One of the main objectives of the 3S-2019-OPS trial is to collect data to test how the distance to the source affects behavioural responses. The experimental protocol is designed to test this by combining data already collected during previous trials in 2016 and 2017 using the SOCRATES source with data collected during this year’s trial using the operational CAPTAS source on a Norwegian Navy ASW frigate (KNM Otto Sverdrup).
The reason why we need an operational source is that the source level is significantly higher than with the SOCRATES source and by combining the two we will get better data coverage with exposures to similar levels at longer distances, and at higher levels at the same distance. However, the transmission scheme used by the CAPTAS on the frigate and the scheme used by the SOCRATES source will not match exactly (table A1). The main difference is the higher maximum source level, but the frequency band and ramp-up scheme will also not be exactly the same.
Table A1. Comparison of the pulsed active sonar signals transmitted by the SOCRATES source on HUS and the CAPTAS source on OSVE. Further details of the frigates transmission scheme are given in Table 5 and 8.
SOC on HUS CAPTAS on OSVE
HPAS HPAS XHPAS
Max Source level 214 dB 214 dB (-6 dB) >220 dB (max)
Pulse duration 1000ms 1000ms 1000ms
Pulse repetition time 20s 20s (12 ky) 20s (12 ky)
Frequency/pulseform 1000-2000 Hz HFM 1280-1920 Hz HFM 1280-1920 Hz HFM
Tow speed 8 knots 8 knots 8 knots
Source depth 100-120 m (min 100m)
100-120 (min 50m) 100-120 (min 50m) Ramp up
power with incremental steps of 1 dB per transmission for 20 minutes). To enable an optimal match, the XPHAS exposure approach was specified as follows.
An important goal is to ensure similar exposure conditions between the distant XHPAS exposure with CAPTAS and the close HPAS exposure with Socrates. The start location in the exposure protocol for the distant XHPAS exposure was chosen as such to match the receiver SPL with that was produced by the close HPAS exposure starting at 7.4 km (4 NM). Due to differences in start location, SPL could not be matched for all periods, therefore a single time needed to be selected. For which we chose to match the received level at the time of the first full power transmission. Assuming a mode-stripping propagation loss (~15*log R spreading law), as suggested by the average SPL on tagged sperm whales and bottom-moored recorders in this environment, this led to a starting distance of 15 km (8.1 NM) for the XHPAS exposure (Fig A1).
Due to differences in approach distance, and related change in SPL and SEL with time, it was decided to match the transmitted sound levels of both HPAS and XHPAS transmissions as closely as possible: First, the last final five XHPAS ramp-up steps (3 dB each) were matched in time to every three HPAS transmissions (with 1 dB each transmission). The timing of the initial large jump from -55 to -15 dB in the XHPAS was then chosen by minimizing difference in SELcum (total transmitted SEL
accumulated to each transmission) for each transmission between the start time and the first -12 dB step (Figure A3). This resulted in the following ramp-up scheme (Table A2) (detailed scheme provided in Table A3):
Table A2. Sonar transmission scheme for the XHPAS and HPAS transmissions of the CAPTAS on OSVE.
The transmitted signal will always be 1000ms sonar transmissions of 1280-1920 Hz HFM with 20s pulse repetition time. The approach speed should be 8 knots, constant course, tow depth 100-120m. Time vs source level is specified in the table as attenuation from full powered source level. Time T0 is the time of the first ping. Only the time of changes of the source level is given. Maximum source level of HPAS is 214 dB (re 1µPa·m) and max source level of the XHPAS is 220 dB.
TIME The resulting XHPAS and HPAS ramp-up scheme, and a comparison with a
hypothetical ramp-up scheme using the Socrates ramp-up at matching source level is given in Figure A1 for XHPAS. The expected exposure range (both in SELcum as well as SPL) is similar over the relevant range of SPL and SELcum for which HPAS
exposures have previously indicated sperm whale responses.
Figure A1: Top panel: Adopted XHPAS ramp-up scheme using the operational source compared with a hypothetical Socrates ramp-up scheme, transmitting at 220 dB re µPa2m2. The cumulative transmitted energy source level during the ramp-up was matched with that of a hypothetical Socrates exposure.
Middle panel: comparison of predicted average SPL and SELcum on the sperm whales, showing that for a large range of SELcum and SPL similar exposure histories are expected using the adopted XHPAS ramp-up scheme.
Table A3: Ping by ping XHPAS and HPAS ramp-up scheme. Following ramp-up both schemes will continue for another 20 min at the level of the last ping of ramp up.
Transmission
40 780 13.00 -15 205 -15 205
41 800 13.33 -15 205 -15 205
42 820 13.67 -15 205 -15 205
43 840 14.00 -15 205 -15 205
44 860 14.33 -15 205 -15 205
45 880 14.67 -15 205 -15 205
46 900 15.00 -15 205 -15 205
47 920 15.33 -15 205 -15 205
48 940 15.67 -15 205 -15 205
49 960 16.00 -12 208 -15 205
50 980 16.33 -12 208 -15 205
51 1000 16.67 -12 208 -15 205
52 1020 17.00 -9 211 -15 205
53 1040 17.33 -9 211 -15 205
54 1060 17.67 -9 211 -15 205
55 1080 18.00 -6 214 -12 208
56 1100 18.33 -6 214 -12 208
57 1120 18.67 -6 214 -12 208
58 1140 19.00 -3 217 -9 211
59 1160 19.33 -3 217 -9 211
60 1180 19.67 -3 217 -9 211
61 1200 20.00 0 220 -6 214
After the 20min ramp-up continue for another 20 min at the level of the last ping (ping 61)
Figure A2: top: Comparison of the SL and SEL of the specified XHPAS ramp-up (with steps), and 3S ramp-up (1dB per transmission) at the same full power SL (220 dB re 1 µPa2m2). Bottom: Assumed sailed ship tracks for distant and close exposures.
Figure A3: Minimization of SELcum over all transmissions to choose time of switch from -55 dB to -15 dB point. Minimum indicates after 10 min switch to -15 dB. Also practical for operator.