Applied physics: Gravity measurements on chips
Gravimeters have applications ranging from oil exploration to the detection of underground tunnels, but size and lack of portability have limited their field use. A device the size of a postage stamp promises fresh opportunities. See Letter p.614
 Gravimeters can measure tiny variations in the local gravitational acceleration, with sensitivity high enough to measure Earth's tides: the elastic deformation of the Earth's crust caused by the changing relative phases of the Sun, Earth and Moon. Existing gravimeters are bulky and costly, but now Giles Hammond and colleagues have developed a compact and inexpensive microelectromechanical gravimeter with a novel 'anti-spring' design. They use it to measure gravitational acceleration with sufficient sensitivity and stability to detect Earth's tides. The small size and low cost of the device mean it could be mounted on a drone for surveying and exploration roles, used to monitor volcanoes or built into multi-pixel density-contrast imaging arrays.
Geophysics From Terrestrial Time‐Variable Gravity Measurements
In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time‐variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground‐based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time‐variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation.
A comparison of gravimetric techniques for measuring subsurface void signals
To demonstrate the potential of gravity gradiometry in the detection of underground structures or voids, an experiment was carried out at an underground missile launch facility at Vandenberg Air Force Base, California. Gravity data were collected using a LaCoste-Romberg Model G gravimeter and gravity gradient measurements were made using a vertical spin-axis Lockheed Martin developed arms control verification gravity gradiometer. After performing all necessary corrections, the data were compared to modelled signatures of the launch facility. Peak gravity and gravity gradient anomalies of approximately -75 µGal (1 µGal = 10-8 m s-1) and 30 E (1 E = 10-9 s-2) respectively were measured over the centreline of the largest capsule. The presence of the buried launch facility was apparent in both the observed gravity and gravity gradient data. The gravity gradients were unambiguous in resolving the underground facility's signature and thus revealing its location. In contrast, identifying the launch complex solely from the gravity anomaly data would have proven difficult due to a strong regional trend across the target area.} \fnm{4}{National Imagery and Mapping Agency, Patrick AFB, FL 32925-3438, USA.} \fnm{5}{MITRE Corporation, Bedford, Massachusetts 01730-1420, USA.