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.
SCIENCE AND EMPIRICISM IN PILE FOUNDATION DESIGN
Scientific approaches to pile design have advanced enormously in recent decades and yet, still, the most fundamental aspect of pile design—that of estimating the axial capacity—relies heavily upon ...