RRS James Cook JC251
Cruise summary report
| Cruise Info. | |
| Ship name (ship code) | RRS James Cook (740H) |
| Cruise identifier | JC251 |
| Cruise period | 2023-08-29 — 2023-09-26 |
| Status | Completed |
| Port of departure | Southampton, United Kingdom |
| Port of return | Southampton, United Kingdom |
| Purpose | Research |
| Objectives | JC251 collected the multi-method geophysical dataset for the NSFGEO-NERC project "Quantifying evolution of magmatism and serpentinisation during the onset of seafloor spreading". The overall aim of the project was to quantify the evolution of serpentinisation and magmatism at a magma poor rifted margin as seafloor spreading is initiated, including whether serpentinisation of exhumed mantle is focused in discrete fault zones, and whether magmatism increases abruptly or gradually.
The cruise acquired a coincident high-quality controlled source electromagnetic (CSEM), mangetoteluric (MT) and wide-angle seismic datasets along a carefully chosen profile at the magma-poor Goban Spur rifted margin. We deployed 49 multi-channel Ocean Bottom Instrument (OBI; 35 Scripps US and 14 OBIC UK) capable of recording simultaneously wide-angle seismic, electric resistivity and magnetotelluric datasets, and recorded a multi-physics dataset along a ~200 km line. 14 OBIC instruments were equipped with 3-component geophones and 2-component horizontal electrode dipoles with 12 m antennae and ten of them had additional 3 component fluxgate magnetometers. 35 Scripps instruments were equipped with 2-component horizontal electrode dipoles with 10 m antennae and 2-component induction coils and hydrophones. Additional two Scripps LEM (Long-wire electromagnetic) devices with a horizontal electrode dipole with a 200 m antenna were also deployed to investigate the deeper electric resistivity structure along the profile. All datasets were recorded at 250 Hz sampling rate. Controlled source wide-angle seismic data were acquired using Bolt LL guns, twice; line 10149-002 (4900 cu.in and 90 s) and 10149-003 (3900 cu.in and 30 s). The electric filed transmission was carried out with the electromagnetic source of the University of Southampton, DASI. DASI was equipped with a 300 m antenna and 30 m copper electrodes, and it is towed at 100 m altitude above the seafloor using the A-frame winch. During the deep-tow, DASI's altitude was controlled by its altimeter, and two USBL beacons (one at the front of DASI and another at 330 m back) were used for positioning, though the latter rarely provided a position. DASI transmitted the waveform-D at 0.25 Hz. During the DASI operation, a fluxgate magnetometer was also towed 550 m behind DASI and operated at 250 Hz to record DASI transmissions in addition to the magnetic field, but the instrument did not record any data due pressure onto its SD card. The initial survey plan included two (northern and southern) profiles, both parallel to the spreading direction. Thanks to a legacy multi-channel seismic profile, most of the structure along the northern line was known. We started the data acquisition with the northern profile because the profile showed unambiguous oceanic crust to the west and stretched continental crust to the east, with a transitional crust in between suitable to study the roles of magmatism and serpentinisation during continental break-up and initiation of seafloor spreading. After the deployment of OBIs, we deployed DASI within which the newly built electronics was used for the first time in JC251. DASI was not tested at sea and required some troubleshooting before successful data acquisition. After the first DASI deployment, to give time to the DASI team, we switched to controlled source seismic. A seismic Bolt LL airgun array of 5700 cu.in was deployed and operated using Big Shot gun controller with a shot interval of 90 s and with a vessel speed of 5 kn. During this first seismic gun deployment (10149-001), two single guns of 700 cu.in volume did not fire and the 500 cu.in volume returned a faulty waveform and had to be turned off, red |
| Chief scientist | Gaye Bayrakci (National Oceanography Centre, Southampton) |
| Cruise report |  (4.55 MB) |
| Ocean/sea areas | |
| General | Bay of Biscay North East Atlantic Ocean (limit 40W) |
| Specific | Western Approaches, Goban Spur rifted margin, Whittard Canyon. |
| Track charts | (0.06 MB) |
| Measurements | |
| Geology and geophysics | |
| Geophysical measurements made at depth | Quantity: number of deployments = 2 Description: Scripps LEM (Long wire Electromagnetic) seafloor instruments with 200 m dipole length - Electromagnetic data, sampling rate - 250 Hz, instruments continuously recorded from 31st August to 12th September 12:00 and recorded DASI's waveform-D (0.25 Hz) transmissions between between 08th September 21:00 and 12th September 04:20. |
| Geophysical measurements made at depth | Quantity: number of deployments = 14 Description: OBIF seafloor instruments - Controlled source electromagnetic data, sampling rate 250 Hz, instruments contained two horizontal electric dipoles and recorded DASI's waveform-D (0.25 Hz) transmissions between 08th September 21:00 and 12th September 04:20 as well as continuous recording from 31st August up to 17th September. 13 of 14 deployments recovered. |
