This module addresses what is an airborne gravity system. The module starts with a reference to gravity variation and acceleration. It takes you through the different types of gravity systems. Simply, there are two types of highly sensitive accelerometers used to measure the earth’s gravity field. The instruments and the calibration required are discussed. The module concludes with applications when airborne gravity surveys are appropriate, and what are the factors required to design an effective survey for the target sought.
After completing this module, participants will come to understand the following topics:
- What is Gravity Variation?
- Units Of Acceleration,
- Different Types Of Gravity Systems Namely Absolute and Relative,
- Gravimeter Calibration,
- Applications for Airborne Gravity Surveys,
- Survey Design, and
- Types Of Flight Surfaces.
Slide 1: Hello and welcome to the VIDL Network. My name is Sol Meyer for the VIDL Network. This is module GS 101 titled: Airborne Gravity Systems and Survey Design.
Slide 2: Today we will be discussing the following topics: What is gravity variation? The units of acceleration, different types of gravity systems namely absolute and relative, gravimeter calibration, applications for airborne gravity surveys, survey design, and types of flight surfaces.
Slide 3: So, what are some of the things that can cause variations in the measured gravity? Gravity is a function of mass, therefore changes in rock density will lead to changes in the measured gravity. In the top image, the central block represents a higher density mass compared to the surrounding area. The black trace shows the recorded gravity trace, with a positive anomaly over the higher density mass. Structural variations, either in the form of topography or occurring below ground, also cause variations in the measured gravity. It is assumed that the gravity data in the bottom images were recorded under the same conditions and at the exact same elevation as in the first image. This shows us that a structural high will have a higher measured gravity compared to a structural low if the density is constant, and this also indicates what effect the terrain can have on the measured gravity.
Slide 4: The main units of acceleration being used to measure gravity are the milligal and micrometers per second squared. We will be using the mGal in this module. This is a very small portion of the m/s2 that we are normally used to seeing. 1 mGal is equal to one millionth of the Earth’s gravity. What this tells us is that the changes we are trying to measure are very very small.
Slide 5: To start, what is a gravity system? To put it simply, it is a type of highly sensitive accelerometer and there are two main types: absolute, and relative. An absolute gravimeter works by measuring the acceleration of a mass during free fall in a vacuum and it needs no other point of reference. A relative gravimeter, as the name suggests, must be calibrated in a location with known gravity. It is what we use for gravity surveys and they are usually spring based or accelerometer based like SGL’s AIRGrav system. There are also superconducting gravimeters, which use a magnetic field to suspend a superconducting niobium sphere. The force required to suspend the sphere is proportional to the Earth’s gravity. Superconducting gravimeters are the most sensitive instruments however they cannot be used on inertial platforms.
Slide 6: Absolute gravimeters are highly sensitive and can achieve gravity readings with high accuracies as low as 0.001 mGal. Absolute gravimeters typically function by measuring the free fall of an optical mass over a vertical distance in a vacuum. Absolute gravimeters are typically used to create gravity reference points, like those in international absolute gravity value databases (IGSN, etc). Because they are so highly sensitive they cannot be used on inertial platforms and therefore are not used for airborne gravity surveys.
Slide 7: Here is an example of the FG5-x absolute gravimeter by LaCoste & Romberg-Scintrex (LRS), Inc. The upper section encloses the dropping chamber, the middle section is the interferometer for measuring the position of the falling mass, and the lower section contains the springs which isolate the meter from Earth vibrations. This particular absolute gravimeter is capable of achieving accuracies of 2 µGal.
Slide 8: Relative gravimeters are typically less sensitive than absolute gravimeters and can achieve accuracies as low as 0.1 mGal along a repeat test line. They need to be calibrated to a known gravity point. There are two main types of relative gravimeters: spring based, which measures the change of strain of a spring which accompanies a change in vertical gravity, or accelerometer based, which uses accelerometers to measure the vertical accelerations due to changes in gravity. There are many different industry-used gravity systems out there which are either spring based or accelerometer based. These relative gravimeters can be used on inertial platforms, therefore they are mounted in aircraft and on ships in order to perform gravity surveys. From this point forward we will be primarily referring to relative gravimeters.
Slide 9: Here is an example of a spring based relative gravimeter. This is the TAGS-7 system made by LaCoste-Romberg Scintrex which is based off of the original LaCoste airborne gravity system. It contains a zero-length spring gravity sensor and is mounted on a gyro-stabilized gimbal platform to isolate it from the movements of the aircraft during flight. It records at 20 Hz and has a repeatability of 0.75 mGal along a repeat test line.
Slide 10: Here is another example of a spring based relative gravimeter. This is the Russian CHEKAN-AM gravity. It contains a double quartz elastic spring system and is mounted on a two-axis gyro-stabilized platform to isolate it from the movements of the aircraft during flight. It records gravity at 25 Hz and has a repeatability of less than 1 mGal along a repeat test line.
Slide 11: Here is one more example of a spring based relative gravimeter. This is the German Bodenseewerke Gravimeter model KSS32. It contains a vertical tension spring system and is also mounted on a two-axis gyro-stabilized platform to isolate it from the movements of the aircraft during flight. It records gravity at 1 Hz and has a repeatability of about 0.5 mGal along a repeat test line.
