Ocean Bottom Seismic Cruise on R/V Helmer Hanssen – Blog by Frances Cooke

Day 1 – A smooth start

We departed Tromsø on the Helmer Hanssen before our first, of likely many, onboard fish lunches. We were glad of the sea breeze after the unusual Arctic summer heat during the past few days. The seas are calm which is always a relief knowing that your journey north is going to be a smooth one and we found the server machine room the perfect place to cool down! We are in transit returning to Vestnesa Ridge – West Svalbard to recover earthquake data from previously deployed ocean-bottom seismic instruments (OBS). OBS are instruments that contain seismometers and hydrophones that sit on the seafloor, listening and recording vibrations from shallow earthquakes.

Last view of the land: Vannøya (photo: Frances)

Onboard we have two of our colleagues from the Alfred Wegener Institute (Vera and Mechita), three from the Seamstress project (Andreia, Frances and Przemek) and two technical engineers (Stormer and Truls). We are busy setting up the GPS required for the active OBS experiment that will take place at the west of Vestnesa ridge. We have Sunil (in-house expert) present through video call while we are near shore! The setup ensures that we have precision in the timing and positioning of each shot fired during the active experiment.

Stormer and Andreia attach GPS antenna to railing (photo: Frances)

Last year during cruise CAGE20_5 we recovered 7 OBS as part of the Seamstress passive seismic experiment. This experiment took place between July 2019 and August 2020. The OBS instruments in the passive experiment record signals generated by earthquakes or other natural sound waves such as whale calls, instead of being actively generated by airguns. During the same cruise, we deployed 10 more OBS to begin a new experiment at the northern termination of the Knipovich mid-ocean ridge. We are now on our way back to recover them.

Whale Tail – photo opportunity while the sea is still calm!  (photo: Frances)

 

Day 2 – Preparing the instrumentation

Temperatures have dropped and the skies are grey. The bathymetry (map of the seafloor) shows a typical Barents Sea display of iceberg plough marks cut deeply into the seafloor as we cross over the ‘Polar Sonen’ – a blue line displayed on the ship’s navigational map extending from Russia through to Iceland. We are still 1 day and 14 hours away from our OBS recovery site, but we have some tests to do along the way.

Barents Sea real time bathymetry swath with iceberg scours (yellow) (photo: Frances)

Preparation: OBS releasers

We are preparing our OBS releasers for testing at ~1250m water depth, away from the shallow Barents Sea continental shelf. It is important to test the instruments at the same water depth as the OBS site to ensure a comparable pressure. The releasers are the most important part of the OBS to test. If they do not function correctly, the OBS will fail to return to the surface. They are called back using an acoustic device that sends a command through the water to the instrument instructing its release. We will lower the cage with all six releasers (2 from UiT and 5 from AWI) and we will talk to them and wait to see if they talk back. Bergen University were lucky this year to recover their ‘lost’ OBS picked up during the AKMA CAGE cruise in May using a remotely operated vehicle.

Top left: all releasers ready to be deployed (in view Kathi, Kunigunde and Kordula). Emma, Diana, Christine and Wojtek – the easily identifiable instrumentation from the Alfred Wegener Institute. (Photos: Frances)

OBS naming

Five of the releasers have names: Kunigunde, Kathi, Kordula, Kaya and Kim. Vera and Machita inform us that all the parts that make up the OBS (hydrophones, frames, beacon, releaser etc.) have names. The names made for the releasers start with a letter K or L. In the labs at AWI, they have in total ~1000 pieces of equipment to manage, including auxillary parts such as laptops, deck units, and cables. They have 80 OBS equipment groups. Each OBS group is comprised of 10 parts. The serial numbers are so similar that naming the equipment makes it much easier to identify them. I ask how they manage to find names for 800+ OBS parts, and they reply that the names are both male and female, and both German and foreign.  The names are also old fashioned such as Kunigunde. Watching the Denmark-England game last night was possibly as intense as the half hour wait time during the recovery of the OBS – at least for some. Others discussed the names of the players:  Boleslaw – an old-fashioned Polish name. A good reserve name for a Beacon.

