MH370 Acoustic Analysis for Implosion with a (retracted) Proposed Search Area

Sept 2, 2015 – While the late implosion scenario still merits investigation, the proposal of a new search area has now been retracted. Please read this update for details. 

N.B.: The Scott Reef timing triangulations here were initially miscalculated, causing a false premise for the full analysis. A careful recalculation using the current method finds that they are consistent with the HA01 triad bearing of 301.6 and the HA01+RCS triangulation.  The event appears as the Curtin researchers deduced on the Carlsberg Ridge West of the Maldives. It was probably seismic, and within 50 km of 4.65N 66.62E. The .kml file linked below shows the event area.

 

August 15, 2015 — by Ed Anderson

Searching continues since the disappearance of Malaysian flight MH370 on March 8, 2014.  Satellite radio communications with the plane when in flight have been analyzed in detail, giving a search path tens of kilometers wide on a long arc with a radius of over 5,000 km, defined by the seventh and last INMARSAT handshake with the plane.  The  narrowed search area along this “7th arc” remains large, with ship soundings since mid-2014 focusing on about 1,000 km of the arc, centered some 2,000 km WSW of Perth. This area is based on the fuel range of the aircraft and estimates of an autopilot flight bearing based on radar, satellite, and cellphone contact signals at the time the flight went missing.

The Australian Transport Safety Bureau (ATSB) coordinating search efforts has requested analysis of acoustic data for either an impact or implosion that might narrow the search region. The possibility of an underwater implosion after the plane was down has been conceded by experts, but there does not appear to be any analysis of available data for that scenario.

Shortly after the loss of the plane, a media release from the UN’s CTBTO nuclear test ban monitoring organization suggested that acoustic location might be possible using their network of sensors, with atmospheric infrasound more likely than underwater hydrophones to detect any surface disturbance. They put out a call for scientific experts in their member states to analyze data, but nothing was initially found due to the difficulty of picking out an event from the environmental noise. The CTBTO operates multiple hydrophone listening stations on the Indian Ocean. Each station has at least one triad of microphones spaced 2 km apart, using the different sound arrival times to get a bearing toward an event. These hydrophones are placed at a depth to detect  sounds that can propagate long distances within the SOFAR (SOund Fixing And Ranging) channel.

The SOFAR or Deep Sound Channel (DSC) is a layer of the ocean where sound propagates at a local minimum speed because it increases with both temperature and pressure. The effect is like a lens for sound that refracts sounds toward the center of the channel. Similar to gradient index fiber optics that allow light signals to travel long distances, the SOFAR channel allows whales to dive down to the channel depth and communicate across oceans.

About two months after the call for acoustic location, the Australian ATSB search organizers reported that hydrophone data was being analyzed by the CTBTO and IMOS.org.au (Integrated Marine Observing System) as a factor in defining the search area. In early June, Curtin.edu.au put out a press release describing how they physically retrieved an IMOS recorder RCS (PAPCA) near Perth and found a candidate event.

Unlike the CTBTO hydrophone arrays that have cables some 100km long to shore and stream live to a data center,  the IMOS devices are remote hydrophones placed to record marine life. They are normally retrieved annually to minimize cost, and to make the battery last they record only about 1/3 of the time on a 15 minute cycle. The Rottnest Continental Shelf (RCS) hydrophone is located on the edge of Perth Canyon. Its depth of about 400 meters near the sea floor is shallower than SOFAR depth, but the slope of the shelf helps to reflect sounds upward from the channel.

The report from the Curtin researchers explains that the timing of the IMOS event allowed a more careful check of the CTBTO hydrophone data from the HA01 array near Cape Leeuwin and a matching event was found. Analysis of arrival time differences among the three HA01 detectors provided a heading to the event that the researchers considered precise, and they report that the Rottnest hydrophone was used to get an approximate range. These hydrophone analysis results were graphed on a map of the Indian Ocean, but the proposed search area was very large, stretching over the Arabic Sea and centered West of India around the Maldives.

