Ligurian Ocean Bottom Seismology and Tectonics Research (LOBSTER)

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The aim of the proposed LOBSTER activity field within SPP 4D-MB is twofold: 1) to deploy a network of French/German ocean bottom seismometers (OBS) in the Ligurian Sea to serve as the marine component of the seismic network of AlpArray, and 2) to correct and process the marine data in order to be compatible with onshore data. AlpArray is a European initiative with the aim to deploy several hundred closely spaced broadband seismometers that will cover the entire Alpine orogen to provide state-of-the-art imaging of subsurface structures. The offshore component of SPP 4D-MB and AlpArray involves deployment of 33 broadband OBS from France/Germany in the Ligurian Sea. The deployment of the German stations is applied for here. To this end, colleagues in France have secured ship time in 2017 for the deployment. Simultenously, a ship time proposal has been submitted in Germany and 14 days of ship time have been granted on RV Meteor/Merian for the spring of 2018 for instrument recovery in order to share ship-related costs between the two nations. OBS recordings will be essential to define subsurface structures at the transition from the Western Alps to the Apennines and improve our understanding of the 3D-geometry of the system and its kinematics. OBS are advanced instrument systems that, as a result of their subsea operating environment, differ significantly in their operation and resultant raw data from their land-based counterparts. As a result of these fundamental differences, the main task of LOBSTER is to provide OBS data compatible with onshore data. Within the framework of SPP 4D-MB a combined analysis of onshore and offshore data requires the timely correction and provision of the marine data so that the two data sets can be merged. As a consequence, a number of processing steps must be prepared and set up prior to instrument recovery so that the work may commence as soon as the data become available during the recovery cruise. The period between the deployment and recovery cruises shall be used for this preparatory phase. The first processing step involves instrumental clock error correction. With no connection to the outside world, OBS are not able to maintain synchronization with standard timing systems (via GPS or network connection). Analysis of the cross correlation of ambient seismic noise provide techniques for nonlinear clock drift corrections. OBS are deployed remotely and without intervention in a free-fall mode - therefore, their actual horizontal orientation on the seafloor is unknown. Consequently, horizontal orientation of the OBS must be determined from measured air gun shots and/or ambient noise cross correlation. In the last step of the project we will characterize the spectral behaviour of the OBS data through probability density functions of power spectral density, which provide insights on the generation and propagating of seismic noise in the Liguria Sea.