Top: The deployment of the releasers cage, bottom: Przymek, Vera and Mechita watching the winch wire go out, waiting for the cage to drop down to 1250m before communication with the releasers begins. (photos: Frances)

While Vera, Mechita and Przymek were preparing the OBS releasers, Truls and Stormer were preparing UiT’s two ‘Digi-birds.’ This is equipment used to control the depth of the cable in the water for seismic operations. The birds are mounted externally on a marine seismic streamer cable. They have acoustic devices that measure actual depth of the cable during operation, through an onboard sensor located in the wing module. Periodic adjustments are made to the angle of the birds wings (or “fins”) to drive the bird (and streamer) back towards the target depth.

Top: Truls and Stormer fix the wings to the birds; bottom: Truls and Stormer attach one of the birds to the streamer cable on deck (Photos: Frances).

 

Day 3 – Adapting to the weather

We are now three days into the cruise and we have to wait another 5 hours, after another fish lunch, until we get to the northern termination of Knipovich ridge. The new weather forecast predicts a wave height of under 2m for our arrival and the winds will drop from Sunday evening. If the wave height is larger than 2m, or if the visibility is poor because of fog, we would have to leave the OBS recovery work for another day.

Truls and Stormer preparing the mini GI airguns for deployment (left), the injector air release increasing internal pressure of the bubble created by the generator (right schematic)

 

While we are waiting to arrive on site, Truls and Stormer prepare the mini GI guns. Final preparations prior to deployment include blowing out the air hose (as the air has to be clean before attaching to the guns), checking the solenoid valves and listening for a click when energizing the firing line, and attaching the gun GPS to the gun float.

An airgun releases a high pressure bubble of air in the water to generate an acoustic pressure wave. The pressure variation in the water as a function of time, caused by the high pressure bubble, is called the airgun signature. Next time you boil a kettle, consider the sounds you can hear of bubbles collapsing. This is what to picture when imagining guns firing under water. Not small explosives, that were used much earlier in marine seismic surveys.

The seismic gun set up requires two mini GI (generator-injector) guns (as pictured). Each mini GI gun is made of two independent airguns that sit within the same casing. The ‘generator’, produces the primary pulse, and the ‘injector’ controls the oscillation of the bubble produced by the generator. The volume of air released by the injector increases the internal pressure of the bubble, and prevents its violent collapse, reducing subsequent oscillations. The bubble oscillations are pressure variations that occur, as the bubble expands and collapses on its way to the surface. These bubble oscillations transmit pressure disturbances outwards into the water. The volume of air released by the injector can act to reshape, reduce or completely remove the bubble oscillations that characterize the airgun signature.

We will begin our shift work at midnight tonight. The plan is to recover one or two OBS, before commencing the seismic work in the early hours of the morning.

Day 4

While the night shift took some rest, we successfully recovered one OBS (no. 10) at the northern termination of Knipovich ridge (NKR). The original plan was to recover two OBS but the visibility was too poor. The breaking waves made the search for the OBS a difficult task!

Clock Drift

‘DIRC’ Clocks in the OBS are temperature sensitive. When the clocks are recovered they need to be synchronized right away before they warm. They are synchronized at the beginning (pre-deployment) and at the end (on recovery). It is known that during the experiment the clocks drift linearly. This clock error is applied automatically which addresses the linear drift problem. However, there is also non linear drift which is a much more time consuming fix (requiring several weeks of processing). Vera tells me that the new manufactured (AWI) seismometers were fitted with clocks that have worse drift than the old seismometer clocks. They are less power consuming but the processing as a result is more time consuming!

The seismic deployment began at midnight, and we were ready to start our first 2D seismic line by 01:30 AM. During the night shift we will complete at least 2 survey lines that will take approximately 3 hours each to shoot (line distances: ~25 km). The sea feels a little rough (2m swell), but by tomorrow evening the wind will drop.

