October 2024
In this insight we discuss all there is to know about Airborne Survey Design and why it is important for achieving reliable results.
The purpose of fixed-wing airborne geophysical surveys is to collect high-resolution data over extensive survey areas for geological mapping and exploration. Good data resolution is achieved by flying at low altitudes along parallel lines with flight line spacing ranging from 100 to 1000 meters and at a typical flight speed of around 200km/h. Flight line spacing and flying height can be adjusted based on project requirements, desired data resolution and on-board geophysical instrumentation.
Fixed-wing airborne survey design involves several critical considerations to ensure accurate data collection and safe and efficient survey operations. Some key aspects of airborne survey design include:
Survey Area:
Clear definition of the geographical extent of the survey which considers the project objectives, geological context, and objectives. Country borders cannot be flown over without appropriate authorisation and permits, and this will usually mean flying right up to a territorial border is impossible. Certain areas will have flight restrictions often meaning no-fly zones such as over densely populated urban areas, airports, military complexes, and other critical infrastructure such as power stations.
Flight Line Spacing:
Airborne surveys are generally flown in a grid pattern of primary lines and tie lines which are usually flown at 90 deg to the main lines. The optimal spacing between adjacent flight lines is determined by pre-survey geological feasibility studies and operational planning, as well as the type of instruments deployed. As a general rule, when it comes to airborne survey design, tighter line spacing provides higher resolution but requires more flight time and hence costs more.
Flight Height:
The primary determinant of survey flight height is always safety first and foremost. If the terrain is generally low lying with good flight conditions, an optimal altitude of 80-120m may be achieved (considering sensor specifications, and desired resolution etc.). In areas of higher topography, often associated with poorer visibility and more challenging flight conditions, a higher average clearance will be required.
Aircraft Performance Parameters:
Each survey aircraft will have different flight performance characteristics based on weight, engine power etc. This makes the available aircraft performance parameters an important aspect to consider when designing an airborne survey. Fixed wing aircraft can handle stronger winds and turbulence than helicopters and generally have better endurance.
Metatek uses twin-engine turboprop aircraft for geophysical surveying. These have redundant systems which provide an additional layer of safety in case of engine failure. Having two engines means that even if one engine experiences an issue, the other can continue to operate, allowing the aircraft to maintain safe flight. This is sometimes a regulatory requirement when flying surveys over the sea.
Rate of climb is a key parameter and safety consideration, particularly when surveying over mountainous areas and is affected by a number of factors, including:
1. Engine power, weight and aerodynamic design – Lighter aircraft with powerful engines will have a higher rate of climb.
2. Altitude – as this increases, the air becomes thinner, which reduces engine performance and lift, leading to a slower rate of climb.
3. Temperature – Higher temperature can also reduce air density, which lowers engine performance and the rate of climb.
4. The best angle of climb (Vx) – is achieved where maximum excess thrust is available, while the best rate of climb (Vy) occurs where the maximum power is available. Vx is used to gain the maximum altitude over a short horizontal distance, whereas Vy is used to gain the most altitude over time.
Fight Drape:
When airborne geophysical surveys are flown, the flying style will vary according to a number of factors. There are three main flight styles which are used in aerial surveying which are described below from the simplest (fixed) to the most complex (2D Drape):
1. Fixed Altitude Surveying:
In this flying style all flight lines are flown at the same altitude, e.g. 120 m above the highest point and it is the simplest way to survey. An advantage of this method is that it can allow the most favourable flying conditions and therefore the lowest instrument noise. A significant disadvantage, especially where there is topography in the survey area is that these surveys tend to have a large ground clearance which means the geological signal is weaker, resulting in lower survey resolution.
2. 3D Draped Surveying:
The 3D draped flight design produces a smooth surface on which all survey lines and tie-lines are placed. All data acquisition therefore occurs on a common surface. It can be thought of as the surface that would result if a tensioned bed sheet was lowered over the topography. This “tension” is set according to the aircraft’s climb and descent rates so that from any point on the surface, the aircraft can follow the surface in any direction. The 3D draped survey gives a well-connected data set which is easy to process since there are no abrupt changes in altitude from line to line. When lines and tie-lines intersect, they do so at a common altitude, and this allows conventional levelling techniques such as mis-tie analysis to be used.
The disadvantage of a 3D draped surface is that it can result in high ground clearances when the draped surface is dominated by a topographic high. Even a small area of high topography can pull up the flying height over the entire survey area and therefore result in unnecessarily high ground clearances for the majority of the acquisition.
3. 2D Draped Surveying:
This flying pattern enables the minimum possible ground clearance, particularly over areas with elevated topography, therefore allowing maximum signal amplitude and resolution. A 2D drape survey is ‘forward looking’ in that it only considers the topography along the line of flight and therefore is not influenced by surrounding topography. This means that the lines do not fall onto a common draped surface, but to minimise the flying height, are disconnected with lines crossing at different altitudes. This has consequences on the methods that can be used to process the data, and in particular, the data cannot be levelled using conventional techniques. Leveling therefore must be accomplished using more advanced equivalent source joint processing which is able to cope with data not collected on a single draped surface. A major advantage of a 2D drape is that in certain topographic environments, it can allow acquisition to occur significantly closer to the ground.
The figure below shows a schematic of 2D drape survey line and corresponding tie lines. The red dots are the survey (tie) lines perpendicular to the black survey line, the blue is the terrain. Note that the red dots are very close to the ground as they approach the two peaks – because they are following strike and a 2D drape allows flying low even when there is high topography to the side. When on the peaks they are much higher than the black line because the topography is higher out of the plane of the diagram. The important factor to note is that with a 3D drape, all survey/tie lines would follow the highest of the black line and the red dots – i.e. would be a connected surface on which to fly but would be much further from the ground than the 2D drape.
Metatek has successfully used 2D drapes to significantly reduce terrain clearance in many mountainous areas worldwide such as the Rift Valley Systems in East Africa and mountainous regions in Turkey and Asia.
Below is an example comparing 2 and 3D drape surveying methods illustrating the impact on flight profile and line intersections between 2 and 3D drape techniques.
Other Fight Lines:
Metatek has developed several methods of data acquisition and processing which can reduce average terrain clearance by careful selection of line orientation and deployment of strategic lines in non-grid format orientation. For more information – please contact us.
Weather:
The right weather window is an essential element to consider for successful fixed-wing airborne survey design. The weather contributes, not only to the survey results but also safety of the flight crew, this makes it the primary consideration in determining when to fly. Visibility is aways a primary factor in determining surveying and turbulence is also a critical factor. The advanced design of Metatek gravity gradiometers means they are extremely tolerant of turbulent flying conditions and can remain within survey parameters when older generation AGG systems stray out of specification and data is rejected.
These airborne survey design parameters are all interrelated and pilots, engineers, geoscientists and HSE managers must consider all these factors when planning survey flights to ensure safety and efficiency.
If you are conducting a geophysical survey and need a partner to work with, contact the team at Metatek today.
