Acoustic Detectability of Bottom Impact Events

By Bruce Rule - August 16, 2013

The signal propagation paths of bottom impact events almost always involves multiple surface and bottom reflections. Signal loss through absorption by the bottom depends on the composition of the bottom. Twenty dB loss per bottom reflection (bounce) is a useful average value; hence, bottom impacts are seldom acoustically detected at ranges greater than several multiples of water depth at the impact site.

An exception to this general rule is known from one impact event that occurred on a relatively steep slope which allowed the signal to propagate laterally toward deeper water and be detected at a range of several hundred nm. In contrast, no bottom impact events were detected from the USS THRESHER at a range of only about 30 nm from the nearest SOSUS array (1411/FOX). The THRESHER wreck-site is located on a nearly level abyssal plain at a depth of 8400-feet.

If the acoustic sensor is located at a range of less than water depth from the impact site (direct-path for surface-deployed sensors), multiple impacts can be – and have been – detected not only from sections of a collapsed pressure hull but also from literally hundreds of small pieces of wreckage, some of which had sink-rates as low as two knots compared to 10-12 knots for the major sections of the pressure-hull.

Imagery of the Soviet GOLF Class submarine K-129 taken by the USS HALIBUT (SSN-587) and the CIA salvage ship Hugh Glomar Explorer Capture Vehicle showed the bottom littered with debris even though the submarine forward of the aft-end of sail is relatively intact. In that case, the debris was the result of the burn-through of the K-129 pressure-hull by the firing to fuel exhaustion of two R-21/D4 missiles, as discussed by an earlier posting on this site. As noted in that posting, the K-129 did not experience hydrostatic collapse events because the pressure-hull was fully flooded before it reached collapse depth.

As discussed in the writer's letter of 10 April 1963 to CNO OPNAV (N97) – also posted as an article on this site – the major sections of collapsed submarine pressure-hulls sink nearly vertically, i.e., depart from the vertical by less than two-degree while lighter debris which sinks at the aforementioned very low speeds can be carried some distance from the hull sections by existing currents thus creating a debris field of many hundreds of feet in extent when the water-depth is about 11,000-feet (SCORPION).

A final note on sink-rates useful to repeat because of the uninformed conjectures that submarine hulk sink-rates and impact velocities can be as high as 200-knots (BLIND MAN'S BLUFF: K-129). When submarine pressure-hulls are destroyed at great depth, the hulls can be torn longitudinally and vertically into distorted shapes (THRESHER) that are hydrodynamically inefficient in the extreme. As they fall toward the bottom, they probably twist and turn - literally "flutter" - and may never stabilize before impact; hence, the relatively slow sink-rates in the 10-15 knot range.

The purpose of this posting - and many others by this writer - is to increase the probability that this information is archived for future availability.

Additional Comment August 17, 2013 in reply to John Ellis Post:

It has been conjectured SCORPION had a depth restriction and was transiting westward to Norfolk at 300 feet – certainly not deeper than 400 feet - when hydrogen outgassed by the main battery exploded twice in half a second at 18:20:44Z killing and/or incapacitating the crew with an atmospheric overpressure estimated to have been 150-200 psi (circa three times fatal value) at the site of the event.

It is assumed propulsion was lost due to that event. SCORPION collapsed at a depth of 1530 feet 21 mins and 50 secs after the battery event. So, we have a relatively slow average sink-rate of 0.6 knots which, due to hull compression (increasing with depth and reducing displacement) would have gradually accelerated, all of which suggests that when the battery exploded, SCORPION had to have been in only very slightly negative trim when propulsion was lost.

Other sink-rate data indicates the collapsed SCORPION hulk sank from 1530-feet to the bottom (11,100 feet) at 10-15 knots. Since, as just posted, bottom impact events are not detected by sea-floor sensors at any range greater than several multiples of water depth, we know the SCORPION wreckage was still above the bottom at the time of the last acoustically-detected event (at a range of 821 nm) which was about 200 seconds after collapse. If you assume the hulk was immediately (only 100 feet) above the bottom at that time, you have it sinking about 9500 feet in 200 seconds for a sink-rate no faster (and probably a lot slower) than 28 knots. That's all you can get from the Canary Island acoustics.

The K-129 was at snorkel depth (imagery of the wreck confirms masts were extended) when the first internal explosion occurred at 11:58:58Z, an event that almost certainly killed the crew. The K-129 pressure-hull was breached at an unknown time after launch tube burn through which estimated to have occurred at 12:00:05Z.

There is no way of estimating how long it took the 5000 degree (F) missile exhaust plume to also burn through the pressure-hull. The first missile fired for 95.2 seconds, the second missile ignited at 12:06:01Z and burned for 95.4 seconds.

The K-129 probably would have flooded slowly initially but that rate would have increased as pressure increased with increasing depth. The published K-129 collapse depth was 300m (985 feet). It is probable that at snorkel depth, all compartments would have been open, a conclusion supported by the failure of any AFTAC hydrophones to detect collapse event signals.

Available data indicates the gradually flooding K-129 would have accelerated to terminal velocity of 10-12 knots by the time she reached a depth of 1200-feet. So, we have the K-129 having taken perhaps as long as five minutes to reach that depth after which she fell toward the bottom at 10-12 knots.

Bottom line: all fully flooded but basically intact submarines have similar sink-rates between 10 and 12 knots – possibly as high as 15 knots. Sink-rates for sections of collapsed pressure-hulls are similar. In fact, based on a very limited data base, the sink-rates for surface ships also is within that range.