 

Bathymetry map displaying NKR survey area with seismic lines and OBS pick up locations (orange), the red cross marks the retrieved OBS no. 10 (recovered late in the evening on day 3)

We completed four seismic lines, finishing the last line just after 18:00. We had to stop work and recover the seismic instrumentation after an incident onboard required a helicopter rescue. We had planned to start OBS recovery at midnight during the calm weather, but now we will head to Longyearbyen and wait for a replacement crewmember to join us.

A very impressive and quick helicopter rescue operation (photo: Frances)

Day 5

We arrived into Longyearbyen 05:50 on what was a very pleasant morning. We tied up to the wharf in view of the seed vault and joining us were three adult common eider ducks (ærfugl) and their 21 ducklings, together with two adult barnacle geese (Hvitkinngås in Norwegian – ‘white cheek goose’). I had initially thought that the eider ducks were pink-footed geese, until Stormer corrected me. He told me a story about an ærfugl he found on the aft of the Kronprins Håkon. On a cold, stormy November day while on the way to Greenland, Stormer moved a stack of pallets to prepare for coring, and there standing before him was a duck. The duck was standing so still, the first thought that came to mind was “who brings a wooden duck onboard?” He took a step forward to take a closer look, to find that the duck was in fact a real duck. An ærfugl. The crew kept her warm and fed, and released her back into the wild in Tromsø.

 

Two barnacle geese, three common eider ducks and their 21 ducklings (photo: Frances)

It was a quick turnaround, and we left Longyearbyen at 15:00 after our new crewmember joined the ship. The plan is to deploy the seismic at 03:00 on our return back to site, however plans can quickly change and the order of activities is dependent on the weather forecast. We left behind bright skies in Longyearbyen, with a striking display of cirrus and cirrocumulus clouds. After eating match worthy food – chili and nachos, and watching the intense euros finals game, we wait three more hours and hope that the waves will calm for the seismic deployment.

 

Prins Karls Forland in the distance with magnificent cloud formations – originally named Prince Charles’ Foreland after King James’ son in 1612 (photo: Frances).

Day 6

Back to site

We arrived back to site at 02:30 and the weather was less than ideal for seismic. We are limited with what we can do in poor weather, but waiting doing nothing is not a favourable option. We deployed the seismic in a slightly uncomfortable but a ‘safe to deploy’ sea state (winds 12-13 m/s). We started the deployment of the equipment at 04:00 then transited to the start of the survey line starting at 05:30. The line was complete at 08:30 just after our fantastic breakfast of pancake topped with berry jam and maple syrup.

The day shift (Vera, Mechita and Przemek) were up earlier than usual at 06:00, as the plan was to start the recovery of the OBS, but as it was too windy they had to wait until later on in the day. We decided the next best thing was to start a small multibeam survey. We plan to use the multibeam map to compare with the same data acquired in previous years, to see if there is any slump movement recorded in the sediments (pictured). We completed three lines of multibeam before cutting the fourth line short to recover OBS #5, and #6 once we had entered a calmer sea state.

Map of the bathymetry (seafloor topography) in the rainbow coloured depth scale, acquired during the small multibeam survey. Orange filled circles are the locations of OBS #5 and #6.

Recovering OBS #5

I hear Vera on the radio at 20:45 “OBS is rising. It’s at 730 metres” so I go up to the bridge to get ready with Truls and Hans (the Captain) to look for the OBS. Outside on the bridge deck are Andreia, Przemek and Mechita ready and waiting for the OBS to come to the surface. Attached to the side of the OBS is a flashing light and radio beacon that sends out a signal in one of four different frequencies, assigned to each OBS. The radio picks up the signal and makes a beeping sound when the OBS is close and Przemek and Mechita are then able to find the direction of the signal in relation to where they are standing on the ship. As the signal can only be picked up close to the ship, relying on sight is sometimes more reliable, so the more eyes on watch the better!

Andreia and Przemek holding the radio, looking and listening for the OBS once Vera has confirmed that it has reached the surface.

Mechita holding her own radio during the search for the OBS.

