JC152: The end and the beginning

JC152 has been a successful cruise with loads of seismic data acquired in the North Sea. The data set acquired should greatly increase our ability to understand fluid flow processes in the sub-surface and across the seabed. Eleven seismic surveys with four different sources were recorded using Ocean Bottom Seismometers (OBS) and surface streamer in many varied combinations! This data set is surely going to keep the scientists occupied for next few years – processing, interpreting and publishing new understandings about chimney/fluid flow structures.

The JC152 team

We thank the Captain of James Cook and the crew for their assiduous efforts in making this acquisition possible. Maintaining the ship on track with speeds below c. 4.5 knots on blustery days isn’t an easy task! The crew did everything to make our living on ship a memorable experience.

We thank the technical support teams for their tireless work – OBIC team, National Marine Facilities, the Airgun team and Sparker team. These individuals ensured that ocean bottom data was properly collected, that seismic sources functioned efficiently and that landers were deployed in the correct way – this cruise would not have been a success without them all. Finally a big thanks to the science party, for their hard work and good humour throughout the 24 hours of a day.

All in all, it was an amazing experience in every respect: James Cook cruise 152

PS. A big thank you to the blogger – Bhargav – pictured above holding the JC152 sign.

Day 16: The final survey!

JC152 is acquiring seismic data along the lines of the final survey. Though it is last, it is still an important component. After collecting the spares for the streamer from Aberdeen, we reached the survey site on 7th September, c. 0500 hours.

The acquisition activities have commenced without any delay. The first to go on was the Sub Bottom Profiler (SPB). After a marine mammal survey, seismic activities resumed. Throughout this cruise, planning has been perfect – excellent coordination between science party and ship crew. Today’s plans were to record the water column velocity using a Sound Velocity Profile (SVP) and deploy 7 OBSs for the final survey.

Sound Velocity Profiler goes into the water

Unlike other surveys, which were acquired using a single streamer, this survey is being acquired by deploying two streamers behind the boat – more recordings per shot! The survey has 50 ships tracks and they are spaced at 25 metres distance, which with two streamers means we should have seismic lines every 12.5 m. Enhanced data collection from the second streamer is well underway with the aim of three dimensional image of the area. Hoping for the best!

More seismic data!

Day 10: All OBS recovered!

Unsettled weather with winds gusting 35 knots and increasing wave height meant activities had to be suspended for 12 hours. When winds reduced, activities on board resumed with the deployment of the Sound Velocity Profile (SVP) at c. 0230 hours during the night. The SVP is used to measure the water column velocity as it descends to sea floor. Soon after it, the ship crew and OBIC team were all set to start with the recovery of OBSs. They started the recovery at c. 0300 hours and successfully recovered all 25 by c. 1130 hours. Quick work!

OBS floating at the surface after triggering the release signal

OBSs are instruments which record seismic signals from the seismic sources while sitting on the sea floor. These instruments are dropped from the surface at designated locations. In our surveys, we deployed them on and around the chimney structures, also few away from the chimney to track the change in geology away from them.

Crew getting hold of the OBS in the water

OBS safely on the deck

We are now heading to the port of Aberdeen (cruise not finished yet!) to pick up a spare part for the recording streamer – back to the connected world for few hours. See you again soon with more updates.

Day 9: Loads of data!

Nine days of the cruise are done and we managed to do 10 different seismic surveys – excellent progress! Since our last update we have continued with our range of seismic sources, including a DuraSpark surface sparker, deep tow sparker and GI airguns. We were careful to follow the protocol for marine mammal observation before and during the survey.

Three survey designs for GI airguns

GI gun deployment

Ship crew deploying streamer into the water for recording seismic signals

Careful planning and a super-efficient crew are making this cruise a big success. Following the GI gun survey we completed a Deep Towed Sparker (DTS) survey, which is high frequency seismic sparker system that can be towed to a depth of c. 500m from the sea surface. In our surveys, we limited the depth to c. 125m as we have a maximum water depth of around 160 m.

Deep towed Sparker

After the seismic reflection surveys we headed to the Mini-lander location to recover it from the water. On the lander we had fixed a passive acoustic recorder floating at a depth of c. 20 m above the seafloor. The aim of the acoustic recorder was to record all the signals that we had generated using different seismic sources. A big well done to University of Southampton undergraduate student Dan Ellis for successful programming the recorder to operate properly. The data look great. The lander signals are going to be very useful in processing of the seismic data (in technical language we have recorded what is known as the far field signature). The recorded data from lander shows echoes from seismic sources, ship noises, and noise from other sources in water – it records any sound in the vicinity!

Signal detected on the acoustic recorder showing Deep Towed Sparker signal and other ship signals. Top panel shows sound detected on the acoustic recorder, the bottom panel shows the frequencies of different arrivals.

