Quantum Technology gravity gradiometer sensors will ultimately be applied in a range of different application areas. For the Gravity Pioneer project, the focus is on building on existing techniques and develop new ways of identifying the unidentifiable.
Existing Technologies and Applications
Building on Existing Technologies
Ground Penetrating Radar (GPR) utilises electromagnetic waves of frequencies between 50MHz and 2.6GHz that are transmitted into the ground or structure that can resolve features from a couple of centimetres up to ~10m below ground level (bgl). The transmitted energy is reflected back to the surface when it encounters the boundary interface between two materials that have contrasting dielectric properties.
The data can be collected as a series of 1D soundings, 2D traverses, or in more complex situations as a 3D volume. The data acquired from GPR can be observed in real-time, or post-processed in more complex environments, displaying a 2D profile of the near-surface, which can help understand features in the near-surface and can aid in making an engineering judgement based on the experience of the user.
However, the inherent limitations that GPR has, such as not being able to penetrate through saturated ground (due to the high dielectric properties of water), being influenced by external electronic interference, or not being able to identifying smaller features at depth, require us to look at other ways of resolving features in these circumstances.
Microgravity is the term that has been adopted for high-resolution gravity mapping. The technique relies on using the extremely small localised variations in the Earth’s gravitation field to detect density variations in the near-surface. The presence of an anomalous feature with a density contrast in the subsurface causes a relative gravitational high or low. These effects can be extremely small, with the limitations of current traditional microgravity equipment (pictured right) have a reading resolution of 0.1μGals. However, the actual practical limitations of this equipment limit the detection of features with signal sizes of 5-7μGals. This technique is typically used to identify karstic and/or solution features, fault zones, mine-workings, basements and tunnels. As this technique measures the potential field, it does not have the inherent problems that are associated with GPR, which lends itself to being able to identify features at a greater depth and through materials that are problematic for other techniques .
However, there are still inherent problems with traditional microgravity data collection, such as instrumental noise, environmental noise (i.e. microseismic noise from human activity, planetary and lunar signals, as well as atmospheric pressure), and location-based noise caused by buildings, latitude, height of the equipment and terrain. Which leads us onto Quantum Technology microgravity sensing.
Gravity Pioneer, funded by Innovate UK, brings together industry and academic experts that are aiming to develop the first commercial Quantum Technology Gravity Gradiometer sensor to reveal what is currently undetectable through existing technologies. To achieve this, both the academic and industrial experts will build upon what has been produced in previous Innovate UK, EPSRC and dstl funded projects by undertaking regular field evaluations to drive the sensor development to identify any possible problems before they arise, which will reduce the risks and ensures the sensor is fit for purpose.
An atom interferometer operates by dropping or throwing up a cloud of cooled atoms, which act as ideal test masses in freefall. To measure gravity, three precisely timed pulses of light are shone onto the atoms. The first light pulse is tailored to give half of the atoms an extra momentum kick, splitting it in two and places the atoms into a quantum superposition of two momentum states. After a time T has passed, a second light pulse is used to invert the momentum difference of the two clouds, causing them to begin to move towards each other once again. Finally, after further time T, a third light pulse is used to close the interferometer. This sequence is analogous to that of an optical Mach–Zehnder interferometer, in which the roles of matter and light have been reversed. The first and last light pulses act as beam splitters, while the intermediate serves as a mirror. The interference pattern is measured by detecting the population of two atomic states instead of optical intensity. During the interferometry sequence, the atoms accumulate a phase difference due to gravity and hence gravity can be measured.
The gravity gradiometer sensor developed by the Gravity Pioneer project comprises two atom interferometers on top of each other interrogating them with the same laser beam. The atom interferometers are separated by a distance referred to as the baseline. It is the gravity gradient over this baseline that is measured by the gradiometer and can be used to learn about the subterranean world. Gravity Gradiometery with cold atoms in which both atom clouds are interrogated with the same beam, allowing for noise common to both atom interferometers can be suppressed, such as vibration. This is of great advantage when operating in real world environments such as those civil engineers operate in and will allow for faster measurement times to be achieved in the field.
With the ever-increasing deterioration of buried assets and increasing pressures to use brownfield sites, it is ever more important to fully characterise our subsurface through site investigations. Traditionally, geophysical, intrusive and, in some cases, destructive methods are used to determine the location and condition of our buried (legacy) infrastructure, which may not identify critical components in a timely manner. By using QT gravity gradiometry, we have the option to detect smaller or deeper assets, thereby increasing the chances of locating a problem before it becomes a problem.
Further applications include the locating of, and identifying the condition of, water leakage and animal burrows. By preemptively locating buried targets and determining their condition, future excavations become safer, more efficient to plan and, ultimately, increase productivity.
Locating buried and unknown Unexploded Ordnance (UXO) can present significant challenges and be a real threat to workers and the public. Most common methods used to locate UXOs require the user to walk over the area, detecting magnetic anomalies. However, in areas with heterogeneous ground, or a congested location, locating them may be difficult. QT gravity gradiometry has the potential to filter out the heterogeneous and congested conditions, to hone in on the target of interest.
Other hazards, for example, include sinkholes, solution features, soft ground or unidentified buried storage tanks. Once again, it is vitally important that these hazards can be identified before they become a significant problem.
With natural resources, such as gas, oil and minerals becoming ever-more difficult to locate and extract, the use of Quantum Technology gravity sensing has the potential to locate and identify resources that are currently undetectable by more traditional methods.
The technology has the potential to significantly decrease the price of prospecting and find the resources in ever-more heterogeneous areas.