Amine Ourabah

Amine Ourabah (Stryde), Sebastien Chevrot (CNRS) , Matthieu Sylvander (CNRS),

Introduction

In recent years, the world has witnessed a surge in research and development of renewable energy
technologies, ranging from solar and wind power to geothermal and biomass solutions. These
innovative approaches undoubtedly hold the key to reducing our carbon footprint, yet several of them
require the extraction and utilization of scarce minerals, leading to potential environmental
degradation and geopolitical complexities. A potentially game-changing alternative has emerged on
the horizon - Natural hydrogen. Mainly generated by geological processes, Natural H2 has been
observed with different degrees of purity as surface seepages or occasionally in wells targeting other
resources. Although some distinct natural processes are known to generate hydrogen, exploring for it
and understanding its presence remain a challenging task due the lack of geological models and
geophysical data.

Context and survey location

Natural hydrogen gas emanations have been observed and measured along the North Pyrenean Frontal
Thrust 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 H2
may be sourced from mantle rocks serpentinization and carried to the surface along major thrusting
faults. To test this theory, CNRS, university of Toulouse and Stryde have collaborated to acquire a
passive seismic data on a 3d grid using the latest generation of compact autonomous nodes, usually
used for very dense active seismic surveys in O&G exploration. Given the very difficult access terrain
and the size of the area to survey, the use of such technology was a major enabler in this project.

Data acquisition and analysis

900 autonomous nodes were deployed over a 10x10km area, plus an additional 2d line along a road
with 100 stations. Each station was made of 3 single sensor nodes (Figure 1). The nodes were left to
record continuously for 1 month and were retrieved during the 2nd week of October 2022. 550 events
were recorded during this period, half of them directly below the grid with magnitudes varying
between -2.1 and 2.3.

Results

The north Pyrenean fault can clearly be identified on the event localization graph (Figure 2). Using
these picks, a local earthquake tomography (LET) of the upper crust was performed. The tomography
velocity model in shows two domains separated by an isovelocity of 5.6 km/s for P waves and 3.3
km/s for S waves (Figure2). Low velocities, characteristic of sedimentary rocks (smaller than 5.6
km/s) represents the decollement level of the Aquitaine Basin. Beneath we observe a Vp between 6.0
and 6.4 km/s, characteristic of hercynian basement rocks. We do not observe any high velocity
anomaly that could suggest the presence of serpentinized mantle in the shallow crust even though this
hypothesis cannot be excluded in the case of a completely serpentinized mantle body which would
have physical properties close to those of a normal upper crust.

Conclusions

We show that passive seismic can be significantly enhanced using a much denser grid of receiver
made possible by the latest generation of autonomous nodes.

In this project, the tomography doesn’t seem to support the presence of serpentinized mantle in the
shallow 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 of
variety 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 and
renewable energy industries in general. This convergence highlights the potential for leveraging
established methodologies to bolster advancements in these evolving sectors.

Figure 1 – left, position of the 3C stations. Right, deployment crew deploying Stryde nodes in a 3C
configuration.

Figure 2 – Left, North-South profile of recorded microseismic events localized in depth. Right,
Tomography inversion results (VP) of the same profile.