Slide 12: An example of an accelerometer-based gravimeter is the AIRGrav (Airborne Inertially Referenced Gravimeter) designed and built by Sander Geophysics Ltd. The gravimeter was built specifically for airborne surveying and can be installed in almost any aircraft, including a helicopter as seen in the right image. The gravimeter itself is contained inside a large case in a fire-resistant cover.
Slide 13: The AIRGrav system consists of a three-axis gyro stabilized platform and three accelerometers. The platform keeps the vertical accelerometer oriented such that it measures changes in the vertical gravity field and minimizes effects from aircraft motion. Highly accurate GPS accelerations are recorded and subtracted from the measured AIRGrav accelerations to compute the gravity. Raw gravity is sampled at 128 Hz and it has a repeatability of about 0.1 mGal along a repeat test line.
Slide 14: Another type of gravimeter is the Strapdown Airborne gravimeter. These gravimeters are similar to the accelerometer based relative gravimeters however the accelerometers are mounted to the aircraft in order to measure all experienced forces and the gyroscopes measure the aircraft motions rather than eliminating the motions of the aircraft as they do with the AIRGrav system and other systems mounted on gyro stabilized platforms. These strapdown gravimeters are simple systems that can be quite small and are used in many applications that expand beyond geophysics. This particular unit, the iCORUS made by iMAR is made for airborne gravity surveying however their accuracies are significantly less than most other relative gravity systems. This system is sometimes combined with gradiometers to measure some of the long wavelength gravity field that gradiometers are incapable of measuring.
Slide 15: As was previously mentioned, relative gravimeters must be calibrated to a known gravity value. There are large databases of values which exist around the world and often these stations are located at airports. This allows for easy calibration by simply parking the aircraft with gravimeter on board next to the known gravity point. The value likely will need to be adjusted for the height of the gravimeter above the ground (inside the belly of the aircraft). If there is no available known gravity point, then an absolute gravimeter can be used to calculate the local g. Absolute gravimeters are sensitive instruments and costly however, so this is not usually the preferred method. A portable relative gravimeter can also be used to transfer the local gravity value from a known point which is not located at an airport, to the parking location of the aircraft at the airport. In a similar fashion, the airborne gravimeter itself can be used to calculate the gravity value at the airport. The airborne gravimeter would need to be calibrated to a known gravity value at a different airport or location and then, with the system continually running, be flown to the airport where the survey will be conducted from and measure the average gravity value in a parked position with little disturbance over a long period of time. Often the survey data itself can be shifted to match existing gravity data in the survey area, which makes a precise gravity tie at the airport less important.
Slide 16: On the left is an example of a portable relative gravimeter which can be used to transfer the gravity value from a known gravity point to a point at the airport where the aircraft can be parked and can perform a gravimeter calibration. The airborne gravimeter uses the measured value of the portable gravimeter as the calibration value. On the right is an example of an IGSN station log. It contains information about where the gravity point is located, directions on how to find it, how it was measured, and the measured gravity value.
Slide 17: Airborne gravity surveys have many applications which include but are not limited to: Exploration (and in particular oil, gas and mineral exploration), Research (environmental surveys interested in mapping the geology or researching climate change in the polar regions, etc.), and Geodesy (using long wavelength gravity data to improve the geoid model which is widely used in many different applications).
Slide 18: Airborne gravity surveys can either be regional or detailed in size and resolution, and have many advantages over ground methods. For example, if there is limited time available for data acquisition, then airborne surveys are typically a faster method than ground surveys. If the region of interest is in an environmentally sensitive area, if there is restricted access to land, difficult terrain, if it’s in a transition zone, deep water, or if the targets are deeply buried, then airborne surveys may be the only viable option.
Slide 19: We have discussed many different types of gravimeters and therefore the question may arise “which gravity system should I choose?”. Different systems may be better for different purposes, and some of the factors to consider include: the target anomaly amplitude and spatial extent (which system has both the necessary resolution and accuracy to measure it with certainty?), the cost (typically the more accurate the system, the higher the cost), do you wish to measure other properties such as the magnetic field and can other systems be installed at the same time as the gravimeter of choice?, what are the weather conditions and other environmental factors of the survey location? Which gravimeter can handle those conditions and can be installed and has proved to be successful in the chosen aircraft? These are all important questions, and should be considered fully before choosing a gravity system.
Slide 20: Once an aircraft and gravimeter have been chosen, how does a survey begin? For commercial surveys, the region of interest is usually provided and possibly some other details such as maximum number of total line kilometers, line orientation, line spacing, etc. Line kilometers are the total number of kilometers along the pre-planned lines. It does not include any kilometers flown during the ferry flight to the survey area or during the turns. Typically, surveys are billed by number of line kilometers proposed or flown. If the line spacing or line orientation is not specified then there are some important factors to consider when choosing these variables.
