Seismic properties of the Vienna basin

Modeling the seismic properties of tectonic active sedimentary basins s vital for localizing geo- resources and assessing seismic hazard. In Austria, the Vienna basin has been a target of studies focused on determining how seismic waves propagate through the basin for oil&gas and geothermal exploration. Moreover, the Vienna Basin was historically struck by moderate and geologically during the Pleistocene even strong earthquakes (Mw=6.3-7.0), posing serious concerns in term of seismic hazard for the city of Vienna. Both these tasks, assessing seismic hazard and basin subsurface exploration, rely on models reconstructed from the interpretation of active seismic data, i.e. where artificial explosions produce seismic P-waves that travel across the basin. Active seismics have been successfully used to map the bottom of the sediments across the entire region. But information on two, more elusive, seismic properties of the sediments, the velocity of the seismic S-wave in sediments and their seismic anisotropy, are not available for the Vienna basin due to the lack of fundamental, complementary data. The velocity of the S-wave through the sediments is the key-element for modeling the shaking of the ground given by seismic waves propagation. On the other hand, seismic anisotropy of the sediments, that implies a different velocity of propagation of the seismic waves as a function of the direction of propagation itself, is related to the presence of fluids and small fractures, a useful information, for example, for geothermal energy production. Thus, the lack of knowledge about these two characteristics severely limits basin investigation and ground-shaking prediction. A classical approach to basin exploration is to reconstruct the shape of the filled basin using different methods, and then combine these results and interpret them simultaneously. Geological and geophysical data are often integrated following such scheme, retrieving the P-wave velocity of the sediments in different part of the basin and the geometry of the bottom of the basin. However, a critical issue must be faced. Simultaneous interpretation of different images is based on expert opinion, and, thus, cannot give any quantification of the uncertainties in the basin model. More sophisticated approaches based on statistics are needed to quantitatively integrate the different data-sets in a coherent structural model of the basin. The goal of this project is to explore the Vienna basin addressing the two issues outlined above in two ways: (a) define a S-wave velocity model for the Vienna basin and map seismic anisotropy within the basin, merging newly acquired passive seismic data with published geological/geophysical data; and (b) adopt a probabilistic approach which allows to integrate information available at sparse points across the basin and estimate the uncertainties on the measured S-wave velocity.

The Lise Meitner project focused on the development of a new algorithm for defining the physical properties of the sediments across the Vienna basin. Physical properties of sediments are critical factors for (a) assessing seismic risk in the urban area of Vienna; and (b) exploiting buried geo-resources (e.g. geothermal energy). The new algorithm makes use of statistical analysis for integrating different models, each one belonging to a different source and representing a single physical property, and for merging them in comprehensive multi-parametric model. At present, three different models have been integrated: a model of rock elasticity derived by passive natural seismicity, a model of sub-surface structures from interpretation of active seismics, and a model of rock resistivity derived by magnetotelluric data. The new algorithm has been proved to produce coherent multi-physics images of a sedimentary basin. The new algorithm has been presented at geophysical scientific community at national (PanGEO 2018) and international (EGU General Assembly 2019) conferences.