Hydroacoustic monitoring

Hydroacoustic technology

SOFAR (SOund Fixing And Ranging) Channel.

The term hydroacoustics describes the study of sound waves in the water and its applications. Hydroacoustic monitoring involves recording signals that show changes in water pressure generated by sound waves in the water.

Sound propagates very efficiently through water so that it can be heard and detected at great distances. There is one layer in the water where sound travel is slower but particularly efficient. This layer is the Sound Fixing and Ranging Channel, SOFAR, which is typically at a depth of about 1000 m. Hydroacoustic monitoring makes use of the unique phenomenon of sound waves being trapped in that layer.

Hydroacoustic technology first evolved at the beginning of the 20th century with the aim of increasing safety of sea travel. Sound waves were emitted and their reflections measured off objects such as icebergs and shoals in the water. Called sonar (sound navigation and ranging), this technology was soon used for submarine navigation and detection.

USS Greeneville, US Navy submarine.

Apart from military application, this technology is of great use in a range of civil and scientific fields. Hydroacoustic technology helps in the research of whale populations and their migration patterns, in climate change studies and in tsunami warning systems. The technology also continues to be used where it first started, namely in increasing shipping safety.

For more details on potential civil and scientific applications, please see here.

Hydroacoustics first evolved at the beginning of the 20th century to increase safety at sea. Sonar technology was soon used for submarine navigation and detection.

Objectives

Underwater nuclear weapons effects test, Operation Crossroads, Event Baker, 25 July 1946, Bikini Atoll, Marshall Islands. Click to see clip.

Hydroacoustic monitoring is one of four technologies used by the International Monitoring System (IMS) to verify compliance with the Comprehensive Nuclear-Test-Ban Treaty (CTBT). Nuclear explosions underwater, in the atmosphere near the ocean surface or underground near a coastline generate sound waves that can be detected by the hydroacoustic monitoring network.

Hydroacoustic technology is used to measure changes in the water pressure caused by sound waves. Data obtained from hydroacoustic monitoring provide information on the location of a nuclear explosion underwater, near the ocean surface or near a coastline.

Due to the efficient transmission of sound through water, even comparatively small signals are readily detectable at very long distances. Thus, eleven stations are sufficient to monitor the Earth’s big oceans, with emphasis on the Southern Hemisphere which is largely dominated by water.

Sound transmission in the water is very efficient. Eleven stations are therefore sufficient to monitor the Earth’s big oceans, with emphasis on the Southern Hemisphere which is largely dominated by water.
Lava emerging from an underwater volcanic vent.

Hydroacoustic monitoring can be used to differentiate signals generated by nuclear explosions from signals caused by human activity or natural events. Those activities or events may include seismic profiling for oil exploration or military exercises, or natural occurrences like volcanic eruptions or underwater earthquakes.

Building and certifying hydrophone stations

Hydroacoustic station HA05, Guadeloupe, Martinique, France.

Both types of hydroacoustic stations, hydrophone stations and T-phase stations, are located on islands or on the coast. The hydrophone stations in particular, involving underwater installations, are among the most challenging and most costly monitoring stations to build. The installations have to function for 20 to 25 years in extremely inhospitable environments, exposed to near-freezing temperatures, huge pressures and saline corrosiveness.

There two types of hydroacoustic stations – underwater hydrophone stations and T-phase stations on islands or on the coast.

The deployment of the underwater parts of a hydrophone station, i.e. placing the hydrophones and laying the cables, is a highly sophisticated and complex affair. It involves the hiring of ships, extensive underwater work and the use of specially designed materials and equipment.

The installation of hydrophone stations is very challenging since they have to function many years in extremely inhospitable environments. They are the most costly monitoring stations to build.
Deployment of hydrophone at hydroacoustic station HA11, Wake Island, USA.

The installation of a station involves numerous challenges. Hydrophones are deployed at a depth of about 750 meters. The length of cables laid between the hydrophones and the shore can exceed 100 kilometres. Depths of up to 5000 meters are encountered along the cable routes.

