Cruise Blog by Frances Cooke: Day 9
July 15th, 2021
Text and photos: Frances Cooke (PhD Candidate in the Gas Hydrates and Free Gas Reservoirs research group at CAGE, working within the SEAMSTRESS project).
We arrived to our new active seismic survey location yesterday late afternoon and started with chirp survey lines closely spaced over a previously interpreted mud diapir and buried glacial meltwater flow. The chirp is also seismic data and classed as ‘single channel.’ It uses a hydrophone array mounted on the hull of the ship, and the transmitted sound is just a ‘ping’ or ‘chirp,’ as opposed to ‘multi-channel’ hydrophones along a seismic streamer, that require a small ‘bang’ from the guns. The frequency is much higher; therefore, with this system we are able to image even finer (decimeter scale) sediment layering near the surface. The chirp typically penetrates ~40 m into the sediment in this area, however the chirp system on this ship penetrates just < 20 m, so we are missing some data deeper down.
3D surface in miliseconds two-way time, using a rainbow colour scale, extracted from 3D seismic volume data, shows a buried glacial feature. The purple line is one of the chirp survey lines that passes south of the feature. At the east of the line, we observe some hard glacial material transported by ice during the last glacial period.
The profile of chirp line 3 shows the buried glacial debris – a hard surface, potentially supersaturated with water or gas.
This morning 5 OBS were deployed for the active experiment. Two belong to UiT and have a short period frequency range for detecting shallow earthquakes and the remaining three are broadband OBS from AWI that also have long period signals that range deeper. The OBS team had a rocky start deploying the instruments, with the first OBS bashing into the side of the ship, and needed some repairs.
Bathymetry map (topography of the seafloor) showing an overview of the work completed in the cruise so far and our ongoing active seismic survey, in the new east location, with Prins Karls Forland in site.
After completing the chirp survey lines and deploying the OBS, we put out the seismic today at 12.15. We will shoot all 12 crosslines and some ‘inlines’ first, then we will move onto the circlular lines. The acquisition survey design is typically in this way for OBS active experiments, with the feature of interest in the centre of the array. The placement of the OBS is also dependent on the topography of the seafloor. We try to avoid landing them on dipping surfaces and away from depressions. We have begun with the crosslines (NE-SW), and by tomorrow we should have come to the end of the in lines (NW-SE). The wind has finally dropped and the sea is calm – for now!
Active OBS experiment seismic array (black lines) and chirp survey lines in gray.
Read more about the SEAMSTRESS project here.