New gravimeter built using smartphone accelerometers

BBC : Gravimeters, which are used to measure local variations in gravity, are generally large and cost about $100 000. Now Richard Middlemiss of the University of Glasgow, UK, and his colleagues have developed a cheap, postage stamp–sized gravimeter based on the tiny accelerometers commonly found in smartphones. The new gravimeter employs a block of silicon suspended between two thin bands. As gravity pulls the block downward, the block’s shadow—cast by light shining through the device—also moves and that movement is detected by a photodiode. Researchers can calculate the strength of the gravitational pull by measuring the amount of current the light-sensitive diode produces. The team tested the device by measuring Earth’s tides over the span of several days. The device was more than sensitive enough to detect the changes in gravity due to tidal forces; it could also detect variations as small as those caused by a 1 m diameter tunnel located 2 m below ground. Distributing a network of the small gravimeters around a volcano could provide continuous localized monitoring for a fraction of the cost of a conventional gravimeter.