| Geophysical measurements made at depth | Quantity: number of deployments = 14 Description: OBIF Seafloor instruments - Magnetotelluric data, sampling rate - 250 Hz, instruments contained three- component fluxgate magnetometers (x,y,z) and recorded continuously from 31st August up to 17th September. 13 of 14 deployments recovered. |
| Geophysical measurements made at depth | Quantity: number of deployments = 2 Description: Valeport CDT-V measurements during LEM launching. These data are used for multi-beam acquisition and for the acoustic navigation for relocating the instruments. The data is archived with the Ship Scientific Systems data. |
| Geophysical measurements made at depth | Quantity: number of deployments = 35 Description: Scripps seafloor instruments - Magnetotelluric data, sampling rate - 250 Hz, instruments contained two horizontal coils and continuously recorded from 31st August to 12th September 12:00. 33 of 35 deployments recovered. |
| Geophysical measurements made at depth | Quantity: number of deployments = 35 Description: Scripps seafloor instruments - Controlled source electromagnetic data, sampling rate - 250 Hz, instruments contained two horizontal electric dipoles, recorded DASI's waveform-D (0.25 Hz) transmissions along the northern line (10149-004) between 8th September 21:00 and 12th September 04:20 as well as continuous recording from 31st August to 12th September 12:00. 33 of 35 deployments recovered. |
| Geophysical measurements made at depth | Quantity: activity duration in days = 3 Description: DASI deep-tow electromagnetic source: altimeter, CTD, waveform recorded for the duration of the controlled source electromagnetic acquisition (between 08th September 21:00 and 12th September 04:20) |
| Seismic refraction | Quantity: number of deployments = 35 Description: Scripps seafloor instruments - Controlled source acoustic data, sampling rate - 250 Hz, Instruments contained one hydrophone and recorded the BOTL LL airgun shots along the northern profile shot twice: with a 90 s shot interval and a gun volume of first 5200 cu.in then 4900 cu.in (10149-002, 4th September 13:56 to 6th September 03:10), and with a shot interval of 30 s and a gun volume of 3800 cu.in (10149-003, 6th September 03:11 to 07th September 08:53). The instruments also recorded the initial seismic line aborted due to the gun volume becoming too small due to misfiring guns (10149-001, 3rd September 21:33 to 4th September 06:01). 33 of 35 deployments recovered. |
| Seismic refraction | Quantity: number of deployments = 35 Description: Scripps seafloor instruments - Continuous acoustic data, sampling rate - 250 Hz, instruments contained one hydrophone and recorded continuously from 31st August to 12th September 12:00. 33 of 35 deployments recovered. |
| Seismic refraction | Quantity: number of deployments = 14 Description: OBIF seafloor instruments: Continuous acoustic data, sampling rate - 250 Hz, instruments contained one hydrophone and recorded continuously from 31st August up to 17th September. 13 of 14 deployments recovered. |
| Seismic refraction | Quantity: number of deployments = 14 Description: OBIF seafloor instruments - Continuous seismic data, sampling rate - 250 Hz, instruments contained 3 component geophones, and recorded continuously from 31st August up to 17th September. 13 of 14 deployments recovered. |
| Seismic refraction | Quantity: number of deployments = 14 Description: OBIF seafloor instruments - Controlled source acoustic data, sampling rate - 250 Hz, instruments contained one hydrophone, recorded the BOLT LL airgun shots along the northern profile shot twice: with a 90 s shot interval and a gun volume of first 5200 cu.in then 4900 cu.in (10149-002, 4th September 13:56 to 6th September 03:10), and with a shot interval of 30 s and a gun volume of 3800 cu.in (10149-003, 6th September 03:11 to 07th September 08:53). The instruments also recorded the initial seismic line aborted due to the gun volume becoming too small due to misfiring guns (10149-001, 3rd September 21:33 to 4th September 06:01). 13 of 14 deployments recovered. |
| Seismic refraction | Quantity: number of deployments = 14 Description: OBIF seafloor instruments - Controlled source seismic data, sampling rate - 250 Hz, instruments contained 3 component geophones, recorded the BOLT LL airgun shots along the northern profile shot twice: with a 90 s shot interval and a gun volume of first 5200 cu.in then 4900 cu.in (10149-002, 4th September 13:56 to 6th September 03:10), and with a shot interval of 30 s and a gun volume of 3800 cu.in (10149-003, 6th September 03:11 to 07th September 08:53). The instruments also recorded the initial seismic line aborted due to the gun volume becoming too small due to misfiring guns (10149-001, 3rd September 21:33 to 4th September 06:01). 13 of 14 deployments recovered. |
| Moorings, landers, buoys | |
| Physical oceanography | |
| Neutrally buoyant floats | 45° 40' 37" N 14° 19' 36" E — Recovery of PAP SO buoy that was detached from its anchor due to bad weather |