Curtin_Map_yellow_polygon_shows_uncertainty_region_for_source_location
Curtin Map: Yellow polygon shows uncertainty region

That search area does not include the 7th Arc, being assumed as too distant to match the timing of last satellite handshake with MH370.  A June 1 letter to the ATSB reported that the acoustic data was incompatible with the generally accepted satellite data. In October, Curtin researchers reported on the analysis of an additional event signal obtained from another IMOS hydrophone retrieved from Scott Reef in September.  The projected location was near the Arabian Sea, and researchers concluded that because of the conflict with satellite timing and different wave envelope, it was most likely a natural event. This resulted in news stories that the hydrophones were not useful for locating MH370.

 

Dismissal of the existing acoustic data appears to be based on both  an assumption that the event would have been at the time of an impact, and the way the analysis was presented on the map.  There is still a real possibility that the detected acoustic event occurred nearly an hour after impact. Due to the propagation speed of sound in water, this would place it much closer to the RCS and HA01 hydrophones.  The experts addressed the possibility of an implosion, but perhaps in the context of the longer distances. The Executive Summary released as Appendix B of the June 24 ATSB Definition of Underwater Search Areas (p 47) concludes:

“… however, should the arc defined by the handshake data be called into question, the various timing and acoustic considerations discussed here would suggest that a reasonable place to look for the aircraft would be near where the position line defined by a bearing of 301.6° from HA01 crosses the Chagos-Laccadive Ridge, at approximately 2.3°S, 73.7°E. If the source of the detected signals was the aircraft impacting the sea surface then this would most likely have occurred in water depths less than 2000m and where the seabed slopes downwards towards the east or southeast. These considerations could be used to further refine the search area. If, instead, the received sounds were due to debris imploding at depth it is much less certain where along the position line from HA01 this would have occurred.

In followup comments to the June 4 report from Curtin, a note on July 21 says:

“With regard to the suggestion of using the arrival time and (reasonably well) known speed of sound to estimate the range, we have done this based on the earliest and latest times the plane is likely to have hit the water, and the results are consistent with the uncertainty box derived from the arrival time differences at Rottnest and Cape Leeuwin. Unfortunately the speed of sound is large (about 1480 m/s) and the uncertainty in the impact time is also large (more than half an hour), so this doesn’t reduce the size of the uncertainty box by much. It also assumes that the sound we heard was the actual impact. It could also have been the implosion of sinking wreckage, but that would have been expected to occur some time later, which would confound this calculation.”

The confounding factors and the probable depth are apparently references to the efficient propagation of sounds only after they enter the SOFAR channel. Surface sounds, like from an impact, propagate mostly downward passing through the channel while portions of the sound traveling more horizontally are refracted back upward toward the surface by the speed gradient. This has a complex effect of causing convergence zones of strong and weak nodes along the surface with a larger attenuation of sound and only a small portion entering the SOFAR channel, plus multi-path ghosting of the signal. A similar effect occurs from an event on the sea floor below the SOFAR channel. Nearly horizontal signals are refracted back downward and attenuated.

The difficulty of sounds entering the SOFAR channel from above or below may be why atmospheric infrasound was initially suggested as a more likely avenue of investigation than hydroacoustics.

SOFAR channel propagation can also be disturbed by variations in the sea floor. It can be completely blocked by a seamount or ridge that nears the surface. Temperature variations along the path can also change the wave speed. Such variables can also cause horizontal refraction of the signal, causing it to take a different path to the hydrophone, or show a false bearing.

These effects may partly explain why the event was not detected at the other CTBTO hydrophone array in the Indian Ocean, HA08 in the British Indian Ocean Territory, South of the Maldives. That hydrophone array was experiencing local noise from a sounding source, but there may still have been windows of detection.

Still, a candidate event was detected by four hydrophones at two locations. It is not unlikely that some piece of the aircraft could have made a buckling or implosion sound from crushing pressure as it sank through the SOFAR channel depth range from about 550 to 1500 meters. Being directly in the sound channel, it would not need to be louder than a whale vocalization to be detectable at long distances. Rather than confounding the calculations, the sound travel in a direct path at a more uniform speed should reduce the error components.

