IntroductionIn recent years, the world has witnessed a surge in research and development of renewable energytechnologies, ranging from solar and wind power to geothermal and biomass solutions. Theseinnovative approaches undoubtedly hold the key to reducing our carbon footprint, yet several of themrequire the extraction and utilization of scarce minerals, leading to potential environmentaldegradation and geopolitical complexities. A potentially game-changing alternative has emerged onthe horizon - Natural hydrogen. Mainly generated by geological processes, Natural H2 has beenobserved with different degrees of purity as surface seepages or occasionally in wells targeting otherresources. Although some distinct natural processes are known to generate hydrogen, exploring for itand understanding its presence remain a challenging task due the lack of geological models andgeophysical data.Context and survey locationNatural hydrogen gas emanations have been observed and measured along the North Pyrenean FrontalThrust and other related faults rooted in the mantle body (Lefeuvre et al, 2021). These results,together with a promising geological setting and evidence of fluid migration at depth, suggest that H2may be sourced from mantle rocks serpentinization and carried to the surface along major thrustingfaults. To test this theory, CNRS, university of Toulouse and Stryde have collaborated to acquire apassive seismic data on a 3d grid using the latest generation of compact autonomous nodes, usuallyused for very dense active seismic surveys in O&G exploration. Given the very difficult access terrainand the size of the area to survey, the use of such technology was a major enabler in this project.Data acquisition and analysis900 autonomous nodes were deployed over a 10x10km area, plus an additional 2d line along a roadwith 100 stations. Each station was made of 3 single sensor nodes (Figure 1). The nodes were left torecord continuously for 1 month and were retrieved during the 2nd week of October 2022. 550 eventswere recorded during this period, half of them directly below the grid with magnitudes varyingbetween -2.1 and 2.3.ResultsThe north Pyrenean fault can clearly be identified on the event localization graph (Figure 2). Usingthese picks, a local earthquake tomography (LET) of the upper crust was performed. The tomographyvelocity model in shows two domains separated by an isovelocity of 5.6 km/s for P waves and 3.3km/s for S waves (Figure2). Low velocities, characteristic of sedimentary rocks (smaller than 5.6km/s) represents the decollement level of the Aquitaine Basin. Beneath we observe a Vp between 6.0and 6.4 km/s, characteristic of hercynian basement rocks. We do not observe any high velocityanomaly that could suggest the presence of serpentinized mantle in the shallow crust even though thishypothesis cannot be excluded in the case of a completely serpentinized mantle body which wouldhave physical properties close to those of a normal upper crust.ConclusionsWe show that passive seismic can be significantly enhanced using a much denser grid of receivermade possible by the latest generation of autonomous nodes.In this project, the tomography doesn’t seem to support the presence of serpentinized mantle in theshallow crust which raises new questions about the origin of natural hydrogen observed at the surface.Natural hydrogen exploration is at its early stages of development and requires the utilization ofvariety of geophysical techniques and instruments for establishment. Similar to the Oil & Gas sector,which has significantly benefited from geophysical tools, employing these technologies promises enhanced efficiency and vital data acquisition for the success of natural hydrogen exploration andrenewable energy industries in general. This convergence highlights the potential for leveragingestablished methodologies to bolster advancements in these evolving sectors.