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

Report No. 6945
BBN Systems and Technologies Corporation
The transmission loss characteristics calculated using the model with the
Bottom Type 2 parameters are shown in Figs. 4.6A through 4.6C. When the TL
characteristics at 100 Hz for the rocky bottom (Fig. 4.6A) are compared with
those for the sandy bottom (Fig 4.5A), the propagation from the source at 10
km offshore can be seen to fall off more rapidly for the rocky bottom than for
the sandy bottom. Normally sound transmission over a rocky bottom would be
expected to be better than that over a sandy bottom. However in this case,
because of the shallow source and receiver positions, most of the sound energy
travels between the source and receiver by downward directed ray paths which
incur a large number of bottom reflections in the case of the rocky bottom.
For the sandy bottom much more sound energy is able to penetrate the bottom
and eventually reflect and refract back out into the water layer to reinforce
sound transmission at the longer ranges. The TL characteristics at 315 Hz
(Fig. 4.6B) and at 1 kHz (Fig 4.6C) are similar to those at 100 Hz in that
they all show a cutoff at a range offshore of 5 to 6 km for the 10,km source
position. For the 3.3 km source position, the differences in TL characteristics
between the Type 1 bottom and the Type 2 bottom are small. The TL near
the beach is somewhat less for the rocky bottom than for the sandy bottom.
Figure 4.6D was developed by interpolation of the model results to obtain
curves of TL versus source distance directly offshore for the Type 2 bottom.
Comparison of the results for a rocky bottom (Fig. 4.6D) with those for a
sandy bottom (Fig. 4.3D) shows that, while the TL is high at 100 Hz for both
types of bottom, it is somewhat lower for the rocky bottom. At 315 Hz the TL
for the rocky bottom is less than that for the sandy bottom for source
distances less than 7 km offshore. For 1 kHz the TL values are similar for
source distances less than 4 km, beyond which the TL for the sandy bottom
condition is smaller. Thus the model results indicate that for the bottom
geometries and parameter values used in the study, a rocky beach has less TL
for nearby offshore sources than a sandy beach. While the transmission
properties of a sandy beach provide less TL for the more distant offshore
sources (>5 km) than a rocky beach, the relatively high losses for both types
of beaches at these ranges probably make the difference academic for most
sources of concern.
The TL characteristics shown in Figs. 4.7A and 4.7B were obtained using
the IFD Model with a Type 3 Bottom and the layer geometry shown in Table 4.2
and Fig 4.4B. The source and receiver depths used were 5 m and 10 m
respectively. Only one source position was used in this case and the figures
show predicted TL versus range from the source toward shore. This analysis
was directeh at the situation of gray whales near shore in 20 m of water with
a source offshore in 70 m. .Because of deeper water, no acoustic cutoff is
obtained within the the modeled range. For the neutral gradient condition
(Fig. 4.7A), the TL from 1 km to 10 km for the 100 and 315 Hz bands can be
seen to be about 15 dB. This is a normal value for propagation in shallow
water over a flat bottom. However, the 1 kHz band shows a loss of only 3 dB
over the same range. The upward sloping bottom seems to have the greatest
effect on the higher frequencies for neutral SVP conditions.
For surface layer conditions (Fig. 4.7B) the predicted TL from 1 km to
10 km can be seen to be higher than in the previous case probably because of
downward refraction and a greater number of bottom reflections per kilometer.