Executive summary: Permitting of the seismic survey and the acquisition of data are the first steps in WP3, the pilot test of the passive reflection seismic technique in the Marathon deposit. The processing and development stages of the Work Package rely directly on the successful acquisition of ambient seismic noise data from the Marathon test site.
Between September 17th and October 26th of 2018, at the Marathon test site, a 1025 sensor passive seismic survey was completed. The sensors equipment was rented from SAExploration. 1024 sensors were successfully deployed; however, only 1019 were recovered. The loss of sensors was due to animal activity or being buried by a rock slide.
The grid design was composed of two overlapping grids, a 416-sensor array and a 609-sensor profile line. The array had a grid spacing of 150m, while the profile line had a grid spacing of 50m. Both grids designs were configured along the main noise source of Lake Superior in the direction of 250deg to the west.
The sensors selected for the survey were ZL and C1, vertical direction sensors with a 10hz range. Once the sensors were retrieved, they were shipped back to SAExploration for download. The data was successfully downloaded and shipped to Sisprobe for analysis.

Train traffic as a powerful noise source for monitoring active faults with seismic interferometry.

Laboratory experiments report that detectable seismic velocity changes should occur in the vicinity of fault zones prior to earthquakes. However, operating permanent active seismic sources to monitor natural faults at seismogenic depth is found to be nearly impossible to achieve. 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 P 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.

Executive summary: This report presents two versions of the flyer produced in the framework of the PACIFIC project. Both documents are included as annexes to this deliverable and can be downloaded on PACIFIC public website: https://www.pacific-h2020.eu/media/ 

The project flyer is a means to introduce PACIFIC to the public and more specifically to stakeholders in the project research domain. The initial version of the PACIFIC flyer produced at the start of the project (see section 2 - first item) sums up the project objectives, background and expected results. It also includes the list of partners
involved, as well as contact details for the project. This version was printed and distributed at partners' premises and during the PACIFIC internal and external events organised in the first two years of the project.

A digital update of the PACIFIC flyer has been prepared at the beginning of the third year (see section 2 ʹsecond item) to provide further information on the challenges addressed by the PACIFIC research activities, the expected results and their expected impact on mineral exploration. The list of project partners, the logo section and the coordination team section have also been updated as required. 

 Charlie Beard's presentation can be viewed on the link https://www.youtube.com/watch?v=g_qdwQGHNhs 

Noise-based passive ballistic wave seismic monitoring on an active volcano

Monitoring temporal changes of volcanic interiors is important to understand magma, fluid pressurization and transport leading to eruptions. Noise-based passive seismic monitoring using coda wave interferometry is a powerful tool to detect and monitor very slight changes in the mechanical properties of volcanic edifices. However, the complexity of coda waves limits our ability to properly image localized changes in seismic properties within volcanic edifices. In this work, we apply a novel passive ballistic wave seismic monitoring approach to examine the active Piton de la Fournaise volcano (La Réunion island). Using noise correlations between two distant dense seismic arrays, we find a 2.4 per cent velocity increase and −0.6 per cent velocity decrease of Rayleigh waves at frequency bands of 0.5–1 and 1–3 Hz, respectively. We also observe a −2.2 per cent velocity decrease of refracted P waves at 550 m depth at the 6–12 Hz band. We interpret the polarity differences of seismic velocity changes at different frequency bands and for different wave types as being due to strain change complexity at depth associated with subtle pressurization of the shallow magma reservoir. Our results show that velocity changes measured using ballistic waves provide complementary information to interpret temporal changes of the seismic properties within volcanic edifices.

Executive summary: The passive seismic survey of the Kaiserstuhl test site was initiated during discussion between partners of the PACIFIC and HiTech AlkCarb H2020 projects in February 2019. The final survey design is similar to an already existing geophysical profile crossing the Kaiserstuhl volcanic edifice surveyed by electrical techniques. A total of 66 3-component nodes were deployed along of the profile, and at the center, above a fault, a few additional nodes were deployed away from the profile to be used for earthquake detection and localization.

The nodes recorded during 25 days in October-November 2019. The raw data was of good quality with stable ambient noise records over the whole duration of acquisition. Cross-correlation showed Rayleigh and Love propagation with weak dispersion, testifying to a homogeneous medium in terms of seismic velocities. However individual correlations showed low signal to noise ratio.

We processed the correlations using a classical method of surface wave tomography, and we jointly inverted Rayleigh and Love wave dispersion curves in order to obtain a 3D S-wave velocity model.

The final Vs model showed three layers parallel to the surface with strong velocity contrasts. This is unexpected in such geological context and could be an edge effect of the inversion near the bottom of the model. Once removed, the Vs Anomaly model shows a homogeneous medium with only weak velocity changes (<5%). A positive anomaly dominates the model and coincides well with the location, size, and shape of the carbonatite pipe found in the existing geological and geophysical models.

Executive summary: Active seismic sources such as explosives, air guns and vibroseis generate energetic P-waves well suited for reflection seismic studies. However, they can have negative environmental impacts and are expensive, both of which have motivated the development of passive seismic methods. Passive seismic methods utilise ambient noise from meteorological and anthropogenic activity. They have been successful for surface wave recovery but extracting body waves for reflection imaging is still a challenge. A key goal of the PACIFIC project is to develop methodologies for extracting body waves from passive seismic data, and for using these body waves for subsurface imaging. This report describes the development of synthetic velocity models that characterise the geological structure and seismic reflectivity at the Marathon Cu-PGE prospect Ontario, Canada. Synthetic seismic signals generated in these models will then be used to develop and test processing procedures for body wave recovery and body wave imaging. A first velocity model consists of two vertical sections obtained by interpolation of lithological contacts identified in drillholes. One section is perpendicular to the dip of the main gabbro intrusion, the other is parallel. A second model is obtained by blind 3D interpolation between drillholes and uses velocities measured on hand samples and drill core. Work in progress uses dedicated geological modelling software to generate a 3D block model that honors geological structures and cross-cutting relationships. A recently acquired downhole acoustic log avoids negative velocity biases from microfractures that can be introduced during depressurisation (e.g. of drill core). This will be used to calibrate a new velocity forward model.

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

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.