to 0.6°, before accounting for other factors. A practical azimuth accuracy for this north-finding instrument is actually around triple the ‘spec sheet’ value.
Dip accuracy shows a different problem. Early MEMS survey gyros offered about 0.2° dip accuracy, whereas modern equivalents claim 0.05°–0.1°, a genuine improvement. Yet, some current north-finding systems report dip accuracies around 0.25°, worse than instruments from fifteen years ago. While this may appear minor, the implications are significant: a 0.25° dip error can correspond to a position error exceeding 0.4% of hole depth, undermining common claims of 0.1% positional precision. This is curious, as precise dip measurements are independent of the north-finding technology and have long been possible at the 0.1° accuracy level.
Key challenges in modern surveying
The current landscape of borehole surveying exhibits several interrelated issues that collectively degrade data reliability. These do not share a single root cause, but rather stem from economic, technical, and procedural shifts and choices within the industry.
Cheap systems
Accurate gyroscopic surveys depend not only on quality sensor packages, but also on comprehensive calibration and robust navigation algorithms. These are resource-intensive to develop and maintain, requiring institutional commitment to scientific rigor. The availability of consumer-grade IMUs makes it tempting to build low-cost gyros, but without sophisticated calibration and algorithmic compensation, their accuracy remains inadequate. There is little ‘secret sauce’ here with proprietary algorithms and the like. The physics and mathematics have been known for a very long time, indeed much of the work came from the early NASA space programs.
Overgeneralized instrument design
Some manufacturers promote a ‘one-size-fits-all’ approach, deploying the same technology across all drilling and measurement contexts. As a result, north-finding gyros, which are bulky and sensitive, are sometimes used in production blast hole applications, where speed and maneuverability are more critical than relying on the most expensive equipment. Furthermore, the north-finding shots are very sensitive to local noise and easily disrupted by underground operations. In such cases, the chosen tool is technically inappropriate and operationally inefficient. But a north-finder excels where a deep hole is drilled vertically to a steering point, and an independent orientation shot at that point from the north-finding module is critical for quality assurance. The reversed situations are true, the ideal lightweight, relatively vibration-insensitive blast hole gyro is not the best choice for a deep vertical hole.
Overreliance on north-finding gyro technology
Historically, north-finding gyros have been marketed as the most advanced instruments available. However, they are not universally superior and have several critical limitations when applied in borehole surveying. A well-calibrated reference gyro system can outperform north-finders in many conditions, particularly in inclined holes at mid to high latitudes. In these conditions, north-finders have a degradation of accuracy [3], whereas a reference gyro has no such limit. North-finders are indispensable for deep, vertical wells where steering accuracy is critical, such as transitioning to horizontal in gas drilling, but less ideal for routine mining surveys, particularly at higher latitudes, surveying in east-west directions, or close to horizontal.
Excessive survey speed
The introduction of continuous gyro surveys has reduced survey times but introduced new challenges. The data sampling rate of the wireline encoder is typically quite low, so at high wireline speeds the resolution of the wireline depth is poor. To obtain good resolution, data interpolation is required, which introduces a new error source. This error source is not accounted for in most instances. Proper quality control would constrain survey speed, or acknowledge the error source and quantify it, or employ a high-frequency depth measurement system, such as the TrueDepth encoderless solution used with the BlastGyro, which eliminates the need for interpolation.
No check points in continuous surveys
When a gyro is running a continuous survey, it is tempting to run from the start of the hole to the end in one motion, with no pauses taken anywhere in between. This approach is optimal for speed, but suboptimal for quality control. If a pause (or pauses) is taken instead, for just a few seconds partway down the hole, then stable inclination and gravity high-side readings can be obtained from the inclinometers of the tool. These measurements would give a direct check on the state of the gyro navigation, since while moving these angles must be taken from the gyros (as with the azimuth). Any significant drift or other error in the gyros will therefore be immediately revealed. However, this is often not done in the race to the bottom, and a simple quality check is missed.
Data smoothing and loss of resolution
Survey data that appear unnaturally smooth often indicate over-filtering during post-processing. While some perceive ‘rough’ data as a sign of poor instrument performance, minor directional variations frequently reflect genuine borehole features. Excessive smoothing conceals these features, degrading spatial accuracy. In contrast, oil and gas micro-dogleg surveys intentionally capture such fine-scale variations to diagnose drilling performance. It appears that the desire for smoothness comes from the earlier generation of survey instruments, based on strain gauges or optical deflection that directly measured the bend of the long survey instrument. This length and method of measurement automatically produced surveys that smoothed out the shorter-scale features of many boreholes. Sometimes smoothing is also applied to try and eliminate the artificial wobbles caused by coning of a poorly calibrated instrument that is forced to rotate downhole in an effort to remove calibration alignment issues.
Magnetic compensation
Some modern gyros attempt to compensate for drift by incorporating magnetometers. Given the magnetic variability in mining environments, this approach introduces additional uncertainty and should be treated with caution. There is virtually no call for a modern gyro instrument to rely on a magnetic reference for error correction during surveying.
