Integrated levitated optomechanical gravimeter

Current highly sensitive gravimeters, such as superconducting spheres, atom interferometers, and torsion pendulums, suffer from high manufacture and maintenance cost (up to £400k), bulky size (as large as 2.5m^3) and slow measurement speed (typically 1 hour).

Here we propose an exciting innovation in quantifying gravity, based on the frequency measurement of the gravity-induced precession in an optically levitated fast-spinning particle. This novel levitated optomechanical systems (LOMS) gravimeter can be fabricated on a silicon wafer with wafer-level vacuum encapsulation, making its footprint as small as one mm^2. The small size device is mass-producible with a fabrication cost potentially less than £4k.

The proposed research uses the analogy of the precession of the Earth, a slow and continuous change in the orientation of the Earth's rotational axis induced by the gravity of the sun, to develop the novel gravimeter. In December 2018, our research for the first time revealed that the precessional motion also appears in sophisticatedly designed LOMS and that optical scattering techniques can precisely measure the frequency of precession [U9]. Our calculation predicts that levitated rotating particles of 10um diameter can achieve the sensitivity of 10^-9 g/sqrt(Hz) and a very fast-spinning particle (GHz reported in 2018 [x19]) can achieve 10^-11 g/sqrt(Hz) sensitivity, respectively.

The novel gravimeter can also measure the acceleration due to the Einstein equivalence principle. Thanks to the ultra-high Quality-factor (7.7x10^11 demonstrated in 2017 [x3]) of the rotating particles, the novel sensor will have the potential to cover 11 orders of magnitude of acceleration measurement.

Moreover, using the advanced silicon fabrication technique, we will be able to differentiate the centre-of-mass and the centre-of-optical-force of the levitated particle, in order to optimise the range of the gravity (or acceleration) induced torque, and correspondingly design the sensing range and sensitivity of the acceleration, e.g. 10^-6 m/s^2 to 10^5 m/s^2 to cover the seismic and mining health monitoring applications or 1 m/s^2 to 10^11 m/s^2 for fundamental physics research. The sensor only requires short integration times (1ns to 100s, depend on the precession frequency). Thus, it can complete the measurement very rapidly. This novel precession sensing principle can also be utilised to measure force, strain, charge and mass, with similar ultra-wide dynamic range and ultra-high sensitivity potentially.

The innovative gravimeter (accelerometer) can be a powerful tool for investigating fundamental physics questions in gravitation, which are pressing and very hard to access experimentally due to the weakness of the gravitational interaction if compared to other interactions. The proposed research can also provide a platform for quantum manipulation of mesoscopic mechanical devices in the nano-scale regime and can serve as a testbed for theoretical predictions.

Furthermore, our novel sensor can equipt the oil and gas industry with its applications in CO2-EOR and exploration. It can track temporal and spatial variations of the gravitational field and provide highly accurate information of mass redistribution below the surface. The prototype on-chip LOMS gravimeter has a small footprint so that it can be installed close to the drilling bit. Based on Newton's law of universal gravitation, the gravimeter has the potential to detect 1.5x10^7 kg mass redistribution above the ground, and 1.5x10^5 kg mass redistribution inside the wellbore. The sensitivity of the novel gravimeters installed inside wellbores can be four orders of magnitude better than that of the existing highly sensitive gravimeters.

Our research also contributes to CSS, mineral exploration, structural safety monitoring for mining, earthquake warning, inertial navigation and geoscience, and can lead to significant cost savings in multiple industries.

Global carbon-intensity of energy has improved every year since 2011 but total emissions still grew in 2018 to record levels, over 55 Gt CO2 emission. The UK Government and Parliament set a target of net-zero emissions of greenhouse gases in the UK by 2050. This project will facilitate the target of the government and equipt the deployment of carbon capture and storage (CSS), and CO2-EOR to reduce CO2 emission by providing ultra-sensitive on-chip gravimeters to quantify gravitational signatures.

For CSS, our study will provide an opportunity to advance CO2 storage surveillance by offering fast-speed monitoring of the temporal and spatial variations of the gravitational field during the CO2 injection into geological formations. The high-pressure injection sometimes causes the rocks to slip on a fracture and therefore induces an earthquake. In the UK, all carbon dioxide storage sites are located offshore (under the seabed) and the undersea seismic activity monitoring is essential to prevent major earthquakes. The outcome on-chip LOMS gravimeter is small, fast and sensitive, having great potential to contribute to these applications.

BP, one of our industrial partners, has a specific requirement for small, high sensitivity gravimeters to both improve the efficiency of oil extraction, and meanwhile reduce the carbon footprint. The CO2-EOR technique, which injects CO2 into oil reservoirs, is adopted to increase the pressure and force out the residual oil, and therefore can enhance oil production by 10% to 25%. It can generate a potential incremental oil recovery of 2.5 billion barrels of oil in the UK Continental Shelf (UKCS), associated with storage of approximately 0.8 Gt CO2. This innovation can extend the life of the oil field, improve its operational efficiency, reduce environmental impact, and address climate change issues.

CO2-EOR requires ultra-sensitive gravimeters to provide highly accurate information of the magnitude and spatial redistribution of mass below the surface. The current small size microelectromechanical-system gravimeters are not sensitive enough, and the current highly sensitive gravimeters are too bulky for the wellbore and consequently, losing at least two orders of magnitude mass resolution when applied above the surface. Our innovative on-chip LOMS gravimeter is small, fast and ultra-sensitive, making an ideal alternative for CO2-EOR applications. It also opens the possibility for the lifecycle emissions from the oil industry to be neutral or even 'carbon-negative'. Thus, this research will promote the Return on Investment of our industrial partners, as well as fulfilling their social, safety and environmental responsibilities.

We will work with Public Policy Southampton (PPS) at the University of Southampton to identify and build connections with colleagues in the Department for Business, Energy and Industrial Strategy to optimise feedback to the oil and gas industry stakeholder. Moreover, we will take advantage of the opportunity presented by the recommendation of the Wood Review 2016 to reduce the complexity of the Code of Practice on Access to Upstream Oil and Gas Infrastructure on the UKCS, by highlighting how this project's key outputs can be put to use on the UKCS to reduce emissions from exploration and extraction, especially with the recent complication with alternative and controversial oil recovery and exploration techniques.

The development of our innovative on-chip levitated sensors is highly desired in several industrial sectors. This proposal has gained £525k in-kind support from five industrial partners. With the remarkable support from the industry community, this project will lead to successful commercialisation and future industrialisation by closely engaging with industrial collaborators. We have a plan to commercialise the outcome of this project by forming a spin-out company in the UK to generate significant economic impact.