experimental protocol and theoretical ramp-up design
5. General description experimental protocol
We will perform the experiments to test 2 distinct hypotheses. The first hypothesis is that animals will avoid the sound source, and that avoidance starts during the ramp-up period.
We will use the procedures already proven in 3S to test whether or not avoidance occurs and at which sound pressure level. The second hypothesis is the central hypothesis of this work: that ramp-up leads to lower total sound exposure level and maximum sound
pressure level because it is effective in allowing animals to move away from the path of a moving source, despite the fact that additional sound is transmitted into the ocean. We will measure both cumulative levels and the level of the single most intense ping, as both of these metrics may be useful predictors of behavioral effects and risk of injury (Southall et al., 2007).
Note that ultimately one would like to demonstrate the beneficial effect on the entire population (or all possible starting positions). This is beyond the scope of the 3S2
16
experiments. However, the results of the experiments will be used to validate or adopt the assumptions in the behavioral model (response thresholds, duration of the pulse, pulse type, speed, direction and duration of response) presented in Section 4 and the model will then be used to assess the effect on the entire population.
Our proposed method is to simply experimentally recreate the two scenarios shown in figures 11 and 12, below. This protocol entails a process of tag attachment and vessel approach similar as in the other 3S experiments. We will monitor the movement and behavior of Dtagged subjects throughout, starting with a pre-exposure period and then by a silent source vessel passby to control for any immediate effects of the source vessel. It is expected that the vessel approach itself should not cause strong reactions by the tagged animal(s), but the silent passby is critical to conduct first to test how reactive humpbacks might be to the vessel itself. Our experiences in the 3S experiments conducted to date indicate that animals may become more sensitive to the vessel following a sonar exposure. While such an observation is interesting, it is not central to our study to fully describe an order effect of silent (with a non-transmitting sonar) vessel approach. Rather our goal is simply to test whether the vessel itself, in the absence of sonar transmission or history of sonar transmission from the vessel, causes avoidance reactions. Thus, the silent passby should be conducted first in all cases, and not randomized.
Figure 11. Animals placed near (but ahead of) the position of the first full-power sonar transmissions are at a higher risk of hearing injury.
Travel path of sonar vessel
Position of first
‘real’ sonar ping
Area of increased and high risk of severe, i.e.
physiological, effects
17
Figure 12. Sonar sounds are started earlier at lower levels and are gradually increased to full power at the planned position. These additional ramp-up transmissions increase the total amount of sound energy transmitted into the ocean, but have the potential to reduce risk by giving animals in the zone of increased risk time to move away.
Following the silent (non-transmitting sonar) passby, two additional passes including transmissions of the 1-2 kHz LFAS will be conducted, one with ramp-up and another without ramp-up (Fig. 13). The order of the two types of passes with transmissions will be changed for each new subject. Each approach will be conducted in precisely the same fashion, with the playback coordinator (blind to the specific exposure signal)
maneuvering the vessel so that the Dtagged subject is directly in the path of the vessel.
Replicates will be needed to control for the inevitable variation in the location of the Dtagged subject(s) relative to the sonar transmissions. It is expected that we can collect 4-5 of these ramp-up tests per 4-week trial, for a minimum total of 12-15 tests, which should be sufficient to provide advice on the effectiveness of ramp-up.
Figure 13. Conceptual diagram of the approach-exposure protocol to be used in the test of the effectiveness of ramp-up. The oval represents a tagged subject whale, and the pointed lines represent the source vessel course. In all three types of pass, the animal is approached as directly as possible, and the course of the vessel is fixed at a pre-determined distance, before the planned start of ramp-up signals. In the silent pass, no sonar transmissions are made. In the ramp-up pass, a ramp-up sequence is transmitted in addition to full-level signals. In the no-ramp-up pass, transmission starts with the first full level ping.
