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We can’t hit targets hundreds (or thousands) of meters below the surface of the Earth if we don’t know where we’re going. To determine the trajectory of a borehole, we use borehole surveying as a measurement tool. This makes it an essential instrument for ensuring accuracy and preventing collisions while drilling.
What is borehole surveying?
Borehole surveying is the process of measuring and mapping the trajectory of a drilled borehole. To survey a borehole means to take measurements that will help you determine where the drill is heading. To collect this information, we use a variety of general and specialized survey tools. For example, magnetic and gyroscopic survey tools are used to determine direction and inclination, while an inclinometer is a specialized tool that provides detailed information about the inclination.
Key data collected
- Azimuth: The horizontal direction of the borehole.
- Inclination: The angle of the borehole relative to the vertical axis.
- Depth: The vertical depth at which measurements are taken.
The data provides insight into the well path, which is crucial information for adjusting while drilling, in addition to analyses.
Also read: Directional drilling 101
Different types of surveying tools
At the beginning of the drilling industry as we know it, people simply assumed that a borehole would follow a straight path. For the most part, they were right, since the early boreholes (mainly for oil wells) were rather short. However, as technology improved and boreholes became longer, it was observed that they tended to deviate from the theoretical path due to geological as well as technological reasons.
And so, the brightest minds of the drilling industry began trying to figure out how to accurately measure the trajectory of their boreholes. For this task, we have a selection of survey tools that we use to measure depth and inclination of boreholes.
- Magnetic survey tools
- Gyroscopic survey tools
- Inclinometers
- Optical and acoustic tools
- MWD and LWD tools
Magnetic survey tools
Magnetic survey tools determine azimuth (direction) and inclination. They rely on the Earth’s magnetic field to calculate azimuth and accelerometers to measure inclination (angle from vertical).
Variations of magnetic survey tools
- Single-shot instruments: Take one reading at a time (inclination and azimuth). Used for simple and shallow boreholes.
- Multi-shot instruments: Capture multiple readings at set intervals in a single run. Faster and more detailed than single-shot.
- Continuous magnetic tools: Provide a near-continuous record of the borehole path as the tool is moved, giving a high-resolution trajectory.
Gyroscopic survey tools
Unlike their magnetic counterparts, a gyroscope measures direction based on the Earth’s rotation rather than magnetics, making them immune to magnetic interference. They can also be of multiple types:
- North-seeking gyros: Detect true north directly with high accuracy.
- Rate gyros: Measure angular rate changes and integrate over time to calculate direction.
Gyroscopic survey tools can be single-shot, multi-shot, or continuous, just like magnetic tools.
Inclinometers or accelerometer-based tools
Inclinometers are specialized tools that primarily measure inclination (dip) using accelerometers. While they don’t measure azimuth directly, they are useful in geotechnical engineering and shallow borehole applications where dip is the most critical parameter.
Optical and acoustic tools
In geotechnical and hydrogeological investigations, optical and acoustic tools are deployed. These tools provide detailed information on not only direction, but also the structure of the wellbore. Structural detail is as important as direction in hydrogeological investigations, where one wants to understand how groundwater moves, and in geotechnical investigations.
What is the difference between optical and acoustic tools?
- Optical tools: Use cameras or light-based systems to capture the borehole’s orientation and sometimes wall features.
- Acoustic televiewer tools: Send sound waves to scan the borehole walls, creating detailed images of fractures, bedding planes, and structural features.
Measurement-While-Drilling (MWD) & Logging-While-Drilling (LWD) tools
Primarily used in oil and gas drilling, these tools are added directly to the drill string.
- MWD tools: Continuously measure inclination, azimuth, and toolface orientation while drilling, transmitting data to the surface in real time (via mud pulse or electromagnetic signals).
- LWD tools: Combine MWD with logging sensors that measure formation properties, helping guide the drill toward productive zones.
Timeline of surveying tools
Early 1900s
The mechanical single-shot survey tool was developed, using a compass and photographic plate to record inclination (dip) and azimuth (direction) at a single depth.
