The spatial resolution of hull-mounted 3.5-kHz echo sounding systems is limited in deep water by the large footprint (Fresnel zone) of the insonifying energy on the seafloor. At Ocean Drilling Program (ODP) Site 1224, we tested a simple system that can be used from the drillship while on station to significantly improve reflection profiling resolution. In 4970 m water depth, a 4-kHz pinger was mounted on the ship's video camera frame and lowered down the drill string to a few meters above the seafloor. The ship's 3.5-kHz transceiver recorded the returns.
Having the source and receiver at differing distances above the seafloor provides two advantages: (1) the area returning reflections is greatly reduced, and (2) the amplitudes of the subseafloor reflections are less affected by the spreading effect since the traveltime into the seafloor is much less than in the water column.
Reflections were observed to 40 ms beneath the seafloor on 10 lowerings at three closely spaced holes. The heave of the ship and camera frame shifts the traveltimes of the reflection sequences. A level-discriminator and correlation routine was used to align the traces and stack them to enhance the signal. The reflections were commensurate with the geotechnical data obtained from the limited number of sediment cores obtained.
These measurements do not take rig time. In addition to use on the drillship, this system could provide a low-cost, shallow-penetration profiling system from remotely operated vehicles and autonomous underwater vehicles deployed while the ship is drilling.
1Bolmer, S.T., Hoskins, H., and Stephen, R.A., 2006. Data report: 4-kHz profiling with vertically separated source and receiver: a mini reflection survey around a deepwater drill hole. In Kasahara, J., Stephen, R.A., Acton, G.D., and Frey, F.A. (Eds.), Proc. ODP, Sci. Results, 200 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/200_SR/005/005.htm>. [Cited YYYY-MM-DD]
2Woods Hole Oceanographic Institution, 360 Woods Hole Road, Woods Hole MA 02543, USA. Correspondence author: hhoskins@whoi.edu
Initial
receipt: 8 July 2004
Acceptance: 13 September 2005
Web publication: 18 April 2006
Ms 200SR-005