The SOFAR channel

Most marine seismic studies fail to record events within the water section.

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Most marine seismic studies fail to record events within the water section. The oceans contain layers of waters of different temperature, strong currents such as the Gulf Stream, and other phenomena like El Nino. Oceans contain different structures:

  • Thermocline (temperature)
  • Halocline (salinity)
  • Pycnocline (pressure)
  • Lysocline (dissolution of calcium carbonate)
  • SOFAR channel, a minimum sound velocity speed layer. This channel is a strong path for communication used by dolphins, whales, and submarines.

Submarine communications were a confidential topic until 1991. More data are now being released. The SOFAR (sound fixing and ranging) channel was discovered during WW II and used by the US Navy to locate downed aircraft pilots. Acoustic waves can propagate thousands of km through SOFAR with relatively little attenuation.

In a 1960 experiment, 130 kg of TNT were detonated off the coast of Perth, Australia. Hydrophone receivers detected sounds generated by the detonation 3.7 hours later near Bermuda, 20,000 km away. A proposal by Dr. Munk resulted in the Heard Island Feasibility Test (HIFT), to assess the feasibility of acoustic thermometry for long-range monitoring of ocean temperatures. The test was conducted 26-30 January 1991 off the coast of Heard Island. This uninhabited volcanic island is located near Antarctica, midway between Africa and Australia.

The location was selected because of the sound channel's close proximity to the surface (175 meters), as well as the multiple, unimpeded paths to receiving stations on both US coasts, 18,000 km away. The HIFT signal was of sufficient strength to be detected, after computer processing, by 17 of 19 monitoring stations in all five oceans, including along both the east and west US coasts.

The speed of sound decreases with decreasing temperature (4 meters/sec per 1°C) and increases with increasing pressure (or depth = 1 meter/sec for 100 meters) and increasing salinity (1 meter/sec for 1%). Temperature in the ocean is primarily controlled by the absorption of sunlight. A total of 99% of light is absorbed in the upper 100 meters of the oceans. The relationship of these factors results in a zone of minimum sound velocity in the ocean referred to as the SOFAR channel.

It is easy to reconstruct such curves with the rules on temperature (4 meters/sec for 1°C) and depth (1 meter/sec for 100 meters) and with the temperature data. For the Pacific Ocean at the equator, there is 27°C at surface, 25°C at 100 meters, 8°C at 500 meters, 4.5°C at 1,000 meters and 1.7°C at 3,000 meters. The minimum velocity is around 1,000 meters depth.

This channel is important for a variety of reasons. Sound is used by a variety of marine organisms to communicate. Acoustic thermometry is the field of study that relies on sound to measure the temperature of the oceans. Scientists have proposed using sound to assess global warming. This field experiment has generated quite a bit of controversy. In order to move forward with the experiment, additional studies were needed to determine the potential impact on marine mammals.

But now the SOFAR channel is used to measure the temperature of ocean and global warming in the Acoustic Thermometry of Ocean Climate program (ATOC), an international program involving 11 institutions in seven nations. The program began transmitting acoustic signals in late 1995 from a low-frequency acoustic source installed on Pioneer Seamount off central California. The signals were recorded on horizontal receiving arrays at 11 US Navy SOSUS stations in the northeast Pacific, and on two vertical receiving arrays, one near the Big Island of Hawaii and the other near Kiritimati (Christmas) Island, just north of the equator in the central Pacific Ocean. A single receiver off New Zealand also recorded signals.

Howard 1998 reports that the scientists were able to detect variations as small as 20 ms in the hour-long time it took pulses to travel some 4,800 km between the underwater speakers and receivers. Those subtle shifts allowed the scientists to estimate average ocean temperatures along the signals' pathways, to within 0.006°C. They also were able to detect an expected seasonal swing in upper ocean temperatures of about 2 °C.

The SOFAR channel should be seen also as a reflector on a vertical path. Very few seismic studies have been recorded on the water column as it is omitted in all surveys as of no interest. This author found only one by Gonella & Michon (1988) on a 100-km track by CGG in the Atlantic west of the Gibraltar Straight, near the Goringe Ridge.

Acoustic reflectors are clearly detected within the water in the 600-1500 meter depth range (around 1 sec). These reflectors seem to correspond not only to the SOFAR channel, but also to the strong water current coming from the Mediterranean Sea. This explains the dipping of the current over the flank of the Goringe Ridge. Unfortunately, no BSR could be seen on the sediment reflections as they are parallel and weak.

The study of the SOFAR channel is in its infancy. It should be interesting for the deepwater oil industry to try to record seismic events around water depths of 600 - 1000 meters in order to evaluate its influence in seismic recordings.;

This channel is important for a variety of reasons. Sound is used by a variety of marine organisms to communicate. Acoustic thermometry is the field of study that relies on sound to measure the temperature of the oceans. Scientists have proposed using sound to assess global warming. This field experiment has generated quite a bit of controversy. In order to move forward with the experiment, additional studies were needed to determine the potential impact on marine mammals.

But now the SOFAR channel is used to measure the temperature of ocean and global warming in the Acoustic Thermometry of Ocean Climate program (ATOC), an international program involving 11 institutions in seven nations. The program began transmitting acoustic signals in late 1995 from a low-frequency acoustic source installed on Pioneer Seamount off central California. The signals were recorded on horizontal receiving arrays at 11 US Navy SOSUS stations in the northeast Pacific, and on two vertical receiving arrays, one near the Big Island of Hawaii and the other near Kiritimati (Christmas) Island, just north of the equator in the central Pacific Ocean. A single receiver off New Zealand also recorded signals.

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Equatorial Pacific seawater sound velocity versus depth, showing the SOFAR channel at 1,000 meters.
Click here to enlarge image

Howard 1998 reports that the scientists were able to detect variations as small as 20 ms in the hour-long time it took pulses to travel some 4,800 km between the underwater speakers and receivers. Those subtle shifts allowed the scientists to estimate average ocean temperatures along the signals' pathways, to within 0.006°C. They also were able to detect an expected seasonal swing in upper ocean temperatures of about 2 degrees C.

The SOFAR channel should be seen also as a reflector on a vertical path. Very few seismic studies have been recorded on the water column as it is omitted in all surveys as of no interest. This author found only one by Gonella & Michon (1988) on a 100-km track by CGG in the Atlantic west of the Gibraltar Straight, near the Goringe Ridge.

Acoustic reflectors are clearly detected within the water in the 600-1500 meter depth range (around 1 sec). These reflectors seem to correspond not only to the SOFAR channel, but also to the strong water current coming from the Mediterranean Sea. This explains the dipping of the current over the flank of the Goringe Ridge. Unfortunately, no BSR could be seen on the sediment reflections as they are parallel and weak.

The study of the SOFAR channel is in its infancy. It should be interesting for the deepwater oil industry to try to record seismic events around water depths of 600 - 1000 meters in order to evaluate its influence in seismic recordings.

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