Bird commonly sighted, flying around the ship – best photo! (photo: Przemek)

Przemek looking the part! (photo: Frances)

Day 7

If it’s not a strong wind preventing the recovery of the OBS it’s poor visibility. We stopped the recovery of the OBS late in the night due to fog, and deployed the seismic in the early hours, ready to go at 01:30. We repeated a seismic line that was shot in bad weather from the day before. This time the line started with fairly low winds (9m/s) and then the wind picked back up midway through. When the line was completed, we had to recover the seismic to transit to the start of the next line. It is a lot of work taking the seismic equipment in and out, but we are unable to transit between lines at a good speed with the equipment in the water. Time is everything when working at sea.

We (the night shift) were let off bringing the seismic back in for the second time in the night. The day shift took over and completed the second seismic line of the day (Line 5). We managed to recover two OBS before lunch, and another at 16:00. All the OBS recoveries went smoothly, and we were joined by a pod of dolphins at 17:40 during OBS # 3 and large number of puffins, in flight circling around the ship. They are the fastest flying birds I have ever seen at sea. They don’t appear to conserve any energy while flying. They look like they are going somewhere in a hurry, and it is almost impossible (using my compact camera) to get a good picture of them.

It took a little longer than usual to spot OBS # 3. Conditions never seem to be completely perfect for the job! This time the glare on the water made spotting the OBS difficult. Przemek was the only one wearing sunglasses, and was able to strain his eyes to spot the OBS some 300-400 metres away from the ship. What made it even more difficult to spot was that the flag was lying flat against the water, instead of upright.

Between OBS sites 3 and 9 we transited for an hour. This was a good opportunity to eat the freshly prepared doughnuts left in the mess. What a treat!

We successfully recovered OBS # 9, and there were hopes to recover the last OBS left on the sea floor, but the fog returned. The day shift OBS team, were exhausted after such a successful day of OBS recoveries and went straight to bed.

 

Andreia watches out for the (not so obvious) OBS near the ship.

The view of the OBS drifting by, moments before catching it, using a hook thrown out to the line to pull it in.

Mechita and Andreia assist with the winch operation as the OBS lifts out of the water.

Day 8

The fog cleared during the night while the OBS team were in bed, and the sun shone briefly. We continued the multibeam survey, started two days ago, all through the night, until breakfast. We had an English breakfast this morning, eggs and bacon, but no beans. The final OBS was recovered mid-morning and OBS preparation for the active seismic experiment has started.

If it’s not the strong wind or the fog preventing OBS work – it’s the sea ice! We planned a seismic experiment before the start of the cruise, in the west of Vestnesa ridge. Here we will most likely encounter sea ice. We discover today that our proposed location for the OBS sits within the open drift ice. It would be foolish to take the risk and deploy the OBS, while the charts show drift ice nearby. I am told that deploying the OBS in sea ice is no problem, but when the OBS surfaces it could get stuck beneath the ice.

The  Norwegian Meteorological Institute (www.cryo.met.no) provides ice charts and satellite images daily that we can use to monitor the whereabouts and movement of sea ice. The satellite imagery is not affected by cloud cover and can acquire data during the day or night and under all weather conditions.

An ice chart overlaid onto the bathymetry (seafloor map) provides location of drift ice in the Fram Strait. The active seismic planned experiment in the west of Vestnesa ridge is within open ice drift.

The latest satellite image used for mapping of the sea ice (cryo.met.no). The sentinel-1 mission includes a C-band Synthetic Aperture Radar (SAR) sensor that has a resolution down to 5m. Red arrow points to west Vestnesa ridge.

The winds were high in the afternoon, so we continued with multibeam mapping, extending away from our first survey area towards the northwest. Once the ship met the sea ice we decided to abort the multibeam survey, change course and head three hours east to prepare ourselves for the active OBS deployment in an ice free conditions.

For the transit there were fresh wienebrød (Norwegian custard centered pastries). De var gode!

Birds pictured on the sea ice, during the last multibeam survey line, heading northwest, towards the western part of Vestnesa ridge (photo: Frances)

Day 9

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.

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.

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!

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. The profile of chirp line 3 shows the buried glacial debris.

