Differential Lidar Catches "Fugitive" Methane on the Fly

At left an aerial image of an industrial area overlaid with a zig-zag plume of green yellow and red in the lower left corner. At right the same image but with the plume as a more detailed mass of red.
Image: Steve Karcher, Ball Aerospace
The methane plume imaged from a light aircraft by Methane Monitor: real-time [left] and processed [right].

In 2015, methane accounted for 655 million kilograms  of the 7.1 billion kilograms of greenhouse gases released into the atmosphere of the United States alone. The energy sector was responsible for just under half of the methane released, about 279 million kg—lost product with a value of hundreds of millions of dollars.

So detecting leaks from the 2.6 million miles of natural gas pipelines snaking across America is properly both a business and an environmental priority. Air surveillance has reduced serious pipeline leaks by 39 percent since 2009, but there have still been 250 serious incidents in the past 8 years. These include a San Bruno, Calif., pipeline blast that killed eight people in 2010 and the Aliso Canyon leak in 2016—which released about 97 million kilograms of methane, essentially doubling the Greater Los Angeles area’s usual volume of methane emissions from all sources for a three month period.

Until now, efforts to detect what the industry calls “fugitive emissions” have been constrained by the instrument sensitivity and response times. Airborne surveillance required low-flying, slow-moving, expensive-to-run helicopters.

A new approach increases sensitivity and tightens control of timing and synchronization to permit the system to operate at higher speeds and higher altitudes—allowing a shift from helicopters to faster-moving, higher-flying single-engine, fixed-wing aircraft, which are less expensive to own and operate. The innovation earned Ball Aerospace & Technologies engineers Steve Karcher, Phil Lyman, and Jarett Bartholomew the Engineering Impact Award for Energy at NIWeek 2017 in Austin, Tex. The award was presented on 23 May.

Their Methane Monitor is a differential-absorption lidar (DIAL) methane detection system that uses two lasers of slightly different infrared wavelengths to map the ground and measure atmospheric methane. Methane strongly absorbs one of the wavelengths (at about 1,645.55 nm, the “on-resonance beam”) and is virtually transparent to the other—at about 1645.4 nm, the “off-resonance beam”). DIAL makes 1,000 to 10,000 measurements per second, firing the off- and on-resonance beams a few nanoseconds apart. The lasers light bounces off the ground and scatters back to the receiver, and the system calculates the intensity differences between the returns to measure the amount of methane in the beams’ paths. Overall, the differential intensity measurement requires signal-to-noise ratios 500 times better than ordinary lidar applications demand.

Return pulses may be sharply reflected by solid ground, distorted by foliage, or, in the case of the on-resonance pulse, completely absent because they have been fully absorbed by high concentrations of methane. The adaptive FPGA-based controller allows the system to compensate on the fly for variations in ground reflectivity, the energies and wavelengths of the two pulses, and aircraft velocity and position. Overall, Methane Monitor gathers data at rates of 2 to 17 gigabits per second.

Cruising in calm conditions at an altitude of 500 to 1000 meters, Methane Monitor can detect methane leaking at 50 cubic feet per hour (a rate about equivalent to what a single person achieves while blowing up a rubber party balloon, noted National Instruments VP Dave Wilson noted during the presentation)—all while sweeping a corridor up to 60 meters wide and providing real-time heat-map images of methane plumes overlaid on ground images from the system itself and such resources as Google Maps.  

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