Could PACSOSUS Have Detected the THRESHER Collapse Event in the Atlantic?
By Bruce Rule - September 7, 2013
The following quote is from the Acoustical Society of America website: http://asadl.org/jasa/resource/1/jasman/v96/i4/p2330_s1?isAuthorized=no
“In January 1991, the Heard Island (53°06'S, 73°30'E) Feasibility Test (HIFT) was carried out to establish the limits of usable, long-range acoustic transmissions. Coded acoustic signals transmitted from a (60-Hz?) source near Heard Island in the southern Indian Ocean were monitored at 16 sites in the North and South Atlantic, the North and South Pacific, the Indian Ocean, and the Southern Ocean. The question posed by HIFT, whether at such global ranges the signals would permit phase-coherent processing and thus yield favorable signal-to-noise levels, was answered in the affirmative.” (The tow-ship was the M/V Amy Chouest, sister ship to the M/V Cory Chouest.)
The following quote is from theThe American Institute of Physics website: http://www.aip.org/pnu/1994/split/pnu205-2.htm
“...the signal took 2.95 hours to reach the Bermuda station 16,000 km ((8,640 nautical miles (nm)) away (from the position of the Amy Chouest). The researchers were generally satisfied with the quality of the received signals, as measured by the signal-to-noise ratio and the stability of the waveform over time. (17 papers in the Journal of the Acoustical Society of America, October 1994; introduction by Walter R. Munk of Scripps and Arthur Baggeroer of MIT.)”
With the above as background, the question that arises is: could the USS THRESHER (SSN-593) pressure-hull collapse signal – equal in energy release to the explosion of 22,500 pound of TNT at a depth of 2400-feet - have propagated from the wreck-site at 41°46'N, 65°03'W through the position of the Amy Chouest to a west coast SOSUS array?
A map function available at http://scilib.ucsd.edu/sio/hist/munk-heard-island-expt.pdf indicates the position of the Amy Chouest for the HIFT was near 54-30S, 73-00E or 9000 nm from the THRESHER wreck-site. The bearing from the wreck-site to the position of the Amy Chouest was 129.5 degrees. The "incoming" bearing of the THRESHER signal at the position of the Amy Chouest was about 270 degrees according to the aforementioned website map. That great circle line of bearing continues past the position of the Amy Chouset and runs out of deep water near the southern Australian coast (33S, 123E) at a range of 10,160 nm from the THRESHER wreck-site.
The great circle lines of bearing that reached the west coast of the US - and through which the Amy Chouest signal successfully propagated - were between about 110- and 135-degrees from the ship, passing north and south of New Zealand and impacting the US coast near Centerville and Coos Bay at a range to Coos Bay of about 9700 nm. (See map at http://scilib.ucsd.edu/sio/hist/munk-heard-island-expt.pdf) ;
Unless the writer has missed something, for the THRESHER collapse event signal to have propagation through the position of the Amy Chouest, the bearing of the incoming signal (270 degree) would have had to continue on an “outgoing” bearing of about 090 for there to have been a single propagation path from the THRESHER wreck-site to Coos Bay. Since this was not the case (by about 20 degrees: 090 vs 110), it does not appear any PAC SOSUS array could have detected the THRESHER event.
Go to http://www.movable-type.co.uk/scripts/latlong.html for calculations and a Google map that shows the great circle plots for the signal transmission paths, and for any two positions you enter.
With reference to the detectability of the THRESHER collapse event signal level above background (S/N) at a range of about 20,000 nm from the wreck-site to Coos Bay, note the following: the THRESHER event S/N can be conservatively estimated from the Antigua FQQ broadband and vernier displays (no tapes) to have been at least 30 dB at a range of 1300 nm. (The eighth harmonics of the bubble-pulse frequency was detected on a vernier display.)
Using a signal loss level of 4.2 dB per doubling of range calculated for Barbados target 27103 (NOVEMBER Class) at a range of 3000 nm in July 1962 with the 30 dB S/N value of the THRESHER event at 1300 nm, it appears the THRESHER signal would still have been a S/N value of about 10 dB at a range of 20,000 nm.
So, the noise level of the THRESHER collapse event would have been sufficient to have been detected by PACSOSUS arrays had a deep-water propagation path existed.
It is instructive to “play around” with the Google map function at http://www.movable-type.co.uk/scripts/latlong.html with respect to long range propagation paths from the THRESHER wreck-site to west coast stations. The writer was unable to adjust latitude and longitude in the southern Indian Ocean to identify a transmission path from the wreck-site to the US west coast that did not "run aground” in either eastern Brazil or on the Antarctic coast. Perhaps members of this site will have better success.