Wee-g: A MEMS gravimeter for precision gravity surveying in Security and the Environment
Lead Research Organisation:
University of Glasgow
Department Name: School of Physics and Astronomy
Abstract
By measuring subtle changes (<10-8) in the acceleration of gravity we can infer the local density of nearby objects. When the density is lower (e.g. a tunnel) the local gravity becomes slightly less. While this technique is readily adopted in the oil & gas industry to (i) search for new resources (ii) perform long term monitoring at active wells, broader uptake of gravimetry in other fields is limited due to the high upfront cost of gravimeters ($80k for a Scintrex CG6), the fragility of devices and the time it takes to undertake field surveys with a single instrument.
There are disruptive opportunities for gravimetry to breakthrough into other fields including environmental monitoring and security & defence. For environmental monitoring, a smaller-lighter-cheaper gravimeter will open-up opportunities for (i) deployment of sensors arrays of volcanos as a technique to image the magma plumbing system and provide resilience against eruptions, (ii) performing rapid field surveys to identify collapsed culverts, sinkholes or underground tunnels. Within the field of Security & Defence there are further opportunities of monitoring ports of entry with underwater sensors, detecting underground tunnels and monitoring compounds. Beyond this, we see opportunities in monitoring dam infrastructure, carbon capture, geothermal engineering detection/monitoring of underground aquifers.
Wee-g is a precision MicroElectroMechanicalSensor (MEMS) that has been developed within the Institute for Gravitational Research (University of Glasgow). It is a spin-off from the Gravitational-Wave research activities led by Prof . Hammond. Wee-g is the world's first gravimeter, capable of monitoring the Earth tides; elastic deformations of the Earth caused by the tidal potential of the Moon and Sun. As typical gravity signals are 10-50% of the Earth ides, this is an essential measurement to show devices have sufficient stability and sensitivity. The Wee-g sensor has the potential to be made much cheaper than existing gravimeters, and thus can open-up these new opportunities in Environmental monitoring and Security & Defence.
Field trials are underway with partners in both the Environmental and Security & Defence fields. Wee-g Mk I systems are being deployed on Mt Etna as part of a H2020 project (Newton-g) to monitor magma intrusion in volcanoes, and we estimate the TRL is 5. We also have a system being trialled by DSTL for underwater monitoring at a port of entry. We will use this proposal to further develop the Wee-g gravimeter (Mk II system) to put us in a prime position to spinout. We will address some of the challenges found in the Mk I system including temperature sensitivity of the MEMS chip, miniaturisation and temperature stability of the front-end electronics, and removing reliance on evaluation FPGA boards which are liable to be discontinued.
There are disruptive opportunities for gravimetry to breakthrough into other fields including environmental monitoring and security & defence. For environmental monitoring, a smaller-lighter-cheaper gravimeter will open-up opportunities for (i) deployment of sensors arrays of volcanos as a technique to image the magma plumbing system and provide resilience against eruptions, (ii) performing rapid field surveys to identify collapsed culverts, sinkholes or underground tunnels. Within the field of Security & Defence there are further opportunities of monitoring ports of entry with underwater sensors, detecting underground tunnels and monitoring compounds. Beyond this, we see opportunities in monitoring dam infrastructure, carbon capture, geothermal engineering detection/monitoring of underground aquifers.
Wee-g is a precision MicroElectroMechanicalSensor (MEMS) that has been developed within the Institute for Gravitational Research (University of Glasgow). It is a spin-off from the Gravitational-Wave research activities led by Prof . Hammond. Wee-g is the world's first gravimeter, capable of monitoring the Earth tides; elastic deformations of the Earth caused by the tidal potential of the Moon and Sun. As typical gravity signals are 10-50% of the Earth ides, this is an essential measurement to show devices have sufficient stability and sensitivity. The Wee-g sensor has the potential to be made much cheaper than existing gravimeters, and thus can open-up these new opportunities in Environmental monitoring and Security & Defence.
Field trials are underway with partners in both the Environmental and Security & Defence fields. Wee-g Mk I systems are being deployed on Mt Etna as part of a H2020 project (Newton-g) to monitor magma intrusion in volcanoes, and we estimate the TRL is 5. We also have a system being trialled by DSTL for underwater monitoring at a port of entry. We will use this proposal to further develop the Wee-g gravimeter (Mk II system) to put us in a prime position to spinout. We will address some of the challenges found in the Mk I system including temperature sensitivity of the MEMS chip, miniaturisation and temperature stability of the front-end electronics, and removing reliance on evaluation FPGA boards which are liable to be discontinued.
Technical Summary
The Wee-g MkI is a portable system with internal batteries that provide 14 hours operation in the field. The device is controlled via a "Xilinx Snickerdoodle" Field-Programmable-Gate-Array (FPGA) which has the parallel computing necessary to modulate/demodulate the capacitive comb sensor on the MEMS and read multiple environmental channels. The system incorporates a Bluetooth interface that allows the user to view MEMS data on a wireless tablet in the field using a custom Graphical-User-Interface (GUI) designed by the Glasgow team.
There are two areas to improve the Wee-g sensor before fully undertaking a commercial spinout.
1. All relative gravimeters are ultimately sensitive to temperature. The spring flexures undergo variation in Young's modulus which means that when the temperature is increased the proof mass sags; which looks like a gravity change. Data have taken on an uncorrected device shows a proof mass sag up to 4mm for an 8 Hz device with a temperature change of 10K. This results in a thermal sensitivity of 120uGal/K and means that temperature must be controlled to better than a few mK.
2. This CLASP proposal will focus on addressing the redesign of the electronics readout. We will further miniaturise the front-end readout of the MEMS and develop a custom Field-Programmable-Gate-Array (FPGA) which will mean not having to rely on the current Xilinx FPGA evaluation board. We already have a nested thermal enclosure for the MEMS sensor and plan to further miniaturise the front-end electronics and position inside the MEMS enclosure, which is controlled to (45C+/-1mK). To de-risk, we have designed and fabricated a test circuit which has on-board ADCs, voltage references and amplifiers, although a further size reduction by x3 is needed for location inside the MEMS enclosure. This system shows an improved temperature sensitivity of x20.
There are two areas to improve the Wee-g sensor before fully undertaking a commercial spinout.
1. All relative gravimeters are ultimately sensitive to temperature. The spring flexures undergo variation in Young's modulus which means that when the temperature is increased the proof mass sags; which looks like a gravity change. Data have taken on an uncorrected device shows a proof mass sag up to 4mm for an 8 Hz device with a temperature change of 10K. This results in a thermal sensitivity of 120uGal/K and means that temperature must be controlled to better than a few mK.
2. This CLASP proposal will focus on addressing the redesign of the electronics readout. We will further miniaturise the front-end readout of the MEMS and develop a custom Field-Programmable-Gate-Array (FPGA) which will mean not having to rely on the current Xilinx FPGA evaluation board. We already have a nested thermal enclosure for the MEMS sensor and plan to further miniaturise the front-end electronics and position inside the MEMS enclosure, which is controlled to (45C+/-1mK). To de-risk, we have designed and fabricated a test circuit which has on-board ADCs, voltage references and amplifiers, although a further size reduction by x3 is needed for location inside the MEMS enclosure. This system shows an improved temperature sensitivity of x20.
