Cruise report RV Pelagia campaign 64PE503 Version 2

This cruise report documents the scientific activities and preliminary results of the RV Pelagia campaign 64PE503 that was carried out 5-22 June 2022 as part of the North Sea Methane Project, carried out by NIOZ and TNO on behalf of Dutch State Supervision of Mines. The goal of this first campaign was to assess the extent of gas ebullition in the Dutch North Sea, by determining the presence of ebullition at a representative selection of wells and natural background sites. A total of 68 locations of interest was surveyed during the campaign, comprising 57 abandoned wells, 7 locations where natural ebullition was expected, 2 ship wrecks, a section of pipeline, and one reference location. The presence of active ebullition was assessed on the basis of water column acoustic backscatter profiles acquired with Kongsberg EM2040 high resolution multibeam. Surveys with G882 magnetometer were conducted to verify the given position of abandoned well heads buried below the seabed. Profiles of the shallow subsurface geology were acquired with Innomar Medium 100 Sub Bottom Profiler. The CTD-Rosette system of NIOZ was used to obtain vertical profiles of a range of water column parameters above abandoned wells and other points of interest, and to collect water samples from discrete depth intervals for chemical analysis. Analysis of gas content in surface and deep water was performed with a laser spectrometer. In addition to analysis of gases in water, air taken in at approximately 10 m above sea surface was continuously analysed for atmospheric CO2 and CH4 with a Picarro G2301 gas concentration analyzer. Preliminary analysis of the acquired data indicates that the most active ebullition, with multiple bubble plumes emanating from the seabed, occurred at the natural seep site B13-BS. Ebullition associated with abandoned wells was observed at only 6 of the 57 wells surveyed during the campaign, at wells A08-01, A14-02, A15-03, F01-01, B17-03 and B17-05. At each of these locations, enhanced concentrations of methane were measured in the water column by laser spectrometry