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Seismological Parameters and INstrumentation

Periodic Reporting for period 1 - SPIN (Seismological Parameters and INstrumentation)

Reporting period: 2021-03-01 to 2023-02-28

We now understand that seismic wavefields alter the material when they pass through it and that these changes are measurable. Traditional seismic sensors - seismometer networks - provide us with high time resolution, but sparse spatial resolution. Right now, new sensing technologies (DAS, large N arrays, rotation sensors) are emerging that can give us much more detailed spatial information about how the seismic wavefield behaves. This means that we can study changes in local material properties, and investigate complex behaviour of materials as they deform under small strain. These sensing technologies open a new era of observations for which new skills need to be developed. In SPIN, we train a new generation of scientists to develop novel views about the dynamic behaviour of Earth materials, and in particular how to observe them with the revolutionary new sensing systems at hand. This research and training will impact the way we understand solid Earth processes, how we interrogate the Earth’s geomechanical behavior, and the way we forecast natural hazards.
The new instrumentation and their application are important, because dynamic material behaviour affects our societies: geomaterial alterations are associated with many natural hazards, such as volcanic eruptions, landslides, earthquakes, and the changing health status of civil structures such as bridges and buildings.

The overarching goal of the SPIN network is to make a major advance in Earth science by fully integrating the latest ground-motion sensing technology and training a new generation of unique researchers who can incorporate new sensor types into widespread, societally-relevant applications. Broken down into work packages the SPIN research objectives are

■ Implement high-quality complete ground motion measurements with new sensing technology for the full range of seismological applications (WP1)
■ Develop models of wave propagation that extend to the nonlinear and transient elastic properties of micro-inhomogeneous materials under low strain, and characterize these nonclassical effects (WP2)
■ Develop novel theory to design experiments and hazard monitoring systems using the new sensing technologies, optimizing ‘heterogeneous’ sensor networks that combine different instrumentation types (WP3)
■ Demonstrate the full impact of the new concepts for observation, instrumentation and interpretation on applications in different hazard settings in volcanology, earthquake physics, structural health monitoring, hazard early warning and permafrost monitoring (WP4)
We are developing strategies for testing and verifying the amplitude of ground motion measurements from new sensing technologies (e.g. dense networks of mechanical seismometers, rotation sensors, fiber-optic cables) which can be used both for dynamic and static observations. This requires fundamental research that combines laboratory and field studies with new sensing technologies.
Progress could be made both with additional rotational motions as well as distributed acoustic sensing (DAS). The latter could be incorporated in waveform inversion schemes. The overall inclusion of gradient observations into full waveform inversion is still in progress but will be one of the outcomes of SPIN.

Material properties:
Different physical models for the observed time-dependent changes in elastic wave propagation are developed and tested in controlled laboratory environments. So far, laboratory experiments were performed to image the transient internal processes at the microscale. On slightly larger scale, the effects of successive cycles of compression and decompression of a sandstone sample are tested. Experiments on a concrete test specimen of civil engineering scale were performed with the goal to characterize the signature of damage formation. All this is being tied together by the integration of experimentally gained knowledge into wave propagation codes.

In order to make efficient and effective use of heterogeneous sensor-arrays, we need to develop methods to optimise their deployment and process their data. So far, a method is being developed and tested to optimally design experiments, such as where to best place different types of sensors. Progress was made on understanding the effect of isolated noise sources on ambient noise correlation measurements, which impacts many applications relying on using seismic noise. Moreover, an approach to use existing fibre optic infrastructure in urban settings to monitor traffic automatically was developed and successfully tested.

The specific choice of Earth science applications in SPIN is driven by the expectation of the highest impact of new ground motion instrumentation. All application-oriented projects have successfully started with data acquisition and data analysis, and have achieved first promising results. Application areas include active volcanoes, where dynamic triggering of small seismic events was detected, and where nonlinear response of the ground was observed with DAS. Another project focuses on a landslide, where an extensive array of different types of sensors is currently recording a unique data set. Finally, an interdisciplinary project investigates damage indicators on a small bridge test-structure.
SPIN addresses a gap in the current doctoral training by fostering a deep knowledge of emerging seismic instrumentation to young scientists who will design and operate the geohazard monitoring systems of the future. Despite a slow start due to the global pandemic, the early career researchers involved in SPIN have already built up a solid network with their peers, and with top-level experienced researchers, as well as with several sensor development companies and other stakeholders. Most ESRs have done at least one secondment, or have one planned in the near future. This has led to a significant increase in collaborations across the different SPIN projects. In several cases, cross-pollination between different projects have started and will come to fruition in the next project phase. As stated above, the PhD projects have already reached a range of results that go beyond the state of the art in the first project phase, publications about these results are currently being prepared.

Understanding the internal physical processes governing the transient elastic and mechanical properties of geomaterials is key to the assessment of hazard related to material failure. By the end of the project, we expect to have made significant steps forward in understanding the macroscopic behavior of materials as damage slowly develops, and how this is expressed in the measurements of seismic gradients.

We are developing and applying new methods to data sets acquired with novel sensor technology, and on application areas relevant to geohazards. The application-driven projects in SPIN are motivated foremost by specific geoscience questions in societally relevant but challenging environments such as volcanoes, high-alpine regions and buildings. In such environments, logistical constraints prevent the installation of a large number of instruments. We expect a significant improvement in geohazard monitoring from full ground motion observations in these conditions particularly.
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