CCS2022-2024 WP1: The Havnsø structure - Marine acquisition report. Offshore seismic acquisition Havnsø-Nekselø 2022, with seismic source from the onshore acquisition

A combined onshore-offshore seismic survey (GEUS2022–HAVNSOE) was conducted in 2022 to map the Havnsø structure in northwestern Zealand. The overall objective of the survey was to define if the structure is suitable for underground storage of CO2. GEUS had the overall project management for the survey and contracted Uppsala University for carrying out the land component of the acquisition that is described in Malehmir & Papadopoulou (2023). The survey used two Vibroseis trucks as seismic source. One of the survey lines (line GEUS22–HVN–P1) had to cross a 2-km-wide waterway (Sejerø Bugt) between the port of Havnsø and the island of Nekselø. To bridge the water, marine receivers were used to record the onshore vibroseismic sources. Marine sources were not an option due to the shallow water and the protection status as Natura2000 area. With the additional marine data, it will be possible to connect the land data from Nekselø with those from Havnsø. In addition, existing offshore seismic lines can be tied to the new onshore data. This report describes the recording and initial processing of the marine data, including data examples. Ultimately, the data from the marine receivers have to be merged with those recorded on land, which is described in Malehmir & Papadopoulou (2023).

For the marine component of the acquisition, 18 ocean bottom seismometers from the national Danish seismometer pool DanSeis were deployed at the seafloor between Havnsø and Nekselø. These short-period instruments are equipped with three-component geophones and a hydrophone. The latter one has the best data quality. Some stations stopped recording before all Vibroseis sweeps on the two segments (Nekselø and road towards the beach in Havnsø) were completed. However, the data quality of the recorded signals is good. After corelation with the source sweep, reflections can be seen from depths below 2 s two-way travel time.

A second type of receivers was used close to Havnsø. Here a 600-m-long marine streamer with 96 channels was deployed from a winch located on the beach. After pulling out the streamer seaward, lead was added to it to lower it down to the seafloor. Data quality is good with exception of the channels close to shore and at the seaward end of the streamer. The poorer data channels are probably caused by increased noise levels due to the surge of waves at the beach and by motion that was transferred from the recovery buoy attached to the tail end of the streamer. Sweeps from the area close to Havnsø produce an almost complete record from the surface to depths greater than 2.5 s two-way travel time, while the larger shot-receiver offsets for the sweeps on Nekselø result in a lack of data in the upper 1.0 to 1.5 s of the record section.