Using horizontal-to-vertical spectral ratios to construct shear-wave velocity profiles

For seismic hazard assessment and earthquake hypocentre localization, detailed shear-wave velocity profiles are an important input parameter. Here, we present a method to construct a shear-wave velocity profiles for a deep unconsolidated sedimentary layer by using strong teleseismic phases and the ambient noise field. Gas extraction in the Groningen field, in the northern part of the Netherlands, is causing low-magnitude, induced seismic events. This region forms an excellent case study due to the presence of a permanent borehole network and detailed subsurface knowledge. Instead of conventional horizontal-to-vertical spectral ratios (H/V ratios) from amplitude spectra, we calculate power spectral densities and use those as input for H/V calculations. The strong teleseisms provide resonance recordings at low frequencies, where the seismic noise field is too weak to be recorded well with the employed geophones and accelerometers. The H/V ratios of the ambient noise field are compared with several forward modelling approaches to quality check the teleseism-based shear-wave velocity profiles. Using the well-constrained depth of the sedimentary basin, we invert the H/V ratios for velocity profiles. A close relationship is observed between the H/V spectral ratios from the ambient noise field, shear-wave resonance frequencies and Rayleigh-wave ellipticity. By processing only five teleseismic events, we are able to derive shear-wave velocities for the deeper sedimentary sequence with a 7% bias in comparison with the existing detailed velocity model for the Cenozoic sediments overlying the Groningen gas field. Furthermore, a relation between resonance frequency and unconsolidated sediment thickness is derived, to be used in other areas in the Netherlands, where detailed depth maps are not available

Assessing soil amplifications in Groningen, the Netherlands

Since the shallow part of the Dutch subsurface is practically always unconsolidated, the elastic waves generated by deeper (~3000 m) seated earthquakes will be subjected to transformation when arriving in these layers. Since, the number of induced seismic events has increased over recent decades, a better understanding of site response of the Dutch subsurface is required. Local site amplification can directly be measured due to the presence of sensors at multiple depth levels in the Groningen borehole network. Amplification factors from 73 local events have been calculated for each borehole location to quantify earthquake site response. Furthermore, horizontal-tovertical spectral ratios (HVSR) from the ambient seismic field are calculated. A relationship has been established between the composition of the upper Holocene sediments and the size of the amplitudes of HVSR and earthquake site response. Highest amplitudes are measured where the Holocene sediments are composed of clay, fine sands, silts and peat. We can conclude that HVSR from the ambient seismic field can be used as a first-order proxy to get an indication for wave amplification during a seismic event. This allows a first assessment on wave amplification at sites without sensors at multiple depth levels and without abundant local seismicity

Designing a Decision Support System for subsurface activities:A meta analysis of the design of a social acceptance motivated decision support system for subsurface activities in the Netherlands

The decision-making process for subsurface activities in the Netherlands has been unable to cope with the driving forces related to social acceptance in several recently proposed subsurface activities. We therefore investigated the possibility to include the triangle of social acceptance in the decision-making process. Our conceptual model relates the stakeholders, their goals and the driving forces to each other. We developed a framework, which describes the interaction between eleven design criteria for a Decision Support System (DSS). This framework will enable us to design a better, from a social acceptance perspective, DSS for subsurface activities in the Netherlands. Since the goals addressed in the decision-making process are very broad and the stakeholders are quite diverse, a single uniform DSS is not able to provide a satisfactory solution. We therefore suggest to design a DSS that is matched with each class of social acceptance