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| Titel |
Shear wave velocity for the upper 30 m: Combining a 3D voxel model and seismic CPTS for the Groningen gas field, the Netherlands. |
| VerfasserIn |
Roula Dambrink, Jan Gunnink, Jan Stafleu, Ger De Lange, Pauline Kruiver |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250129302
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| Publikation (Nr.) |
 EGU/EGU2016-9395.pdf |
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| Zusammenfassung |
| The Groningen gas field in the Netherlands is one of the largest gas fields of Europe
and has been in production since the 1960’s. Due to the progressive depletion of
the reservoir, induced seismic activity has increased in recent years. In 2012, an
earthquake of magnitude 3.6 initiated further research in prediction and management
of risks related to man-induced earthquakes. Last year the government decided
to reduce the gas extraction for this reason. One of the topics of concern is the
large difference in earthquake-related damage to buildings which, in addition to the
distance to the epicenter, appears to be also related to the composition of the shallow
subsurface.
To improve the spatial distribution of Shear Wave Velocities (Vs) in the shallow
subsurface, used for hazard prediction, the Geological Survey of the Netherlands and Deltares
constructed a Vs30 map of the upper 30 m of the gas field. In this map a high-resolution
geological model (GeoTOP) is combined with seismic cone penetration tests (SCPT) from
the area. The GeoTOP model is a 3D voxel model of the upper 50 m, in which each voxel
(100x100x0.5 m) is attributed with lithostratigraphy and the most likely lithological class
(peat, clay, fine sand, etc.). To obtain statistical distributions (with mean and standard
deviation) of Vs for each combination of lithostratigraphical unit and lithoclass, 60 SCPTs
were analyzed. In this way, it was possible to assign a specific Vs to each voxel in the
model. For each voxel in the stack of voxels that covers the upper 30 m (i.e. 60
voxels), a Vs value was randomly drawn from the statistical distribution of the
lithostratigraphical – lithoclass combination it belongs to. The Vs30 for each voxelstack is
then calculated using the harmonic mean of the Vs of the 60 voxels. By repeating this
procedure 100 times, an (average) Vs30 map and the uncertainty in Vs30 has been
constructed.
Using the procedure described above we were able to delineate zones with
distinct Vs30 characteristics: areas containing predominantly soft Holocene deposits
with low Vs30 and areas with predominantly stiff Pleistocene deposits with high
Vs30, complemented with the uncertainty in Vs30. The incorporation of the high
resolution geological model resulted in a large improvement compared to the previously
used Vs30, which was a simple interpolation of Vs30 from a limited number of
CPTs and SCPTs. This new procedure also gives the opportunity to study the role
of the upper subsurface in site amplification predictions. Using this new method,
risks related to gas extraction in urban areas can be better managed and predicted. |
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