The development of a field-portable MEMS gravimeter

Gravimetry is a technique that has existed since the 17th and 18th century and involves the measurement of the acceleration due to gravity. The technique can be used to measure changes in density below the ground where conventional methods cannot. This is because gravity cannot be shielded, and therefore, can always be measured. It has many useful applications, particularly in surveying for oil, gas and minerals but can also be used to detect the precursors to volcanic eruptions and earthquakes, the density contrast of buried walls for archaeology and the detection of subsurface voids. Typically, however, gravimeters can cost in excess of ￡70k and weight over 8 kg. This cost and weight has prohibited some applications of gravimeters where either many of the devices would be required or if weight was prohibitive. In the winter of 2014, an important step forward to a lightweight, low-cost and portable MEMS gravimeter was made. This was the first time that a MEMS based gravimeter had measured the Earth Tides as reported by R. P. Middlemiss et al. The device, however, still required a large array of electronics and vacuum equipment which made it impossible to be able to be taken into the field. Clearly, if the device was to ever become a disruptive technology within the gravimeter industry, significant effort would be required to obtain a device with sufficient sensitivity that was also portable. This thesis highlights the work that was required to bring the MEMS that was a lab-bound device, and make successful measurements of the change in acceleration due to gravity in the field. In this thesis, a miniaturised MEMS gravimeter is presented with an RMS of 13 μGal when averaged to 1000 s, a factor three better than the original set-up from R. P. Middlemiss et al. The system went from a set-up with the approximate dimensions of a typical fridge freezer(including the array of electronic equipment) to a portable platform of dimensions 30cm wide by 30 cm deep by 15 cm high. The platform used a small steel cube as a vacuum container for the device, three micrometer legs for precision levelling, batteries and a custom electronics board. This custom electronics board was designed, tested and improved during the course of this thesis. The board was controlled by a microcontroller from Microchip (the dsPIC33EP512MU810) and is shown to be capable of a large amount of digital filtering that was required for this application, including the use of a decimator and digital based lock-in amplifier. The board measured displacements, temperatures (which were also controlled) and tilts, all of which the microcontroller digitally filtered and down sampled so that they could be sent to a computer for data logging