HR: 08:30h
AN: OS41A-03    [PDF]
TI: R/V EWING seismic source array calibrations: 2003
AU: * Diebold, J
EM: johnd@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AU: Webb, S
EM: scw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AU: Tolstoy, M
EM: tolstoy@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AU: Rawson, M
EM: rawson@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AU: Holmes, C
EM: cholmes@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AU: Bohnenstiehl, D
EM: del@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AU: Chapp, E
EM: chapp@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory, PO Box 1000, Palisades NY, NY 10964 United States
AB: In the Northern Gulf of Mexico, May, 2003, an NSF-funded effort was carried out to obtain calibrated measurements of the various airgun arrays deployed by R/V EWING during its seismic surveys. The motivations for this were several: to ground-truth the modeling upon which safety radii for marine mammal mitigation are established; to obtain broadband digitized signals which will accurately define the full spectral content of airgun signatures; to investigate the effects of seafloor interactions and their contribution to the acoustic noise levels from seismic sources. For this purpose, a digital, remotely telemetering spar buoy was designed and assembled; affording interactive control over the choice of two hydrophone channels, four fixed gain settings and four digitizing rates [6,250 - 50,000 Hz.] Three deployments were planned: a deep-water site, suitable for comparison of actual signals with modeled results; a shallow-water [25 - 50m] site where the effects of bottom interaction would be strongest; and a continental-slope site, which represents the favored habitat of many cetacean species. Methodology was developed which enabled the sequential discharge of four subarrays of 6, 10, 12 and 20 airguns. A separate run was made with two "GI" airguns, the favored high resolution survey source. An Incidental Harassment Authorization and a Biological Opinion, including an Incidental Take Statement were issued for the project by National Marine Fisheries, and a suite of marine mammal observation and mitigation procedures was followed. The deep and shallow water sites were occupied, and some 440 airgun signals were recorded. The slope site work was cancelled due to weather too poor for accurate marine mammal observation, but calibration was subsequently carried out with an exploration industry source vessel in a similar environment. Preliminary results indicate that the mitigation modeling is accurate, though somewhat conservative; that the radiated energy from airgun arrays, known to be strongest at very low frequencies, continues to diminish as frequencies increase up to 25 kHz, and that interactions with the seafloor, while complex, are understandable.
DE: 0394 Instruments and techniques
DE: 0400 Biogeosciences
DE: 0994 Instruments and techniques
DE: 3025 Marine seismics (0935)
DE: 3210 Modeling
SC: Ocean Sciences [OS]
MN: 2003 Fall Meeting