The seismic acquisition was carried out from the Faroese research vessel Jákup Sverri that towed a solid-state streamer (Sentinel SSRD from Sercel) with an active length of 2100 m (336 channels) with a hydrophone group interval of 6.25 m. The streamer was towed at a depth between 3 and 5 m. The seismic source was a GI-gun cluster consisting of two guns. Initially, data were acquired with a small cluster (two GI-guns with a generator chamber of 45 cubic inches volume and an injector chamber of 105 cubic inches each) triggered every 6 s. After some initial onboard date processing, it was decided to switch to a larger array (two GI-guns with a generator chamber of 250 cubic inches and an injector chamber of 105 cubic inches) triggered every 10 to 12 s, to ensure that the target depth is reached. The first ten lines were acquired with the small array, of which five were shot a second time with the large array.
In total, 1450 km of seismic reflection data were acquired in the ten days the ship was in the survey area. There was very little downtime and it was related to compressor maintenance and the switch of the air gun array. There was no downtime related to weather. The study area is characterized by shallow water and a hard seafloor, which is challenging for any seismic acquisition. Nevertheless, the chosen configuration for the seismic experiment was adequate for this type of conditions. Onboard processing and an initial evaluation of the data indicate that there was seismic penetration down to the base of Zechstein (at depths of 3 to 4 s two-way travel time) and that the reservoir rocks (Gassum and Haldager formations) are resolved. One of the main tasks for the post-cruise processing will be the removal of multiple energy.
The survey not only provides a dense grid of seismic lines across the Jammerbugt structure, but also made a tie to the well J-1X to the northwest of the structure that was crossed by three lines. In addition, two lines crossed the Hanstholm structure to the west of the Jammerbugt structure. While the larger Hanstholm structure was not the prime target, it was deemed useful to connect the two structures for future reference. Towards land, the survey was restricted by the 10-m depth contour given the draught of the ship and the setup of the seismic equipment. This leaves a coastal zone, about 10 km wide, that could not be surveyed.
The ship was equipped with the shallow to medium water multibeam echosounder (Kongsberg EM 712) and a sub-bottom profiler (Kongsberg TOPAS PS18). Both systems were in operation in the survey area and can be used to improve the interpretation of the seismic data. The TOPAS data give a high-resolution image of the upper 30 ms two-way travel time below the seafloor and may provide linkages to possible faults at deeper levels. The swathbathymetric data provide information about the morphology of the seafloor and can also aid the processing of the seismic data in the shallow water. For calibration, a multibeam patch test was carried out and two sound velocity profiles (SVP) were measured. An additional 14 SVPs are available from another vessel (Poul Løwenørn) that carried out a bathymetric survey in close vicinity.
To mitigate the risk of potential harm to marine mammals related to the acoustic signals emitted from the seismic survey, the guidelines provided by the Danish energy Agency were followed. Two certified marine mammal observers were onboard, and a passive acoustic monitoring (PAM) system (Sercel QuietSea) was integrated into the streamer configuration. Prior to the start of seismic acquisition, a 30-minute pre-watch was carried out followed by a 60-minute ramp-up procedure of the air gun array. During the survey, two harbour porpoises were sighted but beyond the critical distance of 500 m.
|Published - 10 Nov 2023
|Danmarks og Grønlands Geologiske Undersøgelse Rapport
- Programme Area 3: Energy Resources