ALBERT

Description: Distributed acoustic sensing (DAS) can be deployed on existing submarine fibre optic cables to add sensor capability to the cable infrastructure. This does not require offshore installations and the DAS interrogation can also coexist with data traffic on telecom fibres. In this way, DAS provides a cost-efficient means to monitor marine cables with application to environment studies and seismology, seabed activity from e.g. fishing, and changes to the cable condition. For monitoring of seismic signals, DAS provides several advantages over traditional seismic sensors including a dense spatial sampling of the wavefield (on the order of metres) over long ranges (〉100 km). It also poses some challenges. For example, the single-component measurement (along the cable) can lead to ambiguity in event location due to the unresolved directionality of incoming waves. Here we will present observations from a long-term study on a Tampnet North Sea submarine telecom cable. We have characterized local earthquakes and underwater explosions in addition to anthropogenic activity. After post-processing to suppress marine noise, a detailed image of the earthquake wavefield with clear P- and S-arrivals can be observed. Recordings of underwater explosions are characterized by low-frequency first arrivals refracted through the subsurface followed by slower propagating high frequency and high amplitude direct arrivals through the water column, which can clearly be observed over the full length of the cable. Furthermore, tracking the travel-time moveout along the non-straight cable route allows for positioning of the source, which we demonstrate by back-propagating the recorded wavefield.

Description: Common practice to perform a seismic hazard study at regional scale is to present the results computed at bedrock condition. Site effects are usually not considered because soil information is not always available. Our goal is to carry out a site-specific seismic hazard study for the city of Oslo (Norway), conducting a microzonation study that will include site effects. More specifically, the average shear wave velocity in the upper 30 meters of the ground (Vs30) has been introduced to become the main attribute to characterize the soil type and subsequently account for soil type related seismic amplification Vs30 values are missing for the Oslo area, therefore we propose an integrated approach that estimates Vs30 using Horizontal-to-Vertical Spectral Ratio (HVSR) method together with a combined geology-slope approach. A campaign with 61 H/V measurements has been performed between March and June 2021 at specific locations in Oslo. Topographic slope attributed are extracted from DTM10 (10 m horizontal-scale resolution Digital Terrain Model), provided by the Norwegian Mapping Authority agency. In addition to resulting Vs30 values, this method will also allow us to define depth to bedrock estimates, which will likely be required for an updated Eurocode. This procedure is applied in specific areas in the Oslo urban environment that are susceptible to site effect amplifications.

Description: The Alna district in Oslo, Norway is a high profile (residential, commercial and industrial) area exposed to multiple hazards, in particular to quick clay landslides. Quick clay is a marine clay sediment where the salt that binds the clay together has been washed away over time. Thus, the structure becomes unstable, and increased point loads or erosion of waterways can trigger landslides. In case of a landslide, the quick clay becomes liquefied, and major material damage and danger to life can occur. To support the authorities in selecting the optimal risk-mitigation action, this study develops the multi-criteria decision-making methodology, which accounts for the stakeholders' preferences. The Decision Support System framework includes three key steps: 1) Identify criteria for assessing the impacts of multi-hazard scenario; 2) Identify possible measures that can reduce the impacts; 3) Give objective/subjective weighting of the identified criteria, reflecting the stakeholders’ preferences. The framework is demonstrated in a multi-hazard scenario (i.e., quick clay, flood and fire) of interest to the Oslo stakeholders. The impact of the considered scenario is evaluated in terms of five categories (i.e., life and health, nature and environment, economy, societal stability, and governance and control) and 13 single criteria. Through workshops with stakeholders, possible alternatives as well as the respective cost and mitigated impacts were estimated, and stakeholders’ preferences were collected. The optimal action under given circumstances is identified quantitatively and is highly dependent on stakeholders' preferences. Further details of the impact analysis are needed to understand the risk landscape of different alternatives.

