The seismic stations described in this manuscript are part of the international AlpArray temporary seismic network (www.alparray.ethz.ch). AlpArray is a unique European transnational research initiative: 45 research institutes from 18 countries join their expertise to advance our knowledge about the structure and evolution of the lithosphere beneath the entire Alpine area . AlpArray will shed light on the detailed geological structure and geodynamical evolution of the Alps to answer outstanding questions, e.g. on slab geometry and subduction polarity under the Eastern Alps . While the primary scope of AlpArray is fundamental research, the unique dataset will also improve our knowledge about near-surface geologic structures and help to assess the seismic hazard in the Alpine area. The scientific goals of the AlpArray seismic network are manifold and among others include e.g. Alpine geodynamics, crustal and mantle imaging, seismic anisotropy, as well as regional and local seismic activity. Hence, temporary seismic stations installed in the framework of AlpArray should be multi-purpose stations that perform reasonably well for frequencies from above to below the microseism peaks.

Here we describe the site selection criteria, technical realization and noise performance of thirty temporary seismic broadband stations operated by the Department of Meteorology and Geophysics, University of Vienna in the context of AlpArray in eastern Austria and western Slovakia (see Table  for a complete station list). Few of the stations were already described in more detail by .

Our first temporary AlpArray station was installed in February 2015 (A009A) and we concluded the deployment in November 2015 (A010A). Few stations experienced power losses during the first winter months of 2015 because of insufficient solar power, but this was fixed (by adding a second solar panel or a fuel cell) before christmas 2015. Since January 2016 – which is the official AlpArray seismic network starting date – all of our 30 stations are fully operational. Four stations (A002A, A005A, A011A, A024A) have been moved to new sites in June 2016 because their noise levels were unacceptable (see below). They were replaced by stations A002B, A005B, A011B, A024B, respectively which perform much better than the previous site.

In the following we briefly describe each station in terms of sensor installation, ground type, housing characteristics and the geological setting. Station pictures and aerial views, as well as noise plots of all stations are available in the online material. Please note that stations pictures, aerial and map views, as well as sensor housing and geological descriptions for all stations are also available online at the European Station Book hosted by the ODC: http://www.orfeus-eu.org/opencms/stationbook/index.html. Table  contains a comprehensive list of the stations, including instrumentation and coordinates.

A001A is located inside a small abandoned WW2 bunker, that is built on a limestone formation within a gentle hillside. The sensor is put directly onto the bunker concrete floor in a corner furthest from any window or entry. The site is surrounded by vineyards and lies 700 m outside a small village and 300 m from the nearest road. This station shows the second lowest (best) noise levels of all stations. A001A is powered by two 100 W solar panels.

A332A is installed at surface level of a large cemetery house (20 × 13 m) that contains a church tower with a bell ringing four times a day. The sensor is placed on the tiles of a rarely visited cleaning storage room. The site is surrounded by hilly landscape created by alluvial deposits. Noise limits are kept except for horizontal long periods, which are strong and strongly varying.

A339A is placed 2 m under the surface level in a cellar of an unused storage building (25 × 5 m) of an operating chicken farm. The sensor is placed onto a tile floor. The site lies within the all flat danube flood plain 3 km from the danube and 800 m from a village. Long period vertical noise limits are met, but all other noise requirements are strongly exceeded.

We classify the noise performance of our stations into three groups, according to the noise requirements set by the AlpArray Working Group. Stations that fulfill the noise requirements (see Sect. ) in both frequency ranges (1–20 Hz and 30–200 s) for most of the components at all times fall into group green. If only some components (e.g. the vertical) meet the required noise levels for some of the frequency ranges or if there is strong temporal variation (e.g. anthropogenic noise with day/night cycle) in the noise levels, the station falls into group orange. Group red contains stations which do not meet the noise requirements for most of the components in both frequency ranges. Figure  shows examples of probabilistic power spectral density plots for each group and Fig.  visualizes which stations meet the noise requirement and which don not. Table  summarizes the noise performance of all 30 stations by the individual components. Individual power spectral density graphs (calculated from five months of data, January–May 2016) are available in the online material. Updated monthly power spectral density plots for our AlpArray stations are available at http://imgw.univie.ac.at/en/research/geophysics/projects/aaa/ppsd/. All probabilistic power spectral density graphs were created with the ObsPy toolbox following the procedure of .

To summarize, in the high frequency range (1–20 Hz) 50 % of our stations are at least 20 dB below the NHNM on both vertical and horizontal components. 25 % of the stations meet the limits during quiet times (i.e. at night) and another 25 % are affected by strong high frequency noise and cannot fulfill the noise requirements. The latter stations are almost exclusively located in sedimentary basins and/or close to anthropogenic noise sources.

In the long period range (30–200 s) noise performance clearly separates into vertical and horizontal components. On the vertical component all stations show noise levels of 20 dB below the NHNM or better and thus meet the AlpArray noise requirements. However, many stations are affected by rather strong horizontal long period noise. For most of the stations, horizontal noise is up to several tens of dB higher than the vertical noise level. Still, 30 % of the stations satisfy the requirements on the horizontal components, but another 30 % experience noise levels in the range of the NHNM or higher. 40 % of the stations show strong variations in horizontal noise (see orange example, A020A in Fig. ) and only during unspecifiable quiet times meet the noise requirements.

