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.
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.