Acoustic emitters for use in marine seismic surveying

1. An acoustic emitter for incorporation in a marine seismic streamer having an electrical or electro-optical transmission line extending therealong, the emitter comprising an annular housing adapted to be connected substantially coaxially in series with the streamer, and an annular acoustic emitting element mounted substantially coaxially within the housing, the arrangement being such that the acoustic emitting element surrounds the transmission line when the housing is connected in the streamer, and the housing protects the acoustic emitting element from bending loads during spooling of the streamer.

2. An acoustic emitter as claimed in claim 1 , for use where the streamer comprises a plurality of streamer sections connected together end to end, wherein the acoustic emitter is implemented as an insert adapted to be connected between two adjacent streamer sections.

3. An acoustic emitter as claimed in claim 2 , further comprising first and second end connectors for connecting to respective ones of the two streamer sections, and a short flexible body interconnecting the end connectors, the annular housing forming part of one of the end connectors.

4. An acoustic emitter as claimed in claim 2 or claim 3 , wherein said insert contains a sensor for sensing signals of the kind produced by the emitter.

5. An acoustic emitter as claimed in claim 1 , wherein the emitting element is piezoelectric.

6. An acoustic emitter as claimed in claim 5 , wherein the emitting element comprises an annular support member having a plurality of piezoelectric members mounted on its radially outer surface.

7. An acoustic emitter as claimed in claim 6 , wherein the housing has a plurality of apertures extending radially therethrough, and the piezoelectric members are substantially aligned with the apertures in the housing.

8. An acoustic emitter as claimed in claim 5 , wherein the emitting element comprises a stack of coaxially aligned annular piezoelectric discs.

9. An acoustic emitter as claimed in claim 8 , wherein the piezoelectric discs are mounted on an annular support member so as to be trapped between two annular end members, and a plurality of flexible barrel stave members extend between the end members so as to substantially completely surround the stack of piezoelectric discs and so as to be flexed by contraction and expansion of the stack of discs.

10. An acoustic emitter as claimed in claim 1 , wherein the housing has a plurality of apertures extending radially therethrough.

11. An acoustic emitter as claimed in claim 1 , wherein the emitting element is adapted to operate in the frequency range up to 10 kHz.

12. An apparatus comprising a marine seismic streamer incorporating at least one acoustic emitter in accordance with any one of claims 1 to 3 and 5 to 11 .

13. An apparatus as claimed in claim 12 , the streamer including a multiplicity of sensors suitable for detecting seismic signals along its length, wherein at least some of said sensors are arranged to be used as part of a position determining system to detect acoustic signals of the kind produced by said acoustic emitter.

14. An apparatus as claimed in claim 13 , wherein said at least some of said sensors are arranged to be used to detect both seismic and acoustic signals.

15. An apparatus as claimed in claim 13 , wherein said sensors are hydrophones.

16. An apparatus as claimed in claim 12 , wherein the acoustic emitter is coupled to said transmission line to be energised therefrom.