Suboptimal calibration practices
As detailed in our earlier articles [1] and [2], inadequate calibration is a pervasive problem. Typical shortcomings include:
- Temperature calibration limited to a few discrete points or relying on the sensor manufacturer’s defaults;
- System calibration treated as a nominal alignment or improperly combined with temperature calibration.
Without rigorous calibration across operating conditions, systematic errors persist and accumulate. Typically, the effects are evidenced in the final data as excessive drift, non-physical ‘oscillation’ of the angles or positions, unusually stable azimuth near vertical, and so forth. These problems often lead to ad-hoc pre- or post-survey procedures to try and filter out the problems.
Induced rotation
If a gyro suffers from a poor calibration, then a common tactic is to force it to rotate as it traverses the hole. The idea is to spread the angle errors across the survey in the hope of smoothing out the position errors. Paradoxically, given that gyros are sensors of rotation, this induced rotation will worsen results through amplification of scale factor error. The induced rotation produces artificial oscillations and positional deviations, masking rather than solving underlying inaccuracies [2].
Limitations of current quality control
Modern quality control practices in mining surveys often emphasize convenience over diagnostic value. Many systems provide binary pass/fail results, offering no quantitative measure of survey uncertainty. The most detailed information available is typically a generic accuracy percentage from the specification sheet or a simple misclose comparison between repeat surveys.
A rigorous QC framework should estimate error bounds, flag systematic biases, and provide transparent confidence intervals. Unfortunately, such standards are rarely implemented, leaving end users with limited insight into true data quality.
Lessons from the energy industry
The energy industry has long recognized the importance of standardized accuracy assessment. The Industry Steering Committee on Wellbore Survey Accuracy (ISCWSA), operating under the Society of Petroleum Engineers (SPE), provides a model worth emulating. The ISCWSA’s mission is to:
‘Produce and maintain standards for wellbore survey accuracy; define terminology and accuracy specifications; establish frameworks for modeling and validating tool performance; and raise industry-wide awareness of survey accuracy issues.’
Furthermore:
‘The Industry Steering Committee on Wellbore Survey Accuracy was founded to dispel the confusion and secrecy commonly associated with wellbore surveying. And to enable the industry to produce consistent, reliable estimates of survey-tool performance in today’s wells. We achieve these goals through the production and maintenance of standards covering the construction and validation of tool error models.’
This voluntary, non-partisan group develops scientific models that instrument manufacturers use to generate Instrument Performance Models (IPMs) and associated error models. These can be incorporated into industry software to estimate wellbore position accuracy given the tool’s performance characteristics and hole geometry.
Such collaboration exists because the consequences of poor accuracy in oil and gas, such as missed deposits, well collisions, or environmental hazards, can carry severe legal and financial implications. The mining industry faces lower immediate risks, but the need for data integrity is equally critical, especially as automation and AI-driven decision systems become more prevalent.
Conclusion
Over the past decade, the range of survey gyros available to the mining market has expanded significantly. Sensor technology has improved, yet overall survey accuracy has often stagnated and, in some cases, declined. While low-cost tools will always serve the market segment that is not reliant on high accuracy, the erosion of accuracy among higher-end instruments is both surprising and concerning.
A pervasive misconception persists that ‘a gyro is a gyro’, and that all systems are effectively interchangeable. This attitude, combined with the drive for speed and digital efficiency, risks embedding systematic inaccuracies into mining databases and AI models alike. The long-term consequences are reduced trust in data and suboptimal operational decisions.
There is, however, no need to trade accuracy for convenience. The technology, expertise, and manufacturing capability already exist to produce ‘driller-friendly’ tools without compromising precision. Different applications such as blast hole surveying, core drilling, directional steering, etc., each demand tailored solutions optimized for their operating environments and latitudes.
We therefore advocate for the creation of a non-partisan industry body, analogous to the ISCWSA, dedicated to developing and maintaining standards for borehole survey instrument performance. Such a framework would:
- Define consistent terminology and performance metrics;
- Enable quantitative estimation of uncertainty for each survey;
- Encourage transparency as well as comparability across manufacturers;
- Foster a lasting corpus of knowledge to guide instrument design and validation.
Establishing such a standard would strengthen confidence in survey data and enhance the integrity of modern mining operations. We invite discussion and collaboration from industry stakeholders to help make this vision a reality.
References
- The importance of instrument calibration, Part I, Coring Magazine, Issue 23, 2023.
- The importance of instrument calibration, Part II, Coring Magazine, Issue 24, 2023.
- Similarities between magnetic and north-finding survey tools, Coring Magazine, Issue 9, 2019.
- Comparing multishot & continuous surveys using the TwinGyro, Coring Magazine, Issue 12, 2020.
For more information
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Get in touch with Duncan and Dag on LinkedIn