After lander recovery, we started with the Airgun grid, but we had to halt the survey due to poor weather on 4th September.

Airguns into the water

Next on the cards is Ocean Bottom Seismometer recovery (hoping to recover all OBSs without losing none). The recording of swath bathymetry (seafloor topography), sub-bottom profiles (30m below sea bed) and echosounder (water column) have been going on throughout all the surveys. The profiles from the three recordings show very clearly the gas flares emitting from pockmarks and chimney structures into the water!

Echo sounder (EK60) showing gas flare (yellowish) from pockmark– each panel shows a different signal range

See you later with more updates 🙂

Day 6: High resolution imaging continues!

Fantastic weather for surveying!

The day begin with the morning meeting after breakfast, in which it was decided to deploy a different high frequency sound source – the Dura Sparker – in place of the Squid Sparker source. We are fortunate to have Matt Poxon from Applied Acoustics (AA) Ltd on board; AA are the manufacturer of both systems. The Dura Sparker source contains the latest Sparker technology which uses negative discharge and is more sustainable. It uses a power of 2000 Joules/shot. It should be capable of giving more insights about the chimney structures!

The aim is to acquire high resolution data using the Dura source along the asterisk survey lines. This will enable us to look at the chimney structures at different scales of resolution and get a better understanding of their evolution and nature. So far we have data from Airguns, Squid Sparker and Dura Sparker – all of which create sound signals that penetrate through the sub-seafloor strata, but they operate at different frequencies which enables us to pick up different details in the geological structures. More high-resolution airguns called GI guns will also be deployed in the coming days, widening the range of frequencies of our seismic data. It is going to be excellent data for frequency-dependent studies!

Deployment of the Dura Spark system

Excellent job by Squid Sparker!

The asterisk survey is designed in such a way to image the chimney structures at different orientations. This will help us image the fracture patterns and understand how the physical properties of the chimney structures change with direction. In scientific terminology, this is called anisotropy.

Asterisk survey almost complete!

See you tomorrow with more updates 🙂

Day 5: Higher resolution imaging

The Surface “Squid” Sparker – great for higher resolution

The Bolt Airgun asterisk survey was completed early in the morning, c. 0600 hours. We call this survey an asterisk because of the orientation of the profiles on a map (see below). Airguns produce low frequency sound signals (c. 5 – 200 Hz) which penetrate deep beneath the seabed (c. 2 km depth). In geophysics when doing seismic reflection surveying there is a trade-off between penetration and resolution of the sound sources. The lower the frequency, the deeper the penetration, but the lower the resolution. Airguns are our strongest sound source, but lowest resolution. The aim of the airgun “asterisk” survey was to create sound energy that would be recorded by the Ocean Bottom Seismometers sitting on the seabed (see previous blog entries).
However, it was decided in the morning meeting to temporarily abandon the Airgun survey and get our highest resolution data collection going while weather was good (hopefully for next 72 hours) using a “Squid” surface sparker. The frequency range of the sparker is much higher than the airguns and is c. 300 – 2000 Hz.

Meeting of ship and scientific crews happens at 08.30 every morning.

The Sparker sound source and streamer (recording sound arrivals from the sub-surface – pictures will be in future blog entries) were deployed at c. 1000 hours and we have acquired 5 lines since then of the Sparker “Asterisk” survey (repeating lines done by the Airgun sound source).

Sparker “Asterisk” survey. Bold lines mean completed! The colours are depth to seabed.

High frequency seismic data coming in!

Lots more data to collect.. See you tomorrow with more updates!

Day 4: Acquiring data!

Before getting on with 25 Ocean Bottom Seismometer (OBS) deployments, water column sound speed measurement and patch tests were performed. Very accurate water column sound speed profiles are required for recording correct bathymetry. Patch tests are done to calibrate various sensors of the multi beam bathymetry acquisition.
The first OBS to go into water was OBS_19. The deployment started at c. 0130 hours and the last one to deploy was OBS_1 at c. 0700 hours.

OBS deployment

Following OBS deployment, we prepared for the Sparker (High frequency seismic source) acquisition. However, data acquisition with the Sparker had been halted for some technical problems.
Soon after it, the air guns were ready to go into the water. Mammal observation protocol has been followed before air guns started firing with full power. Also, one watchkeeper was assigned the duty of mammal observation during the survey. We have three airguns firing at full power of 700 cubic inch volume. With this start, we begin the Bolt Asterisk Survey.

Three guns firing

Asterisk survey – bold lines are those that have been completed

We are acquiring the last line of the survey at 0530 hours, 29th August. The next survey to follow is the grid survey.
See you soon with grid survey updates 🙂