Airborne methane emission inventories are built by systematically flying a sensor-equipped aircraft over a target area, measuring gas column concentrations in real time, and then converting those spatial measurements into quantified leak rates at the individual source level. Those individual figures are then aggregated across all detected sources within a defined boundary to produce a site-level or network-level emission total. The process combines physics-based measurement, atmospheric dispersion modelling, and structured data management, and the sections below walk through each step in detail.
What data does an airborne methane campaign actually collect?
An airborne methane campaign collects spatially referenced gas concentration measurements, precise aircraft position and altitude data, and real-time meteorological readings, all time-stamped and recorded continuously along the flight path. Together these inputs form the raw dataset from which leak locations, plume shapes, and ultimately emission rates are derived.
The core measurement is a column-integrated methane concentration, expressed in parts per million metres (ppm·m). This tells you how much methane is present in the vertical column of air the sensor is looking through at each point along the track. Alongside this, GPS and inertial navigation data anchor every measurement to a precise geographic coordinate, while onboard anemometers or integration with meteorological models supply wind speed and direction, which are essential for the calculations that follow.
High-quality campaigns also log background concentration levels upwind of the survey area. Establishing a clean background reference is critical: without it, you cannot separate the methane that was already in the atmosphere from the methane attributable to the infrastructure being inspected. The combination of foreground plume signal and background baseline is what makes quantification possible rather than just detection.
How is a single leak rate calculated from aerial measurements?
A single leak rate is calculated by integrating the measured methane concentration enhancement across the downwind cross-section of a plume and multiplying that integrated value by the wind speed at the time of measurement. This mass-balance approach converts a spatial concentration pattern into a volumetric or mass emission rate, typically expressed in litres per hour or grams per second.
The method works because a continuous emission source creates a plume that is carried downwind. If you fly a transect perpendicular to the wind direction through that plume, you capture a cross-sectional slice of it. Integrating the concentration enhancement across the width of that slice, and multiplying by the wind speed, gives the total flux of methane passing through the transect per unit time, which equals the source emission rate under steady-state conditions.
Accurate wind data is therefore not optional; it is a primary input to the calculation. Small errors in wind speed propagate directly into the emission rate estimate. This is why well-designed campaigns combine onboard meteorological sensors with independent ground-level or radiosonde wind measurements, and why surveys are ideally conducted under stable, moderate wind conditions rather than in calm or highly turbulent air.
How are site-level emissions aggregated into an inventory?
Site-level emissions are aggregated by summing the quantified leak rates of all individual sources identified within the site boundary during the campaign, then applying quality filters and, where needed, statistical corrections for sources that may have been below the detection threshold or missed due to wind conditions during the overflight.
In practice, this means the data processing workflow moves through several stages. First, individual plumes are identified in the concentration dataset and attributed to specific source locations using the wind vector and plume geometry. Second, a leak rate is calculated for each attributed source. Third, all source-level rates within the defined site perimeter are summed to produce a total site emission figure.
A robust inventory also accounts for measurement uncertainty at each stage. Concentration measurement precision, wind speed variability, and the probability of detection at a given leak rate all introduce uncertainty, and a credible inventory quantifies these contributions rather than ignoring them. The result is not a single number but a value with an associated confidence range, which is exactly what regulators and independent verifiers need to assess compliance with obligations to measure methane emissions accurately.
What role does flight planning play in inventory completeness?
Flight planning directly determines inventory completeness because the coverage pattern, altitude, speed, and timing of the survey define which sources are detectable and which are not. A poorly planned campaign can miss entire sections of infrastructure or survey under wind conditions that push plumes outside the measurement swath, creating gaps in the final inventory.
Good flight planning starts with the wind forecast. The aircraft needs to fly transects that are roughly perpendicular to the prevailing wind so that plumes are carried across the flight path rather than along it. If the wind direction is variable or shifts during the survey, the flight plan may need to be adapted in real time to maintain the geometry needed for reliable quantification.
