What is a Directional Survey?
A directional survey is a survey method that can be used to map a variety of underground drilling applications. Directional surveys are used to map a planned underground route by using aiding data provided from any combination of directional survey stations.  Directional survey stations serve as a reference point that corresponds to a measured depth along the actual bored path known as the actual well path. While the drill bores a hole along a planned route, data regarding the borehole inclination and azimuth is transmitted back to a computer that is controlled by an operator. This data allows the operator to control the speed and direction of the drill as it advances. Directional surveying enables the user to carry out operation with minimal disturbance to natural habitats and helps to reduce accidents that can occur from directional drilling, which is any drilling is not directly downwards.
Measurement While Drilling (MWD)
Proper data-logging instrumentation has been forced to be implemented into operations as drilling has become more complex and as directional drilling has increased in popularity. MWD is a type of logging that incorporates measurement tools that provide data in real time. This data serves as the building blocks for a successful directional survey.  MWD is designed to overcome the challenges of logging while drilling at extreme angles. It uses gyroscopes, magnetometers and accelerometers to determine borehole inclination and azimuth angles. Data from these devices is decoded at the surface, and the data can be transmitted to an offsite location with minimal delay. With such precise drilling, MWD aids drilling engineers with real-time information so that they can make important decisions while drilling. As a result, MWD systems are trusted for there ability to provide accurate directional survey data, such as borehole attitude and trajectory, in many types of drilling environments.
Extract, Transform, Load (ETL)
An ETL process can be used for a variety of projects, but in the context of a directional survey, it is typically applied to the mapping phase of the procedure. The mapping process begins with defining the start location. From there, a magnetic or grid correction is applied to azimuth values in the survey, so they are normalized to true north. Once the values are normalized, calculation methods are used to determine the change in position between a point and the location of the well. 
At the end of any successful directional survey, the survey measured depth (1) along the well path, borehole inclination, and borehole azimuth is known regardless of the survey method. Todays software platforms offer many suites to help end-users with these calculations. The most popular calculation methods include the tangential survey method, the average angle method, the radius of curvature method, and the minimum curvature method. 
(1) The summation of course lengths, used as a distance reference between survey points
Tangential Survey Method
The tangential survey method is the oldest and least sophisticated directional survey calculation method. Presently, many newer methods have proven to be more accurate than the tangential survey method. This method calculates a straight line representing the wellbore (2) passing through the lower end of the course length (3) by using the inclination and hole direction. The inaccuracy of this method lies within the assumption that the wellbore is a straight line throughout the course length. 
(2) A type of borehole that is used to extract oil or gas. (3) The actual distance drilled by the well bore from one point to the next as measured.
ΔV = Change in the projection of the measured depth along the well onto the Z axis; β = Azimuth in degrees; ΔE = Change in East; ΔN = Change in North; α = Inclination in degrees;
Average Angle Method
The average angle method is commonly used in the field and can be done relatively easily via a handheld calculator. This method focuses on the upper and lower ends of the course length and measures the average inclination and hole-direction angles at these sites. The average of the two datasets is assumed as the inclination and direction for the course length. Simple trigonometric calculations are then applied to determine the well path. 
Radius of Curvature Method
Like the average angle method, the radius of curvature method is typically applied for field use. The radius of curvature method is more widely used than the average angle method. This method generates a circular arc by measuring the upper and lower ends of the course length and assumes that this is the wellbore. The curvature of this arc is determined by the inclinations and azimuths of the upper and lower parts of the course length. Unlike the previously mentioned methods which used straight line approximations, the radius of curvature method's arc approximation is more accurate to the actual well course as it does not assume that the well path is a straight line. 
Az = Azimuth; I = Inclination; ΔMD = Change in measured depth between surveys; ΔTVD = Change in the projection of the measured depth along the well onto the vertical axis; ΔHD = Change in the projection of the measured depth along the well onto the horizontal axis;
Minimum Curvature Method
DL = Absolute Change in direction; Az = Azimuth; I = Inclination; ΔTVD = Change in the projection of the measured depth along the well onto the vertical axis; ΔMD = Change in measured depth; ΔHD = Change in the projection of the measured depth along the well onto the horizontal axis;
The minimum curvature method is the most accurate method but does not lend itself well to users that require on-site hand-calculations. Like the radius of curvature method, this method utilizes the inclination and hole direction at the upper and lower ends of the course length. It compares each point to the next point and references the changes in azimuth, inclination and measured depth to compute displacement between points in the X, Y, and Z directions. It then adds a ratio factor to smoothen the spherical arc. This ratio factor is the key difference between the minimum curvature method and the radius of curvature method. These changes are summed and give the total change in position from the origin and consecutively acquired data point thereafter. These values give the user the position of each point in relation to the well. 
Underground Utility Mapping
Directional surveying can be used to map out plans for underground utility construction such as sewage pipelines, water pipelines, gas pipelines, and electrical cables. These surveys map the planned underground route by considering the soil type and detecting any obstacles. Directional surveying is especially helpful in cross boring prevention, which is a hazard that will be touched on later in this paper.
