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The depth dependence of ambient noise coherence in the Challenger Deep

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The depth dependence of ambient noise coherence in the Challenger Deep

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Introduction
A series of experiments aimed at measuring the power spectrum
and vertical and horizontal coherence and directionality of
ambient noise in the deep ocean were carried out between 2009
– 2014 using the family of “Deep Sound” autonomous acoustic
recording platforms. In December 2014, an expedition was
mounted aboard the Schmidt Ocean Institute’s R/V Falkor to
the Challenger Deep in the Mariana Trench, with the objective
of proling the ambient noise in the deepest known spot in the
world’s oceans. e depth at the site is 10,916 m. When compared
with previous measurements made in the Philippine Sea to a
depth of 5,940 m (Barclay and Buckingham, 2013a), in the Tonga
Trench to a depth of 8,500 m (Barclay and Buckingham, 2014),
and in the Sirena Deep, Mariana Trench to a depth of 8,900 m,
the results show that below the critical conjugate depth (depth
at which the sound speed is higher than any sound speed above
it), the second-order noise eld statistics depend on the relative
contributions of locally generated surface noise and distantly
generated propagating noise. e depth dependence of the
sound eld varies with local surface conditions where near
constant noise power and coherence (directionality) is found
at moderate wind states and above, while a drop in noise level
below the critical depth is found during calm sea states. It is also
shown that the vertical noise coherence is well described by
the Cron & Sherman surface noise model (Cron and Sherman,
1962) with some depth-dependence explained by the local sound
speed variations.
Deep Sound
Deep Sound is a free-falling (untethered) acoustic recorder
designed to descend from the ocean’s surface to a pre-assigned
depth where it drops an iron weight and returns to the surface
under its own buoyancy with a speed of ~0.5 m/s in either
direction. ree variants of the instrument family have been
built, the Mk I, II and III. e data recorded at the Challenger
Deep was done by the Mk. II and as such this variant will be
briey described here. More complete descriptions of all three
are found in previous publications (Barclay et al., 2009; Barclay
and Buckingham, 2013b; 2014). e instrument comprises of a
3.6 cm thick Vitrovex glass sphere with a 43.2 cm outer diameter
and a depth rating of 9 km, four external hydrophones, a conduc-
tivity-depth-temperature (CTD) sensor, and recovery beacons: a
strobe, radio beacon, and satellite beacon.
Inside the sphere an embedded microprocessor monitors
and records the acoustic and oceanographic data, six degree of
freedom inertial motion and magnetometer (compass) data, and
system data such as internal temperature and remaining battery
charge. e ensemble of data is used to trigger a drop weight when
a set of pre-determined conditions are met. In practice, this is
typically when the instrument reaches a pre-programmed depth,
or when a certain deployment time limit has elapsed, but may also
be due to a low battery warning, overheating, or communications
failure with one of the external sensors.
Pressure time series are recorded on four High Tech Inc.
HTI-99-DY hydrophones arranged in an ‘L’ shaped array with
three vertical and two horizontal spacing. e hydrophones
are mounted on arms to places them outside the motion-in-
duced turbulence of the main instrument package. e acoustic
bandwidth of this hydrophone is 5 Hz – 30 kHz, and all four
channels are simultaneously sampled by the data acquisition
board at an over-sampled rate of 204.8 kHz and a dynamic range
of 24 bits. e CTD provides real time depth and vertical speed
data, and is used to calculate temperature, salinity, density, and
sound speed depth proles.
Battery recharge and data transfer are accomplished through
an external housing throughput connections and Wi-Fi connec-
tivity, allowing the system to be recovered and rapidly redeployed.
Sound speed profiles and the conjugate depth
e sound speed in seawater is driven by an empirically derived
equation of state linking temperature, pressure, salinity, and
density. In the deep ocean, the sound speed prole is typically
described by warm (fast) surface waters, with decreasing tempera-
ture driving sound speed down with depth until pressure becomes
the dominant term, now raising sound speed with depth. is
results in a sound speed minimum, typically around 700 m depth
in temperate Pacic waters, known as the sound channel axis.