Slide 21: Airborne surveys are typically flown using a series of parallel lines with a second set of orthogonal lines often referred to as “tie” lines. The orthogonal tie lines are usually spaced at 5 to 10 times the line spacing and the primary purpose of them is to level the data using the intersections. The line spacing for the “traverse” lines which are the primary set of flight lines is dependent on the desired resolution. Closer line spacing leads to higher resolution and more accurate data, and is therefore a critical parameter in determining the eventual size of the anomalies that can be resolved for a particular survey.
Slide 22: A few factors should be considered when selecting the primary line orientation. These factors include the geology: if there are prominent geological features which have a regular strike-direction throughout the survey area, the survey lines should be oriented perpendicular to the strike direction. Terrain is another factor. If the survey area contains steep terrain edges, it’s best to fly parallel to them. This allows the aircraft to maintain a low altitude right up to the edge of the terrain, rather than needing to climb further from the edge and increasing ground clearances. It is sometimes beneficial to fly parallel to the edges of the survey area which usually leads to longer consistent lines. International borders also sometimes need to be considered. If the survey area is near the edge of an international border, the flight permits may not allow the aircraft to cross the border. If lines are parallel to the border the aircraft can fly closer to it without worrying about the space it needs to turn around.
Slide 23: This illustration shows why flying parallel to steep terrain allows for lower ground clearances than flying perpendicularly to it.
Slide 24: Once the lines have been planned, the desired flight surface has to be determined. There are different types of flight surfaces which can be flown: constant altitude with respect to mean sea level or the ellipsoid. Certain gravimeters which cannot withstand horizontal or vertical accelerations due to aircraft motion are required to fly along a constant altitude flight path. This ensures smooth flying, however it leads to higher ground clearances and therefore lower resolution data. Some surveys are flown using a constant height above ground. This is usually achieved simply by the pilot checking their altitude gauge and trying to stay at the desired height above ground. The desired height above ground is dependent on the aircraft’s capabilities, local aviation regulations, safety, and the desired gravity data resolution. The final type of flight surface is called a drape surface. This is a digitally created surface which the pilots or autopilot use as vertical guidance when flying. The surface in general follows the topography however it is smoother and therefore reduces the vertical and horizontal accelerations due to aircraft motion that are experienced by the gravimeter. It also ensures a consistent altitude at the intersections between the primarily survey lines and the tie lines which results in better leveling and less error in the final data set.
Slide 25: This diagram illustrates the difference between constant altitude (above the mean sea level or ellipsoid surface), constant height above ground (which is just a raised version of the topography and includes the sharp rises and falls), and the draped surface which is a smoothed version of the topography raised up to the desired flying height. It is evident that a constant altitude results in larger ground clearances and therefore lower resolution data. Flying a constant height above ground can result in abrupt aircraft movements which can result in increased noise levels in the gravity data. Draped surfaces are usually the preferred method.
Slide 26: Besides the smoother flying that the drape surface provides or the consistent altitude at line intersections, drape surfaces provide lower ground clearances than surveys with a constant altitude above the mean sea level or ellipsoid surface. The images illustrate this effect on gravity data, clearly showing that the data acquired on a drape surface is higher resolution than on a constant altitude surface.
Slide 27: So how do you calculate a drape surface? You begin by sourcing a high quality digital elevation model of the topography. For most areas in the world, Shuttle radar topography mission data can be used. Be aware that SRTM data sometimes has small gaps in the data which need to be filled and to use caution when interpolating those areas. The digital elevation model is then filtered to create a smoother surface which will be easier for the aircraft to fly. The amount of filtering depends on the aircraft specific climb rates and maximum climb gradients as well as how much movement is too much for the gravimeter itself.
Slide 28: The images on the left illustrate the digital elevation model for a specific area. On the right are two different views on the corresponding drape surface. You’ll notice it is a much smoother version of the terrain.
Slide 29: In summary, relative gravimeters (not absolute gravimeters) are used for airborne surveys. There are two main types: spring based or accelerometer based. Researching the different systems and understanding how each one differs is important before choosing one. Each system obtains different levels of accuracy and some systems have limitations on the way they are flown or the weather conditions that they can withstand. All relative gravimeters need to be calibrated to a known absolute value of which there are many listed in the numerous absolute gravity value databases (for example, IGSN). Airborne gravity surveys have many applications which include exploration, research, and geodesy. Airborne surveys are often faster and less impactful then ground surveys.
Slide 30: The size of a survey is typically expressed in total number of line kilometers which refers to the number of kilometers along the planned flight lines, not including turns or the ferry to the block. The survey is typically planned in a series of parallel flight lines with orthogonal tie lines at a spacing 5 to 10 times that of the primary flight lines. There are different options when it comes to the flight surface that can be followed during surveying, and the chosen type depends on the type of gravimeter being used and the type of aircraft. The preferred method for a standard gravity survey is to fly a drape surface which combines smooth flying with optimal flying heights. This summarizes the initial steps for planning a survey from understanding the different types of gravity systems, airborne gravity applications, as well as understanding the factors which need to be considered when designing a survey and how a survey is typically designed.
Slide 31: Thank you for joining me for this module. VIDL Network is available for questions at any time.