Analysis and Ranking of the Acoustic Disturbance Potential of ...

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Report No. 6945 BBN Systems and Technologies Corporation The IFD Model was also used to obtain transmission loss estimates for the North Aleutian Area and the Shumagin Area since only a limited number of empirical data are available for these areas also. Long range model estimates were not made for the Chukchi Area because data are available for both winter and summer conditions for this area (Greene 1981). The additional model results and the Greene data are presented in Appendix C. 4.21 The effect of a sloping bottom The habitat of many species of marine mammals includes near shore and beach areas. Sound transmission is strongly influenced by the bottom slope present in most near-shore areas. When sound is transmitted upslope, as is the case for a source passing near a haulout area, two effects occur. If the bottom reflection loss is low, sound levels tend to be higher than those predicted by geometric spreading because the sound energy becomes concentrated in a smaller water volume as it travels upslope. However, if bottom loss is high, sound levels are reduced at a greater rate than expected from geometric spreading since sound undergoes more bottom contact than would occur for transmission over a constant depth bottom. These effects are further complicated by sound transmission and refraction in bottom material which often is an important means of sound transmission in very shallow water. For a rigid, impermeable bottom theory predicts that sound transmission is not possible at frequencies for which the depth of water is less than 1/4 wavelength. Thus for sound transmission upslope from a broadband source, the how frequencies will be cut off or attenuated heavily at shorter ranges than the high frequencies. However, since most bottom material is not rigid and impermeable, this frequency-selective cutoff characteristic is not always observed. The presence of water-saturated sediments often permits significant sound transmission to occur up into the surf zone. Because of the sloping bottom capability of the IFD Model, it was used to predict sound transmission characteristics for propagation toward shore in the Alaska Peninsula and Unimak Island areas. The profiles used for the model in this study are shown in Fig. 4.4. The geometry features a beach profile which has a constant slope connecting a flat region offshore with a flat region near shore. There are also two sloping bottom layers which have range-dependent thickness. Two types of potential sound impact situations were considered. An analysis of noise transmission from small craft offshore to a pinniped haulout area was made for a study conducted by LGL (Johnson et al. 1988). An analysis of an offsho+e vessel or oil rig noise transmission to whales near shore was made for this study. An example of the procedure and results for each analysis is presented here. Table 4.2 lists the parameter values used in modeling the sound transmission for three different bottom types. Bottom Type 1 represents near-shore conditions at Port Moller and Cape Seniavin on the north shore of the Alaska Peninsula. It features a relatively thick layer of fine sand over a deep layer of coarser sand and gravel. Information for this model is based on data obtained from a NOAA survey made by Ertec Western Inc. (1983) and sand properties data reported by Stoll and Bryan (1970). Bottom Type 2 represents near-shore conditions at Pribilof Island sites and features a thin layer of
Report No. 6945 BBN Systems and Technologies Corporation source Pos. 2 .r t Source Water I I I Pos. 1 A. Beach Profile TRANSMISSION PATH PROFILE (Vert 1ca1 Scale Exaggerated) Figure 4.41. IFD M e 1 Geometry for Transmission from an Offshore Source to a Receiver Near the Beach. TI 20m B. Shelf Profile I Source Pos. 7 I Uat er I I Layer 1 Figure 4.4~. IFD Model Geometry for Transmission From an Offshore Source to a Receiver Over a Coastal Shelf.
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FIG. D. 1 HUMAN VOCALIZATION AND AU
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