We cannot relate the susceptibility of a station to long period horizontal noise to a specific housing type. Among the well performing stations are those in well-isolated underground shelters as well as surface stations inside big or small aperture buildings or huts. Similarly there is no clear relation between sensor shelter and strong or strongly varying long period noise. We do note, however, that for the majority of stations vertical noise levels are much lower (up to several tens of dB) than horizontal noise levels. Additionally, for some sites we observe up to 20 dB difference in noise levels between the two horizontal components. This may reflect local site conditions or effects which should be studied further. Since our stations are not pressure sealed or installed on rigid baseplates they might be affected by pressure variations and long period surface tilt . The latter may in particular cause the elevated horizontal noise levels (compared to vertical) on our stations since almost all of them are surface or near surface installations. Additionally, most stations are located inside 1–2 floors buildings which may pick up e.g. wind or temperature variations and thus create additional long period noise. However, even some of the well-performing sensors are located on surface level inside buildings. Convection in the surroundings of the sensor seems unlikely to be a dominant long period noise source since even some basement stations with almost constant temperature are affected. Convection inside the styrofoam box should be minimal due to the silicon sealing and little free space between the textile cover of the sensor and the styrofoam insulation box (see Fig. ).

determined the vertical self-noise of our Reftek 151 60s A generation sensors. They show self-noise levels below the NLNM for the flat part of the instrument velocity response down to the specified effective eigenperiod of 60 s. At 200 s the sensor self-noise is 15 dB higher than at 60 s and exceeding the NLNM model. During personal communication they report occasional sequences of instability for generation A sensors with elevated self-noise levels for periods longer than the effective sensor eigenperiod. Still, the measured self-noise for the Reftek 151 60s A sensors for periods between 60 and 200 s is well below the horizontal long period noise levels we encounter in our deployments. Additionally, for stations affected by strong horizontal long period noise, the noise levels are elevated for all periods below 10s and the stations in question are equipped with both Reftek 151 A and B sensor generations. Hence, we conclude that the horizontal long period noise levels are not due to exceeding the sensor specified frequency range but rather due to housing and surface effects.

The majority of our stations is installed within sedimentary surface layers of unknown thickness and only few stations have direct contact to host rock. point out significant diurnal variation in horizontal long period noise for shallow temporary vault installations inside thick layers of soft sediments. They relate the noise variations to the soil responding mainly to atmospheric pressure variations. We did not perform a detailed study of potential diurnal variations in noise levels for our stations, yet, but the strong variability seen in some of the probabilistic power spectral densities (calculated for 5 months, winter to spring) may reflect a similar effect. report similar variations for long period horizontal noise and suggest that for for sites in sedimentary geologies direct burial of broadband sensors may be favorable over vault emplacement.

The Department of Meteorology and Geophysics of the University of Vienna started deploying temporary broadband stations for the AlpArray project in early 2015. Since January 2016 thirty seismic stations equipped with Reftek 151 60s sensors and Reftek 130(S) data loggers are fully operational and sending 100 sps waveform streams to the ODC in real-time, using the cellular network. Since January 2016 we retrieve almost 99 % of the data in real-time. Our stations follow a low-cost installation design and are usually placed inside basements of uninhabited houses, huts, cellars or castles. The sensor, insulated with a microfleece bag and a styrofoam box, is commonly put on concrete ground or tiles and in few cases on top of a constructed concrete base in soil. Most stations are powered through the power grid, while few stations feature a 2 × 100 W solar panels installation or one 100 W panel and a fuel cell.

With such design 50 % of the stations show high-frequency (1–20 Hz) noise levels which are 20 dB below the NHNM or less on all components. Another 30 % reach the NHNM – 20 dB noise performance during quiet times (i.e. at night). All stations perform well for vertical long period (30–200 s) signals with noise levels 20 dB below the NHNM or less. However, horizontal long period noise levels are generally much stronger than the verticals and only 30 % of our stations meet the NHNM – 10 dB requirements for horizontal long period noise. 40 % of the stations are affected by strongly varying long period horizontal noise. This is likely due to the fact that all stations are surface or near surface stations inside or near the footprint of an artificial structure such as a building. Hence, our stations are affected by long period surface tilt introduced by e.g. atmospheric pressure variations and little or no efforts to minimize such effects were made for these temporary deployments.

Overall our low-cost deployment provides reliable and continuous seismic data of good quality in real-time and forms one integral part of the greater AlpArray seismic instrumentation. In particular, data from our stations will help to resolve outstanding questions and debates about the geological structures under the Eastern Alps, which is one of the key scientific targets of AlpArray.

Seismic data used for this manuscript is currently not publicly accessible by decision of the AlpArray Working Group. Currently, all waveform data is exclusively available to registered members of the core group of the AlpArray seismic network. Please visit http://www.alparray.ethz.ch/seismic_network/backbone/data-access/ for further information on data access.