Accurately detecting, locating, and quantifying leaks of methane — the main component of natural gas and a major fuel source worldwide — is critically important for both environmental and economic reasons. Unfortunately, traditional methods are slow, labor-intensive, limited to small coverage areas, and expensive to operate over time.

Illustration showing how trace gases are detected in the field using a mobile, dual-frequency comb laser spectrometer. The spectrometer sits in the center of a circle ringed with retroreflecting mirrors. Laser light from the spectrometer (yellow line) passes through a gas cloud, strikes the retroreflector, and is returned directly to its point of origin. The data collected are used to identify leaking trace gases (including methane) as well as leak locations and their emission rates. (Credit: Stephanie Sizemore and Ian Coddington/NIST)

Current approaches to methane detection rely heavily on inspectors using infrared (IR) cameras to look for gas plumes one-by-one across large sites such as well fields with hundreds of potential leaks. This is time-consuming, requires skilled operators, and may only be done once a year or less because of the high cost of monitoring expansive, remote, or otherwise difficult-to-survey areas. Aircraft- and vehicle-mounted IR cameras or spectrometers offer another option; however, this method also is expensive and may be ill-suited for continuous monitoring.

A solution was developed that continuously and cost effectively monitors leaks of methane and other trace gases with extreme precision and over large areas. The observing system combines an in-the-field, dual-frequency, comb laser spectrometer and an array of corner cube retroreflectors — special mirrors that send light striking them straight back to their source.

An optical frequency comb is a very precise tool for measuring different colors — or frequencies — of light. Scientists start with lasers that emit a continuous train of very brief (femtosecond, or one-millionth of one-billionth of a second) pulses of light containing millions of different colors, also known as frequency spectra. The evenly spaced, individual lines of the spectra look like the teeth of a comb, giving the tool its name. The laser pulses — with their millions of highly defined teeth — can serve like the marks on a ruler for measuring the spectral signature of any material through which they pass with incredible precision.

The new trace gas monitoring system uses an evolution of the frequency comb technology: a dual-comb laser spectrometer where a second comb is added. When paired, the combs can act like hundreds of thousands of laser spectrometers working in unison and yield a device that is 10 to 100 times better than a traditional spectrometer, and very sensitive to leaks, even at a great distance.

For more information, contact Michael E. Newman at This email address is being protected from spambots. You need JavaScript enabled to view it.; 301-975-3025.


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This article first appeared in the September, 2019 issue of Tech Briefs Magazine.

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