Sound rays refract towards this axis and remain in this ocean
wave-guide, allowing sound to propagate over thousands of kilo-
meters with no attenuation due to interactions with the surface
and bottom (Munk et al., 1994).
Sound speed proles are shown in Fig. 1, collected by
Deep Sound at the ve deployments sites visited: Challenger
Deep (11°21.599’ N, 142°27.249’ E), Challenger Deep East
Basin (11°22.130’ N, 142°35.172’ E), Sirena Deep (12°43.680’ N,
145°47.820’ E), Tonga Trench (16°31.8760’ S, 172°12.0930’ W),
and Philippine Sea (22°13.572’ N, 126°13.807’ E). Despite their
disparate locations, the proles are remarkably similar with
the sound channel axis between 700 - 850 m. Additionally, the
conjugate depth, the depth at which the sound speed is equal to
the sound speed at the surface, is at ~ 5 km for all deployments,
with the exception of the Philippine Sea, which has a critical
depth just below 4 km due to its higher latitude and cooler surface
waters. e conjugate depth is signicant, since no surface-gen-
erated sound propagating in the deep sound channel may
penetrate below it and, according to Weston (Weston, 1980),
the ambient noise from surface sources is thought to decay to a
Figure 1. The ensemble of sound speed profiles collected by the Deep Sound
instruments 2009 – 2014 (deployment location indicated by colour), with the
solid black line depicting the sound channel axes and the dashed coloured
lines showing the conjugate depths.
The depth dependence of
ambient noise coherence in
the Challenger Deep
By David Barclay, Michael Buckingham and Dieter Bevans
Technical Contributions
Acoustics Bulletin July/August 2017
36
negligible level. Prior to the development of Deep Sound, meas-
urements of ambient noise below the critical depth to conrm
Weston’s prediction were sparse due to the relative diculty of
deploying moored arrays in such deep waters (Morris, 1978; Gaul
et al., 2007).
Power spectrum, vertical coherence,
and directionality
As Deep Sound descends and ascends through the water column,
each hydrophone records a pressure time series. From the Fourier
transform, X
i
(w), of the time series at the i
th
hydrophone, the
power spectrum
(1)
is formed, where T is the record length of each transform, the
brackets denote an ensemble average, w is angular frequency and
the asterisk indicates a complex conjugate. e spatial coherence
function between hydrophones i and j, dened as the cross-spec-
tral density normalized to the geometric mean of the power
spectra at the two sensors, is also constructed as a measure of the
directionality of the noise eld:
(2)
(i) Cox’s equation for vertical noise directionality
In a plane wave noise eld, such as ambient ocean noise, the
vertical noise coherence can be related to the vertical noise
directionality through an integral expression derived by Cox
(Cox, 1973)
(3)
where i=(-1), q is the polar angle measured from the zenith,
and t
d
=d/c is the acoustic travel time between sensors separated
by a distance d with a local sound speed of c, and F(q) is the
noise directional density function, representing the noise power
incident on the sensor per unit polar angle. e integral relation-
ship is linear, thus the noise directional density function may
represent a superposition of source spatial distributions.
(ii) Cron and Sherman model of wind driven surface noise
A theoretical model of wind driven surface noise in an innitely
deep ocean with an iso-velocity sound speed prole was derived
by Cron and Sherman (Cron and Sherman, 1962), where a
random uniform distribution of monopole sound sources is
placed on a plane immediately beneath the at, pressure-release
sea surface. In eect, each monopole and its negative image in the
sea surface acts as a vertical dipole with power directivity function
cos
2
q
. From this geometry, the depth-independent vertical noise
coherence on a pair of sensors has the closed form solution
(4)
representing a directional density function for downward
traveling noise, consistent with the lack of an ocean bottom reec-
tion, of the form
(5)
where u(.) is the Heaviside unit step function.
