HR: 1340h
AN: T33B-1385    [Abstracts]
TI: Assessing Acoustic Sound Levels Associated with Active Source Seismic Surveys in Shallow Marine Environments
AU: * Bohnenstiehl, D R
EM: del@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964 United States
AU: Tolstoy, M
EM: tolstoy@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964 United States
AU: Thode, A
EM: thode@mpl.ucsd.edu
AF: Scripps Institution of Oceanography, 9500 Gilman Drive, La Jolla, CA 92093 United States
AU: Diebold, J B
EM: marscico@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964 United States
AU: Webb, S C
EM: scw@ldeo.columbia.edu
AF: Lamont-Doherty Earth Observatory of Columbia University, 61 Route 9W, P.O. Box 1000, Palisades, NY 10964 United States
AB: The potential effect of active source seismic research on marine mammal populations is a topic of increasing concern, and controversy surrounding such operations has begun to impact the planning and permitting of academic surveys [e.g., Malakoff, 2002 Science]. Although no causal relationship between marine mammal strandings and seismic exploration has been proven, any circumstantial evidence must be thoroughly investigated. A 2002 stranding of two beaked whales in the Gulf of California within 50 km of a R/V Ewing seismic survey has been a subject of concern for both marine seismologists and environmentalists. In order to better understand possible received levels for whales in the vicinity of these operations, modeling is combined with ground-truth calibration measurements. A wide-angle parabolic equation model, which is capable of including shear within the sediment and basement layers, is used to generate predictive models of low-frequency transmission loss within the Gulf of California. This work incorporates range-dependent bathymetry, sediment thickness, sound velocity structure and sub-bottom properties. Oceanic sounds speed profiles are derived from the U.S. Navy's seasonal GDEM model and sediment thicknesses are taken from NOAA's worldwide database. The spectral content of the Ewing's 20-airgun seismic array is constrained by field calibration in the spring of 2003 [Tolstoy et al., 2004 GRL], indicating peak energies at frequencies below a few hundred Hz, with energy spectral density showing an approximate power-law decrease at higher frequencies (being ~40 dB below peak at 1 kHz). Transmission loss is estimated along a series of radials extending from multiple positions along the ship's track, with the directivity of the array accounted for by phase-shifting point sources that are scaled by the cube root of the individual airgun volumes. This allows the time-space history of low-frequency received levels to be reconstructed within the Gulf of California. At various times or positions along the ship's track, the predicted mean and maximum sound level in the water column are contoured. By reconstructing the possible positions of the whales during the survey, based on the time of their stranding and reasonable swim velocities, we constrain the sound levels that they may have been subjected to for a series of scenarios. It is hoped that this work will facilitate a better understanding of acoustic propagation during future airgun experiments in similar environments.
UR: http://www.ldeo.columbia.edu/res/pi/tphase/
DE: 4251 Marine pollution
DE: 4259 Ocean acoustics
DE: 3025 Marine seismics (0935)
DE: 3094 Instruments and techniques
DE: 3099 General or miscellaneous
SC: Tectonophysics [T]
MN: 2004 AGU Fall Meeting