Together, Duncan McLeod and Dag Billger, specialists in inertial micro-sensor systems (MEMS), have more than 30 years of experience of developing commercial positioning and tracking systems.
Duncan and Dag have a strong focus on wellbore survey instruments. They work with a wide range of applications, which are relevant for the following fields: Mining, Oil and Gas, Geotechnical and Civil Engineering.
Duncan and Dag founded Inertial Sensing in 2010, and its goal is to create modern surveying technology that will give their customers a competitive edge.
How do magnetic survey tools work?
The simplest description of a magnetic electronic multi-shot
survey (EMS) tool is that it is at heart an electronic compass that can be lowered into a borehole and stopped at a series of locations to measure where it is pointing with respect to magnetic north, to give the azimuth. It also contains inclinometers to measure how the tool is tilted away from the horizontal, to give the dip angle. The borehole path/coordinates are reconstructed from the measured angles of dip and azimuth in combination with the known depths of the stations, using a method such as minimum curvature interpolation.
EMS tools can also measure more information that is sometimes not presented, as it is not needed to reconstruct the borehole path. This includes the magnetic field strength and the magnetic dip/ inclination, which can be used as survey quality indicators.
The magnetic field strength, measured in nano-Tesla (nT), is simply how strong the magnetic field is. The strength of the Earth’s magnetic field typically ranges from 25 000 to 65 000 nT depending on the location. A quality EMS tool should be calibrated to read magnetic field strength in the range of zero to 100 000 nT.
The Earth’s magnetic field is a vector, which means that at any location it has a direction as well as a strength. The magnetic field usually points in a north direction, which is, of course, why compasses can be used for navigation. But the exact direction can vary significantly around the Earth because of the geology of local rocks and the exact conditions in the Earth’s core, which ultimately generates the magnetic field. The strength and direction of the magnetic field also vary over time, with the geographical positions of the poles changing over the years.
The direction of the magnetic field can be broken down into two parts of interest. The first is the part that defines the compass heading, the direction of magnetic north. The second is the magnetic dip, which describes how the magnetic field is tilted up or down from horizontal. At the equator, the magnetic field is ideally horizontal and has a magnetic dip of 0°. At the magnetic North Pole, it is vertical and straight down, or -90°. At the magnetic South Pole, it is vertical and straight up, or +90°. In between these locations, it varies roughly according to latitude. Note that the Earth magnetic field does not vary exactly with latitude as there are large regional variations in different parts of the world.
Many EMS tools contain a full set of three perpendicular magnetometers to measure the complete magnetic field around the tool. They can then also measure the magnetic field strength, the magnetic dip and the azimuth. Apart from being interesting for investigating the geology of the local rocks, the magnetic field strength and the magnetic dip can also be used as quality indicators for survey results. This is because a magnetic disturbance in the borehole will most likely affect the magnetic field strength and the magnetic dip values, as well as the azimuth.
Magnetic declination is one of the quantities of interest in magnetic surveys which are not measured by the tool. The declination describes how the magnetic field at a location points away from the true geographic north. For a local field pointing to the east of the true north, the declination value is positive. Since the Earth’s magnetic poles wander over time, the declination at a location also changes. It is therefore important to remain updated on these changes when surveying.
Various scientific bodies monitor the Earth’s magnetic field globally and locally for its strength, dip and declination. There is a variety of sources of information and online calculators are provided by various organizations, for instance by NOAA [1] and The British Geological Survey [2].
Solar activity can also influence the Earth’s magnetic field and cause disturbances that are large enough to affect the quality of a magnetic survey conducted during a solar storm. Solar activity is kept under scientific observation and it is possible to receive space weather alerts warning of conditions that might have a geomagnetic effect on Earth [3].
Is there a difference in the quality of sensors in magnetic survey tools that are available on the market?
EMS tools are a very mature technology and the quality of sensors and results from a reputable supplier should not be in question, nor should they differ significantly. Most manufacturers quote a general calibrated azimuth accuracy in the range of ±0.35 to ±0.5°.
However, that says nothing about other quality issues that may arise. The sensors used in a tool are only a part of a system that must come together to give good results. This includes the length of time that a good calibration holds for a tool, power consumption, ease-of-use, ruggedness, tendency to fail in certain situations and so on.
How can a surveyor assess the quality of magnetic survey tools, based on their results?
In general, it is difficult to use survey results to assess tool quality, unless you already have a good reference survey to compare with. But one can always compare two surveys of the same hole with the same tool to check for repeatability.
A tool manufacturer should be able to specify the accuracy of the tool, assuming it is in good condition and recently calibrated. One way to verify performance is to do tests in a magnetically undisturbed area. For instance, setting up an accurate reference line using GPS, gyrocompass or similar and repeatedly measuring the line with the EMS tool. By rotating the tool around its long axis and taking measurements with different gravity highside positions one can also test for any ‘coning’ of the azimuth results which would indicate poor calibration or a bent tool. In addition to checking the azimuth results, the values of the local magnetic field and magnetic dip can be verified against the expected values.
Frequently, the reference value of the magnetic field strength (in nT) of a project is assumed to be the value taken from several undisturbed borehole survey stations. Is this good enough or is there a more precise and reliable way to find the magnetic reference value?