The depth-dependence of noise across the
conjugate depth
Philippine Sea, Sirena Deep, and Tonga Trench
Measurements made during the PhilSea experiment in May 2009
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Acoustics Bulletin July/August 2017
36 Acoustics Bulletin July/August 2017 37
in the Philippine Basin found that the Cron and Sherman model
described in Eq. (4) and the experimentally measured vertical
coherence function agree remarkably, both above and below the
critical depth, with some ltering need at low-frequencies due to
turbulent ow-noise generated on the surface of the hydrophones
by their motion descending and ascending through the water
column, and at high frequencies due to system noise (see (Barclay
and Buckingham, 2013a) for more detail). e agreement between
the data and model at all depths, with both amplitudes and
zero-crossings well matched in the real and imaginary compo-
nents of the coherence, is shown in Fig. 2, taken from (Barclay
and Buckingham, 2013a). In computing the theoretical curves
in Fig. 2, the locally measured CTD sound speed was used in Eq.
(4). It follows from this result that the vertical directionality of the
wind noise in the band 1 – 10 kHz must be independent of depth
and well approximated by the cosine law in Eq. (5). e sound
speed prole has a negligible eect on the vertical directionality
of surface-generated wind driven noise since it is predomi-
nantly all downward traveling at steep angles, even at the sound
channel axis. Similarly, any bottom reections make a negligible
contribution to the noise eld, even at depths near the seabed.
As a result, the noise below the critical depth shows no change
in directionality.
Additionally, measurements of the power spectral density
across the wind driven noise band show no depth dependence
across the critical depth, as illustrated in Fig. 3. e wind speed
at 10 metres above the sea surface, U
10
, had a steady mean of 10
m/s during the deployment. e remarkable consistency of the
second-order noise eld statistics across all depths is consistent
with the noise originating from local, wind driven surface sources,
with a negligible contribution from distantly located noise sources
propagating in the sound channel.
Figure 4. (a) Real and (b) imaginary parts of the vertical noise coherence in
the Challenger Deep, as a function of depth over the wind noise band and
plotted in dimensionless frequency, wtd=wd/c, where d is the sensor spacing
and c is the local sound speed. The black line indicates the conjugate depth
and the black dashed line is sound channel axis.
Figure 3. The depth-dependence of ambient noise power in the wind driven
noise band in the Philippine Sea, with centre frequencies at 1, 3, 5, and 8 kHz
with a 5% bandwidth. The solid line indicates the sound channel axis depth
while the dashed line shows the conjugate depth.
Figure 2. (a) Real and (b) imaginary parts of the filtered coherence function
versus frequency at 500 m depth intervals on the ascent in the Philippine Sea.
The jagged lines represent data and the smooth curves are the filtered version
of Cron and Sherman’s expression in Eq. (4). To separate the curves, 1 has
been added (subtracted) for each depth increment above (below) 3000 m.
P37
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Acoustics Bulletin July/August 2017
38
ese results are true for the typical deep-water sound
speed prole recorded in the Philippine Sea, as well as the
other deep-water locations where Deep Sound was deployed,
such as the Sirena Deep, and the Tonga Trench (Barclay and
Buckingham, 2014).
Challenger Deep
In December 2014, Deep Sound was deployed in the Challenger
Deep, Mariana Trench, as part of a multi-disciplinary research
cruise aboard the R/V Falkor that included the deployment of
demersal and benthic animal and microbe collecting landers, and
a video camera lander.
Deep Sound returned the depth-dependent vertical noise
coherence from the surface to its maximum depth rating of 9 km,
as illustrated in Fig. 4. In contrast to previously collected data,
a clear dierence between the coherence above and below the
conjugate depth is apparent. Below the conjugate depth, at 7.5
km, the vertical coherence ts a modied Cron and Sherman
model, illustrated in Fig. 5, conrming that the noise is due to
locally generated surface sources, and predominantly downward
propagating with no apparent reection from the seabed. Note
that below 1 kHz, the wind noise begins to roll o in strength
due to the nature of the sources (Wenz, 1962), while inco-
herent turbulent ow noise dominates the signal, causing the
real component of the coherence to drop to zero instead of the
predicted unity.
e model has been modied to t the data by the addition
of a frequency independent scaling factor to account for the
broadband mixture of turbulent ow noise with characteristic
frequency slope f-5/3 (Lighthill, 1954) and the ambient noise on
the trailing hydrophone. Neither of these model modications
has relocated the zero crossings of the coherence curves, and
a more complete discussion is given in previous publications
(Barclay and Buckingham, 2013a). Lastly, the model has been
given a small DC oset corresponding to zero time delay noise
across the entire band, which may be due to either electronic
system noise, or to noise arriving broadside to the array. e
contribution is small overall, and based on previous observations
with this instrument, most likely due to system noise.
