Pendulum MEMS gravimeters for semi-absolute gravimetry

By measuring tiny variations in the Earth’s gravitational acceleration, g, one can infer density variations beneath the ground.  Since magmatic systems contain rock of differing density, changes in gravity over time can tell us when/where magma is moving. Traditional gravity sensors (gravimeters) were costly and heavy, but with the advent of the technology used to make mobile phone accelerometers (MEMS – Microelectromechanical-systems), this is changing.

At Glasgow University we have already developed the first MEMS gravity sensor and we are now working with several other European institutions to make a network of gravity sensors around Mt Etna – NEWTON-g. It will be the first multi-pixel gravity imager – enabling unprecedented resolution of Etna’s plumbing system.

While this work is ongoing, a second generation of MEMS gravity sensor is now under development. The first-generation sensor comprises a mass on a spring, which moves in response to changing values of g. This, however, can only ever be used to measure changes in gravity, which means it can be difficult to tell the difference between a geophysical signal and instrumental drift. If we could measure absolute values of gravity, then instrumental drift would become less of a concern, and we could remove the need to calibrate the sensors against commercial absolute gravimeters.

One way of making absolute measurements of gravity is to use a pendulum. This method was used for hundreds of years until the scientists and engineers essentially ran out of fabrication tolerance about 100 years ago. But now nanofabrication is at our disposal, so pendulums are a valid approach to gravimetry again. Such a gravimeter is now being designed and fabricated at the University of Glasgow. It consists of a pair of coupled pendulums, who’s oscillation period is monitored to measure gravity. Here we present the intricacies of the gravimeter design, discuss the expected performance of this new tool, and propose some implications that this sensor could have on the field of volcano gravimetry.