SURTASS LFA

In order to minimize the impact of the SURTASS LFA system on marine mammals and other wildlife, the Navy has undertaken a variety of measures to restrict how and where the system will be operated. These include careful monitoring, both visual and acoustic, of marine wildlife during the operation of the system and strict geographic limitations on the areas where the system will be used.

Geographic Restrictions

The following geographic restrictions apply to the employment of SURTASS LFA sonar:

SURTASS LFA sonar-generated sound field will be below 180 dB within 22 km (12 nm) of any coastlines, and in offshore areas outside this zone that have been determined by NMFS and the Navy to be biologically important;
When in the vicinity of known recreational or commercial dive sites, SURTASS LFA sonar will be operated such that the sound fields at those sites will not exceed 145 dB; and
SURTASS LFA sonar operators will estimate sound pressure levels (SPL) prior to and during operations to provide the information necessary to modify operations, including the delay or suspension of transmissions, in order not to exceed the 180-dB and 145-dB sound field criteria cited above.

The system will use three monitoring techniques:

Visual monitoring for marine mammals and sea turtles from the SURTASS LFA sonar vessel during daylight hours;
Passive (low frequency) SURTASS array to listen for sounds generated by marine mammals as an indicator of their presence; and
High frequency (HF) active sonar to detect/locate/track potentially affected marine mammals (and possibly sea turtles) near the SURTASS LFA sonar vessel and the sound field produced by the SURTASS LFA sonar source array.

Visual Monitoring

Visual monitoring will include daytime observations from the SURTASS LFA sonar vessel for potentially affected species.

Formal record logging

Delay/suspend transmission if marine mammal sighted within LFA mitigation zone

Restricted to daylight

Formal training by marine biologist

Passive Acoustic Monitoring

Passive acoustic monitoring for low frequency sounds generated by marine mammals will be conducted when SURTASS is deployed.

High Frequency Active Acoustic Monitoring

The Navy will conduct high frequency (HF) active acoustic monitoring (through the use of an enhanced HF commercial-type sonar) to detect, locate, and track marine mammals (and possibly sea turtles) that pass close enough to the SURTASS LFA sonar transmit array to exceed the 180-dB mitigation criterion. This HF Marine Mammal Monitoring (HF/M3) sonar operates with a similar power level, signal type and frequency as HF “fish finder” type sonars used worldwide by both commercial and recreational fishermen.

HF/M3 Sonar, LFA Mitigation Zone, and Sound Propagation

The extent of the LFA mitigation zone (i.e., within the 180-dB sound field) is based on onboard acoustic modeling and environmental data collected in situ. Factored into this calculation are SURTASS LFA sonar source physical parameters of tow speed, depth, vertical steering, signal waveform/wavetrain selection, and peak transmit source level (SL).

The HF/M3 sonar is located near the top of the SURTASS LFA sonar vertical line array. The HF/M3 sonar computer terminal for data acquisition/processing/display is located in the SURTASS Operations Center (SOC). The general characteristics of the HF/M3 sonar are:

Frequency: 30 to 40 kHz;
Bandwidth: variable (1.5 to 6 kHz nominal);
Duty Cycle: 3-4 percent (nominal);
Nominal Source Level: 220 dB re 1 microPascal at 1 m;
Pulse Length: variable (10-40 msec nominal);
Pulse Repetition Rate: set by maximum search range (3-4 sec nominal);
Source Ramp-Up: five-minute period;
Detection Volume: 4 equally spaced swept 8º (horizontal) x 10º (vertical) beams making up a 10º (vertical) sector sweep through full 360º (horizontal) around the source (i.e., omnidirectional in the horizontal, 10º vertical beamwidth); nominal time for full 360º sweep 45 to 60 seconds;
Maximum Detection Range: nominally 2 km (1.08 nm).
Operational Depth Capability: compatible with maximum deployed depth of SURTASS LFA sonar source array;
Vertical Steering: ±10^o; and
Receiver Gain: 23 dB (nominal vs. omnidirectional noise).

HF/M3 with Tow Body

The HF/M3 sonar will operate 30 minutes prior to commencement of LFA transmissions and continuously while the SURTASS LFA sonar is deployed. Detection of a marine animal by the HF/M3 sonar automatically triggers an alert to the SURTASS Operations Center Watch Supervisor, who has the HF/M3 tracking team immediately evaluate the detection.

Analysis and testing of the HF/M3 sonar operating capabilities indicates that this system substantially increases the probability of detecting marine mammals within the LFA mitigation zone, and provides an excellent monitoring capability (particularly for medium-large marine mammals) beyond the LFA mitigation zone, out to approximately 2 km (1.08 nm).

Monitoring Protocols

Visual Monitoring

There are two potential visual monitoring scenarios.

First, if a potentially affected marine mammal or sea turtle is sighted outside of the LFA mitigation zone (approximately a 1 kilometer radius around the transmit array), the observer will notify the Officer-in-Charge (OIC). The OIC will then notify the HF/M3 sonar operator to determine the range and projected track of the animal. If it is determined that the animal will pass within the LFA mitigation zone, the OIC will order the delay or suspension of SURTASS LFA sonar transmissions when the animal enters the LFA mitigation zone. If the animal is visually observed within 2 km (1.08 nm), the OIC will order the delay or suspension of SURTASS LFA sonar transmissions. The observer will continue visual monitoring/recording until the animal is no longer seen.
Second, if the potentially affected animal is sighted within the LFA mitigation zone, the observer will notify the OIC who will order the immediate delay or suspension of SURTASS LFA sonar transmissions.

Passive Acoustic Monitoring

Passive acoustic monitoring will be conducted when SURTASS is deployed, using the SURTASS towed array to listen for vocalizing marine mammals as an indicator of their presence. If the sound is estimated to be from a marine mammal that may be potentially affected by SURTASS LFA sonar, the technician will notify the OIC who will alert the HF/M3 sonar operator and visual observers. If prior to or during transmissions, the OIC will then order the delay or suspension of the SURTASS LFA sonar transmissions when the animal enters the LFA mitigation zone.

Active Acoustic Monitoring

There are two potential scenarios for mitigation via active acoustic monitoring.
First, if a contact is detected outside of the LFA mitigation zone, the HF/M3 sonar operator will determine the range and projected track of the animal. If it is determined that the animal will pass within the LFA mitigation zone, the sonar operator will notify the OIC. The OIC will then order the delay or suspension of transmissions when the animal is predicted to enter the LFA mitigation zone.
Second, if a contact is detected by the HF/M3 sonar within the LFA mitigation zone, the observer will notify the OIC who will order the immediate delay or suspension of transmissions.

Resumption of Transmissions

SURTASS LFA sonar transmissions can commence/resume 15 minutes after there is no further detection by the HF/M3 sonar and there is no further visual observation of the animal within the LFA mitigation zone.