However, above the conjugate depth, the vertical noise
coherence changes in character, as illustrated in Fig. 4. At the
sound channel axis, the vertical coherence appears to be nearly
Figure 5. (a) Real and (b) imaginary parts of the vertical noise coherence
below the conjugate depth, at 7.5 km in the Challenger Deep. The blue
line is the data and the black line is the modified Cron and Sherman model
calculated with the local sound speed.
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constant across the entire wind noise band, with the character-
istic roll o below 1 kHz due to ow noise, shown in Fig. 6a. On
closer inspection, illustrated in Fig. 6b, the coherence is the linear
superposition of a frequency independent constant and a scaled
Cron and Sherman coherence function. is combination can be
related to directionality by considering Eq. (3),
(6)
where is the scaled Cron and Sherman contribution,
is a scaled version of Eq. (5), Γ
0
is the frequency independent
constant coherence contribution and F
0
(
q
) is its corresponding
vertical noise directionality. Eq. (6) is linear, so we can solve for
the unknown noise directionality, as it is clear that that Γ
0
and
F
0
(
q
) are Fourier transform pairs and thus unique. erefore F
0
(
q
)
must be due to noise arriving broadside, or exactly horizontally on
the vertically separated sensors, and the total noise directionality
at the sound channel axis must be given by
(7)
where A and B are scaling constant that ensure the normaliza-
tion condition,
(8)
is satised, plus the requirement that the coherence must be unity
at zero frequency (or zero separation).
Since this horizontal contribution to the noise eld is inde-
pendent of frequency over the wind driven band of 1 – 10 kHz,
we can conclude that wind generated surface noise, not distant
shipping noise or any other narrower band source, is propagating
in the sound channel axis. Although wave induced surface sound
sources have dipole radiation patterns oriented in the downward
direction, and are therefore very inecient at coupling into the
sound channel, bathymetric features such as the continental shelf
slope, sea mounts, and islands are known to contribute surface
noise to the channel via scattering (Carey and Evans, 2011). e
Challenger Deep is 300 km from the island of Guam, providing the
appropriate bathymetry for such a scattering mechanism.
e U
10
wind conditions during the Challenger Deep deploy-
ment were light, with a mean of 4 m/s at the beginning of the
deployment, slowly rising to 8 m/s by the end of the recording
period. e contribution of locally generated surface noise was
therefore small, relative to the distantly generated noise in the
sound channel, giving rise to a noise directionality that more
heavily favors the second term in Eq. (7).
From this conclusion, can divide Fig. 4 into two regions.
Above the conjugate depth, where the noise eld is comprised
of a relatively large component due to distantly generated noise
propagating in the sound channel and a small locally generated
surface noise component, and below the conjugate depth, where
the ambient noise is solely due to locally generated surface
wave action.
Conclusions
e importance of local surface conditions in determining the
noise coherence in a deep-water environment is demonstrated
by the collection of these data sets. When winds are 10 m/s or
greater, the noise eld is dominated by downward propagating
locally generated noise, as was found in the Philippine Sea, Sirena
Deep, and Tonga Trench data. For winds lighter than 8 m/s, the
noise eld above the conjugate depth comprises mostly of a hori-
zontally propagating component of distantly generated surface
wave noise with a small downward propagating locally generated
component found both above and below the conjugate depth, as
found in the Challenger Deep. In both cases, reections from the
seaoor do not contribute signicantly to the noise eld.
e Cron and Sherman model describes the locally generated
component of the ambient noise eld which contributes with
constant power at all depths, while the distantly generated noise
can be described as a purely horizontal contribution to the noise
directional density function, or as a broadband constant value to
the vertical noise coherence.
Acknowledgements
e Schmidt Ocean Institute, UC Ship Grants, and the Oce of
Naval Research North Pacic Acoustic Laboratory (NPAL) group
provided ship time. is research was supported by the Oce of
Naval Research, Ocean Acoustics Code 322OA, under Grant Nos.
