Tomographic imaging of methane leaks via chirped laser dispersion spectroscopy

Abstract:

With the rapid growth of natural gas (NG) infrastructure, detecting, localizing, and quantifying emissions have grown in importance because the primary constituent of NG is methane (CH₄), a potent greenhouse gas. Therefore, it is important to have effective emissions monitoring systems for leak detection and repair. This study utilizes near-infrared chirped laser dispersion spectroscopy (CLaDS), which operates by measuring the dispersion of light in a medium using a multi-frequency laser beam. Because the CLaDS technique relies on beat frequency variations rather than amplitude changes, concentrations can be extracted over a broad range of return signal intensities. Additionally, CLaDS enables path-integrated concentration measurements with simultaneous ranging information using a swept sideband technique. The current system has a ranging sensitivity of 0.2 m/Hz^1/2 and sensitivity to methane of 6 ppm-m in the field.

We show that direct CLaDS can be extended to localize natural gas leaks utilizing a modified horizontal radial plume mapping technique. A controlled methane release study was conducted with a 4 x 4 array of retroreflectors positioned on a 50 x 50 m grid with the CLaDS sensor located 25 m away from the grid transverse to the mean wind direction (Fig. 1a). The measurements across the array were acquired over 6-15 minutes with a leak rate of 0.13 g CH₄ s^-1 or 0.42 SCFM, or approximately 25 times lower than typical leak rates from compressor stations. The tomographic reconstruction of the methane distribution identified the leak location and downwind plume enhancements by using a non-negative, linear least squares regression. The field test showcases the system’s ability to locate and map small-scale emissions, with the grid cells immediately downwind the release showing the highest concentrations, and the grid cells upwind and crosswind showing the lowest values. The mean plume position and concentrations were further verified by an in-situ point sensor downwind of the source. Ongoing efforts include tracking a retroreflector flown on a drone in real-time with the CLaDS system for high spatial resolution mapping of emission sources in both the horizontal and vertical.