Top left banner Top right banner
Bottom left banner Bottom right banner



Why Active Sonar?

As submarine quieting technologies improved, the ability to detect them with passive sonar decreased and they become harder to “see”. When SURTASS LFA is used, the Navy will regain the ability it had to “see” quieter submarines at greater distances.

Underwater objects are detected based on sound and distance. The quieter the underwater 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 introduces sound into ocean waters. Active sonar sound pulses (or pings) transmit through ocean waters, bounce or reflect off objects, and return as an echo sound signal that can be detected on a sound receiver like a hydrophone (or an array of hydrophones). The transmission of active sonar signals is what allows even quiet objects like a submarine to be detected, identified, and ranged.

Passive Sonar versus Active Sonar

Passive Sonar
Submarine emitting sound waves detected by a ship-towed hydrophone
Active Sonar
Sound waves emitted by a ship’s active sonar bounce off a submarine, and are detected by a hydrophone

Environmental Concerns

An environmental concern associated with operating LFA sonar is that low frequency (LF) sound may disturb and/or injure marine life.

What is disturbance?

Technically, disturbance ranges from any noticeable minor 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 an animal’s internal organs. However, resonance does not necessarily result in tissue damage, and tissue damage is not always linked to resonance.

Permits to incidentally “take” marine animals

  • The Navy implements procedures or mitigation 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 around the transmitting LFA sonar.
  • Using these monitoring measures allows the Navy to detect marine mammals or sea turtles near the transmitting LFA sonar and LFA vessel (within 2,000 yards) and shut down LFA sonar transmissions if any animals are detected in the mitigation zone, which prevents them from being exposed to LFA sonar transmissions and potentially being 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 use, taking of marine animals principally 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 mammal biologists and acousticians that included:

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

The Navy undertook a scientific research program (SRP) lead by these scientists to study the possible effects that exposure to SURTASS LFA sonar transmissions might have on marine mammals, since they are more easy than sea turtles 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 SRP. Each phase of the SRP was conducted in a different ocean location (southern California, central California, and Hawaii waters) and were permitted by the National Marine Fisheries Service (NMFS) to study LFA sonar’s potential to effect different species of baleen whales.

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

Research approach diagram. The research vessel emits sound underwater. The sound is picked up by a listening array towed by one observation vessel. A second observation vessel and an airplane are in the area. The vessels are separated by 8 to 12 nautical miles.

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
“Risk function against all measured ping received levels” chart thumbnail
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 I map thumbnail
Phase II
Phase II map thumbnail
Phase III
Phase III map thumbnail


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

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

HF/M3 sonar outside water, attached to two chains

When deployed, 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 (at or above 180 dB) in the LFA mitigation zone by immediately detecting the marine mammals, which allows the Navy to suspend LFA sonar transmissions. The HF/M3 sonar was tested and its performance validated using trained bottlenose dolphins in August 2000 off the southern California coast.