Quantum Gravity Gradiometers for Urban Underground Mapping

Understanding what lies below the ground surface and what condition the ground itself is in is vital for the development of urban underground spaces. As the subsurface is not visible, several geophysical sensing technologies exist to look through the ground. One of these technologies measures variations of gravity but is limited by its sensitivity and relatively slow mode of operation. To overcome this, quantum technology (QT) cold-atom gradiometers have been successfully demonstrated for underground cartography in a field environment. These novel sensors require testing in controlled environments whilst benchmarking them against existing technology. This paper presents a feasibility study on using a gravity gradient sensor in a controlled, large-scale monitoring test using the National Buried Infrastructure Facility ( www.birmingham.ac.uk/nbif ) at the University of Birmingham, UK. Over a 30-minute period, a 3 m3 reservoir was drained of water resulting in a drop of 98 ± 32 Eotvos as measured by two Scintrex CG6s in gradient configuration.