Data Services Newsletter

Volume 14 : No 1 : Spring 2012

The Relevance of Seismic Fields for Gravitational-Wave Detection

A first generation of large-scale laser-interferometric gravitational-wave (GW) detectors was operated until 2010, each detector measuring differential strains between two orthogonal directions with strain sensitivities of a few times 10-23/rtHz around 100Hz. Above a few tens of a Hz, the detectors were isolated from ground motion with the hope to detect weak strain signals caused by propagating space-time perturbations known as GWs from merging binary neutron stars or black holes and other potentially unknown astrophysical sources. The two NSF funded LIGO detectors are located in Washington State and Louisiana. These detectors were part of a world-wide network including the GEO600 detector in Germany and the Virgo detector in Italy. The Virgo and LIGO detectors are currently being upgraded and scheduled to resume operation in 2015 with about 10x better strain sensitivity. The new Japanese underground detector KAGRA will join the network a few years later. Whereas the first generation of detectors has not detected any astrophysical signals, it is predicted that the next generation of detectors will see many of these events per year.

Ambient seismic fields and teleseismic disturbances play a very important role for GW detectors. Ground motion below 10Hz needs to be filtered out from the system to keep the interferometers at their designated operation point. Ground motion above 10Hz would be a direct limitation of the detector sensitivity if it was not suppressed by a sophisticated multi-stage active and passive isolation system. With a third generation of detectors, we would try to measure at even lower frequencies maybe down to 1Hz, which makes the seismic isolation problem increasingly difficult. Future detectors will also be sensitive to an entirely new noise source originating from seismic fields; the gravity perturbations caused by surface displacement and density fluctuations from compressional waves. Many people consider this so-called Newtonian noise as impenetrable noise wall that dominates instrumental noise below 10Hz. Other people however have accepted this challenge and conceived a few strategies to mitigate the Newtonian noise.

There are two promising mitigation strategies. The first one is to search for new detector sites that have very weak ambient seismic fields. For this reason, an underground site was chosen for the Japanese detector KAGRA, and it seems likely that all new third-generation detector sites will be underground. This is not only a favorable choice because of ambient seismic fields, but also because atmospheric perturbations cause gravity noise that would be highly suppressed underground. The second strategy is to monitor ground motion with seismic arrays, and to construct an estimate of the Newtonian noise from the seismic data that is coherently subtracted from the GW detector data. A similar scheme could also be applied to atmospheric perturbations using other sensors as for example microphones. In either way, we need a much better understanding of ambient seismic fields especially in the range 1Hz to 30Hz.

Screenshot of the Google Earth ambient seismic noise map
Figure 1: Screenshot of the Google Earth ambient seismic noise map. Each dot represents a seismic station and its color indicates the seismic spectral densities at 1.1Hz. The slider in the lower left corner can be used to change the frequency. A station is selected showing additional information about the station including a spectral histogram.

As the site-selection problem is potentially relevant for a few regions including the US and Europe, we have started to study ambient fields everywhere on Earth. The first step was to draw ambient noise maps. The results can be downloaded in Google Earth kmz format from

http://www.ligo.caltech.edu/~jharms/data/GoogleEarth/

The kmz files also serve as interface to spectral plots that are shown when selecting individual stations on the globe. Figure 1 shows a screenshot of the US map. A station is selected displaying additional information including a spectral histogram. Colors of the station markers represent the average seismic amplitudes. The corresponding frequency can be changed with the Google Earth video function. The quality of the US map is greatly enhanced by contributions from the Transportable Array that provides broadband data from a very dense and extensive seismic network when combined over the past years. We included all available broadband (BH?) channels from IRIS, Orfeus and F-net servers covering the past 5 years. Now it is possible to identify low-noise sites, also including other relevant information as remoteness, geology and topography.

In the context of Newtonian noise, the next problem is to study ambient fields in detail at specific sites. It is possible that the Advanced LIGO detectors, i.e. the second generation, will already see this noise at the lowest end of the detection band, but their sensitivity would only be limited by it during relatively noisy times. However, before considering the LIGO sites as interesting candidates for yet another (third) generation of detectors in the US, much better understanding of the ambient fields is required than we currently have. For example, there has not yet been an array measurement at the LIGO sites allowing us to analyze the wave content of the seismic field and to identify the weaker seismic sources at frequencies between 1Hz to 30Hz. Propagation and scattering of seismic waves need to be characterized to estimate its impact on seismic gravity perturbations, and we need to know if borehole seismometers are required to provide an accurate Newtonian noise estimate. All these issues are very important in order to predict the performance of Newtonian noise subtraction schemes. Despite the great challenges, we are optimistic to solve these problems in time so that further upgrades of the LIGO detectors will become an interesting option in the not so distant future. The outcome of these seismic studies could have a great impact on astrophysical science.

by Michael Coughlin (Carleton College) and Jan Harms (Caltech)

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