RRS Discovery D215
Cruise summary report
| Cruise Info. | |
| Ship name (ship code) | RRS Discovery (74E3) |
| Cruise identifier | D215 |
| Cruise period | 1995-07-18 — 1995-08-20 |
| Status | Completed |
| Port of departure | Barry, United Kingdom |
| Port of return | Barry, United Kingdom |
| Purpose | Research |
| Objectives | To conduct a geophysical survey to characterise in detail the variations in crustal and upper mantle structure in the transition zone in the Iberia Abyssal Plain of the west Iberian Atlantic Margin
First leg: 18th-28th July 1995, Barry-Lisbon 1) 200 km long deep towed three-component magnetometer profile extending from the transitional zone onto the continental slope to the east, and a shorter profile close to the continental slope, using the IFREMER SAR vehicle. 2) The SAR recorded side-scan sonar and high resolution seismic profiles Second leg: Second leg: 29th July-22nd August, Lisbon-Barry 3) 2 deployments of ocean bottom seismographs (OBSs) deploying 21 instruments along a 340 km line for the first experiment (14 good OBS's) and 18 instruments (13 good OBS's) in a 60 by 60 km for the second. Shots were fired into these from a 12-gun 6346cu in airgun array towed at 20 m depth and fired every 40 or 50s. During the second experiment, the full 2.4 km, 48-channel hydrophone streamer at a depth of 20 m and recorded ~ 1500km of deep seismic reflection profiles 4) An extensive grid of bathymetric, gravity and surface magnetic data was collected 5) A variety of triggering systems were used, in each case, the trigger was sent to the Reftek gun controller, which then fired the airguns/waterguns 50ms after the trigger time. The same trigger also triggered the Sercel 48-channel acquisition system, the SAQ four-channel system, and various EPC's and Waverley recorders used for display purposes |
| Chief scientist | Tim A Minshull (University of Cambridge Department of Earth Sciences) |
| Cruise report |  (0.54 MB) |
| Ocean/sea areas | |
| General | North East Atlantic Ocean (limit 40W) |
| Specific | Eastern Atlantic |
| Track charts | (0.28 MB) (0.24 MB) (0.32 MB) |
| Measurements | |
| Physical oceanography | |
| Bathythermograph | Quantity: number of profiles = 7 Description: Sippian XBT probe types T5 and T7, deployment during the cruise, data transmitted via satellite after each launch. 15 XBTs launched, only 7 operated |
| Meteorology | |
| Occasional standard measurements | Quantity: number of deployments = 19 Description: Sparton Electronics AN/SSQ-57A disposable sonobuoys: one during the first OBS deployment and the remainder during the second |
| Geology and geophysics | |
| Geophysical measurements made at depth | Quantity: number of deployments = 22 Description: Ocean bottom seismographs 5 different types were brought on the cruise. 2 Cambridge OBSs in deep-water tubes, 2 Cambridge OBSs in glass spheres, 6 Cambridge Minidobs, 4 IOS (Digital Ocean Bottom Seismographs using Portable Digital Acquisition Systems. Each DOBS was deployed twice), and 6 Dalhousie OBSs. |
| Geophysical measurements made at depth | Quantity: track kilometres = 1500 Description: SAR composed of a side-scan sonar, a 3.5kHz profiler, a 3 component magnetometer, and a 30 m single channel hydrophone. 200 km long roughly east-west profile was acquired in the Iberia Abyssal Plain continental margin across the transitional and thinned continental crust from 5200 m to 1400 m water depth. The mean towing depth was 200 m above the sea floor. A 65 km long complimentary profile was acquired parallel to the previous one 1.5 km north. 565 line km were acquired in 92 hours, with continuous acquisition of side-scan, seismic and magnetic data |
| Single-beam echosounding | Description: Simrad EA500 single beam echosounder operating at 10 Hz to log water depths. It was also used for detection of pingers on the IOS (10kHz) and Dalhousie (12kHz) OBSs. |
| Multichannel seismic reflection | Quantity: number of track lines = 35 Description: Multichannel seismic measurements. Line CAM126 was recorded in format on the SAQ four-channel digital acquisition system. Otherwise, multichannel data were recorded in SEGB multiplexed format on an SN358 with 2 tape drives (A and B) and a spare (drive C). Sample interval was 4 ms. |
| Multichannel seismic reflection | Quantity: track kilometres = 1500 Description: Multichannel streamer 48x50 m active sections, 5 spring sections, a tow table and a tail rope, with 14 individually addressable birds to control the depth, 8 calibrated depth sensors to monitor the depth, and 5 retrievers to provide buoyancy if the streamer sunk below the predefined depth. The streamer was towed at a nominal depth of 20 m. |
| Gravity measurements | Description: The gravity field was sampled every 10 s using a LaCoste and Romberg shipboard gravimeter. Differential GPS navigation was used in data processing during times the differential corrections were enabled. Gravity base ties were taken at Barry at the beginning and end of the cruise. Drift was +2.34mGal in 35 days. |
| Magnetic measurements | Description: Varian V75 Magnetometer, with the fish towed about 160 m astern of the ship on the starboard quarter |
| Other geological/geophysical measurements | Description: Hamworthy compressors Seismic sources. The ship was equipped with Hamworthy compressors with a nominal capacity of 400 cfm each. 12 gun array on 4 beams with a total capacity of 6346 cu.in |
