A gravimeter is an instrument used for measuring the local gravitational field of the Earth. "A gravimeter is a type of accelerometer, specialized for measuring the constant downward acceleration of gravity, which varies by about 0.5% over the surface of the Earth. Though the essential principle of design is the same as in other accelerometers, gravimeters are typically designed to be much more sensitive in order to measure very tiny fractional changes within the Earth's gravity of 1 g, caused by nearby geologic structures or the shape of the Earth and by temporal tidal variations. This sensitivity means that gravimeters are susceptible to extraneous vibrations including noise that tend to cause oscillatory accelerations. In practice this is counteracted by integral vibration isolation and signal processing. The constraints on temporal resolution are usually less for gravimeters, so that resolution can be increased by processing the output with a longer "time constant". Gravimeters display their measurements in units of gals, instead of ordinary units of acceleration. Gravimeters are used for petroleum and mineral prospecting, seismology, geodesy, geophysical surveys and othergeophysical research, and for metrology." More at http://en.wikipedia.org/wiki/Gravimeter
