Harmonic response of soil radon-222 flux and concentration induced by barometric oscillations

Frédéric Perrier (1) , Frédéric Girault (1)

1 IPGP - Institut de Physique du Globe de Paris

Radon-222 flux and concentration in the soil are sensitive to changes in atmospheric pressure, and in particular to periodic signals, such as the semi-diurnal barometric tide S2. The response of radon flux and concentration to barometric oscillations is calculated analytically for all harmonic degrees in the case of a horizontal layer over a half-space, representing the situation of a soil layer over homogeneous bedrock, taking into account air and water phases and the presence of static vertical advection. The calculations show that the presence of an interface changes dramatically the response to barometric oscillations. Large amplitudes at the forcing frequency (fundamental) are concentrated in the vicinity of the interface, while amplitudes remain negligible at the same depth in a homogeneous half-space. A significant negative phase shift in the surface radon flux is introduced when a shallow interface is present, while radon flux is almost in phase opposition with atmospheric pressure over a homogeneous half-space. While, in most situations, the amplitudes are small and difficult to detect, situations can be exhibited where the amplitudes of harmonics 2 and 3 of radon concentration are larger than 100 Bq m −3 , leading, for example, to possible detection of 4-hr peaks in the radon power spectra due to barometric tide S2. Optimal position of the radon sensors appears to be a few centimetres in the bedrock below the soil. Amplitudes of radon concentration and surface flux are sensitive to underlying bedrock permeability, porosity, water saturation and effective radium concentration, and depend also on the presence of advection. At large carrier gas velocities, a more precise calculation valid for multilayered media, is presented, which can be used in volcanic and hydrothermal areas. While the amplitudes of all harmonics for radon concentration in the soil become negligible, the fundamental in radon surface flux reaches a constant and, possibly, observable value dominated by the parameters of the deepest medium. A better knowledge of the response of radon flux and concentration to barometric oscillations is important to interpret the presence or absence of peaks in the power spectra of radon time-series collected for environmental and geodynamical purposes. This study provides further support to the relevance of long-term radon monitoring to constrain the transport properties of the subsurface.

Mots clés: Non-linear differential equations; Gas and hydrate systems; Hydrothermal systems; Permeability and porosity; Fracture and flow

Domaines: Planète et Univers [physics]

Frédéric Perrier, Frédéric Girault. Harmonic response of soil radon-222 flux and concentration induced by barometric oscillations. Geophysical Journal International, 2012, 195 (2), pp.945-971. ⟨10.1093/gji/ggt280⟩. ⟨insu-01298238⟩