The deep ocean temperature derived from CTBT hydroacoustic recordings of deterministic transient signals and ambient acoustic noise
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The IMS hydroacoustic stations
H02
H06
H03
H05
H07
H10
H09
H04
H08
H01
H11
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Hydroacoustic stations consist of underwater hydrophone arrays (red) and T-phase stations (gray)
Hydrophone arrays consist of three elements, located in the deep ocean
T-phase stations are seismometers on land, on steep-sloped islands, to measure hydro-seismic coupled wave
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The Sound Fixing and Ranging SOFAR Channel
I Sound speed profile shaped by pressure and temperature gradients
I Low velocity layer due to minimum in sound speed
I Discovered in the 1940s, important in underwater detection and communication
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Sound propagation in the SOFAR Channel
I Efficient waveguide for low frequency sound waves
I 40 kg of explosives can be detected at 16,000 km
I Wide variety of sources: seismic exploration, mammals, icebergs, earthquakes, underwater volcanoes
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H10 Ascencion Island and earthquakes
20˚
0˚
-20˚
-40˚
-60˚
80˚
60˚
40˚
H10
280˚ 300˚ 320˚ 340˚ 0˚ 20˚
2000
1000
0
-1000
-2000
N3
N2 H10N
N1
-1500 0 1500 x(m)
H10S
S1
S3
S2
-1500 0 1500 x(m)
H10N
H10S km
0 10 20
I H10 consists of two triplet arrays (H10N and H10S)
I Earthquakes from the Mid-Atlantic Ridge, Puerto Rico Trench and South Sandwich Islands
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Deterministic transient signals from earthquakes
Ocean surface
SOFAR channel
Ocean floor
Hydrophone
Earthquake
I Time and location of underwater earthquakes are known from seismic recordings
I Earthquakes near spreading ridges, like those at the
Mid-Atlantic Ridge, have similar source mechanisms and shallow depths of about 3 km
I Traveltime ( t ) and distance ( d ) give propagation velocity
( v = t / d ), which is a function of temperature ( T )
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Temperature difference from earthquake data
1510
1500
1490
1480
1470
1460
1450
−60 −40 −20 0
Source latitude(deg)
20 40 60
I Error due to inaccuracy in earthquake location and time
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High velocity (warm) in the North Atlantic, low (cold) in the southern part
10 m/s difference corresponds to about 3
◦
C temperature difference
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Interferometry with ambient noise
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Few deterministic transient sources in oceans with ground-truth
Use the ambient acoustic noise field to retrieve propagation velocity and hence temperature
Interferometry: the lag time between a pair sensors can be retrieved from cross correlating the ambient noise field
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Interferometry with H10 hydrophones
I Hydrophone N2 and S2 are used for cross-correlations
I With: distance of about
126 km at 800m depth, over 9 years of data
(2005-2014)
I Are correlations in the ambient noise present to determine temperature over such long ranges?
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Cross correlation with H10 hydrophones
I There is coherence in the ambient noise field over 100+ km.
Promising!
I Cross correlations show distinctive peak at a lag time of 85.5s
I Acoustic velocities of 1477 m/s are retrieved, corresponding to about 3
◦
C at 800m depth
I Variations as a function of time (seasonal, long term trends,..) to be investigated
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Concluding remarks
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IMS hydroacoustic station are a source of excellent data
Both deterministic transient signal, e.g., earthquakes, and ambient noise can be used to probe the deep ocean temperature
Continuous records over long periods are rare and important for understanding the changing Earth’s environment
Future research will concentrate on refining methodologies and retrieving global temperature variations at depths where few in-situ measurements are possible
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