Description: Introduction Since the discovery of ‘bright spots’ associated with hydrocarbon deposits, ever increasing interest in determining lithological subsurface parameters has been a driving force for technological development in the hydrocarbon exploration industry. Quantification of lithological parameters is of utmost importance for reservoir prediction and monitoring. Amongst various attempts to determine these, attribute analysis of pwave data and the direct observation of shear wave data are the most visible and successful methods applied. The direct observation of shear waves in the marine environment has been attempted by several means, mainly using ocean bottom cables (OBC) that have three-component geophones (3C) and a hydrophone in addition (thus 4C in total). Some manufacturers offer two component geophones with only one horizontal component. These cables are laid out on the seafloor, sometimes even buried using specialized tools like ROVs (remotely operated vehicles). Data transfer is through the cables as in streamers or land operations, recording is made on a boat or platform where the cable terminates. Geophones are housed in tubes with a self-levelling gimballed mounting system, damped by a viscous fluid. This technique is regarded as proven technology and has been widely accepted. Especially in production areas with many man-made obstacles, this technique also offers a safe operation, and is especially suitable for monitoring purposes (4D–4C seismic). Any desired geometry and density of receivers can be laid out. Direct shear wave observations have been made by several academic institutions, both for active seismic exploration as well as for passive seismological monitoring of earthquakes. These institutions have built ocean bottom seismometers (OBS), which are also four component, two sensor instruments. Unlike OBC, they are autonomously lowered to the seafloor, record within specified time windows, and are later brought back to the surface. Amongst the various instruments designed over the past decades is the OBS range built at GEOMAR, which – due to its modular design – has been used for a wide range of applications.

Description: In 1964, exploration drilling in the German Sector of the North Sea hit a gas pocket at ∼2900 m depth below the seafloor and triggered a blowout, which formed a 550 m-wide and up to 38 m deep seafloor crater now known as Figge Maar. Although seafloor craters formed by fluid flow are very common structures, little is known about their formation dynamics. Here, we present 2D reflection seismic, sediment echosounder, and multibeam echosounder data from three geoscientific surveys of the Figge Maar blowout crater, which are used to reconstruct its formation. Reflection seismic data support a scenario in which overpressured gas ascended first through the lower part of the borehole and then migrated along steeply inclined strata and faults towards the seafloor. The focused discharge of gas at the seafloor removed up to 4.8 Mt of sediments in the following weeks of vigorous venting. Eyewitness accounts document that the initial phase of crater formation was characterized by the eruptive expulsion of fluids and sediments cutting deep into the substrate. This was followed by a prolonged phase of sediment fluidization and redistribution widening the crater. After fluid discharge ceased, the Figge Maar acted as a sediment trap reducing the crater depth to ∼12 m relative to the surrounding seafloor in 2018, which corresponds to an average sedimentation rate of ∼22,000 m 3 /yr between 1995 and 2018. Hydroacoustic and geochemical data indicate that the Figge Maar nowadays emits primarily biogenic methane, predominantly during low tide. The formation of Figge Maar illustrates hazards related to the formation of secondary fluid pathways, which can bypass safety measures at the wellhead and are thus difficult to control.