For an example of this process, please see station HA11, Wake Island.

The process of building a hydroacoustic station, involves the same four steps as the establishment of all other International Monitoring System (IMS) stations: site surveys, installation, certification and operation.

First, a site survey is conducted to assess the suitability of the site to host a station and identify any specific conditions that would impact station design. The Treaty lists the approximate geographical coordinates for each station, but only a site survey determines the exact location of a hydroacoustic station and its sensors.

Overview of a hydroacoustic station's under water system.

Second, for the manufacture and installation of the station, a contractor is generally selected through an international tendering process. This contractor is responsible for the station design, manufacture, installation and testing. The Preparatory Commission for the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) provides guidance for station construction work and reviews all aspects of the process to ensure that it meets all criteria so that it can be certified as a valid station within the IMS network.

Building a hydroacoustic station involves the same four steps as for all other International Monitoring System (IMS) stations: site surveys, installation, certification and operation.
Hydrophone recording of a nuclear explosion.

IMS stations must ensure that data received at the IDC are authentic. This is achieved through a special digital “signature” embedded in the data flow from each station. Tamper-detection devices are placed on enclosures for station equipment to discourage tampering with the hardware.

Third, the IMS station must be certified to ensure that all of its equipment, infrastructure and settings meet the technical specifications set by the CTBTO and that all data are transmitted to the Vienna-based IDC through the Global Communication Infrastructure in a timely manner.

Fourth and finally, once certified, operation and maintenance agreements are established between the CTBTO and a station operator. Long-term quality monitoring is then undertaken to ensure high standards of data quality, data availability and station performance.

The hydroacoustic network contains eleven stations located in the world’s oceans - six hydrophone stations and five T-phase stations.

How the hydroacoustic monitoring network works

Spectogram of whale songs recorded by a hydrophone - click for listening to WAV-file.

The hydroacoustic network contains eleven stations located in the world’s oceans with emphasis on the Southern Hemisphere with its large ocean areas. Two different sensing techniques are employed. Six  hydrophone stations use hydrophone sensors, which effectively cover large ocean areas. These stations are quite complex and costly. Five T-phase stations on small, steep-sloped islands are equipped with seismometers. T-phase stations are less effective, but also considerably simpler and less costly to build.

Hydrophone station

Hydrophone station model.

The hydrophone sensors of hydrophone stations are underwater microphones converting the changes in the water pressure caused by sound waves into electric signals which can then be measured. Hydrophone stations are based on islands in the three major oceans, Atlantic, Pacific and Indian.

The actual measuring equipment consists of two sets of three hydrophones on opposite sides of an island. A single set of hydrophones would not suffice as the island would create a shadow in which sound waves from the opposite direction would be blocked by the island. A second set of hydrophones deployed in that area eliminates the acoustic shadow.

The hydrophones are located in the SOFAR channel at a depth of 600 to 1200 m, depending on location. To hold the sensors at the required depth, they are suspended from sub-surface floats and attached to ocean-bottom anchors.

Hydrophone stations employ underwater microphones converting the changes in the water pressure caused by sound waves into electric signals which can then be measured.
T-phase station model.

Hydrophones can be deployed as far as 100 km from the island which hosts shore station equipment such as data formatters, data processing computers and satellite communication equipment. Specially protected cables lead from the hydrophones to the island. At some near-shore location, the cables pass through a pipe beneath the seabed to avoid exposure to the particularly rough surf zone.

T-phase station
The T-phase stations are located on oceanic islands with steep slopes. They use seismic sensors to detect waterborne acoustic energy, which is converted to seismic waves (T-phases) when hitting land. The stations are mostly three-component stations with one or more seismometers.

Both types of station include data acquisition systems and communication equipment. The hydroacoustic stations transmit continuous data in real time via satellite to the International Data Centre (IDC).

Next Chapter: Infrasound monitoring