Noise-based ballistic wave passive seismic monitoring – Part 2: surface waves

Abstract

SUMMARY We develop a new method to monitor and locate seismic velocity changes in the subsurface using seismic noise interferometry. Contrary to most ambient noise monitoring techniques, we use the ballistic Rayleigh waves computed from 30 d records on a dense nodal array located above the Groningen gas field (the Netherlands), instead of their coda waves. We infer the daily relative phase velocity dispersion changes as a function of frequency and propagation distance with a cross-wavelet transform processing. Assuming a 1-D velocity change within the medium, the induced ballistic Rayleigh wave phase shift exhibits a linear trend as a function of the propagation distance. Measuring this trend for the fundamental mode and the first overtone of the Rayleigh waves for frequencies between 0.5 and 1.1 Hz enables us to invert for shear wave daily velocity changes in the first 1.5 km of the subsurface. The observed deep velocity changes (±1.5 per cent) are difficult to interpret given the environmental factors information available. Most of the observed shallow changes seem associated with effective pressure variations. We observe a reduction of shear wave velocity (–0.2 per cent) at the time of a large rain event accompanied by a strong decrease in atmospheric pressure loading, followed by a migration at depth of the velocity decrease. Combined with P-wave velocity changes observations from a companion paper, we interpret the changes as caused by the diffusion of effective pressure variations at depth. As a new method, noise-based ballistic wave passive monitoring could be used on several dynamic (hydro-)geological targets and in particular, it could be used to estimate hydrological parameters such as the hydraulic conductivity and diffusivity.

Seismic Ambient Noise Analyses Reveal Changing Temperature and Water Signals to 10s of Meters Depth in the Critical Zone

“…We made use of not only the 36 station pairs within the SPMS subarray, but also of longer offset pairs within a set of 10 additional stations around it (orange triangles in Figure 1), which are approximately (within 0.5 m) on the same topographic plane as the SPMS stations and could therefore be rotated into the same slope-aligned coordinate system. In total, we considered a range of interstation distances from 4.6 to 57.6 m. Cross-correlations with similar interstation distances (binned in 0.25 m increments) were stacked together to help increase signal-to-noise ratio (Boué et al, 2013;Mordret et al, 2014Mordret et al, , 2020Nakata et al, 2015). Cross-correlations were then high-pass filtered at the frequency that would produce a wavelength equal to the interstation distance-typically the limit for FTAN analysis (Luo et al, 2015)-at a velocity of 350 m/s (a value chosen empirically after initial analysis).…”

Toward Using Seismic Interferometry to Quantify Landscape Mechanical Variations after Earthquakes

“…Where boreholes are not available, active or passive seismic experiments may help to constrain the shallow velocity structure, and to adequately quantify the magnitude of shallow changes. New methods focusing on correlating specific types of waves may also allow to better locate shallow changes (Mordret et al, 2020).…”

Physics-Based Relationship for Pore Pressure and Vertical Stress Monitoring Using Seismic Velocity Variations

“…At a depth of 50 m, used by Fokker and Ruigrok [16], motion of the first overtone dominates over the motion of the fundamental mode. In Mordret et al [19], it is shown that, in the Groningen setting, the first overtone of the Rayleigh wave has a higher sensitivity to velocity changes than the fundamental mode.…”

“…In another study, a dense surface network of stations was used to detect velocity changes in refracted waves over a one-month period [18]. Using the same dense array of 417 stations, a novel implementation of passive image interferometry was tested [19]. Following heavy rain, they found a velocity reduction that propagates downward with time.…”