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Soundings Archive

NMFS Seeks Comment on a Take Application for a Liquefied Natural Gas Deepwater Port in the Gulf of Mexico
On March 1, 2011, the U.S. National Marine Fisheries Service published Federal Register notice of an application from Port Dolphin Energy LLC for authorization for the take, by Level B harassment, of marine mammals incidental to construction and operation of the Port Dolphin Deepwater Port off the Gulf coast of Florida for the period June 2012-June 2017. If granted, the Take authorization would be issued by NMFS pursuant to the Marine Mammal Protection Act in the form of 5-year regulations and a Letter of Authorization.

Any public comments and information on this Take application must be received by NMFS no later than March 31, 2011, in order to be considered.

A copy of Port Dolphin's application may be obtained at

The application requests authorization to take marine mammals incidental to construction activities, which would take approximately 11 months to complete, and which include: buoy installation; offshore hammering; horizontal directional drilling; HDD vibratory; pipeline laying offshore; pipeline laying inshore; offshore plowing; and inshore plowing. Operational activities include: SRV maneuvering and docking and regasification.

Appendix C to the application states that JASCO's Marine Operations Noise Model was used to predict the directional transmission loss footprint from one or more sound source locations. The application explains that

    "For continuous noise sources (e.g., vessel noise), MONM predicts RMS sound pressure levels (SPL) upon which U.S. safety radius requirements are based. For impulsive noise sources (impact hammering) MONM predicts sound exposure level (SEL) over a nominal time window of 1 second. For in situ measurements of impulsive sound sources, SPL is related to SEL via a simple relation that depends only on the RMS integration period T:

    SPLRMS90 = SEL - 10log10(T) - 0.458.

    Here the last term accounts for the fact that only 90% of the acoustic pulse energy is delivered over the standard integration period (Malme et al. , 1986; Greene, 1997; McCauley et al. , 1998). The pulse duration at any given point in the sound field is highly sensitive to the specific multi-path arrival pattern from an acoustic source. In the absence of in situ measurements, accurate direct forecasting of the pulse duration at any significant range from the source is computationally prohibitive at present. The best alternative is to use a heuristic value of T, based on field measurements in similar environments, to estimate an RMS level from the modeled SEL. Safety radii estimated in this way are approximate since the true time spreading of the pulse has not actually been modeled. For this study, the integration period T has been assumed equal to a pulse width of 0.1 s, resulting in the following approximate relationship between RMS SPL and SEL:

    SPLRMS90 = SEL + 10

    In various studies where the SPLRMS90, SEL, and duration have been determined for individual airgun pulses, the average offset between SPL and SEL has been found to be 5 to 15 dB, with considerable variation dependent on water depth and geo-acoustic environment (Austin et al. 2003; MacGillivray et al. 2007)."
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