Fróðskaparrit - 01.01.1979, Page 97

Marine Seismic Investigation
105
basalts on the sea floor, the Faeroese government and the
Geological Survey of Denmark carried out a seismic survey.
The survey was carried out on the research vessel »Dana«,
6—22 July in 1979, with equipment on loan from the Geo-
logical Survey of Greenland. About 2300 km of shallow seismic
profiles were obtained (Fig. 1).
This initial report describes the survey and presents some
preliminary results.
Methods
The sea bottom was investigated chiefly by seismic methods,
by using the same principle as a recording echo sounder. A
10—20 cu inch airgun (Bolt) operated at 120 atm or a 4 KJ
nine electrode sparker array (EG & G 402) was used as
sound source. The sound signal reflected from the sea floor
or layers in the substrate was detected by a short hydrophone
streamer (EG & G 265). After amplification and filtering of
the seismic signal, the reflections were recorded on a facsi-
mile recorder (EPC 3200 and 4500). An example of the re-
cords is shown on Fig. 4. The depth was measured by a Simrad
Fig. 3 (left). Side-scan record from Djúpini (line 57).
The side-scan emits a narrow sound beam perpendicular to the course
of the ship just above the sea floor. The sound reflected (or rather
scattered) from the obstacles on the sea floor is recorded. The record
resembles a negative photography of the sea bottom as illuminated by a
projector just above the sea floor.
The darker areas mark the parts of the sea bottom which protrude
or for other reasons reflect more sound energy than average. The white
area in the upper central part marks the acoustic shadow behind a pro-
truding feature. The lower half shows the bottom on the port side (west)
of the ship. Interpreted geologically the picture probably shows an out-
crop of basalt, with an outline controlled by intersecting joints. The dark
areas between the outcrops possibly indicate a rather coarse grained
sediment.
3. mynd: Dømi um Ijóðm'ynd (side scan mynd) av botninum (siglingar-
linja 57), Djúpini, sum vísir basalt, sum stingur seg upp við legugrýti
ímillum (svart).
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