Trying to understand the true accuracy of a gyro survey tool can sometimes be difficult. It doesn’t help when the information is displayed in a confusing manner. The purpose of this article is to bring clarity to the subject so that you are confident when you choose the next north-seeking gyro for your mining operation.
One area of difficulty that we’ve had many customers ask us to explain is why some manufacturers quote their azimuth accuracy to 1 sigma (σ) or in other words 1 standard deviation. When it comes to accuracy, quoting to 1 sigma is not best practice, the correct way would be to quote to 3 sigmas.
For example: If a manufacturer declares that they have an azimuth accuracy of ±0.75° quoted at 1 sigma, this means that only two-thirds of their measured values will actually have an accuracy of ±0.75°. Therefore, 1 in 3 data points will have an accuracy of up to ±2.25° (0.75° multiplied by 3), which is below industry standards.
What SPT considers the best practice and what should be industry standard is to quote to 3 sigmas because this will give you a 99.7% assurance that the measured value will be what is stated. The quick way to calculate this is to multiply any figure quoted at 1 sigma by 3.
Inaccurate gyro survey tool specifications can lead to project failure
Let’s use an example to demonstrate how crucial accuracy can be to the success of a project.
To calculate how far off target your tool could be, you can use the following simplified formula:
Borehole depth x sin(Azimuth accuracy) x sin(90° – Inclination in horizontal reference)
Using this calculation, the tool could be off-target by up to 19.6 meters. This is a very large discrepancy and could be disastrous to a project. To give you a comparison, the north-seeking GyroMaster™ would only be off-target by max 4.4 meters (Table 1).
Let’s visualize this:
- Borehole depth: 1000 meters
- Inclination: -60°
- The inclination is quoted from horizontal where vertical is -90°
As you can see there is a significant difference between quoting at 1 sigma vs 3 sigma. This difference in accuracy could lead to a profitable orebody being classed as waste rock.
The key to our high precision is our continuous survey mode, while most tools establish data points every 30 meters, our proprietary Navibore™ solid-state gyro technology registers data points at every centimeter to determine the exact position along the entire run.
How to confirm the accuracy of your downhole gyro survey tool?
With the amount of investment that goes into a mining project, transparency of tools should be a standard in the industry. Too long we’ve seen manufacturers giving false data because the user has no way to confirm specifications.
Here are a few ways that you can confirm your tool is as promised:
- Perform an in-run/out-run survey: This means that you will complete 2 independent surveys of the borehole to ensure that the data matches. This is a standard procedure and quality control measure incorporated into our Navibore™ Technology.
- Land surveyor verification: Have an external surveyor complete a triangulation to confirm accuracy.
- Ask your manufacturer: They should be required to show the ability to measure vertically -89 degrees, in continuous mode and compare that difference in-run (downward) and out-run(upward) measurements does not exceed 0.1% of measured depth in N-S/E-W coordinates.
- Test your tool: Your tool should be tested to the environmental conditions that it will be used in.
Stockholm Precision Tools has been a leader and authority on downhole accuracy for over 20 years and we want to ensure that your project is successful. Our team of experienced engineers are happy to answer any questions that you might have about your project or current tools that you are using.
How does Navibore™ gyro survey tool technology work?
Borehole survey equipment or trajectory measurement technology is composed of an angular velocity sensor with 2 sensitivity axes. The sensors form a system of orthogonal coordinates directed along the axis of the tool. The angular velocity of the Earth’s rotation and gravity vector are used to determine the angles of navigation with post recalculation into the bore path.
Navibore™ gyro tool technology measures the projected angular velocity of the Earth and the accelerometers measure the projection of the gravitation force. The angle of inclination and gravity tool face are calculated from the signals of the accelerometer.
Due to its operation being based on the angular velocity of the Earth and not on magnetism, Navibore™ technology has multiple applications in geology, civil engineering, mining, oil and gas.
How to ensure project success?
Some companies are looking for short term savings via low-cost tools, when, over the long term they are losing much more money than these small initial savings.
We recommend doing your own research and understanding exactly what you are investing in. The importance of knowing the precise direction of your borehole can be the difference between classifying orebody as waste rock.
If you have any questions about downhole gyro survey tool technology and the suitability for your project, please feel free to contact our team any time at [email protected].