Noise-based ballistic wave passive seismic monitoring. Part 1: body waves

Abstract

SUMMARY Unveiling the mechanisms of earthquake and volcanic eruption preparation requires improving our ability to monitor the rock mass response to transient stress perturbations at depth. The standard passive monitoring seismic interferometry technique based on coda waves is robust but recovering accurate and properly localized P- and S-wave velocity temporal anomalies at depth is intrinsically limited by the complexity of scattered, diffracted waves. In order to mitigate this limitation, we propose a complementary, novel, passive seismic monitoring approach based on detecting weak temporal changes of velocities of ballistic waves recovered from seismic noise correlations. This new technique requires dense arrays of seismic sensors in order to circumvent the bias linked to the intrinsic high sensitivity of ballistic waves recovered from noise correlations to changes in the noise source properties. In this work we use a dense network of 417 seismometers in the Groningen area of the Netherlands, one of Europe's largest gas fields. Over the course of 1 month our results show a 1.5 per cent apparent velocity increase of the P wave refracted at the basement of the 700-m-thick sedimentary cover. We interpret this unexpected high value of velocity increase for the refracted wave as being induced by a loading effect associated with rainfall activity and possibly canal drainage at surface. We also observe a 0.25 per cent velocity decrease for the direct P-wave travelling in the near-surface sediments and conclude that it might be partially biased by changes in time in the noise source properties even though it appears to be consistent with complementary results based on ballistic surface waves presented in a companion paper and interpreted as a pore pressure diffusion effect following a strong rainfall episode. The perspective of applying this new technique to detect continuous localized variations of seismic velocity perturbations at a few kilometres depth paves the way for improved in situ earthquake, volcano and producing reservoir monitoring.

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

“…We are able to measure temporal changes of apparent velocities from both fundamental mode and first overtone Rayleigh waves, which allows us to discriminate between changes localized in the shallower part or the deeper part of the subsurface, in good agreement with the P-wave monitoring results (Brenguier et al 2019).…”

“…This procedure effectively approximates the propagating medium as a 1-dimensional medium. During the deployment, the main noise source in the considered frequency band comes from the direction of the North Sea, but significant energy arrival covers about 180 degrees, from South-West to North-East (Spica et al 2018;Brenguier et al 2019).…”

“…(for the fundamental mode and the first overtone, respectively) following the approach of Brenguier et al (2019). In this companion paper, Brenguier et al (2019) showed that the relative velocity change can be computed as the (weighted) linear regression of the time-shifts δt along the offset x (Fig. 4) multiplied by the velocity of the considered phase:…”

“…In this work and a companion paper (Brenguier et al 2019) we propose to overcome these limitations by using a new complementary method for monitoring seismic velocity variations based on the ballistic waves of the noise cross-correlations, instead of their coda waves. The first paper (Brenguier et al 2019) deals with body waves while this paper focuses on surface waves application.…”

“…In this work and a companion paper (Brenguier et al 2019) we propose to overcome these limitations by using a new complementary method for monitoring seismic velocity variations based on the ballistic waves of the noise cross-correlations, instead of their coda waves. The first paper (Brenguier et al 2019) deals with body waves while this paper focuses on surface waves application. Using ballistic waves means that, contrarily to coda-waves, we have accurate models for their propagation and therefore we can project the observed temporal perturbations of seismic velocities to specific regions at depth (Voisin et al 2016(Voisin et al , 2017.…”