THE DOCUMENT IN BRIEF
• Accurate measurements of vertical gravity gradients can be used to detect density inhomogeneities beneath the Earth’s surface.
• In an article in NatureWander et al.1 report a practical quantum sensor that uses atom interferometry to measure gravity gradients quickly and with high sensitivity.
• The sensor is able to detect a tunnel with a cross-section of two square meters under a roadway between two multi-storey buildings, located in an urban environment.
NICOLA POLI: A quantum sense for what lies beneath
Astronomical observations offer us in-depth knowledge of what is above us thanks to both electromagnetic and now gravitational2 signals – even those from sources a billion miles away. But, in many ways, we don’t have the same detailed knowledge of what lies beneath our feet, even a few meters below the Earth’s surface. Although several geophysical monitoring techniques exist, most of the time digging is still the best way to learn about small features below the ground. However, quantum sensors are gaining ground as a viable alternative to conventional geophysical sensors.
Atomic gravimeters are quantum sensors that use a technique called atomic interferometry to measure local gravitational acceleration based on how the gravitational field affects a cloud of free-falling atoms. In a typical setup, pulses of light are used to generate, separate, and recombine matter waves (each particle can be described as a matter wave), allowing them to interfere with each other. The interference pattern detected in a gravimeter is then linked to the local gravitational field. Measurements based on this principle can be incredibly accurate, but they are still subject to the effects of noise. Atomic gradiometers overcome this problem to some extent by measuring gradients in such gravitational fields, instead of absolute values.
Since their first demonstration as gravimeters and gradiometers over 20 years ago3, atomic interferometers continued to improve their performance. At the same time, research focused on how to make these instruments compact and reliable enough to be used outdoors for real-world applications.4,5. Stray and his colleagues’ instrument is a notable advance in this line of research.
The team developed an hourglass configuration for their gradiometer, with which they carried out differential measurements on two clouds of ultracold rubidium atoms, separated vertically by one meter. This configuration provides rugged, compact optics that stay properly aligned over a period of months.
The instrument was able to nondestructively detect a large cavity buried beneath the Earth’s surface, measuring only the tiny gravitational signal from the cavity (Fig. 1a). The sensitivity displayed by the device is about 20E (1E is 109 per square second) for a measurement taken over 10 minutes, making it about 30 times less sensitive than the most sensitive interferometer reported6. However, the authors’ sensor is a step forward in making atomic gradiometers practically useful in real-world situations.
With natural long-term stability and very low sensitivity to environmental effects such as ground tilt and vibration, as well as a lack of mechanical parts, atomic gravimeters and gradiometers possess a clear advantage over their conventional counterparts. Stray and his colleagues’ lead shows that they could soon be more portable and user-friendly.
PAŠTEKA ROMAN & PAVOL ZAHOREC: Practical Solutions for Surface Gravity Mapping
Our fascination with gravity dates back to the ancient Greeks, and the measurement of gravitational acceleration was one of the earliest activities of modern science. 18th century geophysicists used pendulums to make such measurements7. But, since then, gravimetric tools have undergone intensive development – from simple spring-loaded devices to current instruments based on quantum technology. In physical geodesy and applied geophysics, gravity measurements are now used to determine the size and shape of the Earth, and to identify inhomogeneities in the density of the Earth’s interior. Such measurements can reveal near-surface objects or aid in the study of the lithosphere, the rocky outer edge of the Earth’s structure.
Gravitational acceleration gradients are more useful than direct measurements in this regard: they are sensitive to shallow density distributions and can detect objects more accurately (Fig. 1b). In terrestrial gravity surveys, vertical gradients of gravity can be approximated using conventional spring-loaded gravimeter measurements taken at different heights. But this procedure is time-consuming, requiring tens of minutes for each data point, and its uncertainty depends on the accuracy of the gravimeter.
Wander et al. estimated that the uncertainty of measurements taken with their instrument is better than that of commercial gravimeters. Perhaps more importantly, they note that 10 data points can be collected in just 15 minutes. From this point of view, the results of the team, as well as those of other research groups8,9could radically change applied gravimetric research — lending weight to the authors’ claim that the work constitutes a kind of “gravimetric mapping.”
In general, gravity values (and especially gradients) reflect the distribution of density inhomogeneities below the Earth’s surface, but they are also influenced by the effects of nearby terrain and buildings.ten. The key factor in determining the magnitude of this effect is the nearby topography, which is underestimated in some geophysical studies and must be taken into account. The gravitational pull to nearby buildings contributes a smaller, but measurable addition to the gravity field (and its gradients), and therefore must be estimated and removed from the data using numerical methods, which are well developed .
Although the gravity gradient method is extremely useful for detecting subterranean objects across density inhomogeneities, its limitations must be recognized. The likelihood of detecting a subterranean structure depends on the size and depth of the structure, as well as the degree to which its density differs from that of the surrounding soil or rocky environment. From our experience in the detection of underground cavities in archaeological prospecting11we can infer the probability of identifying such cavities under common natural conditions when using an instrument with the uncertainty reported by Stray and colleagues.
We estimate that the maximum amplitude of the vertical gradient of gravity resulting from a tunnel one meter in cross-section diameter, located one meter below the Earth’s surface, is more than six times this uncertainty threshold. For a tunnel with a diameter four times as wide, we calculate that the same maximum amplitude would be measured even if the tunnel was up to four meters below the surface. Such sensing capability looks very promising for many engineering and environmental applications.
The authors declare no competing interests.