Seismic refraction shooting in an area of the Eastern Atlantic

Abstract

For the experiments described in this paper a new method of seismic refraction shooting was developed. With this method hydrophones suspended at a depth of about 100 ft. below the surface of the sea acted as receivers for the compressional waves developed by depth charges exploding at a depth of approximately 900 ft. The hydrophones were connected with sono-radio buoys which radio-transmitted the electrical signals to a recording system in the ship from which the charges were dropped. Four buoys were in use simultaneously, distributed at differing ranges from the ship. The experiments were carried out at three positions in an area of the eastern Atlantic around the point 53° 50' N, 18° 40' W, where the water depth is approximately 1300 fm. (2400 m). The results showed that the uncrystalline sedimentary layer in this area varied in thickness from 6200 ft. to 9700 ft. (1900 to 3000 m), and that the velocity of compressional waves in it increased from the value for sea water, 4900 ft./s (1.5 km/s), at the surface with an approximately constant gradient of 2.5/s to a limiting value of 8200 ft./s (2.5 km/s). Below the sedimentary layer there was a crystalline rock with compressional wave velocity of approximately 16500 ft./s (5.0 km/s) and of thickness varying between 8800 ft. (2700 m) and 11100 ft. (3400 m). The base of this layer was in both determinations at approximately 25500 ft. (7800 m) below sea-level. The lowest layer concerning which information was obtained gave a value for the compressional wave velocity of about 20500 ft./s (6.3 km/s), but was of undetermined thickness. The characteristics of the sedimentary layer were such as might be expected for a continuous succession of deep-sea sediments, the thickness on this basis being such as to indicate the long existence of the ocean in this area. On the other hand, it is possible that it represents a downwarped continental shelf. The layer below the sedimentary layer has a compressional wave velocity which is low for an igneous rock at this depth, and it is probable that it represents a crystalline sedimentary rock. From the evidence it is not possible to determine whether this rock is of continental or deep-sea origin. The lowest layer of these experiments is unlikely to have a constitution similar to that of the European granitic layer, since the compressional wave velocity in it would, on this hypothesis, be exceptionally high. The value is, however, close to that calculated by Jeffreys for the intermediate layer.

The Crustal Structure of Rockall Plateau Microcontinent

“…The thickness of 500-700m calculated for the unconsolidated sediments corresponds to the thickness of Miocene to recent deposits sampled at hole 116 on Leg XI1 of the Deep Sea Drilling Project (DSDP Scientific Staff 1970). At both ends of line B refraction multiples (Hill 1952) which have been once, twice and three times reflected between the sea surface and the sea floor are observed. Similar arrivals were recorded by Francis (1964) The deeper layer has a true velocity of 4.54 km s-' and lies at a depth of 2 km beneath the sea bed.…”

Seismic refraction measurements in the southwestern Indian Ocean

“…The reduced times for buoy 3 are greater than those for buoy 1, implying a greater thickness for layer 3 at, or in the vicinity of, buoy 3. Discussion in the sediment as described by Hill [1952]. The observations can only be explained satisfactorily in terms of one or the other of the structures shown in Figure 5 (b and c).…”

“…A system of this type was employed by Hill [1952] [Gane et al, 1956;Hales and Sacks, 1959]. There are two ways in which the buoy system can be used: in the first, the buoy information is returned via a radio link to a central recording system on the ship; in the second, the information is stored in the buoy for subsequent recovery.…”

Wave Generations in a Superficial Layer Resting on a Semi-infinite Lower Layer.

“…HILL (1952) to be very useful in studying the crustal structure under the ocean. Typical records obtained by him are shown in Fig.…”