Why Active Sonar?
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
Passive Sonar versus 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
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
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
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:
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
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
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
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
Phase III: Humpback Whales Breeding
About 50% of humpback whales stopped singing when LFA sonar
- All interrupted singers resumed singing within 10 seconds
Some humpback whales songs were statistically longer during LFA sonar
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
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.
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)
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.