Remote sensing of atmospheric gas concentrations is an important activity, especially the monitoring of greenhouse gas levels on a global level.
This monitoring is currently carried out via satellite sensing and by laborious ground-based measurements. The ability to measure concentrations over a focussed areas (circa 1km2) and over more immediate timescales, is a pressing need.
Satellite time is precious and expensive.
Current techniques require complete studies to be funded, or data to be borrowed from other studies; here the data collection may not be exactly in sync with the requirements of an analysis. There is also a trade-off
between special and spectral resolution in space-based platforms; generally the higher the special resolution, the lower the corresponding spectral resolution must be.
Ground based sampling is used most often in the initial stages of construction and infrastructure projects, although it is also found in use around facilities where gas monitoring is important, such as in the oil and gas industry or for waste landfill sites.
In these arenas, the analyst’s need can’t be overstated to assess the potential release of ground gases and the ability to monitor the movement of that gas following release.
Sampling occurs at either fixed sites (boreholes) or with handheld sensors. The main issue with accuracy is that the atmospheric concentrations are generally inferred from indirect measurement of gas accumulated in a concentrator/borehole or measured instantaneously for only a certain number of times by handheld instruments.
The advantage of aerial measurement is that a wider area can be measured efficiently, with repeat measurements of days, weeks and months possible to get time-series data for a given area.
Methane (CH4) was the gas chosen to run trials against. Historically both a greenhouse gas and a ground gas commonly released by both earthworks projects and pastureland farming, methane is a colourless, odourless compound which is non-toxic but extremely flammable. It can form explosive mixtures in air at the right concentrations.
The sensor incorporated into the BGS/QuestUAV airframe uses an open-path gas mass spectrometer (a fibre-guided laser beam which is passed laterally across open atmosphere on top of the drone to a reflector and then back to the sensor itself.)
The collector is tuned to a particular gas type (for this study two sensors were used – one tuned for CH4 and one for CO2.)
Signals from the sensor are fed into a multi-core processing unit (also onboard the drone.) All readings were stamped with time and location information provided by the standard GPS and flight units in the QuestUAV Q200.
The equipment was built into a custom QuestUAV QPOD – the beauty of this system is the ability to customise sensors and layouts within the QuestUAV Q200 airframe.
Workflow / Trials
Trial flights of the custom Q-200 Gas Sensor drone took place over several months, with initial integration flights consisting of QuestUAV flight crews and later flights including team members from BGS.
The completed drone was commissioned in March 2017, running trial flights over gas releases initiated manually on the ground in locations over the test site.
Each set of flights recorded sensor data which was processed immediately on return to base. The resultant harmonised and raw data were passed to BGS for analysis and appraisal.
The project had a fully successful outcome.
The UAV flies well and the sensors performed their tasks correctly.
There is still work to be done to fine tune the operational workflow and there are maintenance tasks to be designed for regular scheduled missions.
“It’s been a very challenging project. It took a lot of work by both team – BGS and QuestUAV working in very close partnership. From beginning to end we’ve succeeded – we have a brilliant end-game with vehicles that fly well with gas sensors [integrated] in them. A great experience”