N00014-10-1-0092 and N00014-10-1-0286, sponsor Dr. Robert
Headrick, and N00014-15-1-2629, sponsor Dr. Kyle Becker.
Dr David Barclay is the Canada Research Chair in Ocean
Technology Systems in the Department of Oceanography,
Dalhousie University. He was an Oce of Naval Research post-
doctoral fellow in Ocean Acoustics at Woods Hole Oceanographic
Institution, and received a PhD. in 2011 from the Scripps Institution
of Oceanography, UC San Diego. Michael J Buckingham is a
professor of ocean acoustics in the Marine Physical Laboratory at
Scripps Institution of Oceanography, University of California, San
Diego. His research focuses on ocean acoustic propagation, ambient
noise in the marine environment, and acoustic imaging. Dieter
Bevans is a PhD student at the Scripps Institution.
References
A full list of references can be obtained from the Editor,
CharlesEllis at charles.ellis@ioa.org.uk .
Figure 6. Real part of the vertical noise coherence in the sound channel axis,
at 1 km depth in the Challenger Deep over (a) the full range of coherence,
showing the frequency independent constant coherence due to horizontally
propagating noise, and (b) over a limited, zoomed range to show the details
of the locally generated noise coherence. The data are plotted in blue and
the modelled coherence corresponding to the directionality given by Eq. (7) is
plotted in black.
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Acoustics Bulletin July/August 2017
40
... obtained from Matthew's tables of sound speed (Matthews, 1939;Carruthers and Lawford, 1952;Mantyla and Reid, 1978;Zheng, 2015), comprising a compilation of historical measurements of water column chemistry by latitude and longitude. In modern times, direct measurements of the sound speed profile over the Challenger Deep have been made with conductivity, temperature, depth (CTD) sensors mounted on either deep submersibles or free-falling vehicles (Piccard and Dietz, 1967;Bowen et al., 2009;Taira et al., 2004;Barclay et al., 2017;Dziak et al., 2017;van Haren et al., 2017) and with expendable bathythermographs (XBTs; Fujioka et al., 2002;Gardner et al., 2014). Ideally, for depth estimation, such determinations of the sound speed profile should be concurrent with the acquisition of the time-of-flight data; otherwise, significant errors can arise in the acoustic estimate of the ocean depth (Beaudoin et al., 2009). ...
... Several autonomous, deep-diving instrument platforms were deployed in the cen-tral and eastern basins of the Challenger Deep with varying scientific objectives, including water sampling and CTD profiling throughout the water column, collection and video recording of hadal amphipods, and recording the broadband (5 Hz to 30 kHz) ambient sound over the full ocean depth on vertically and horizontally aligned pairs of hydrophones. Some of the findings from the expedition have been previously reported in the popular (Nestor, 2014) and scientific (Barclay et al., 2017;Lan et al., 2017) literature. ...
Article
Full-text available
Since HMS Challenger made the first sounding in the Mariana Trench in 1875, scientists and explorers have been seeking to establish the exact location and depth of the deepest part of the ocean. The scientific consensus is that the deepest depth is situated in the Challenger Deep, an abyss in the Mariana Trench with depths greater than 10,000 m. Since1952, when HMS Challenger II, following its namesake, returned to the Mariana Trench, 20 estimates (including the one from this study) of the depth of the Challenger Deep have been made. The location and depth estimates are as diverse as the methods used to obtain them; they range from early measurements with explosives and stop watches, to single- and multi-beam sonars, to submersibles, both crewed and remotely operated. In December 2014, we participated in an expedition to the Challenger Deep onboard Schmidt Ocean Institute’s R/V Falkor and deployed two free-falling, passive-acoustic instrument platforms, each with a glass-sphere pressure housing containing system electronics. At a nominal depth of 9,000 m, one of these housings imploded, creating a highly energetic shock wave that, as recorded by the other instrument, reflected multiple times from the sea surface and seafloor. From the arrival times of these multi-path pulses at the surviving instrument, in conjunction with a concurrent measurement of the sound speed profile in the water column, we obtained a highly constrained acoustic estimate of the Challenger Deep: 10,983 ± 6 m.
ResearchGate has not been able to resolve any references for this publication.