A High-Sensitivity, Low-Drift MEMS Relative Gravimeter for Multi-Pixel Imaging Applications

Novelty / Progress Claim(s)

This paper reports a capacitive readout-based MEMS relative gravimeter which can detect sub-Hz microseismic and slowly varying gravitational Earth tide signals. The gravimeter has a noise floor of 6-7 uGal/rt(Hz) at 1Hz and a linear drift of <250 uGal/day, metrics which are on a par with the commercially available gravimeters, and are leading in the field of MEMS accelerometers. The gravimeter is packaged in a standard ceramic-carrier and interfaced to a low-power, advanced FPGA-based readout. This setup is housed within a bespoke thermal enclosure, making the platform ideal for multi-pixel array-based implementation in the field.

Background/State-of-the-Art

Gravimeters are used to measure the local acceleration due to gravity (g). One of the emerging applications of gravimetry is in volcanology where gravimeters can be used to understand magma plumbing, providing information on volcanic activity/unrest events. However, this requires multi-pixel ‘gravity-imaging’ around volcanoes, a feat which is not possible using the expensive, complex, and large form-factor commercially available gravimeters.

Recently, researchers have developed MEMS-scale accelerometers which have excellent sensitivities but not yet demonstrated good long-term stability, making them non-viable for long-term monitoring of slow gravity changes (such as produced by magma flow). In a previous work, the authors have demonstrated an optical shadow-sensor readout based MEMS gravimeter with a sensitivity of 40 uGal/rt(Hz). Building on the work, a portable version of the gravimeter was also reported previously. The devices in both the setups were limited by the displacement noise of the optical shadow-sensor and the packageability of the setup.

In this paper, we are reporting a novel gravimeter which uses a capacitive-readout for sensing the proof-mass displacement, is embedded in a MEMS IC package, and uses advanced FPGA-based electronics for signal conditioning. The improved displacement sensitivity of the capacitive readout allows designing stiffer suspension-springs making the device more robust for operations in extreme environments. The acceleration sensitivity achieved using the new gravimeter is around 6-7 uGal/rt(Hz) at 1Hz, which is a significant improvement over the previous versions of the gravimeter. The device is currently being readied for field trials in the sectors of volcano gravimetry and oil & gas, showing the maturity of the technology.

Methodology

The reported gravimeter has a microfabricated silicon proof-mass which is suspended from thin flexures. Metal-combs are patterned on top of the proof-mass and a fixed glass layer with complementary combs is assembled to be at a close separation from the proof-mass. The overlapping combs act as a capacitor, the magnitude of which is dependent on the proof-mass displacement. The multi-layered gravimeter is embedded within a standard 32-pin ceramic DIP chip-carrier and wire bonded. The MEMS package is interfaced with analog signal conditioning electronics and a digital lock-in implementation is employed for converting the capacitance change into useful units (uGals).The electronics noise of the setup is measured to be <1 uGal. To reduce temperature-related effects, a mK active temperature control is implemented around the device. The packaged device is housed within a prototype thermal enclosure making the platform field-portable.