Editing SOFAR channel (section)

== Principle ==
[[File:Rays test.gif|thumb|600px|Acoustic pulses travel great distances in the ocean because they are trapped in an acoustic [[wave guide]].  This means that as acoustic pulses approach the surface they are turned back towards the bottom, and as they approach the ocean bottom they are turned back towards the surface.  The ocean conducts sound very efficiently, particularly sound at low frequencies, i.e., less than a few hundred Hz]]

Temperature is the dominant factor in determining the speed of sound in the ocean. In areas of higher temperatures (e.g. near the ocean surface), there is higher sound speed. Temperature decreases with depth, with sound speed decreasing accordingly until temperature becomes stable and pressure becomes the dominant factor. The axis of the SOFAR channel lies at the point of minimum sound speed at a depth where pressure begins dominating temperature and sound speed increases. This point is at the bottom of the [[thermocline]] and the top of the deep isothermal layer and thus has some seasonal variance. Other acoustic ducts exist, particularly in the upper [[mixed layer]], but the ray paths lose energy with either surface or bottom reflections. In the SOFAR channel, low frequencies, in particular, are refracted back into the duct so that energy loss is small and the sound travels thousands of miles.<ref name=Kaharl/><ref>{{cite report |last1=Helber |first1=Robert |last2=Barron |first2=Charlie N. |last3=Carnes |first3=Michael R. |last4=Zingarelli |first4=R. A. |title=Evaluating the Sonic Layer Depth Relative to the Mixed Layer Depth  |location=Stennis Space Center, MS |publisher=Naval Research Laboratory, Oceanography Division |url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a484791.pdf |archive-url=https://web.archive.org/web/20210911142832/https://apps.dtic.mil/dtic/tr/fulltext/u2/a484791.pdf |url-status=live |archive-date=September 11, 2021 |access-date=26 September 2020}}</ref><ref>{{cite thesis |last=Thompson |first=Scott R. |date=December 2009 |title=Sound Propagation Considerations for a Deep-Ocean Acoustic Network |type=Master’s Thesis |location=Monterey, CA |publisher=Naval Postgraduate School |url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a514346.pdf |archive-url=https://web.archive.org/web/20210911135704/https://apps.dtic.mil/dtic/tr/fulltext/u2/a514346.pdf |url-status=live |archive-date=September 11, 2021 |access-date=26 September 2020}}</ref> Analysis of Heard Island Feasibility Test data received by the [[Ascension Island]] [[Missile Impact Locating System]] hydrophones at an intermediate range of {{cvt|9200|km| mi nmi}} from the source found "surprisingly high" [[signal-to-noise ratio]]s, ranging from 19 to 30&nbsp;dB, with unexpected phase stability and amplitude variability after a travel time of about 1 hour, 44 minutes and 17 seconds.<ref name=AOML/>