It is not ideal to use results from a survey in the hole, as we cannot be sure what is happening there. Often the best way to measure the local magnetic field strength is to use an EMS tool to take multiple measurements on the surface in an undisturbed area and average the results to get an estimate of the field strength. Of course, this depends on the tool being in good working order and properly calibrated.
If this is not possible or practical to do, there are world magnetic models that are maintained by various scientific agencies, such as those from the aforementioned NOAA and the British Geological Survey.
Magnetic data values within a survey can always be used to identify anomalies appearing locally over a short distance in the borehole. However, if there are disturbances of a more global nature affecting the whole body surrounding the borehole, this would affect the whole survey and then an independent exterior reference value is important.
What is considered as an acceptable fluctuation of the magnetic field strength that does not interfere with the azimuth reading?
Typically, in the borehole surveying industry, acceptable fluctuations are usually in the range ±1000 to ±1500 nT for the magnetic field strength and ±0.5° for the magnetic dip angle.
Is it possible to have azimuth and magnetic field strengths that appear stable and valid on all the records of the survey data but still differ from the actual ones? What could cause this to happen?
The azimuth calculated from the magnetic field components does not depend on the overall strength of the magnetic field, only its direction. If the magnetic field is disturbed so that its strength is very nearly correct, yet the direction is moved from what it should be, then the azimuth would be incorrect, but the field strength would not indicate the problem. In this case, the magnetic dip value can also be checked at each station, since this should be affected to some degree as well and indicate a possible error in the azimuth.
Contrary, for the same reason as mentioned above, in rare cases the value of the magnetic field can appear anomalous, but still its corresponding azimuth values may appear true (undisturbed).
It can be noted that while the azimuth does not depend on the strength of the magnetic field, the spread of repeated azimuth measurements at the same point is sensitive to the field strength. In a location with a weaker Earth magnetic field, the azimuth results will have a higher spread. For instance, see Accuracy prediction for directional MWD’, Williamson, SPE 56702, Society of Petroleum Engineers Annual Technical Conference and Exhibition, 1999.
In vertical surveys, the tool is not pointing in any compass direction, but straight down. The magnetic field readings will be stable, but the azimuth results could also appear to be stable, depending on the setup of the system in the hole. But the azimuths themselves will be essentially meaningless.
This also is true for near-vertical surveys, where the hole has a dip within about -87° to -90° of vertical. As the borehole dip comes close to vertical a lot of care must be taken in interpreting the azimuth results.
Another condition that should be kept in mind is that as the survey location moves closer to the Earth’s magnetic poles, the accuracy of the azimuth results decreases. This is the nature of the azimuth itself, as it is undefined at the poles. To visualize this, imagine standing at the North Pole… every direction from there is south and there is no single azimuth value! The same problem is encountered with north-finding gyroscope tools and is discussed in a previous article [4].
The azimuth is determined by using the local gravity vector and the Earth magnetic field vector as reference directions. The higher the magnetic dip angle is at a given location, the closer it will be to the local gravity vector. The closer these vectors are to each other, the more they represent the same information and gradually lose their significance as independent reference directions. The consequence is that a higher magnetic dip means lower azimuth accuracy, while a low magnetic dip angle means a higher degree of azimuth accuracy.
Finally, as with north-finding gyroscopes, magnetic survey tools are less accurate when surveying in an East/West direction and most accurate when surveying North/South. Thus, a stable magnetic field and azimuth result in an East/West direction is more likely to be in error than the same stable results taken from a hole pointing North/South. This is also discussed in the previous article mentioned in this section.
How can the magnetic dip be used to judge the quality of the survey data; can it improve data analysis and what is the added value from this additional parameter?
At any given point on the Earth, the magnetic dip should be reasonably well known and constant over a distance. So, if in a survey the magnetic dip varies too much outside a standard range (usually ±0.5°), we can be confident that there is magnetic interference in the ground and the survey results are probably unreliable.
In principle, all EMS tools containing a full set of three magnetometers should be able to give a value of the magnetic dip at every station, along with the azimuth and magnetic field strength, since the magnetic dip is just another number derived from the same calculations.
It has become a common practice to do control gyro surveys considering survey data collected from magnetic tools. Often the check survey results in 2-to-3° difference in the azimuth values and the gyro data is accepted valid. Can there be cases when the error is not in the magnetic tool?
There are no perfect survey tools, nor is it possible to guarantee that a survey tool has always been used correctly. So, there stands a chance that a gyro can give a survey that differs from an EMS tool, and for the EMS tool to be more correct.
If we assume the gyro is functioning properly then some sources of error that might be overlooked are:
- For a north-finding gyro, incorrect latitude information was supplied to the tool.
- For a reference gyro, the initial reference direction was incorrectly measured using GPS, a three-axis gyrocompass, optics or similar.
- The gyro was over-rated (turned too quickly) at some point in the survey, which typically adds a constant azimuth error.
- The gyro was rotated excessively during the survey, allowing scale factor errors to build up and cause an azimuth error.
- The magnetic declination was incorrectly applied when comparing magnetic and gyro survey data.
It should be noted that it is possible for both the gyro and the magnetic tool to be incorrect. This is particularly true for gyros that use FOG (fiber-optic gyro) technology, as these gyros are also magnetically sensitive through the Faraday Effect and can be disrupted by magnetic ore bodies or magnetized drill rods.
When comparing two surveys from any tool type it is very important to keep an open mind about where the differences may come from. There is no such thing as a survey tool that is immune to any human error or misuse!
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