T. Nielsen, J. Foeken, J. Taylor, A. van der Molen, N. Kenyon, M. Horstink-, T. Mikkelsen
Zntroduction
Ten lines of OKEAN sidescan sonar data were collected (PSAT-22 to 31), together with a single OREtech line (ORAT 15) which cross-cuts the OKEAN mosaic approximately N-S (Fig. 99).
The OKEAN data form a mosaic, extending SW-NE. The mosaic is approximately 50 km in width and 350 km in length.
OKEAN mosaic
Five acoustic zones are tentatively identified on the OKEAN mosaic (Fig. 104) on the basis of backscatter levels and features.
Zone 1 occurs in the far NE and is high backscattering and featureless, except for vague lineations which are oriented N-S. The second zone occurs to NW of this and is of medium backscatter. This is also generally featureless. There is a faint band of lineations associated with a channel approximately 5 km wide in the central region of this zone (01:OO PSAT-26 and 05:45 PSAT- 28). The third zone forms the majority of the study area and is an area of medium backscatter, grading to high to the NW. The area is subcircular and bounded by narrow high backscattering surfaces associated with a channel (zone 5).
The fourth zone is SW of a channel seen as a band of low backscatter, approximately l-2 km wide. The zone has a train of low backscatter features extending NW. These latter are wave-forms with their crests oriented SW-NE. The belt of features bends to the west as it progresses NW. Beyond this distinct area, there are several low backscatter features without regular shape of orientation.
OREtech data
In the north of the line (12:45 to 13:30), a channel is clearly imaged. The bed is high backscattering and has parallel longitudinal bedforms within it, oriented SE-NW. The SE bank of the channel is low backscattering, and the profiler data show stratification in this location (Fig. 105).
From 13:30 to 17:00 there is a southward dipping surface which is dissected by a 50 metre deep channel. The north side of the channel is a small level plain, which appears as a low backscatter area on the sonar data. The southern boundary of the channel is steep-sided, and the surface beyond is flat and level. There is stratified sequence from 16:O0.
From 17:00 to 17:30, on the starboard side of the data, there is an area of high backscatter and rough surface texture that corresponds to a subcircular high backscatter feature on the OKEAN.
Ramped against this is a smooth high backscatter surface. South of this area, the surface becomes low alternating to medium backscatter. Some sections (e.g. 18:00 to 18:30) show waveforms with crests trending N-S, which pass across the profile at the edge of a low backscatter zone. The northern-most low backscatter area shows some penetration on the profiler record.
Interpretation of seismic and sidescan sonar data
The data collected in this area are intended to look at and map sediment pathways associated with the spreading of Norwegian Sea Overflow Water (NSOW) once it leaves the fast-flowing confinement of the Faeroe Bank Channel.
The seismic data allow the identification of two units, the acoustic basement and the surface unit. The acoustic basement outcrops in the NE of the survey region and appears as a region of high
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61" 45'
61'30'
61"15'
-12' -11'
Fig. 104. OKEAN mosaic of the study area. Numbers indicate zones with different backscatter patterns described in
IOC Technical Series No. 52 Page 166
backscatter on the OKEAN mosaic. Elsewhere the acoustic basement forms a gently dipping surface, with a central high. Occasionally the unit also outcrops within the high and at its margins. The high is bound to the NE and SW by two basins, it is seen from the OKEAN mosaic that the high forms a subcircular feature. The acoustic basement is thought to be basalt formed subaerially during the opening of the North Atlantic. This is because of the generally basaltic nature of the surrounding region and the strong reflector. By analogy with the nearby banks, it is suggested that the central high is a result of compression (Boldreel and Andersen, 1993).
12:40 1340
Fig. 105. Fragment of ORAT- sonograph and profile showing channel in the NE of the survey area with linear longitudinal bedforms. Note the northern bank is of low backscatter and stratified on the profiler record. Arrow indicates sediment transport.
The surface unit infills and smooths the acoustic basement topography. It is present both around and on top of the central high. On top of the latter the surface unit is hummocky, whilst around the high the topography is smooth. The smooth surfaces are cut by several channels of a variety of forms and depths (75 - 150 m). The channels to the south and west are U-shaped or asymmetrical, whilst those to the far north are shallow V-shaped notches. Some of the channels in the S and E show progradation, indicating a flow direction.
The sonar data shows the complete pathways of the channels through the survey area.
Channels enter the region from the SE, flow around the basement high and exit to the NW. There are several other sections of channel in the NE, which do not appear directly connected to the principal channel around the high. These also flow SE-NW (from the OREtech data) but are more linear in plan. The banks of most of the channels are often low backscattering, indicating the accumulation of sand material. The channel to the SW of the central high also has a set of waves to the left of it. These are interpreted to be overbank mud waves.
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IOC Technical Series No. 52 Page 167