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The Most Complex But Also the Most Potentially Important Posting to Date

By Bruce Rule - Oct 4, 2016

Subj: Distinguishing Between Hydroacoustic Detections of Explosions and Implosions, a Critical Analytical Capability


If underwater explosions or implosions do not vent to the surface, they produce cycles of expansion and compression of the area of reduced pressure – basically a vacuum bubble - created by the involved energy release. The oscillations (movement) of the water displaced by both types of events generate high levels of acoustic energy that – unless blocked by bathymetric features - can be detected at extreme ranges.

The ability to distinguish acoustic detections of explosions from implosions can provide critical intelligence. In the case of the loss of the USS SCORPON (SSN-589) on 22 May 1968, that capability would have prevented the Navy's SCORPION Court of Inquiry from erroneously concluding SCORPION was lost because of “the explosion of a large charge weight external to the pressure-hull.” That assessment was generatively responsible not only for the basic misapprehension of why the disaster occurred but also for subsequent conspiracy theories that SCORPION was sunk by a Soviet torpedo.

(SCORPION was lost because of an explosion of hydrogen out-gassed by the main storage battery. That explosion - which was contained within the pressure-hull - disabled and/or killed the crew, who were unable to prevent SCORPION from sinking to a depth of 1530-feet (680 psi) where the pressure-hull imploded with an energy release equal to the explosion of 13,200 lbs of TNT at that depth.)


Hot gases produced by an underwater explosion are contained by hydrostatic pressure within a bubble which rapidly expands. As the bubble expands, the pressure inside decreases. The momentum of the displaced water continues the expansion of the bubble beyond the point at which the internal pressure falls below the external hydrostatic (sea) pressure. When this pressure-difference becomes sufficient to overcome the momentum, the bubble contracts, compressing the gas within until its pressure is sufficient to halt the motion of the water, whereupon the cycle repeats, each time with diminished intensity and lengthened duration because of the loss of energy due to friction. The oscillating bubble generates a series of pressure pulses known as “bubble pulses” which are characteristics of deep underwater explosions. (1)

Unlike the expansion phase of the cycle of the explosion-generated bubble, the duration of the contraction phase is halted abruptly when the basically non-compressible water “meets” at the ”focal point” of the event: the site of the explosion. Therefore, the expansion phase of an explosive-generated event – which occurs first – will have a greater duration than the subsequent contraction phase which is truncated as just discussed.

The exact opposite occurs for implosions. The initial phase is the contraction of the “bubble” which is essentially the air contained within the imploding object - in the context of this discussion - the collapse of a submarine pressure-hull. When that compression reaches the maximum value – which may produce heat sufficient to generate steam – the bubble expands until – as was the case discussed above for the expansion phase of an explosive event, it is overcome by ambient sea-pressure. Therefore, the expansion-compression phase relationship of a implosion is the reverse of an explosion.

Since, the expansion phases of both explosive and implosive events have greater durations than the compression phases – and occur in a reverse order – extremely accurate measurements of the durations of these phases – and the sequence in which they occur – may permit identification of the source of an acoustic event, i.e., distinquish hydroacoustically detected explosions from implosions.


Only one acoustic event analyzed in the required temporal resolution is available to the writer for evaluation of the above discussed assessments: the SCORPION pressure-hull collapse event shown as the figure in Chapter 4 of reference (2). That display has an effective time-resolution of 0.003 seconds ((three milliseconds (ms)).

The temporal characteristics of the first collapse-expansion cycle created by the collapse of the SCORPION pressure-hull – as detected at a range of 821 nautical miles - had a duration of 0.224 seconds. The displayed changes in signal level during that period are consistent with the following assessment: the collapse phase of the event had a duration of about 35ms (1/29th of a second) while the expansion phase had a duration of about 200ms (1/5th of a second); hence, the asymmetry between the durations of the compression and expansion phases of a submarine pressure-hull collapse event may be a great as one-to-six, a value consistent with the travel-distance of the compression phase: a maximum of half the diameter of the pressure-hull or about 15-feet. (Fifteen feet in 35ms requires a speed of 290 mph.) Additionally, the noise-level generated during the expansion phase was significantly higher than during the collapse phase.

If similar relative temporal and amplitude characteristics can be identified - but in reverse sequence - for signals known to have been of explosive origin, these differences can be used to reliably identify acoustic detections of such events as either explosions or implosions.


There were no recordings of the collapse of the THRESHER pressure-hull. The most-accurate THRESHER bubble-pulse frequency of 3.4 Hz was derived from analysis of paper displays (AN-FQQ-1(V) vernier LOFARgrams) from the Antigua SOSUS station at a detection range of 1,300 nautical miles. That frequency value indicated the THRESHER pressure-hull collapsed at a depth ot of 2400-feet with an energy release equal to the explosion of 22,500 lbs of TNT at that depth. If the same ratio of compression duration to expansion duration existed for THRESHER as for SCORPION, the THRESHER pressure-hull collapsed – was completely destroyed - in about 47ms (1/20th of a second). Sea pressure at the THRESHER collapse depth was 1070 psi.


This assessment is yet another example of the axiom: the more carefully you measure something, the more you find out about related subjects: in this case, measurement in the time domain to an accuracy of several milliseconds. The resulting loss of frequency resolution from such signal processing is of no consequence because the frequency of the bubble-pulse can be derived with extreme accuracy as the reciprocal of the duration of the event: 4.46 Hz for SCORPION as shown by the above discussed figure.