Day 10

We continued shooting seismic for the active survey experiment. Some inlines were dropped, to make up time so that we would be ready by 16:00 to start the circular lines. The weather conditions remain perfect for seismic, the sea is calm and there is a moderate breeze.

One thing is certain while working at sea; you can never be too confident that things will go to plan! We had planned to recover the OBS tomorrow morning after completion of the seismic lines, however a large number of whales showed up within 500m of the ship. As there were so many whales showing up so close to the vessel, we shut down all our acoustic systems, including the seismic guns. On the bridge they saw plenty of finwhales and Knølhval (horn whale) breaching the surface. The ‘Havforskningsinstitutet’ (Institue for marine research) specify that the Greenland whale and Narwals are species that require the most protection.

The mini GI air guns we use for the seismic are 15 and 30 cu.in. The guns are significantly smaller than conventional seismic guns used in industry. Higher powered guns are used to send signals much deeper into the subsurface. We are only interested in high detail in the near surface sediments (top ~300m).

While on the bridge deck looking out in awe, we spoke about how long we should wait before starting the seismic back up. This depends on the water depth. Whales spend up to 45 minutes beneath the surface in deep (+1000m) water, so we could be waiting an hour before knowing whether the whales are still close by. I worked on a seismic vessel where we had a marine mammal observer onboard. However, she would not observe the whales. Instead, with her own hydrophone, would spend the whole shift with headphones on, listening for whale calls. Relaxing shift?

As time is precious, we decided to move away from the active seismic survey site and move south to Svyatogor ridge to complete some seismic lines for our colleagues not on the cruise – in whale free areas.

Whales as far as the eye can see. (photo: Vera Schlindwein)

 

Day 11

We completed two seismic lines, at Svyatogor ridge in the night and there were no whales in sight. The wind speed was low for the first line and had gradually picked up by the start of the second line. We then recovered the guns and streamer just before breakfast, before heading back to our NKR active seismic site. During the first half of the day, the winds reached up to 18 m/s, which was far too much to redeploy the seismic at NKR, so we waited. There was no whale watching.

When the winds had dropped down to 11m/s, we decided to deploy the seismic and finally finish what we had started two days ago! We are nervously waiting for calm weather windows to appear so that we can get back the OBS. The OBS each have a programmed automatic release time, which is 16:00 tomorrow.

The wind dropped to a gentle breeze, and we swiftly recovered three out of the five OBS. The remaining two OBS from UiT are more difficult to see, and the fog had started to set in. We decided to leave them until the morning.

We shot our final seismic line after taking the break to recover the OBS, and after dinner, planned a water column survey over the NKR active seismic study area. Microbial mats discovered on the seafloor, during a previous cruise, suggest release of methane gas and we might expect bubbles escaping from beneath the water bottom.

Fog and stillness (photo: Frances)

Above: EM302 multibeam water column swath (30 kHz), and below: EK60 echosounder (18 kHz)

The water column survey completed in the early hours of the following morning did not show any acoustic flares in the water column, but we saw plankton scrolling across the screen in the single beam echo sounder (fish finder), and bright spots appearing mostly above 500 metres in the multibeam swath. I suspect that we imaged more fish bladders than gas bubbles!

 

Day 12

This morning we successfully recovered the last two OBS. It was a relief to have them all back onboard! Shortly afterwards, we left Knipovich ridge and started our two day journey back to Tromsø. On the way, we will stop at a study site called Bellsund, we presume named after Bellsund fjord, situated on the west coast of Svalbard. The site is actually much further south from Bellsund. It is opposite the Hornsund fjord. Here we will shoot two final seismic lines for our Italian colleagues.

After half an hour in the fresh sea air, watching the birds flying around the ship, I spotted a feeding frenzy. From a distance, it looks like the water is bubbling. Dolphins surface, one after the other, and dive back down into the water. The birds hover above, waiting for the scraps. After their feed, the dolphins could not resist coming towards the ship to play in the waves beneath the bow.

Dolphins playing beneath the bow of the ship (photo: Frances)