Ambient Seismic Noise Tomography: Some of Sisprobe Work

Noise-based ballistic wave passive seismic monitoring. Part 1: body waves  F Brenguier,  R Courbis,  A Mordret  et al. (2019) Geophysical Journal International, Vol 221, 683-691

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.

Virtual Sources of Body Waves from Noise Correlations in a Mineral Exploration Context. P. Dales, L. Pinzon-Ricon, F. Brenguier, et al. (2020). Seismological Research Letters (in. press)

The extraction of body waves from passive seismic recordings has great potential for monitoring and imaging applications. The low environmental impact, low cost, and high accessibility of passive techniques makes them especially attractive as replacement or complementary techniques to active‐source exploration. There still, however, remain many challenges with body‐wave extraction, mainly the strong dependence on local seismic sources necessary to create high‐frequency body‐wave energy. Here, we present the Marathon dataset collected in September 2018, which consists of 30 days of continuous recordings from a dense surface array of 1020 single vertical‐component geophones deployed over a mineral exploration block. First, we use a cross‐correlation beamforming technique to evaluate the wavefield each minute and discover that the local highway and railroad traffic are the primary sources of high‐frequency body‐wave energy. Next, we demonstrate how selective stacking of cross‐correlation functions during periods where vehicles and trains are passing near the array reveals strong body‐wave arrivals. Based on source station geometry and the estimated geologic structure, we interpret these arrivals as virtual refractions due to their high velocity and linear moveout. Finally, we demonstrate how the apparent velocity of these arrivals along the array contains information about the local geologic structure, mainly the major dipping layer. Although vehicle sources illuminating array in a narrow azimuth may not seem ideal for passive reflection imaging, we expect this case will be commonly encountered and should serve as a good dataset for the development of new techniques in this domain.

Ambient noise multimode Rayleigh and Love wave tomography to determine the shear velocity structure above the Groningen gas field, Chmiel, M., Mordret, A., Boué, P., et al. (2019). Geophys. J. Int.

To better understand earthquake mechanisms and volcanic eruptions we propose a new passive seismic monitoring approach based on detecting weak temporal changes of velocities of ballistic waves recovered from seismic noise correlations. We used a dense network of 417 seismometers in the Groningen area of the Netherlands, one of Europe’s largest gas fields. Over a 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 attribute this unexpected high velocity increase for the refracted wave to loading associated with rainfall and possibly canal drainage. A 0.25 per cent velocity decrease for the direct P-wave travelling in the near-surface sediments might be biased by changes in time in the noise source properties. This new technique may detect seismic velocity perturbations at a few kilometres depth paving the way for improved in situ earthquake, volcano and gas reservoir monitoring.

We summarise techniques used in mineral exploration and describe how passive seismic techniques can help to locate new ore deposits or determine the structure of known deposits. The basic principles are outlined and the advantages and disadvantages of the technique are discussed. The feasibility of the technique is evaluated by processing a combination of synthetic and real datasets to detect and locate known reflectors.

Train traffic as a powerful noise source for monitoring active faults with seismic interferometry Brenguier, F., Boué, P., Ben‐Zion, Y., Vernon, F., Johnson, C. W., Mordret, A., Coutant, O., Share, P.-E., Beaucé, E., Hollis, D. and Lecocq, T. (2019) Geophysical Research Letters, 46.

We show that seismic noise generated by vehicle traffic, and especially heavy freight trains, can be turned into a powerful repetitive seismic source to continuously probe the Earth’s crust at a few kilometers depth. Results of an exploratory seismic experiment in Southern California demonstrate that correlations of train‐generated seismic signals allow daily reconstruction of direct body waves probing the San Jacinto Fault down to 4‐km depth. This new approach may facilitate monitoring most of the San Andreas Fault system using the railway and highway network of California.

We have used freight trains as a noise source at the Marathon site in Ontario, as part of the H2020 PACIFIC project.