The much longer baseline of 343 km between the RCS hydrophone and the center of HA01 should provide a far more accurate target direction than the shorter 2 km spacing of the HA01 hydrophone array. With the event time unknown for triangulation, the close range acoustic search path defined is not a straight bearing, but a shallow arc that bends toward the closest hydrophone.  This is not indicated in the earlier analysis, which clearly shows a straight great circle path ending directly at HA01, the furthest hydrophone. No explanation was given for the shape of the Curtin search area, but the Southern limit does appear to start on the Southern error limit of the HA01 bearing and arc toward RCS before being truncated where it intersects the Northern error limit of the HA01 bearing. Had that full arc been drawn, it would closely match this proposed search path.

The time difference between signal arrivals is the key element. Without access to the original acoustic data, it can be visually cross correlated from the graphic display of the two sound envelopes provided by the researchers. An optimistic estimate is 66.6 seconds, with an accuracy within about a second. The researchers say the uncertainty is about four seconds, which could be taking into account long term timing accuracy of the clock on the underwater Rottnest recorder. [8/26 update – IMOS PAPCA is checked for clock drift against GPS timing at deployment and recovery.]

This time difference can be turned into a path length difference given the local speed of sound in water along each path.  The speed of sound in the SOFAR channel varies mainly by locality and seasonal water temperature change, but not over a very large range.  Different researchers have estimated at 1460 m/s (used recently by LANL), around 1472 m/s on a bearing of 301.6 to the Arabic Sea, 1480 m/s in a comment from Curtin, and past papers with maps showing computed SOFAR sound speeds for the Indian Ocean.

Using an estimated 1488 meters/second, the path length difference from the event to each sensor is approximately 99.1 km. Plotting the possible locations generates an acoustic search path that crosses the NorthEast side of the Batavia sea mount.

Triangulation can be done by intersecting the acoustic search path with the 7th arc as the most probable final location of the aircraft.  This intersection is at 25.77 S 101.50 E which is about 1 degree (60 nautical miles) due East of the Batavia sea mount peak.  The search bearing from that point is a relatively narrow arc heading 306.71° NW and 126.72° SE.  The sea depth in this area slopes from 4000-4500 m (14,000 ft). Recent bathymetric readings neighbor this location along the 7th Arc, but the wider bathymetric scan map stops about 15 nautical miles SW.

This acoustic search path runs roughly parallel to the estimated path on the earlier map but 180 km NNE, and about 150 km from where the previously suggested straight bearing of 301.6 degrees from where HA01 intersects the 7th Arc at 26.7 S 100.7 E.  It just happens to be nearly perpendicular to the 7th Arc, which minimizes the resulting cross sectional search area and simplifies error accounting.

The acoustic search path passes within 1/4 degree (15 nm) of the 25S 101N location where the first ultrasonic pings from an Underwater Locator Beacon (ULB) were detected, about 50 nm from the 7th Arc intersect.  Tracing back a flight path from the intersect location crossing the satellite handshake rings at the correct times, that path passes within 1 degree (60 nm = 111 km) of the Cocos Island airport with a 10,000 ft runway. This may be significant because although the distance may have been too great for any sighting reports, the CTBTO has an infrasound listening station there. Given the location of Cocos Island about midway between the 5th and 6th handshake arcs, the infrasound data from IS06 could be analyzed for detection of the jet passing nearby.

The width of the acoustic search path depends on an accumulation of errors in the variables.  An error in the estimated 1488 m/s does not create a large deviation from the path because the seafloor topology is very similar for both paths. Substituting the slowest 1460 m/s speed causes an error of about 5 nm SW perpendicular to the path. Is is unlikely that the SOFAR speed is much higher than the chosen estimate. The commonly published locations of the hydrophones are not accurate. The HA01 hydrophones are located about 4 km ENE of the 34.9S 114.1E listing. The center of the triad at 34.890S 114.143E is used, assuming that the three separate signals were correlated and summed for noise reduction. The 31.885S 15.0195E location used for the Rottnest RCS hydrophone is fairly accurate, based on a map showing placement of the hydrophone after retrieval from 2009-2013.   Hydrophone coordinates are probably within 1 km.