1930s
The multi-shot camera system was developed. It could capture several directional readings at different depths in a single run, without the need to pull the tool up after every measurement.
1940s – 1950s
The decade saw the introduction of magnetometers and inclinometers, which allowed for more precise and direct measurements of inclination and direction compared to earlier mechanical and photographic methods.
1960s – 1970s
This decade saw a giant leap in the surveying field with the development of gyroscopic surveying. Until its introduction, magnetic interference with the steel drilling pipes, as well as inconsistencies due to the local anomalies of the Earth’s magnetic field were common. The gyroscope measures the «true north» by measuring Earth’s rotation. Since it doesn’t seek the magnetic north, there is no need to take magnetic declination into account, nor to worry about magnetic interference.
1980s – 1990s
Measurement-While-Drilling (MWD) and Logging-While-Drilling (LWD) were introduced in the oil industry. They allowed data to be transmitted to the surface in real time, greatly improving the efficiency of drilling programs.
2000s – Present
Though little major stand-alone breakthroughs have been made since the 90s, gyroscopic and magnetic surveying tools have been greatly improved through advances in technology, and the addition of computer modeling have added an extra level of accuracy to borehole control and planning.
Borehole surveying in resource exploration
Mining
In the mining industry, borehole surveying plays a critical role in determining the exact position and orientation of drillholes. By accurately mapping the borehole’s path, it can be ensured that samples taken from different depths and directions are properly located in three-dimensional space. This allows for reliable correlation of core samples with geological models, which is vital for assessing the size, shape, and grade of an orebody.
Read more about: Ore mining
Oil and gas
In the oil and gas sector, precision in borehole surveying is equally important, as hydrocarbons are often located in complex subsurface structures. A reservoir might be just a few meters thick, and reaching it requires drilling along a carefully designed trajectory. Borehole surveys provide the necessary directional control to confirm that the drilling path is aligned with the planned design. This minimizes the risk of missing the reservoir and avoids unnecessary drilling costs.
Borehole surveying also contributes to a more efficient extraction of the resource. Engineers can design optimized drilling patterns that minimize overlapping between holes and maximize coverage of the deposit. This reduces wasted effort and ensures that mineral or hydrocarbon recovery is as complete as possible. Over time, precise surveys also build a dataset that can be used for long-term mine planning or field development in oil and gas.
Borehole surveying in geotechnical investigations
When planning a tunnel, dam, bridge, or high-rise building, engineers must know how the rock and soil layers behave at different depths. Borehole surveys allow them to accurately plot the orientation of drillholes, ensuring that collected data such as rock strength, soil samples, and groundwater levels are spatially accurate. This provides a clear picture of subsurface conditions, which can then be used to design stable and safe structures.
For example, in tunnel construction, borehole surveys help identify zones of weak rock, fault lines, or areas where water might pose a risk. With precise borehole positioning, engineers can model the tunnel alignment more effectively and anticipate challenges before excavation begins. Similarly, for dam foundations, borehole surveys ensure that the core samples taken truly represent the subsurface conditions along the dam axis. Without this accuracy, the risk of undetected weak zones or seepage paths could compromise the structure’s stability.
Surveying also improves efficiency during geotechnical investigations. If boreholes are inaccurately logged, the data may not correlate with nearby investigations, making it difficult to build a coherent geological model. This can lead to costly design errors, delays, or even structural failures if the wrong assumptions are made. Borehole surveys ensure that every data point is tied to a known location in space, allowing engineers to confidently interpret and integrate findings from multiple boreholes into one reliable model.
Benefit from our advanced borehole steering technology
Highly reliable estimations and calculations on azimuth, inclination, and ore properties are essential when drilling. That is why you should invest in equally reliable drilling solutions, such as the service, rental, and software products for directional drilling we offer. These are centered around our advanced borehole steering technology, which reduces environmental footprint through fewer drill pads, less operational time, and reduced CO₂ emissions. Our borehole steering is precise, does not require special rods or rigs, and has no depth limitations.