Trenchless Construction & Renewal
Directional surveying can be used in a variety of trenchless construction methods including horizontal earth boring methods such as horizontal directional drilling (HDD). It can also be used for pipe jacking methods for installing underground pipes, ducts and culverts. In addition to trenchless construction methods, trenchless renewal methods can be used in close-fit pipes, slip lining, localized repair, and sewer manhole renewal. Directional surveys can aid these techniques by identifying structural damage, misalignment, corrosion problems, and capacity problems.
The last and most obvious use of directional survey is well drilling. In both directional and straight holes, the orientation of the wellbore must be known with reasonable accuracy to ensure the correct wellbore path. This is also important if a relief well must be drilled. Directional survey data must be completed with no missing wellbores, be spatially correct, and have integration-ready data to ensure accuracy. 
Cross Bore Prevention
A cross bore is when a utility intersects a second, existing utility when using trenchless technology. This intersection of utilities compromises the integrity of either the utilities themselves, or the underground structure in the vicinity. This can result in very dangerous scenarios, for example if a gas pipeline crosses through a sewer pipe, natural gas could enter the sewer system if a pipeline is hit. The use of directional survey can help erase this possible hazard by making the drill operator aware of the drill's surroundings and the orientation of the drill at a given time.
Directional Well Planning
Directional well drilling is a broad term for describing any wellbore that does not go straight down. Directional drilling is very valuable for an application like drilling wells to collect oil and gas deposits. By drilling at an angle, more of the reservoir gets explored as reservoirs tend to form horizontally. Directional drilling can become dangerous as it can be difficult to know where the drill is or the orientation in which it is drilling.
Without this knowledge, hazards like cross bores or extreme equipment stress can occur. With sensors like the Inertial Labs miniAHRS, users can determine the direction of the drill through directional survey with MWD technology as mentioned previously. This data, as well as seismic and magnetic data can be put into computer simulation programs to create 2D and 3D models of the drill’s environment. This process of modeling is known as directional well planning. 
With directional drilling sweeping over the mining industry, it is imperative that employees can be properly trained on how to implement the different drilling techniques as well as understand the hazards that come with each one of them. As techniques such as horizontal directional drilling continue to rise in notoriety, it becomes more important to understand MWD and directional survey. Many services offer training classes in which students can learn safety in the oil and gas industry, directional drilling, MWD, and advanced surveying. 
When directional drilling and performing directional surveys there are some specific equipment that should be used to ensure efficacy and safety. Specialized drill bits are used to improve performance and reduce the chance of failure. Downhole steerable mud motors get positioned near the drill bit. When the correct depth is reached, the drill string stops rotating and drilling fluid is pumped through the mud motor so that the drill bit can turn due to the force of the liquid. Rotary steering is important as it allows the system to drill and steer at the same time, which gives access to new, previously unreachable formations. Multi-shot cameras are set to take regular pictures on a time-lapse setting and can be used as aiding data. Lastly, it is important to monitor the state of important components like the drill string, if its integrity is not maintained, the string can snap or jam resulting in a need for new costly equipment. 
The Inertial Labs miniAHRS
The Inertial Labs miniAHRS utilizes 3 axes each of precision accelerometers, magnetometers and gyroscopes. It is a turnkey solution for systems utilizing MWD, as it is a high performance strapdown system that determines absolute orientation for the device that it is mounted on. The miniAHRS provides all the fundamental information necessary to create directional surveys.
The accuracy of a device's magnetometer is paramount in applications such as well drilling where GPS is not available. In the case of the miniAHRS, Inertial Labs' mini fluxgate magnetometer serves as the basis for orientation, and more specifically, heading data computed by the miniAHRS. Inertial Labs supports multiple 2D and 3D calibration methods that cater to a variety of different applications within the oil and gas industry. The main difference between the two methods is the pitch and roll angles. Users that require a full range of pitch and roll angles should opt for 3D magnetometer calibration, while those that require a limited pitch and roll range should choose 2D calibration.
Compact, Lightweight Design
A major advantage of the miniAHRS over other attitude and heading reference sensors is it’s small size and weight. With dimensions of 53 x 19 x 13 mm and a weight of 20 grams, the miniAHRS is sure to conform to the size and weight requirements of any project. Multiple interface options and a sleek design allows the miniAHRS to be an easy integration into any multifaceted solution.
The drilling aspect of the Oil and Gas industry can be highly dangerous, as there are a variety of environmental hazards present in directional drilling. The lightweight and compact miniAHRS is a key device for MWD, as users can perform directional surveys to be aware of the drill's surroundings. As a result, surveyors can be confident that they are minimizing environmental harm and are fully aware of the potential hazards that can occur in the immediate area of the drill. With Inertial Labs’ 20 years of experience in the navigation, orientation, and positioning industry, customers can rely on Inertial Labs’ ability to provide an excellent solution along with comprehensive customer service regardless of the application.
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