Underwater objects are detected based on sound and distance. The quieter the sound, the more challenging it is to detect. Passive sonar emits no sound into the ocean but “listens” for noises emitted underwater while active sonar injects sound into ocean waters. Active sonar sounds (or pings) bounce off objects, and the returned sound signal allows even quiet objects to be identified and their distance determined.

Passive Sonar versus Active Sonar
Passive Sonar
Active Sonar

Environmental Concerns

The environmental concern is that low frequency (LF) sound may disturb and/or injure marine life.

What is disturbance?

Technically, disturbance ranges from any noticeable change in an animal’s behavior to severe avoidance where an animal actively avoids the underwater sound.

What is injury?

Injury includes tissue damage, permanent threshold shift in hearing, and in some cases, resonance affects on internal organs. It should be noted that resonance does not necessarily equal damage and damage is not always linked to resonance.

Permits to incidentally “take” marine animals

  • The Navy implements measures whenever SURTASS LFA sonar is transmitting to protect marine animals from disturbance or injury. These mitigation measures include visual monitoring, passive acoustic monitoring, and active acoustic monitoring for the presence of marine animals such as marine mammals and sea turtles.
  • Using these preventative measures allows the Navy to shut down LFA sonar transmissions if any marine mammals or sea turtles are detected, which prevents them from being exposed and potentially injured or disturbed.
  • Since the beginning of the SURTASS LFA sonar program, the Navy has applied and been authorized for permits under the Endangered Species Act and Marine Mammal Protection Act to take marine mammals and sea turtles incidental to the use of SURTASS LFA sonar.
  • Some concerned individuals misunderstand “take” to mean kill or harm marine animals. Although the legal definition of take does include the harm or killing of marine mammals, the Navy is not authorized to kill or injure any marine mammals or sea turtles during use of SURTASS LFA sonar. In the context of SURTASS LFA sonar, taking of marine animals means that they may be behaviorally harassed, which in plain language means their behavior may be changed or disturbed.

The Navy’s Assessment of Harm to Marine Life

At the beginning of the SURTASS LFA sonar program, the Navy assembled a team of independent marine biologists that included:

Dr. Chris Clark
Dr. Kurt Fristrup
Dr. Peter Tyack

The Navy undertook a scientific research program lead by these scientists to look at the possible effects exposure to SURTASS LFA sonar might have on marine mammals, since they are more easy to observe at sea and because they rely on sound for a wide variety of critical functions (much as terrestrial animals use light). Since baleen whales use LF sound for communication and other functions, they were selected as indicator species for a 3-phase Scientific Research Program (SRP). Each phase of the SRP was conducted in a different ocean location (southern California, central California, and Hawaii waters) so that different types of baleen whales could be studied.

The four species of baleen whales studied in the Navy’s SRP were:

Blue Whale
Fin Whale
Gray Whale
Humpback Whale

Using a SURTASS LFA sonar ship, the independent scientific team transmitted SURTASS LFA sonar signals into ocean waters under controlled experimental conditions and observed any changes in whale behavior.

Scientific Research Program

Research Approach

Phase I: Blue and Fin Whales Feeding

  • 19 animal observations
  • No overt behavioral responses
  • No changes in whale distribution could be related to SURTASS LFA sonar operations.

Phase II: Gray Whales Migrating

  • Whales only changed their migratory behavior and exhibited avoidance responses when the LFA sonar source was directly in their migratory path
  • Whales did not alter their migratory path at all when the sonar source was not in their direct path, even though the sonar sound levels were the same

Phase III: Humpback Whales Breeding

  • About 50% of humpback whales stopped singing when LFA sonar transmissions began
  • All interrupted singers resumed singing within 10 seconds
  • Some humpback whales songs were statistically longer during LFA sonar transmissions
  • No change in distribution or abundance of singing whales or of cow-calf during experiments

Overall Research Program Results:

  • SRP experiments exposed baleen whales to received levels ranging from 120 to about 155 dB re 1 μPa (rms) (SPL)
  • Exposure to the LFA sonar exposures resulted in only minor, short-term behavioral responses. Short-term behavioral responses do not necessarily constitute significant changes in biologically important behaviors.
  • Based on SRP results, scientists developed the behavioral risk continuum for SURTASS LFA sonar

  • The risk continuum function is used to evaluate the potential for biologically significant behavioral responses of marine mammals

More details on the research that was conducted can be found on the Scientific Research page.

Phase I
Phase II
Phase III


Based on the best available scientific information, the risk of injury (including that from resonance effects) from SURTASS LFA is confined to a relatively small area very close to the vessel. The Navy implements mitigation and monitoring measures in this small area, known as the mitigation zone, to prevent or minimize the risk to marine mammals or sea turtles. The mitigation zone is 2,000 yards surrounding the LFA sonar vessel.

To mitigate the possibility of injury, the Navy designed, developed, tested, and validated a High Frequency Marine Mammal Monitoring (HF/M3) sonar.

The HF/M3 sonar is positioned above the LFA sonar (transmit) array. The HF/M3 sonar ensures a very high probability that no marine mammal would be exposed to high sound levels in the LFA mitigation zone (at or above 180 dB.) The HF/M3 sonar was tested and its performance validated with controlled bottlenose dolphins in August 2000 off the southern California coast.