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.
Highlights
Large earthquakes and volcanic eruptions result from long-lasting, steady, pressure buildup on faults and magmatic reservoirs
We propose a complementary monitoring approach that uses ballistic waves reconstructed from noise correlations
The new approach is based on measuring temporal changes of apparent slowness of specific ballistic waves that have been reconstructed from noise correlations using dense arrays of seismic sensors (Boueet al. 2013b; Mordret et al 2014; Nakata et al 2015, 2016)
Summary
Large earthquakes and volcanic eruptions result from long-lasting, steady, pressure buildup on faults and magmatic reservoirs. Anthropogenic activities such as hydrocarbon extraction, wastewater disposal, CO2 storage and geothermal production induce fluid pore-pressure related deformation that can lead to the triggering of induced seismicity (Talwani et al 2007; Ellsworth 2013; Chang & Segall 2016) Monitoring these stress and porepressure perturbations continuously in time with high spatial accuracy at depth is critical to foresee forthcoming catastrophic tectonic and volcanic events and to improve reservoir management. The standard coda-based technique suffers from shortcomings that limit our ability to detect localized seismic velocity perturbations at depth This technique referred to as coda-wave interferometry (Poupinet et al 1984) has the advantage of being very stable thanks to its low sensitivity to noise source property changes (Colombi et al 2014). By providing direct observations of the rock mass response to stress changes at depth, this new passive seismic approach paves the way for improved in situ earthquake, volcano and producing reservoir monitoring
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