No-ramp up pass
First ‘real’ sonar ping Silent pass
Ramp up pass
First ‘ramp-up’ ping Tagged whale Travel path of sonar vessel
Position of first
‘real’ sonar ping
Area of increased and high risk of severe, i.e.
physiological, effects
Ramp-up period
Animal responds to ramp-up sounds by moving away
t
RUt
FP18
The two red lines in figure 13 indicate two crucial points in space: 1) the distance from the animal when the ramp-up period starts (or would start, in the case of silent or no-rampup passes), and 2) the distance from the animal where the full-power transmission starts (or would start, in the case of a silent pass). To make the passes as similar as possible, the course of the source vessel should not change after tRU (time of ramp-up start) is reached, unless the course imposes a risk of collision. As the speed of the vessel will be fixed, the time and distance from CPA will be related to each other in a
straightforward matter.
The following protocol is adopted for approaching the tagged animals:
• see chapter 8
6 Summary
6.1 Conclusions
We investigated the efficacy of ramp-up during an LFAS sonar operation by modeling the risk of direct physical injury. The ramp-up was specifically designed for humpback whales that are target species for the 3S2 trials. From the simulations we conclude the following for the experimental set-up:
• The ramp-up can significantly decrease the risk both in terms of PTS as well as TTS for a population of humpback whales as well as the risk for a single animal with known location.
• For humpbacks the general risk of actual direct physical injury (PTS) is very low (practically zero), unless the full power start is close (~10 m) to the animal and animals are unresponsive (RLav > 160-170 dB re 1 μPa2). Typical avoidance thresholds from the literature appear to be well below these values (RLav ~ 120 – 140 dB)
• An optimal ramp-up was chosen based on the impact area of TTS for humpback whales. This choice was made because the risk on PTS is below simulation resolution.
• For a single animal at CPA = 100 m, the optimal ramp-up (in terms of Area(TTS) for the humpbacks) provides a drop in SEL and maximum SPL of approximately 10-15 dB compared to the no-ramp-up scenario. These are the actual values that are expected to be measured during the 3S ramp-up experiment.
• The difference in SEL/SPL between ramp-up/no-ramp-up can be increased for a single animal by increasing the ramp-up time. However, the ramp-up time becomes too long to be operationally relevant, and also increases the exposure to the total population (more animals affected).
19
• The optimal ramp-up leads to only a small increase (~1%) in area where animals are affected. I.e. the increase of risk of inducing indirect injury through e.g. DCS is likely to be small.
6.2 Ramp-up scheme for use during the 3S2 experiments
Based on our operational limitation and considerations of naval operational relevance as well as the above analysis we arrive at the following ramp-up scheme to be tested on humpbacks during the 3S-trials. This ramp-up was found to theoretically minimize the overall risk on receiving TTS (and PTS) to a humpback population under the conditions of the experimental set-up.
The transmission program during the ramp up experiments is depicted in Figure 14.
Tables 5 – 6 summarize the parameters adopted for the experimental set-up.
Figure 14: Optimal ramp-up scheme for 3S2 experiment with humpback whales.
20
Table 5: Ramp-up parameters for the 3S2 experiment with humpback whales
Parameter values used
Tramp (ramp-up time) 300 s
nramp (steepness) 4
PRT (pulse-repetition time) 20 s
Tpulse (pulse duration) 500 ms
HFM up-sweep 1.3-2 kHz
SLmax 214 dB re 1 μPa2m2s (@ 1m)
SLmin 152 dB re 1 μPa2m2s (@ 1m)
vs (ship speed) 4 m/s (~ 8 knots)
vm (mammal avoidance speed) 1 m/s (~2 knots)
Tpulse 0.5 s
After the ramp-up we will immediately switch to full power, still considering operational relevance:
Table 6: Operation
parameter values used
Top 5 min*
SLop 214 dB re 1 μPa2m2s (@ 1m)
PRTop 20 s
Tpulse 1000 ms (including two 50 ms
ramps) vs (ship speed) 4 m/s (~ 8 knots) HFM
up-sweep
1300-2000 Hz tow depth Minimal 60 m, because of
cavitation limitations