**4. Conclusions**

This paper analyzes whether pressure- or temperature-driven air flow can explain the temporal variations in hydrogen concentration measured at 1 m depth along the perimeter of a 550 m diameter, largely barren depression in the Sao Francisco Basin in Brazil. Although the temporal variations could be caused by other processes such as solid earth tides or temperature-dependent bacterial *H*2 generation, etc., we find:

1. The variations in hydrogen concentration measured at 1 m depth could be caused by propagation of a pressure wave into the subsurface, but not by the propagation of a thermal wave. Diurnal

temperature changes penetrate less than a meter into the subsurface and produce only weak perturbations of gas flow above the one-meter sensor depth.


**Author Contributions:** L.C. was responsible for the modeling, A.P. for field geology and framing the problem to be addressed. Both contributed equally to the writing of the paper. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work reported here was not supported by any gran<sup>t</sup> by any agency.

**Acknowledgments:** We thank the Scientific Direction of Engie S.A. for having launched the Natural H2 Project. The sensors installed in Brazil that allowed monitoring measurements were developed by the Engie Laboratory CRIGEN. We specially thank Fabian Rupin for the last version of these sensors and Joao Françolin (Georisk) for his geological collaboration in the field and for data acquisition.

**Conflicts of Interest:** The authors declare no conflicts of interest.
