SM2.1

The AlpArray Seismic Network (AASN) is a large-scale multidisciplinary seismic network in Europe that consists of over 600 3-component (3C) broadband stations with mean inter-station distance of 30-40km. This dense array allows the recording of the seismic wave propagation of distant earthquakes at a resolution of typical body and surface waves.

By animating the spatially-dense seismic recordings of the AASN, we can visualize seismic waves propagating across the European Alps as a function of space and time. Our 3C ground motion animations illustrate the full spatial-temporal evolution of global body and surface waves and demonstrates how a dense array allows the transformation from translation measurements at single stations to spatial gradients of the wavefield at the surface, capturing both small- and large-scale wave propagation phenomena. The addition of travel-time estimation, ray path illustration, and array-specific information such as slowness vector of incoming waves facilitate identification of seismic phases and their arrival-angle deviations. We will highlight some interesting observations of different seismic wave types in the animations of a few example teleseismic events during the course of the AASN between 2016-2019. Application for future research and education will also be discussed.

How to cite: Ling, O. K. A., Stähler, S., Giardini, D., and Group, T. A. W.: Visualizing the Seismic Wavefield with AlpArray, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-859, https://doi.org/10.5194/egusphere-egu21-859, 2021.

The Streckeisen STS-1 set a very high performance and lasting broadband (VBB) sensor standard that has been hard to match by other instruments, but these sensors also required a very careful emplacement and shielding from environmental changes and conditions, along with the high costs of ensuring the conditions for this level of instrument performance.  Recent developments have demonstrated equivalent and broader bandwidth sensors that enable deploying these types of sensors in most any terrestrial environment.  These new instruments, in many types of form factors, all magnetically shielded, open up new opportunities for continuing and expanding these VBB observations, democratizing the observations of these long period signals and opening up the possibilities of better performance through deep boreholes and observations of less developed sites that have harsher environmental conditions, along with recapitalizations of sites where STS-1s are no longer supported.   We will describe recent testing results of Trillium 360 GSN vault, borehole, and posthole sensors as well as the Horizon 360 from many observatories and new potential use cases, some in polar environments that were impractical until now, and discuss development of the new Horizon 360 OBS.

How to cite: Bainbridge, G., Hamilton, V., and Parker, T.: Democratizing and Densifying Low Noise Long Period Broadband Stations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13490, https://doi.org/10.5194/egusphere-egu21-13490, 2021.

A new type of portable six-component seismometer is invented and used in the development of a seismic imaging method for shallow subsurface anomaly detection. This new six-component seismometer contains a mini-MEMS-array for acceleration and rotational velocity measurements. The self-noise for acceleration measurement is about 8 µg/√Hz, and the self-noise for rotational velocity measurement is about 5 µrad/s/√Hz. The frequency band is DC-1000 Hz. Different from the traditional seismic imaging methods that require the deployment of an array of either one-component or three-component seismometers, our imaging method is established based on the data recorded at individual six-component seismometer. Because the rotational field (i.e., the curl field) gives information about the spatial gradient of a seismic wavefield, so the translational field together with the rotational field can be used to derive the frequency-dependent velocity (i.e., dispersion) of the formation right beneath a seismic station. This single station velocity inversion approach delivers localized subsurface velocity information, making it suitable for imaging of small-scale underground anomalies. Especially, the Rayleigh wave dispersion is used in our method as Rayleigh wave is generally the dominant signal in surface seismic data. An underground velocity model can be immediately constructed by consolidating the dispersion curves derived from individual receivers. In our study, we first demonstrate the accuracy of our imaging method through numerical modeling of various scenarios of subsurface anomalies and then conduct an experiment to further verify the performance of our self-invented six-component seismometer and the field applicability of our imaging method.

How to cite: Fang, X.: Application of a new type of six-component seismometer for underground anomaly detection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8551, https://doi.org/10.5194/egusphere-egu21-8551, 2021.

The field of rotational seismology has only recently emerged. Portable 3 component rotational sensors are commercially available since a few years which opens the pathway for a first use in volcano-seismology. The combination of rotational and translational components of the wavefield allows identifying and filtering for specific seismic wave types, estimating the back azimuth of an earthquake, and calculating local seismic phase velocities.

Our work focuses on back-azimuth calculations of volcano-tectonic and long-period events detected at Etna volcano in Italy. Therefore, a continuous full seismic wavefield of 30 days was recorded by a BlueSeis-3A, the first portable rotational sensor, and a broadband Trillium Compact seismometer located next to each other at Mount Etna in August and September of 2019. In this study, we applied two methods for back-azimuth calculations. The first one is based on the similarity of the vertical rotation rate to the horizontal acceleration and the second one uses a polarization analysis from the two horizontal components of the rotation rate. The estimated back-azimuths for volcano-tectonic events were compared to theoretical back-azimuths based on the INGV event catalog and the long-period event back-azimuths were analyzed for their dominant directions. We discuss the quality of our back azimuths with respect to event locations and evaluate the sensitivity and benefits of the rotational sensor focusing on volcano-seismic events on Etna regarding the signal to noise ratios, locations, distances, and magnitudes.

How to cite: Rosskopf, M., Eibl, E. P. S., Currenti, G., Jousset, P., Wassermann, J., Vollmer, D., Larocca, G., Pellegrino, D., Pulvirenti, M., and Contrafatto, D.: Performance of a rotational sensor at Etna, Italy focusing on back-azimuth estimations of volcano-seismic events, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10927, https://doi.org/10.5194/egusphere-egu21-10927, 2021.

To install a seismometer with a properly defined orientation - inside a vault or into a borehole - as a single station including various instruments or as a part of an array - an ‘adequate’ tool and an ‘absolute’ reference are needed.

In the past, and sometimes it persists nowadays, magnetic North have been used as a reference for Z-orientation of seismic station. Several studies have extensively measured the orientation error that have been made with this method, using an optical gyrocompass providing True-North as a reference, and their work will be summarized here.

In these studies, optical Gyrocompass is said to be the good solution, even if it is too heavy, expensive, and difficult to export. This paper will explain how iXblue has overcome these limitations to design the new-born Seistans Optical Gyrocompass.

Moreover, to aim True-North with a reliable accuracy is not the only think you need to do on the field. The method to transfer the North-line from the gyrocompass to the instrument to aligned must not induce errors that ruined the accuracy obtained using state-of-the-art gyrocompass. So an exhaustive study of the different ways to transfer the orientation from the compass to the aligned sensor will be presented, and corresponding added uncertainty will be evaluated, which is a good way to promote good practice on the field.

Finally, some figures will be gathered and shared from literature to quantify the precision needed for the alignment of a seismic sensor. There are today so few papers about this important matter that it is worth to spread their information.

How to cite: Guattari, F., Auregan, P., de Toldi, E., Laudat, T., and Mattio, L.: Short Review of True-North Alignment Method on the Field Demonstrating the benefits of the use of an Optical Gyrocompass, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16524, https://doi.org/10.5194/egusphere-egu21-16524, 2021.