Abstract
The seismic hazard and risk analysis for the onshore Groningen gas field requires information about local soil properties, in particular shear-wave velocity (VS). A fieldwork campaign was conducted at 18 surface accelerograph stations of the monitoring network. The subsurface in the region consists of unconsolidated sediments and is heterogeneous in composition and properties. A range of different methods was applied to acquire in situ VS values to a target depth of at least 30 m. The techniques include seismic cone penetration tests (SCPT) with varying source offsets, multichannel analysis of surface waves (MASW) on Rayleigh waves with different processing approaches, microtremor array, cross-hole tomography and suspension P-S logging. The offset SCPT, cross-hole tomography and common midpoint cross-correlation (CMPcc) processing of MASW data all revealed lateral variations on length scales of several to tens of metres in this geological setting. SCPTs resulted in very detailed VS profiles with depth, but represent point measurements in a heterogeneous environment. The MASW results represent VS information on a larger spatial scale and smooth some of the heterogeneity encountered at the sites. The combination of MASW and SCPT proved to be a powerful and cost-effective approach in determining representative VS profiles at the accelerograph station sites. The measured VS profiles correspond well with the modelled profiles and they significantly enhance the ground motion model derivation. The similarity between the theoretical transfer function from the VS profile and the observed amplification from vertical array stations is also excellent.
Highlights
Induced earthquakes due to gas production in the Groningen field in the northern Netherlands has prompted the development of seismic hazard and loss estimation models in order to allow risk-informed decision-making with regard to mitigation options
The ground motion model for the Groningen field is comprised of predictive equations for spectral accelerations and peak ground velocity at a reference rock horizon and non-linear frequency-dependent amplification functions reflecting the dynamic response of the overlying soil layers (Bommer et al, 2017)
The ground motion model derivation has benefited from a database of recordings of ground motions obtained from accelerograph and borehole geophone networks installed in the Groningen field
Summary
Induced earthquakes due to gas production in the Groningen field in the northern Netherlands has prompted the development of seismic hazard and loss estimation models in order to allow risk-informed decision-making with regard to mitigation options. A key element of the seismic hazard and risk models for the Groningen field is a ground motion prediction model to estimate surface motions due to each possible earthquake scenario. The ground motion model for the Groningen field is comprised of predictive equations for spectral accelerations and peak ground velocity at a reference rock horizon (located at about 800 m depth) and non-linear frequency-dependent amplification functions reflecting the dynamic response of the overlying soil layers (Bommer et al, 2017). The first stage of the model building process is to deconvolve the recorded surface motions to the reference rock horizon The uncertainty in this process is greatly reduced by the accurate characterisation of dynamic properties of the soil column, in the uppermost tens of metres that exert the strongest influence on the site response. The paper describes how these tests were conducted and the procedures followed to reconcile the different measurements to construct the final profile for each station
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