On Sunday, April 27, 2025, we travelled to the island of Cres to install the first underwater seismological devices, specifically Ocean Bottom Seismometers (OBS) from the company K.U.M. Umwelt- und Meerestechnik Kiel GmbH, in the Republic of Croatia. OBS is a specialised instrument placed on the seabed to record seismic waves. It contains a seismometer, hydrophone, GPS system and data storage system with a maximum operation time of up to 13.7 months.
OBS instruments were deployed by CROSSNET project associates: Viktorija Milec, Valentina Gašo, Anamarija Tremljan Milun, Iva Kostanjšek, Antonio Brcković, Marko Kapelj, Goran Čorak, Marko Pervan and Bruno Mravlja, with the project manager Tomislav Fiket.
Upon arrival at Cres, we prepared, assembled and calibrated the instruments to ensure the highest quality data collection possible. First, it was necessary to assemble the device itself, which consists of several separate components: a cylindrical container containing the seismometer, digitizer and batteries, a KUMQUAT acoustic releaser, which is detached from the anchor when the instrument is collected and allows the instrument to be launched, a hydrophone, a large anchor, and an XMF flash and XMB signal transmitter.
The next day we headed to the locations where the underwater instruments were to be laid. Upon arrival at the location, the instrument was lifted above the sea surface by a crane and released into the sea. During the lowering of the instrument, its depth was checked using the P.A.C.S. deck unit, and the depths at which the instruments are located are from 55 to 70 m. The instruments will record data on the seabed for several months, and since direct data transfer is not possible, data processing will follow after the instrument is collected.