Description: Distributed Acoustic Sensing (DAS) is becoming increasingly popular for different seismological applications due to its high spatial sampling and potentially long cable lengths. So far, the ability of DAS records for seismic array processing has not been assessed systematically with observed data. We use one month of continuous data recorded on the newly deployed NORFOX DAS array in southern Norway, which is co-located with the 16-element NORES broadband seismic array. NORES has a current aperture of approximately 1.4 km. NORFOX consists of a total cable length of approximately 8 km arranged in 5 arms, with an array aperture of 3 km. Data were acquired from three branches of the array using two DAS interrogators for the full month of February 2023. We analyze this unique data set by first comparing waveforms and signal to noise ratios (SNRs) of local, regional and teleseismic events detected on NORES with the DAS records. We then apply beamforming to the DAS data to evaluate SNR improvements for P and S wave beams deployed for all recorded events. Finally, results of Frequency-Wavenumber analysis of NORFOX and NORES data are compared. We test the findings of a previous theoretical study on DAS array processing which considers the broadside sensitivity of the cable and give a final evaluation of the potential of using DAS arrays for continuous seismic monitoring.

Description: We present an outstanding record of local, dense Large-N seismic and distributed acoustic sensor observations of a meteoroid from July 2, 2021 in Iceland. Our dataset includes high-quality observations from seven small aperture arrays of few hundred meters, an infrasound array, and a rotational station, all located within the distance range of 300 km. The high-frequency data show a variety of different phases associated with the source process along the atmospheric trajectory, including impulsive negative 1 first ground motions, a complex coda wave train about 2.5 s long thereafter, an azimuth-dependent stopping phase with reversed polarity between 1-25 s after the first arrival, which is resolved over only a few kilometers. The ground motion amplitude between the first and last arrivals is generally elevated. We associate the waveform in the 2.5 s coda with meteor-atmosphere interactions and nonlinear plasma processes that produce an oscillating shock-wave source pulse. Our data suggest a small azimuth-dependent deflection or dispersion of this source pulse, which may be related to the meteoroid’s deceleration in the atmosphere. We present a finite-length kinematic line-source pulse model that consistently explains the different phases inside and outside the Mach cone segment of our images, their wave amplitude variations, and a polarity change between the first phase and the terminating phase. The previously undiscovered rich directivity effects can also explain seemingly contradictory, time-dependent wave energy beam-directions at the various small aperture arrays and along the DAS cable. A combination of conventional locations and a Bayesian inversion of first and stopping phase arrivals led to a precise localization of the meteor trajectory.

Description: A common challenge in acoustic meteoroid signal analyses is to discriminate whether the observed wavefield can be better described by line-source or point-source models. This challenge typically arises from a sparse availability of observations. In this work, we present an outstanding record of ground-coupled waves from local large-N seismic and distributed acoustic sensing (DAS) observations of a meteoroid in Iceland. Our complete data set includes additional regional stations located within 300 km of the meteoroid’s trajectory. The dense large-N and DAS data allow identification of acoustic phases that are almost impossible to discriminate on sparser networks, including a weak late arrival resolved mostly only by DAS. Using this data set with a new Bayesian inversion model, we estimate the trajectory parameters of one fragment from the meteoroid. With these results we investigate its orbit in the solar system and propose a classification of the Icelandic event as a slow meteoroid of asteroidal origin with an energy on the order of 4–40 GJ, a probable size on the order of centimeters, and an orbit range consistent with the main asteroid belt.

Description: Microseismic monitoring represents a key surveillance technology to verify the integrity of subsurface CO storage sites. The precise location of microseismic events is first and foremost a direct and immediate indication of caprock and seal behavior but could also provide insight into CO plume migration. Tiny precursor movements provide diagnostic information about injection-related reservoir and caprock dynamics long before potential seal failure occurs. We present a case study from the Quest CCS facility in Canada, where a variety of different monitoring technologies are employed. We present the different microseismic sensor technologies and array configurations currently installed at the site and compare them against each other with respect to their reliability and effectiveness in providing the required verification information.