In October 2021, GFZ installed together with INGV Catania, Iraci and ASIR Ltd (Advances Seismic Instrumentation & Research) the very first seismic borehole broadband seismometers at two selected sites at Mt. Etna, Sicily (see Fig. 1). The installation was completed under the EU-funded project ‘SiC nano for PicoGeo’ (http://www.picogeo.eu/). Site one is located next to the Astrophysical Observatory at Serra La Nave (SLN) and site two is located in the city of Mascalucia (MAS). At each site one borehole broadband seismometer was permanently installed (cemented) at approximately 70 m depth. In approx. 1-2m distance, a second ground-level borehole 4.5 Hz Geophone was temporarily installed (sanded) at 1 m depth until July 2022 (see Fig. 2). The ground-level geophones served as a local surface reference sensor to better evaluate the increase of signal quality from surface to depth. Test data were evaluated between October 2021 and July 2022. Sensor settings were adjusted during this time period to obtain the best possible data resolution at both test sites. This data publication compiles a segment of waveform recordings utilized for the assessment of data quality from the two installed broadband borehole seismometers, along with noise plots (Fig. 3-5) illustrating the enhancements in the data quality of frequency ranges compared to surface sensors at Mt. Etna.
Description of Seismometers
Broadband Borehole Seismometer AFB4.5:
The ASIR borehole seismometer AFB4.5 is a combination of an active and passive sensor with an outer diameter of 61 mm. It consists of a 3-channel short-period seismometer and a 3-channel broadband accelerometer. The short-period component is a standard passive 4.5 Hz sensor and is mostly sensitive to frequencies above 4.5 Hz (earthquakes smaller than magnitude 4). The broadband component is an active Silicon Audio (SiA) interferometer-based optical accelerometer produced for shallow-to-deep boreholes. These two combined components realize the coverage of a broad span of earthquake magnitudes. The AFB4.5 sensor has a +/- 3 dB frequency-response bandwidth of 120 s – 1300 Hz, a clip level of +/- 0.5 g, and a dynamic range of 172 dB over a 1 Hz band centered at 1 Hz. The flat response spectrum (uniform bandpass) ranges from 40 s – 200 Hz. In accordance with the expected magnitudes and distance to main earthquake locations at Mt. Etna, the sampling frequency is set to 200 Hz. According to the settings, the current reliably resolution is in the frequency range of 40 s to 80 Hz.
4.5 Hz Borehole Geophone AG4.5:
The additional and only temporarily deployed ground-level AG4.5 seismometer is of the same type as the short-period component in the AFB4.5 seismometer. It also operates at a sampling frequency of 200 Hz. The current reliable resolution lies within the frequency range of 4.5 Hz to 80 Hz.
File description
Borehole broadband seismometers:
The file ‘Borehole_sensors_information.txt’ provides information on the permanently and temporarily installed borehole sensors at both installation sites. Information on gain, true bitweight and sensor sensitivity can be used to correct the raw data records. The file ‘Borehole_sensors_poles_and_zeros.xlsx’ provides poles and zeros of both sensor types. Note: The polarities of the vertical SiA (accelerometer) component of the AFB4.5 seismometer at MAS and SLN appear to be reversed compared to the vertical Geophone components of the AFB4.5 and AG4.5 seismometers.
The data loggers store raw data in 1-hr Pascal files, which have been converted into mseed files. Each channel is saved as an individual mseed file. Provided are selected test data from January 2022 (for AFB4.5 and AG4.5 in MAS) and April 2022 (for AFB4.5, AG4.5 in SLN and AFB4.5 in MAS) in the zip-folders labeled ‘Waveforms_xxx_xxx’ (see below).
• Zip Folder: Waveforms_SLFB_AFB45
• Zip Folder: Waveforms_SLSP_AG45
• Zip Folder: Waveforms_MAFB_AFB45
• Zip Folder: Waveforms_MASP_AG45
Co-located surface IV-ESLN sensor:
The INGV IV-ESLN station is a surface broadband station with 3 components co-located at SLN. 1hr waveform files were downloaded in mseed format from EIDA Italia (source: https://eida.ingv.it/en/getdata). All 3 components are stored in one mseed file. Data that was used for comparison with the borehole sensors is stored in zip-folder ‘Waveforms_SLN_ESLN’. The respective station information is stored in a text file ‘IV_ESLN_station_info.txt’ (source: https://eida.ingv.it/en/networks/network/IV/station/ESLN).
Noise Plots
Description:
Figures 3-5 show representative average noise levels for the vertical components of the 68-65 m deep borehole seismometers AFB4.5 at SLN (red, magenta) and in MAS (yellow, magenta), the surface broadband seismometer IV-ESLN at SLN (black), and the short-period ground-level borehole seismometers AG4.5 in SLN and in MAS (brown). The horizontal components follow similar characteristics and are not shown here. The thin black lines show the Peterson (1993) model indicating the global average noise level (dashed line) of seismic sensors.
To generate Figures 3 – 5, selected hour-windows were used, which were divided into ten 5-min time-windows to estimate the Power Spectral Density (PSD) by using a multi-taper method. The mean PSD was derived from these ten windows, providing a mean noise level for the 1-hour file. This procedure was applied for different week-days and day times between October 2021 and April 2022. The test data from April 2022, which correspond to a period of reduced volcanic activity at Mt. Etna, exhibited the highest data quality in the recording period and an example is presented in Figure 3 and 4. Due to technical issues and the earlier deinstallation of the ground-level AG4.5 sensor in MAS, a representative average noise level from January 2022 is presented in Figure 5 for the short-period AG4.5 seismometers in MAS.
The findings suggest that deploying a deep borehole broadband seismometer offers the advantage of improved high-frequency resolution compared to surface or ground-level sensors. Borehole sensors enhance the detectability of high-frequency seismic and volcanic signals at Mt. Etna. Specifically, in urban setting or locations prone to weather interference, borehole sensors are a recommended choice. It is important to note that the test data did not indicate enhanced resolution of low frequencies < 0.05 Hz at a depth of approx. 70 m in comparison to the IV-ESLN surface broadband seismometer. Deeper installations may be necessary to achieve a better resolution.
