The generalized stratigraphy to about 2000 m is as indicated in Table 3.2. It is a 

 2 m layer of sand overlying about 2000 m of stiff, hard clay and mudstone, which in turn 

 overiies an oil-producing sandstone layer considered to be the acoustic basement. 



3.2 Strait of Juan de Fuca 



The local area of this site, 48°-49°N, 125°-126°W, is on the continental shelf west 

 of the Strait of Juan de Fuca (see Figure 3.3). The shallow La Perouse Bank is in the upper 

 part of the area while deeper canyon areas are to the south and southwest. The data sample 

 contained sound speed profiles with maximum depths from 50 m to 200 m. The average 

 water depth at the site was taken to be 130 m. Figure 3.4 shows characteristic sound speed 

 profiles for this site; 75 percent of the fall profiles were of the positive gradient type. Thus, 

 a representative fall profile is shown with a surface duct to 30 m. 



Table 3.3 summarizes the sound speed profiles and gives the relative number of 

 positive versus non-positive gradient profiles. 







Positive 



Positive 



Season 



Total 



Gradient, # 



Gradient, % 



Winter 



42 



41 



98 



Spring 



44 



15 



34 



Summer 



22 











FaU 



44 



11 



75 



Fall + Winter 



86 



74 



86 



Spring + Summer 



66 



15 



23 



Table 3.3. Strait of Juan de Fuca sound speed profiles. 



The occurrence of surface ducts in this sample was only 23 percent in the spring or 

 summer (April to September) while the probabihty of encountering a surface duct in the 

 period of October-March was increased to 86%. 



The properties of the sediments for this site are given in Table 3.4. The location of 

 the geoacoustic model is 30 nmi west of the entrance to the Strait of Juan de Fuca. The 

 area is a relatively flat part of the continental shelf. Acoustic reflection records and coring 

 data indicate 40 to 50 m of sandy mud or muddy sand overlie thick, folded and eroded 

 sedimentary rocks. 



Compressional 







Thickness, 



Depth, 



Velocity, 



Attenuation, 



Density, 



Layer 



Material 



m 



m 



m/s 



(dB/km)/Hz 



g/cm^ 



Sea Floor 









R*= 1.03 







1 



Sand - silt 





Sfc 



1525 



0.15 



1.61 





clay 



47 



47- 



1586 



0.16 



1.67 



2 



Sedimentary 





47+ 



3000 



0.10 



2.30 



*R = the ratio of the surface sediment sound velocity to the speed of sound in the bottom water. 

 Table 3.4. Sediments at the Strait of Juan de Fuca site. 



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