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Derivation of a near-surface damping model for the Groningen gas field

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Title: Derivation of a near-surface damping model for the Groningen gas field
Authors: Ruigrok, E
Rodriguez-Marek, A
Edwards, B
Kruiver, PP
Dost, B
Bommer, J
Item Type: Journal Article
Abstract: Seismic damping of near-surface deposits is an important input to site-response analysis for seismic hazard assessment. In Groningen, the Netherlands, gas production from a reservoir at 3 km depth causes seismicity. Above the gas field, an 800 m thick layer of unconsolidated sediments exist, which consists of a mixture of sand, gravel, clay and peat strata. Shear waves induced at 3 km depth experience most of their anelastic attenuation in these loose sediments. A good estimate of damping is therefore crucial for modeling realistic ground-motion levels. In Groningen, we take advantage of a large network of 200 m deep vertical arrays to estimate damping from recordings of the induced events. As a first step, we apply seismic interferometry by deconvolution to estimate local transfer functions over these vertical arrays. Subsequently, two different methods are employed. The first is the ’up-going’ method, where the amplitude decay of the retrieved up-going wave is used. The second is the ’up-down’ method, where the amplitude difference between retrieved up- and down-going waves is utilized. For the up-going method, the amplitude of the up-going direct wave is affected by both elastic and anelastic effects. In order to estimate the anelastic attenuation it is necessary to remove the elastic amplification first. Despite the fact that elastic compensation could be determined quite accurately, non-physical damping values were estimated for a number of boreholes. Likely, the underlying cause was small differences in effective response functions of geophones at different depths. It was found that the up-down method is more robust. With this method, elastic propagation corrections are not needed. In addition, small differences in in situ geophone response are irrelevant because the up- and down-going waves retrieved at the same geophone, are used. For the 1D case we showed that for estimating the local transfer function, the complex reverberations need to be included in the interferometric process. Only when this is done, the transfer function does not contain elastic transmission loss and Q estimation can be made without knowing the soil profile in detail. Uncertainty in the estimated damping was found from the signal-to-noise ratio of the estimated transfer function. The Q profiles estimated with the up-down method were used to derive a damping model for the top 200 m of the entire Groningen field. A scaling relation was derived by comparing estimated Q profiles with low-strain damping profiles that were constructed using published models for low-strain damping linked to soil properties. This scaling relation, together with the soil-properties based damping model, allowed up-scaling of the model to each grid-cell in the Groningen field. For depths below 200 m, damping was derived from the attenuation of the microseism over Groningen. The mean damping model, over a frequency band between 2 and 20 Hz, was estimated to be 2.0% (0-50 m depth), 1.3% (50-100 m), 0.66% (100-150 m), 0.57% (150-200 m) and 0.5% (200-580 m).
Issue Date: 1-Sep-2022
Date of Acceptance: 14-Feb-2022
URI: http://hdl.handle.net/10044/1/95882
DOI: 10.1093/gji/ggac069
ISSN: 0956-540X
Publisher: Oxford University Press (OUP)
Start Page: 776
End Page: 795
Journal / Book Title: Geophysical Journal International
Volume: 230
Issue: 2
Copyright Statement: © The Author(s) 2022. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Keywords: Science & Technology
Physical Sciences
Geochemistry & Geophysics
Downhole methods
Induced seismicity
Seismic attenuation
Seismic interferometry
Site effects
Wave scattering and diffraction
Geochemistry & Geophysics
0403 Geology
0404 Geophysics
0909 Geomatic Engineering
Publication Status: Published
Online Publication Date: 2022-02-18
Appears in Collections:Civil and Environmental Engineering
Faculty of Engineering

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