Andrew Lockwood, Xcalibur Multiphysics
Charlie Ironside, Curtin University
Merv Lynch, Curtin University.
Natural hydrogen and abiotic methane seeps are known from antiquity, the famous flames of the Chimera in Turkey, and the Tanjung Api (fire bay) seeps in Sulawesi being two well-known examples. Mapping the full extent and occurrence of these and similar regional geological hydrogen generating systems is a problem well suited to conventional airborne geophysical mapping, because of the large areas involved. Adding a hydrogen sensor to existing geophysical survey platforms to identify large scale atmospheric anomalies corresponding to features visible in conventional aeromagnetics, radiometrics or gravity data should further inform geological models of natural hydrogen generation. In the absence of gas flares, direct detection of hydrogen will be very challenging, as H2 dissipates rapidly from the ground surface. A review of existing hydrogen detection technologies, coupled with a numerical study of atmospheric dissipation from the ground identified Raman spectroscopy as a promising approach.
Raman scattering is a weak inelastic interaction between diatomic gases such as H2, N2 and light, where some energy is lost to the vibration modes of the molecule. For example, a UV laser at 355 nm irradiating a sample of H2 will result in elastic scattering of 0.1%-0.01% of the incident light at the same frequency as the laser. This is called Rayleigh-Mie scattering. However, an additional 0.0001% of these photons will be scattered back at a Raman shifted wavelength of 415 nm. Raman spectroscopy uses this frequency shifting effect to measure the presence and abundance of gases in the path of the laser by looking for peaks in the scattered light spectrum corresponding to known Raman shifts of the species present, such as N2, H2O and H2. Where all three gas peaks are measured simultaneously, the system is automatically calibrated against the percentage of nitrogen in the atmosphere.
In principle, given a large enough laser and optical system, the sensitivity of existing Raman spectrographs can be increased to the level required for airborne hydrogen mapping. By carefully timing the light pulses it is also possible to use Raman spectroscopy in a light detection and ranging (LiDAR) mode so that the atmospheric composition 60m below the aircraft is sampled, rather than at flying height. There are already examples in the literature of Raman LiDAR being used to detect hydrogen concentrations associated with nuclear waste disposal of 0.1% at 40 m and resolving spatially the extents of anomalous H2 concentration. (Stothard et al, 2022, Choi et. al, 2020).
The numerical models of the H2 Raman response we have developed are being calibrated against published results and our own laboratory work at Curtin. These go beyond demonstrating the feasibility of the technique, as they allow the sensitivity of the system to different design elements to be explored and optimised. Our current setup uses single photon avalanche diodes (SPAD) being driven by a time correlated single photon counting system with 45ps resolution to detect the Raman scatter from each laser pulse, but other photon detector systems will be evaluated in the near term to reduce the measurement time.
Young Choi, Sung Hoon Baik, and Young Soo Choi. Remotely measuring the
hydrogen gas by using portable Raman lidar system. Optica Applicata, 15(1):37–49,
David J. M. Stothard, Matthew S. Warden, Roman Spesyvtsev, Ellis Kelly, John
Leck, Alexander Allen, Jon Squire, Stephen Hepworth, Simon Malone, Jo Tunney,
Frank Allison, David Armstrong, David Li, and Loyd McKnight., 2022. Long-range, range-resolved
detection of H2 using single-photon ‘quantum’ Raman: a condition monitoring
tool for long-term storage of nuclear materials. In CBRNE Sensing XXIII, volume PC12116, page PC1211608. International Society for Optics and Photonics, SPIE, 2022.