The width of the acoustic search path is determined by the error limit of the calculations, with the highest probability toward the center. That error range can be computed by figuring the shortest and longest likely path differences. Since the path difference is a product of wave speed and time, errors in both are easily summed. Sound speed values of 1460 and 1514 m/s are used for the minima and maxima (though the value of 1488 is probably within 2 m/s).  A one second uncertainty has been estimated for the signal timing. It is hoped that the hydrophone signals could be matched to within milliseconds (as within the HA01 triad). An additional 4 km of error has been added to account for errors in hydrophone placement and sound speed variation between the paths. The result is a search path that is about 23 nm wide (42 km or 26 miles). Even with conservative estimates of the errors, this search area is rather narrow because it is much closer to the hydrophones than previously assumed, and the baseline is 160 times longer. The acoustic search path error is still relatively small compared to the 200 nm breadth of the 7th Arc search zone, and tiny compared to the length along it being searched since October.

The errors described can potentially be reduced to within the width of a single sonar pass by experimental post-calibration of the data. Sound speed over the path could be verified by detonation of more easily detectable test charges at a calibrated location and time anywhere near the 7th Arc search path intersection. A pattern of strong sound impulses at SOFAR depth could calibrate all hydrophones in the area to narrow the signal observation window.  The accurate bearing to the source could be used for filtering of various acoustic signals to reduce noise.  For the RCS and Scott Reef hydrophones to record the test signal, it would need to be timed for reception when they are both active, then they could be retrieved once more. A hydrophone could be lowered near the Rottnest location during the test.  Surface and seafloor detonations could be useful in characterizing the signal so that other MH370 acoustic events might be found. A surface explosion would also be detectable by atmospheric infrasound.  The timing of the acoustic event indicates that it may have occurred about 58 minutes after the time of the 7th Arc, with a current estimate of 01:17:10 UTC at 25.773S 101.500E.

The timing of the candidate signal at Scott Reef is not consistent with a near field detection at either RCS or HA01. It is curious because although it is the furthest of the three hydrophone locations from the 7th Arc and the sound speed is shallower over more of the path, that signal arrived at Scott Reef before the others.  It could possibly have been a different MH370 event that was not detected at RCS because both IMOS hydrophones only record for 5-10 min every 15 minutes.

N.B.: The Scott Reef timing triangulations here were initially miscalculated, causing a false premise for the full analysis. A careful recalculation using the current method finds that they are consistent with the HA01 triad bearing of 301.6 and the HA01+RCS triangulation.  The event appears as the Curtin researchers deduced on the Calsberg Ridge West of the Maldives. It was probably seismic, and within 50 km of 4.65N 66.62E. The .kml file linked below shows the event area.

Some speculative possibilities arise given the proximity of the search path to both the Batavia sea mount and the discounted ULB ping detections. Part of the reason for dismissal of those signals was that they were heard beyond the likely range of the ULBs on the recorders. If the resting place of the plane were on the NE skirt of the seamount, that location might have a reflective focusing effect on the sound waves toward the NE. While this location NW of the 7th Arc intersect seems slightly early, it is possible that the plane was already down by the time of the partial handshakes, or that it spiraled back along its path after a loss of power. A flight path crossing over the Cocos island runway incidentally crosses the sea mount just outside the search region.

The location of Cocos Island between the 4th and 5th arcs (21:41:27 and 22:41:22) gives an interpolated flight path time there of 22:19:49 UTC. If the path were 80 km East, the closest approach for doppler drop would be at 22:27:29 UTC. Flight paths to the Southern search region would pass within 300-500km before the timing of the 4th arc. This creates a broad infrasound listening window from 21:30 to 23:00 UTC. Speeding up the recorded signals to the audible range and listening binaurally could reveal a distance and general flight bearing if any doppler shift is detectable.

To expedite the search of the acoustic data for other events and derive a more detailed analysis, it would be very helpful to have the various sound files and datasets (including infrasound for the flight duration) posted online and accessible to the public.

It is hoped that any researchers or organizations involved will take this analytical critique in the constructive way it is intended, using it to refine the available information.

Above all, heartfelt condolences go to the families and friends of those presumed lost on flight MH370. Please excuse any insensitive wording.

Beware that the embedded map above and mobile devices may show rhumb lines rather than great circle paths. Please use Google Earth on a desktop computer to load the .kml file, which contains additional notes.