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EXPLOITING THE UNIQUE NEAR-FIELD BACKSCATTETED ARA SAN JUAN COLLAPSE EVENT SIGNALS TO POSITION THE WRECK
DEFINITION OF THE TERM “SEAFLOOR BACKSCATTER”
Seafloor backscatter is defined as acoustic energy received by a sensor after a complex interaction of a signal with the seafloor.
DISCUSSION
The Comprehensive Nuclear Test-Band Treaty Organization (CTBTO) Report of 29 Nov 2017 provides time-vs-amplitude graphs of the ARA SAN JUAN collapse (implosion) signal as detected by CTBTO hydrophones at Ascenion Island in the tropical Atlantic (H10N) and at the Crozet Islands in the Southern Indian Ocean (H04S).
Detections of multiple reflections of the SAN JUAN implosion-generated acoustic signal were made at a distance from the wreck-site of 6040km (3262 nautical miles) by station H10N from bathymetric features along the South American coastline to the north and of the wreck-site and by the South Georgia Bank to the southeast of the site. Delays as great as 14 minutes and four seconds were observed at N10N between the detection times of the event signal by direct path and the reflection of the signal by the South Georgia Bank. Similar reflections were detected from the Mid-Atlantic Ridge when the USS THRESHER collapsed in the western North Atlantic.
The unique feature of the N10N – and to a lesser extent H04S – detections of the SAN JUAN collapse event was the presence of five or more additional signals detected with delays of less than half-a-second from the actual collapse signal. These sources are assessed to have been backscatter – reflections or echoes – of the collapse event signal by bathymetric features with significant vertical relief (effective reflecting surfaces) to the southwest of the collapse site by no more than several hundred meters.
No such “near field” reflected signals were associated with any of the other five collapse events the writer has analyzed because those events occurred above relatively flat abyssal plains, i.e., no effective reflecting surfaces. Collapse events produce – within the signal integration time of the data display systems used – a single pulse of acoustic energy followed only by further bubble-pulse oscillation periods (BPOP) with energy-decay reduced periods and thus increased frequency (reciprocal function). In the case of the SAN JUAN, the initial BPOP was 4.68 Hz which decayed to 6.0 Hz and thus identified the signals as an implosion rather than sources of independent origin. This frequency relationship - and the energy level of the event implied by the signal to background sea noise evident in the CTBTO data at ranges as great as 7730 km (4175 nautical miles) – was the basis for the writer's assessment made upon receipt of the CTBTO report that the signal had been produced by implosion (total destruction) of the ARA SAN JUAN at great depth.
Note: although the collapse of the USS SCORPION (SSN 589) on 22 May 1968 produced more than 17 follow-on acoustic signals from the collapse of structures internal to the wreckage – such as the torpedo tubes, none occurred earlier than 27 seconds after collapse of the pressure-hull. SCORPION collapsed above an abyssal plain.
Recognition of the SAN JUAN signals addition to the collapse event as reflections of the collapse event source by bathymetric features with significant vertical dimensions in the immediate vicinity of the collapse site implied the SAN JUAN was lost above or near an irregular seafloor surface: an undersea canyon. This information was provided to the search team. The SAN JUAN was found in an area with significant bathymetric relief consistent with the characteristics implied by the backscattered (reflected) acoustic signals.
And finally, detection of a weak signal by N10N 23 mins and 53 seconds after the collapse event detection time on the 217.5 degree bearing of the wreck-site from N10N is assessed to have been a reflected path signal produced by collapse of a small more pressure-resistant structure within the SAN JUAN wreckage that survived at the wreck depth of about 907m (2975-feet) for about 20 minutes after the wreck impacted the bottom at about 1353 Greenwich Mean Time (GMT). Collapse depth was 468m (1535-feet) at 1351 GMT.
It is concluded the 907m depth of the wreck was below “the local underwater horizons” which blocked any direct path detection of this “late arriving” acoustic source by the CTBTO sensor H10N. A bathymetric feature above (shallower than) and to the southwest of the wreck-site was the reflecting surface. This source – and many of the other locally reflected signals - were not detected by CTBTO sensor at H04S to the east of the wreck-site; hence, the assessment that the involved bathymetric features were to the southwest of the wreck-site at a distance required by the delay in detection time relative to the collapse event signal: less than several hundred meters.