Cruise Report of the SIGNAL 2009 Refraction Seismic Cruise (Hudson 2009-019)

The SIGNAL (Seismic Investigations off Greenland, Newfoundland and Labrador) cruise onboard the Canadian Coast Guard Ship Hudson (cruise Hudson 2009-019) started in Halifax, Nova Scotia, on June 5, 2009 and ended in St. John’s, Newfoundland and Labrador, on July 8, 2009. The project was a collaboration between the Geological Survey of Canada (GSC) and the Geological Survey of Denmark and Greenland (GEUS) with scientific participation of Dalhousie University (Halifax, Nova Scotia). For both geological surveys, the cruise was part of their respective continental shelf programs under UNCLOS (United Nations Convention on the Law of the Sea) Article 76.

During the cruise, wide-angle reflection and refraction (R/WAR) seismic data were acquired south of Greenland, in central Labrador Sea and in Orphan Basin using an airgun array consisting of 12 G-guns with a volume of 520 cubic inches each (total volume 6240 cubic inches) as seismic source. The shot interval was one minute at a nominal speed through the water of 4.5 knots, which resulted generally in 4.0 to 6.0 knots over ground. The signals were recorded by ocean bottom seismometers (OBS) equipped with externally mounted three-component geophones and a hydrophone. The number of instruments along the five lines varied between 10 and 24. A total of four OBS were lost during the cruise, three of the instruments did not come back to the sea surface while a fourth instrument came back on the pre-programmed backup release time but the ship was too far away from the OBS to recover the instrument. In addition to the OBS, a total of 29 sonobuoys were deployed to decrease the receiver spacing, which was most relevant on the longest line of the survey in Orphan Basin. Eight of the sonobuoys did not function properly, either they did not re-surface after deployment or no signals were transmitted back to the ship.

Knowledge of the velocity of sound in water is important to apply corrections for the drift of the sonobuoys but also for the calculation of the location of the OBS at the seafloor. For this reason, a total of ten XBT (expendable bathythermograph) measurements were carried out during the recovery of the OBS. All R/WAR seismic lines were collected along pre-existing reflection seismic lines collected by GEUS, GSC, and the German Federal Agency of Geosciences and Natural Resources (BGR).

Fig. 1 shows the location of the five R/WAR seismic lines. Line 1 in Orphan Basin is coincident with reflection seismic line GSC 84-3. Main objective of line 1 is to determine the crustal structure between Newfoundland and Orphan Knoll. In case of crustal continuity, Orphan Knoll could be considered as a natural prolongation of the Canadian continental shelf. The line is 632 km and a total of 22 OBS were deployed complemented by 18 sonobuoys. Due to the relatively wide instrument spacing, one critical segment of the line around the shelf break was shot a second time (line 1A, 208 km) with an additional deployment of five OBS. Two OBS could not be recovered.

Line 2 is a 227-km-long line crossing the Eirik Ridge in a NW-SE orientation, while line 3 (297 km) is located on the crest of the ridge in a SW-NE orientation. Lines 2 and 3 are coincident with reflection seismic lines GEUS 2006-16 and GEUS 2003-1, respectively. Line 3 had 1 sonobuoy and 24 OBS deployments. All OBS were recovered but two instruments did not record. Line 2 had two sonobuoy deployments and 18 of the 20 OBS along the line were recovered. Lines 2 and 3 were designed to define the crustal character and thickness of the Eirik Ridge. The ridge is known as a sediment drift feature but its deeper structure is unknown.

Line 4 is coincident with reflection seismic line BGR 77-17 and crosses the extinct spreading axis in Labrador Sea. Line 5 is a cross line that is coincident with reflection line GEUS 2003-4 and runs along the extinct spreading axis. Line 4 is 255 km long and here three sonobuoys and 18 OBS were deployed. Line 5 is 143 km long with five sonobuoys and 10 OBS. All OBS worked without fault and provide a very good data set from which the crustal structure within and across the extinct spreading axis can be determined. Of particular interest are possible changes in the crustal thickness as the seafloor spreading ceased in Labrador Sea and possible serpentinization processes in the mantle. The data should also be able to detect any possible seismic anisotropy.