[[File:PARKA I Track Acoustic Enfironment - Kaneohe-Alaska SOFAR Channel.png|250px|thumb|right|Profile showing sound channel axis and bottom at critical depth. Where bottom profile intrudes into the sound channel propagation is bottom limited.]]
Within the duct sound waves trace a path that oscillates across the SOFAR channel axis so that a single signal will have multiple arrival times with a signature of multiple pulses climaxing in a sharply defined end.<ref name=HSC-URI/><ref group=note>The "History of the SOFAR Channel" reference has a recording and sonogram of the effect.</ref> That sharply defined end representing a near axial arrival path is sometimes termed the SOFAR finale and the earlier ones the SOFAR symphony.<ref>{{cite journal |last=Spindel |first=Robert C. |year=2004 |title=Fifteen years of long-range propagation experiments in the North Pacific |journal= The Journal of the Acoustical Society of America |volume=116 |issue=4 |page=2608 |doi=10.1121/1.4785400 |bibcode=2004ASAJ..116.2608S |url=https://asa.scitation.org/doi/10.1121/1.4785400 |access-date=26 September 2020|url-access=subscription }}</ref><ref>{{cite journal |last1=Dzieciuch |first1=Matthew |last2=Munk |first2=Walter  |last3=Rudnick |first3=Daniel L. |year=2004 |title=Propagation of sound through a spicy ocean, the SOFAR overture |journal=The Journal of the Acoustical Society of America |volume=116 |issue=3 |pages=1447–1462 |doi=10.1121/1.1772397 |bibcode=2004ASAJ..116.1447D |url=https://asa.scitation.org/doi/10.1121/1.1772397 |access-date=26 September 2020}}</ref> Those effects are due to the larger sound channel in which ray paths are contained between the surface and critical depth.<ref group=note>The term also has a [[Critical depth|biological oceanography application]].</ref> Critical depth is the point below the sound speed minimum axis where sound speed increases to equal the maximum speed above the axis. Where the bottom lies above critical depth the sound is attenuated, as is any ray path intersecting the surface or bottom.<ref>{{cite journal |last1=Williams |first1=Clare M. |last2=Stephen |first2=Ralph A.  |last3=Smith |first3=Deborah K. |date=15 June 2006 |title=Hydroacoustic events located at the intersection of the Atlantis (30°N) and Kane (23°40′N) Transform Faults with the Mid-Atlantic Ridge |journal=Geochemistry, Geophysics, Geosystems |volume= 7|issue= 6|pages=3–4 |publisher=American Geophysical Union |doi=10.1029/2005GC001127 |s2cid=128431632 |doi-access=free }}</ref><ref>{{cite report |last1=Fenner |first1=Don F. |last2=Cronin |first2=William J. Jr. |year=1978 |title=Bearing Stake Exercise: Sound Speed and Other Environmental Variability |location=NSTL Station, MS |publisher=Naval Ocean Research and Development Activity (NORDA) |page=3 |url=http://apps.dtic.mil/dtic/tr/fulltext/u2/c017390.pdf |archive-url=https://web.archive.org/web/20160304064913/http://www.dtic.mil/dtic/tr/fulltext/u2/c017390.pdf |url-status=live |archive-date=March 4, 2016 |access-date=26 September 2020}}</ref><ref>{{cite report |last1=Baggeroer |first1=Arthur B. |last2=Scheer |first2=Edward K. |year=2010 |title=Oceanographic Variability and the Performance of Passive and Active Sonars in the Philippine Sea |url=https://www.onr.navy.mil/reports/FY10/oabagger.pdf |archive-url=https://web.archive.org/web/20161230045918/https://www.onr.navy.mil/reports/FY10/oabagger.pdf |url-status=dead |archive-date=December 30, 2016 |access-date=27 September 2020}}</ref><ref group=note>Figure 2 on page three of the Williams/Stephen/Smith reference is helpful in understanding critical depth, the SOFAR channel, the entire channel and the ray paths involved.</ref>

[[File:Bathymetry-SOFAR channel axis—Heard Island Feasibility Test.png|250px|thumb|right|Bathymetry profile with SOFAR channel axis depth, Heard Island to Ascension Island.]]
The channel axis varies most with its location reaching the surface and disappearing at high latitudes (above about 60°N or below 60°S) but with sound then traveling in a surface duct. A 1980 report by Naval Ocean Systems Center gives examples in a study of a great circle acoustic path between [[Perth, Australia]] and [[Bermuda]] with data at eight locations along the path. At both Perth and Bermuda the sound channel axis occurs at a depth of around {{cvt|1200|m|ft|0|abbr=on}}. Where the path meets the [[Antarctic Convergence]] at 52º south there is no deep sound channel but a {{cvt|30|m|ft|0|abbr=on}} in depth surface duct and a shallow sound channel at {{cvt|200|m|ft|0|abbr=on}}. As the path turns northward, a station at 43º south, 16º east showed the profile reverting to the SOFAR type at {{cvt|800|m|ft|0|abbr=on}}.<ref name=Dushaw>{{cite report |last=Dushaw |first=Brian D |date=10 April 2012 |title=The 1960 Perth to Bermuda antipodal acoustic propagation experiment: A measure of a half-century of ocean warming? |url=https://www.ntnu.edu/documents/14687435/14716676/SSPA_2012_Dushaw_Acoustic_Propagation_Experiment_6p.pdf |access-date=26 September 2020}}</ref><ref name=Northrop>{{cite report |last1=Northrop |first1=J. |last2=Hartdegen |first2=C. |year=1980 |date=August 1980 |title=Underwater Sound Propagation Paths Between Perth, Australia and Bermuda: Theory and Experiment |pages=3–6 |location=San Diego, CA |publisher=Naval Ocean Systems Center |url=https://apps.dtic.mil/dtic/tr/fulltext/u2/a094165.pdf |archive-url=https://web.archive.org/web/20201109031445/https://apps.dtic.mil/dtic/tr/fulltext/u2/a094165.pdf |url-status=live |archive-date=November 9, 2020 |access-date=24 September 2020}}</ref>