Abstract
Several strong-motion networks have been installed in the Groningen gas field in the Netherlands to record ground motions associated with induced earthquakes. There are now more than 450 permanent surface accelerographs plus a mobile array of 450 instruments, which, in addition to many instrumented boreholes, yield a wealth of data. The database of recordings has been of fundamental importance to the development of ground-motion models that form a key element of the seismic hazard and risk estimations for the field. In order to maximise the benefit that can be derived from these recordings, this study evaluates the usability of the recordings from the different networks, in general terms and specifically with regards to the frequency ranges with acceptable signal-to-noise ratios. The study also explores the consistency among the recordings from the different networks, highlighting in particular how a configuration error was identified and resolved. The largest accelerograph network consists of instruments housed in buildings around the field, frequently installed on the lower parts of walls rather than on the floor. A series of experiments were conducted, using additional instruments installed adjacent to these buildings and replicating the installation configuration in full-scale shake table tests, to identify the degree to which structural response contaminated the recordings. The general finding of these efforts was that for PGV and oscillator periods above 0.1 s, the response spectral ordinates from these recordings can be used with confidence.
Highlights
The Groningen gas field, the largest in Western Europe, was discovered in 1959
As part of Nederlandse Aardolie Maatschappij B.V. (NAM)’s data acquisition plan, a second network was installed consisting of instrumented boreholes, each co-located with a surface accelerograph
The shake table tests show a consistent and clear pattern: the Fourier spectra from the ground-level and wall-mounted instruments are very similar for frequencies up to about 15 Hz and the response spectral ordinates are very similar at oscillator periods greater than about 0.1 s
Summary
The Groningen gas field, the largest in Western Europe, was discovered in 1959. Gas production from the field began in 1963 and the reservoir is estimated to be about three-quarters depleted. The purpose of this paper is to present some more details about the characteristics of the networks and the instruments with a view to providing insight into the usability of the growing number of recordings, which form an exceptional database of ground motions from induced earthquakes. Such an evaluation is of interest because it is a rather unique situation to have so many networks operating simultaneously in a relatively small region.
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