Seismic Noise

The radiation pressure noise decreases rapidly with frequency, so we focus our attention on the worst case, the low frequency domain. Because of the seismic noise which is very large at low frequency, it will be very difficult to detect gw below a few hertz, so we put this as a limit. Equation (46) exhibits interesting features ... 

Field, E. H., Jacob, K. H. (1993). The theoretical response of sedimentary layers to ambient seismic noise. Geophysical Research Letters, 20, 2925-2928. doi 10.1029/93GL03054... 

In addition to other science facilities, the South Pole Station includes a seismological observatory that has operated since 1957. This facility was greatly improved in January of 2003 when a new observatory was placed into service about 8 km from the main building. The sensor of the seismograph at the new facility is located in a borehole at a depth of 300 m below the surface where it is not affected by vibrations caused by the diesel-powered electric generators and other activities at the main station. As a result, the South Pole seismological observatory is now virtually free of locally-generated seismic noise and therefore is ideally suited to detect earthquakes that occur elsewhere on the Earth. 

This entry reviews the topic of downhole seismometers. The entry is split into three sections. The first section reviews the different types of downhole seismometers. The second section provides an overview of seismic noise sources and their impact on downhole installations. The last section discusses the downhole installation methods. 

There are three main motivations for deploying seismometers in a downhole environment. The first is to provide quieter seismic noise environment to improve signal-to-noise ratios. The second motivation is to lower the cost of deployment of a seismic station. The third motivation is to reduce the environmental impact and land use costs by minimizing the physical footprint of a seismic station. 

Seismic noise can be broken down into different types or sources of noise. These include wind-induced seismic noise, cultural seismic noise (noise generated by human activities), microseis-mic noise, moving-water-induced seismic noise, and tilt-induced noise (Fig. 3). Each of these noise sources will be briefly described. 

Seismic noise is generated in watercourses from small streams to large rivers. As water flows downhill, turbulence is created which imparts... 

Nevertheless, the effect of seismic noise attenuation with depth results in improved signal-to-noise ratios espteciaUy when the final layers of earth are highly lossy (Young et al. 1996). Ideally, a downhole seismometer should be placed in hard rock below a highly lossy overburden. 

Typically, there is a trade-off to be made between a rural site which has low cultural seismic noise but is less secure due to its isolation and a secure urban site, such as police or fire station, which has higher cultural seismic noise. Ideally, an operator wants a low cultural noise site that is also secure. With downhole installations, a secure urban site enables the reduction of seismic noise by providing vertical separation from seismic noise which originates at the surface. The degree of attenuation of seismic noise with depth will depend... 

A network operator will have to trade security and ease of access for uptime and data quality based on the local geology, cultural seismic noise, economic conditions, cultural norms regarding property, and cost. 

In the past, downhole seismic stations were limited to deep borehole installations which were so expensive that they only justified the deployment of very costly borehole seismometers. These stations demonstrated the reduction of seismic noise and the improvement of signal-to-noise ratios with depth. However, the cost of ownership of these stations limited their widespread adoption. 

In the last few years, posthole and direct burial seismometers have been introduced, a better understanding of seismic noise and its dependence with depth has been developed, and more economic installation methods, such as those using water-well drilling equipment, have been established. These instruments and methods have made downhole seismometer installations cost competitive with vault installations, while at the same time improving on the signal-to-noise ratios. In some cases, downhole installations can cost much less than vault installations of similar performance. 

Sleeman R, van Wettum A, Trampert J (2006) Three-channel correlation analysis a new technique to measure instrumental noise of digitizers and seismic sensors. Bull Seismol Soc Am 96(1) 258-271 Sorrells GG (1971) A preliminary investigation into the relationship between long-period seismic noise and local fluctuations in the atmospheric pressure field. Geophys J Roy Astron Soc 26 71-82 Williams G, Gold L (1976) Ground temperatures. Can Build Dig, CBD-180... 

Eruption forecasting Piton de la Foumaise Volcano Seismic imaging Seismic noise Volcano monitoring... 

By using continuous records of ambient seismic noise from the PdF Volcano observatory short-period seismic network, Brenguier et al. (2007) were able to measure Rayleigh wave velocity dispersion curves obtained from the cross-correlations of 18 months of ambient seismic noise. The tomography of these measurements as well as the depth inversion led to a high-resolution image of the shallow sfructure of PdF Volcano (Fig. 2). The results show a high seismic velocity body located in the central part of the... 

Noise-Based Seismic Imaging and Monitoring of Volcanoes, Fig. 2 3D tomographic image of the shallow central part of Piton de la Foumaise Volcano using crosscorrelations of ambient seismic noise (Brenguier... 

Baig A, Campillo M, Brenguier F (2009) Denoising seismic noise cross correlations. J Geophys Res Solid Earth (1978-2012), 114(B8)... 

Shapiro N, Campillo M, Stehly L, Ritzwoller M (2005) High-resolution surface-wave tomography Ifom ambient seismic noise. Science 307(5715) 1615 Snieder R, Gret A, Douma H, Scales J (2002) Coda wave interferometry for estimating nonlinear behavior in seismic velocity. Science 295(5563) 2253... 

Brune JN, Oliver J (1959) The seismic noise at the earth s surface. Bull Seismol Soc Am 49 349-353 Havskov J, Alguacil G (2010) Instrumentation in earthquake seismology. Springer, Dordrecht, 358 pp LaCoste LIB (1934) A new type long period seismograph. Physics 5 178-180... 

Although the NLNM was constructed from the records of land-based seismometers, it may also represent a hypothetical quietest site under the sea. Seismic noise surveys on the ocean bottom have not yet been undertaken on the scale that they have been on land. It is clear from the work which has been done so far, however, that the noise levels are generally significantly higher. Most of the ocean floor is covered with sediment which is more compliant than sediment on land, and seismic velocities are lower. Seafloor compliance means that excess site noise due to deformation and tilt can result from both variations in pressure due to waves at the surface and currents traveling along the bottom of the ocean (Webb and Crawford 2010). When precautions are taken to mitigate these effects, site noise can approach the levels observed on land. 

AppUcation of INPAR to tectonic events in Antarctica represents a suitable procedure in an environment such as the Scotia SearegitMi, where (1) the high level of seismic noise, typical of the oceanic environment, hampers the use of standard inversion techniques based oti first-arrival polarities, (2) a low number of operating stations are present due to logistic, (3) the structural models are grossly known, and (4) the epistemic uncertainty is thus considerable. 

---