Coverage completeness also depends on line spacing. Flying transects too far apart means plumes from small or diffuse sources may pass between measurement lines undetected. For network-level inventories covering large pipeline corridors, the balance between line spacing, flight speed, and the minimum detectable leak rate determines the lower bound of what the inventory can reliably capture. Documenting these parameters as part of the campaign metadata is essential for interpreting the resulting methane emission factors and emission totals with appropriate confidence.
How do airborne inventories satisfy EU Methane Regulation requirements?
Airborne inventories satisfy EU Methane Regulation 2024/1787 requirements by providing the source-level and site-level emission quantification that the regulation mandates, delivered through an independent, verifiable measurement process with documented uncertainty. The regulation requires operators of fossil energy infrastructure to quantify methane emissions, have them verified by third parties, and report them annually, and a properly executed airborne campaign produces data that meets all three obligations.
The EU Methane Regulation distinguishes between component-level leak detection and repair (LDAR) and site-level emission quantification. Airborne surveys address the site-level requirement directly: by measuring total emissions from a defined boundary, they produce the aggregate figures that feed into annual emission reports. They also support the source attribution needed to prioritise repair work, which links back to the LDAR obligation.
For underground infrastructure in particular, the regulation sets sensitivity thresholds that conventional walking surveys struggle to meet efficiently across large networks. Airborne systems capable of detecting small leak rates across hundreds of kilometres per day are therefore well positioned to help operators comply without disproportionate cost or disruption to operations. Crucially, the measurement data, uncertainty estimates, and flight metadata together form the auditable evidence trail that independent third-party verifiers require.
How accurate are emission inventories built from airborne surveys?
Emission inventories built from airborne surveys are generally accurate to within a factor of two for individual source quantification under good measurement conditions, with site-level totals typically achieving better relative accuracy because random errors at the source level tend to partially cancel when summed across many sources. Accuracy depends most heavily on wind measurement quality, atmospheric stability, and the detection threshold relative to the actual leak size distribution at the site.
It is worth being precise about what „accuracy“ means in this context. For a single small leak measured once under variable wind, the uncertainty can be substantial. But for a site-level inventory that aggregates dozens of sources measured under controlled conditions, the aggregate figure is considerably more reliable than any individual source estimate. This distinction matters when interpreting inventories for regulatory reporting.
Repeated surveys of the same site under different conditions, and cross-validation against ground-based measurements at selected locations, are both effective ways to build confidence in inventory results. Transparency about detection limits is equally important: an inventory that clearly states its minimum detectable leak rate and the proportion of the source population likely to fall below that threshold is far more defensible to regulators and verifiers than one that presents a total without qualification.
How ADLARES supports methane emission inventory campaigns
We help operators build defensible, regulation-grade methane emission inventories from airborne measurement campaigns, combining the sensitivity and speed of our CHARM® DIAL technology with structured data workflows that produce results ready for regulatory submission and third-party verification. Here is what we bring to each campaign:
- High-sensitivity detection: CHARM® detects leak rates from 150 litres per hour at survey speeds of up to 180 km/h, ensuring that even small sources are captured and included in the inventory rather than missed below the detection threshold.
- Source-level and site-level quantification: Our methane emission quantification service delivers both individual leak rates and aggregated site totals, with documented uncertainty estimates at each level.
- EU Methane Regulation compliance: As the world’s only DVGW-approved gas remote detection system, CHARM® provides the verifiable, auditable measurement record that regulators and independent third-party verifiers require under EU Methane Regulation 2024/1787.
- Secure Web GIS delivery: Survey results are delivered through a secure Web GIS platform accessible on desktop and mobile, so your team can verify indications, prioritise repairs, and extract the data needed for annual emission reporting without delay.
- Extensive network experience: With over 250,000 km of gas pipelines inspected across Europe, we bring proven campaign planning, flight execution, and data processing expertise to every project.
If you are preparing for your first EU Methane Regulation reporting cycle or need to upgrade your current measurement approach, get in touch with us to discuss how an ADLARES airborne campaign can deliver the emission inventory your compliance programme requires.