Ambient Noise Tomography for Natural Hydrogen Exploration: A Case Study from the Gawler Craton, South Australia

The prospect of natural hydrogen, also known as geologic or gold hydrogen that is produced in the sub-surface, is rapidly gaining interest as a potentially cheap, clean, and renewable energy source for the future. However, no standardised approach to natural hydrogen exploration yet exists. Traditional exploration activities in sedimentary basins do not easily translate to natural hydrogen, which is often linked instead to non-sedimentary cratonic settings.

Here we present a study of ambient noise tomography within the natural hydrogen exploration licence area (PEL 691) of H2EX Ltd. This area of exploration is focused on the eastern Eyre Peninsula in South Australia, beneath which lies the crystalline basement rocks of the Gawler Craton. A regional passive-seismic survey of 150 nodal seismometers was conducted over an area of 2500 km² with average site spacing of 3-4 km. The area encompasses several major known faults as well as a zone of heightened intra-plate seismicity. The quality of the noise cross-correlations was found to be excellent, resulting in a 3D shear-wave velocity model that is well resolved down to 15 km depth. Prominently fast crustal velocities are found typical of Proterozoic crystalline crust, with average velocities of 3.17-3.43 km/s at 0-3 km depth and large lateral variations of up to 8%. At shallow depths (~1 km) alternating fast and slow anomalies are found to correlate with gravity indicating local variations in density and the sub-surface geology. Low velocity regions likely correspond to sedimentary/meta-sedimentary units, while high velocities reflect where the underlying crystalline basement is closer to the surface. These velocity variations are thus interpreted as reflecting the depth of the unconformity between the basement and the meta-sediments above. At deeper crustal depths lateral velocity variations beneath the major fault zones are assessed and considered for their potential as migration pathways for natural hydrogen. Overall ambient noise tomography is found to be helpful in detailing the sub-surface structures relevant to the natural hydrogen exploration system.