The ambient noise of a seismic station is the sum of acoustic signals in the background environment, The noise level is influenced not only by the site's proximity to human activity and infrastructure, but also by its geology, temperature, pressure stability and even its distance from the sea. Since CHNet uses very sensitive instruments intended to measure background noise as opposed to sensor noise, it is important to minimise ambient noise to enable the detectability of even weak seismic signals. The site selections for the SDSNet must be particularly meticulous, since the aim is to minimise ambient noise across the frequencies of interest, ranging from 100Hz to 1,000s. However, there has to be a trade-off between guaranteeing low ambient noise and ensuring that historical 'legacy' sites remain occupied. Many SDSNet sites were selected decades ago when sensor capability was very different and there was far less nearby infrastructure. Although some of these sites may now be considered noisy, they are important to maintain for the purposes of comparing records from events at the same site, even occurring decades apart, because recordings from the same site can help to identify recurrent events and better understand seismicity patterns. Another important consideration when selecting a site for a seismic station is ensuring adequate security and reliable power and communications. Sometimes sites with high ambient noise are chosen because it is important to measure earthquakes at a particular location: for example, we need to have sensors near Basel, even if the noise level there is high. In a bid to guarantee low noise levels over long periods (this is a key requirement for broadband sensors), tilt, temperature and pressure variations need to be kept extremely low, and the stations should be set up on solid rock.
Many SDSNet stations are located in former military bunkers and near dams, where excellent infrastructure already exists, levels of security and environmental protection are high and 'cultural noise' is often low. Natural caves and man-made tunnels also constitute high-quality sites. In the Alpine foreland, solid rock can be difficult to find, so sensors are placed in 100 to 150m-deep boreholes or in shallow concrete vaults.
SSMNet stations are often sited near old city centres or close to critical infrastructure like hospitals or schools. These sensors are placed outside buildings in shallow concrete vaults.
For any potential new stations, seismic noise measurements are taken continuously over a period of at least two weeks and the sites are only approved if the noise conditions there are deemed acceptable. Higher noise levels are expected in the Alpine foreland, where thick layers of sedimentary deposits and anthropogenic and/or cultural noise cannot be avoided.
The network's daily performance in terms of noise spectra for all continuously monitored SDSNet and SSMNet stations is updated every day and published on the PQLX webpages. More information on the local conditions at each CHNet site can be found in the Site Characterization Database for Seismic Stations in Switzerland.