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
Active Sonar


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.


Marine Mammals: Scientific Research Program

The Navy undertook the Scientific Research Program (SRP), lead by a team of independent marine mammal biologists and acousticians, to study the possible effects that exposure to SURTASS LFA sonar transmissions might have on marine mammals. The SRP at-sea experiments focused on marine mammals, baleen whales specifically, because they depend upon sound for a wide variety of critical biological functions.

The SRP was divided into three experiment phases that were conducted in the North Pacific Ocean from 1997 through 1998. Phase I was conducted in waters off southern California and focused on foraging blue and fin whales, while Phase II was conducted in the Pacific waters off central California to observe the effects on migrating gray whales. Phase III was conducted off the coast of Hawaii to observe the effects on humpback whales on their breeding and calving grounds. The independent scientific team transmitted SURTASS LFA sonar signals into ocean waters under controlled experimental conditions and observed any changes in whale behavior.

Blue Whale
Fin Whale
Gray Whale
Humpback Whale

All phases of the SRP at-sea experiments were conducted under scientific research permits authorized by the National Marine Fisheries Service (NMFS). These at-sea experiments are important contextually because an operational LFA sonar system was deployed from a SURTASS LFA sonar ship in oceanic conditions.

Phase I: Blue and Fin Whales Feeding Results:

  • 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 Results:

  • 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

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

Fishes: Controlled Exposure Experiments

The Navy sponsored a $3 million series of controlled exposure experiments from that were conducted to determine the effects of SURTASS LFA sonar signals on fishes. This independent research was conducted by a team of eminent scientists in the fields of fish biology and fish bioacoustics at the Navy’s Lake Seneca sonar test facility. The experiments focused on five species of fishes (rainbow trout, channel catfish, hybrid sunfish, largemouth bass, and yellow perch) and exposed these fish species to relatively high LFA sonar sound received levels (up to 195 dB re 1 µPa [rms] transmitted by a single LFA sonar element. The experiments examined the impacts on fish hearing, physiology (sensory hair cells in the fish’s ears), and behavior. In all species, up to 96 hours post-exposure, there were no indications of any damage to sensory cells or to any other tissues and only limited impacts to hearing, and only in some but not all of the fish species; hearing loss was temporary, with recovery within 72 hours after exposure. Fish behavior after sound exposure was no different than behavior prior to or after the experimental exposures.


Based on the best available scientific information, the risk of injury (including that from resonance effects) from exposure to SURTASS LFA sonar transmissions is confined to a relatively small area surrounding the LFA sonar vessel and transmitting sonar that is known as the mitigation zone. The mitigation zone is a 2,000-yard radius surrounding the LFA sonar vessel and transmitting LFA sonar array.

During training and testing activities using LFA sonar, the Navy implements mitigation and monitoring measures in the mitigation zone to prevent or minimize the exposure risk to marine mammals or sea turtles. The mitigation monitoring that the Navy implements during LFA sonar training and testing transmissions are visual, passive acoustic, and active acoustic monitoring. Detection of marine mammals or sea turtles in the LFA mitigation zone by any of the mitigation monitoring methods results in the immediate suspension or delay of LFA sonar transmissions to prevent possible exposure of the detected marine animal.

Passive acoustic monitoring uses SURTASS (the passive part of the SURTASS LFA sonar system) to listen for sounds generated by marine mammals as an indicator of their presence. Active acoustic monitoring is conducted using the High Frequency Marine Mammal Monitoring (HF/M3) sonar system, which is a Navy-developed and enhanced HF commercial sonar used to detect, locate, and track marine mammals and, to some extent, sea turtles.

When deployed, the HF/M3 sonar system (pictured above) 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.

The Navy implements other operational and geographic mitigation measures to protect marine life. See the Preventive Measures page for more information on all mitigation the Navy implements for use of SURTASS LFA sonar.