Full-tensor gravity gradiometry (FTG) is not necessarily a new technique—it was developed by Bell Aerospace Textron (later acquired by Lockheed Martin) in the 1970s and deployed by the U.S. Navy to aid in covert navigation. In 1990 the technology was declassified as an exploration tool, driving its commercialization into the energy industry.

Various notable improvements to reduce noisy gravity data have been made to FTG technology over the years, making it a viable technology for natural resource exploration. However, noise in the measurements still limits what can be inferred about the geology being explored.

A new player has entered the multiphysics acquisition field. NEOS, which was previously a multiphysics processing and interpretation company, announced its plan to acquire the Multi-Physics group of CGG in April; the deal is expected to close in the near future. NEOS recently announced a new generation of FTG sensor that it’s developing with Lockheed Martin called FTG Plus. It promises to be 20 times more powerful than current technology. The technology, which reduces sensor noise to the point where it is no longer a limitation on the use of gravity data, can be deployed on a vessel or aircraft.

E&P talked to Mark Dransfield of NEOS, the chief scientist for the program.

E&P: Can you explain how current FTG methods work?

Dransfield: ‘FTG’ is a name for a type of gravity gradiometer. Gravity gradiometers provide very sensitive measurements of the changes in the Earth’s gravity field while traveling in an aircraft. Accelerometers are used to measure the gravitational acceleration, but achieving sufficient accuracy to be useful for exploration requires overcoming several challenges.

The first challenge is that the accelerometers cannot distinguish the accelerations due to motion from those due to gravity. This is overcome by measuring the difference in signal between two accelerometers joined rigidly together but spatially separated. Because they are rigidly connected, the two accelerometers experience the same acceleration due to aircraft motion, and this cancels out when the difference is taken. Since they are spatially separated, one accelerometer will generally be closer to the gravitational source than the other and will have a larger gravitational measurement. The result is a residual gravitational signal when the difference is taken, and this is the gravity gradient. The greater the spatial separation, the greater the sensitivity to gravity.

The second challenge is that rotations also cause accelerometer signals not distinguishable from gravity. This is tackled in two ways. Firstly, the use of a second pair of accelerometers oriented orthogonally to the first pair provides for cancellation of centrifugal accelerations. Secondly, mounting the set of accelerometers on a rotationally stabilized platform allows the accelerometers to be kept pointing in a constant direction while the aircraft rolls and pitches as it travels. This second approach also is used to keep cameras pointing in a constant direction so that movies can be shot from moving vehicles. Current gravity gradiometers operate mounted in a nested set of three rotationally stabilized gimbals.

E&P: What tweaks are necessary to make FTG Plus 20 times more powerful?

Dransfield: Making a gravity gradiometer with 20 times more sensitivity requires first increasing the separation (called the “baseline”) between accelerometers. This must be done without increasing the size or weight of the gravity gradiometer so it remains light enough to be carried on small aircraft, including helicopters. Current gravity gradiometers have the accelerometers mounted on a rotating wheel inside three nested gimbals, and we need to have the rotational control moved to the center with the accelerometers on the outside. This reversal of the relationship provides a large increase in baseline and hence sensitivity.

The current nested-gimbal technology is a limiting factor on improving sensitivity, and the FTG Plus replaces the rotational control geometry with a single-point air bearing. The air-bearing technology is the key to allowing us to perform rotational control inside the accelerometers. Air bearings are essentially frictionless and will provide better rotational control than gimbals. This better rotational control means less error due to the pitch and roll of the aircraft in flight and so allows additional sensitivity for the FTG Plus.

E&P: What are NEOS and Lockheed Martin bringing to the development?

Dransfield: NEOS is bringing several things. Based on our knowledge of industry needs and our decades of experience flying airborne geophysical surveys, we set the specification goals for the technology. We are responsible for the interface designs that determine how the instrument integrates with the various other systems onboard the aircraft, and we are responsible for the necessary improvements in survey design and logistics to maximize the value of the FTG Plus. We also will lead the development of the data processing software and will spearhead turning the processed data into information (for example, interpretations and 3-D earth models) that is of direct value in exploration.

Lockheed Martin brings decades of experience in building gravity gradiometers. It is the only company to build a working commercial airborne gravity gradiometer, and it has incrementally improved many times on the first working gradiometers to build better systems. For the FTG Plus technology, Lockheed Martin’s engineers are driving a major step change in gravity gradiometer technology. Lockheed Martin is designing and building the entire gravity gradient sensor.

We will be working closely together on the entire project. FTG Plus is the first time Lockheed Martin has specifically built a sensor for our precise use and needs. That is a fundamental change; it is an entirely new design for us, and we will have exclusive rights to use it.

E&P: What benefits do you expect the oil and gas industry to realize from this new technology? Are there places where it’s unlikely to work?

Dransfield: The FTG Plus sensor will provide more geological information at greater accuracy in every application from near-surface to deep basement. The greater sensitivity will allow the detection of small and subtle fault structures at greater depths than is possible today. This will improve the mapping of small deep faults, salt features and intrusives such as volcanics. At intermediate depths, sedimentary geological features below basalt layers and subsalt will be better resolved, providing more certain identification of their size, shape and density. The better spatial resolution will allow mapping of very shallow features such as karsting, shallow compaction, velocity anomalies, geohazards and so forth. There will be value in structural mapping, seismic survey planning, shallow reservoir estimation, joint seismic-gravity inversions, velocity models, salt modeling and many other aspects of oil and gas exploration. With this technology gravity gradiometry will become useful in deeper basins and will supplant detailed ground gravimetry for mapping shallow features.

E&P: On a cost-per-kilometer basis, how will this compare to 2-D and 3-D seismic, current FTG technologies, electromagnetic technology, etc.?

Dransfield: Airborne geophysical technologies are dramatically more cost-effective than seismic, typically by about two orders of magnitude or more. This also will be true for FTG Plus. Of course, actual prices vary depending on survey design, operational logistics, location and the type of aircraft. Furthermore, acquisition from the air is more environmentally friendly and can be completed faster than traditional seismic.

We look forward to commercializing FTG Plus in the market and providing greater value to our customers than possible with other gravity gradiometer technologies.

Contact the author, Rhonda Duey, at rduey@hartenergy.com for more information.

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