Temporary CRONOS accelerographic network

Component 2 of the CRONOS project focuses on development of hybrid-empirical attenuation relations (Ground Motion Prediction Equation, GMPE) by combining empirical data from recorded earthquakes and stochastic modelling, using characteristic regional seismic parameters for the researched area. For the successful execution of this task, it was necessary to establish a new network of accelerographs in a selected seismically active area, covering different types of soil, and to collect a sufficient number of records of ground acceleration caused by a sufficiently strong earthquake.

Slika 1. Locations of installed accelerographic systems as part of Component 2 of CRONOS project

Accelerographs are instruments that excel at recording moderately strong and strong earthquakes at short epicentral distances, unlike highly sensitive seismographs that are not suitable for this purpose. Thus, accelerographs perfectly complement the classic seismographic network in case of a strong and nearby earthquake.

Borehole accelerographic systems

Three locations were chosen for the installation of borehole systems – in Vrlika, Sinj and Imotski; based on differences in local soil effects. After detailed geophysical and geological survey and confirmation of the adequacy of the locations, geotechnical boreholes were drilled for the installation of the accelerographs themselves. The wells were drilled by continuous rotary method with the use of protective columns, to a depth of 17-40 m, depending on the location, i.e. the depth to the bedrock. The borehole bottom is cemented with about 1 m of concrete plug. Cementation of the space between the borehole wall and the pipe was also carried out in order to stabilize and transfer the stress of the soil and rock mass to the borehole walls. The water from the plastic pipe was exhausted. Control of cementation, stability of the inserted plastic pipe and water level in the pipe was carried out.

Slika 2. Up: The process of drilling a geotechnical borehole for the installation of the borehole accelerographic system in Imotski; Down: an example of a geotechnical borehole core

Following the drilling, preparatory work for the installation of accelerographic systems was carried out. Specifically, a well protection system (cabinet, fence) was installed and a solar panel was mounted where needed. Finally, a final inspection of patency and safety of the installed pipe was performed by lowering the borehole camera. In all locations, the pipe was well fixed, stable, without groundwater penetration and ready for the installation of borehole sensors.

A borehole accelerographic system was installed. First, the borehole sensor was lowered into the borehole, and then connected to the rest of the wellhead system, which consists of a surface sensor (accelerograph), a Kinemetrics Quanterra 330+ digitizer and data logging system, wireless data transmission equipment, and other related equipment. The data is sent via an LTE modem to a server located at the Geophysical Department of the Faculty of Science in Zagreb. In addition to the LTE modem, the station is also equipped with a Starlink satellite in case of emergency situations where the internet connection is lost due to major earthquakes or the interruption of work of local operators. Data is also recorded locally in the system for a certain period.

Slika 3. Up: Borehole accelerographic system in Imotski; Down: surface equipment of the borehole accelerographic system

Surface accelerographic stations

A total of 11 accelerographic stations equipped with surface accelerographs were installed at the following locations: Knin, Drniš, Neviđane, Pirovac, Split, Trilj, Aržano, Imotski, Veliki Prolog, Opuzen, Makarska; based on differences in local soil effects. After a detailed geophysical and geological survey and confirmation of the adequacy of the locations, installation of the stations began.

Each location had to meet several basic requirements for the installation of the accelerograph: connection to electrical grid, the availability of an LTE signal for data transmission, a clear sky visibility from the room (for the output of the GPS antenna).

At each location, a Kinemetrics Obsidian accelerograph, which is both a digitizer and data logger, is installed in the concrete floor. A battery power supply system, equipment for wireless data transmission, a metal protective cabinet and other accompanying equipment were also installed. The data is sent via an LTE modem to a server located at the Geophysical Department of the Faculty of Science in Zagreb. In addition to the LTE modem, the station is also equipped with a Starlink satellite in case of emergency situations where the internet is lost due to major earthquakes or the interruption of work by local operators. Data is also recorded locally in the system for a certain period.

Slika 4. Up: Accelerographic station at the location in Knin; Middle: equipment located in a protective cabinet; Down: accelerograph Kinemetrics Obsidian

Acknowledgements

Prepared by Bruno Mravlja