Oc679 Acoustical Oceanography - PowerPoint PPT Presentation

Title: Oc679 Acoustical Oceanography


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• Sonar Equation
• Parameters determined by the Medium
• Transmission Loss TL
• spreading
• absorption
• Reverberation Level RL (directional, DI cant
  improve behaviour)
• Ambient-Noise Level NL (isotropic, DI improves
  behaviour)
• Parameters determined by the Equipment
• Source Level SL
• Self-Noise Level NL
• Receiver Directivity Index DI
• Detector Threshold DT (not independent)
• Parameters determined by the Target
• Target Strength TS
• Target Source Level SL

Oc679 Acoustical Oceanography
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terms are not very universal!
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• for comparison
• atmospheric pressure is 100 kPa
• pressure increases at the rate of 10 kPa per
  meter of depth from the surface down

• p/p0 dB 20log10 p/p0
• 1 0
• v2 3 (double power level)
• 2 6
• 4 12
• 10 20
• 20 26
• 100 40
• 1000 60

compare p/p01/v2, I/I0 ½ dB -3 we might say
the -3dB level or ½ power level
Oc679 Acoustical Oceanography
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1 ?Pa is equivalent to 0 dB
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1 ?Pa is the reference standard for water 20 ?Pa
is the reference standard for air
20log20 26 dB
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Scattering
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Scattering
scattering of light follows essentially the same
scattering laws as sound but light wavelengths
are much smaller than sound - O(100s of
nm) almost all scattering bodies in seawater are
large compared to optical wavelengths and have
optical cross-sections equal to their geometrical
cross-sections ? Large Targets ? the sea is
turbid to light on the other hand, acoustic
wavelengths are typically large compared to
scattering bodies found in seawater (at 300 kHz,
?? 5 mm, 4 orders of magnitude larger) -
acoustic scattering is dominated by Rayleigh
scattering ? Small Targets by comparison the sea
is transparent to sound - what limits the
propagation of 300 kHz sound is not scattering
but absorption
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TL

• Absorption
• in a homogeneous medium, a plane wave
  experiences an attenuation of acoustic pressure
  ? the original pressure and to the distance
  traveled this is represented by a bulk
  viscosity in the N-S equations which applies only
  to compressible fluid (this is distinct from a
  shear viscosity MC sec 3.4.2)
• this is due To wave absorption (energy lost to
  heat)
• 
  ?e is the amplitude decay coefficient
• absorption losses are due to ionic dissociation
  that is alternately activated and deactivated by
  sound condensation and rarefaction
• the attenuation by this manner in SW is 30x that
  in FW
• dominated by magnesium sulphate and boric acid

acoustic absorption absorption
typical profiling frequency _at_S0
_at_S35 range 3000 kHz 2.4 dB/m
2.5 dB/m 3-6 m 1500 kHz 0.60 dB/m
0.67 dB/m 15-25 m 500 kHz 0.07
dB/m 0.14 dB/m 70-110 m source
Sontek ADCP manual
Oc679 Acoustical Oceanography
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TL
10
TL
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TL
relate spherical spreading range attenuation
due to absorption in terms of sonar equation
spherical spreading ? 1/R2, that is log10
I/I0 log10 R02/R2 log10R02 log10R2 but
since typically I0 is referred to 1 m range from
source, log10R020 and 10log10 I/I0
20log10R range attenuation related by p
p0e-?R, or I I0e-2?R then 10log10 I/I0 -
20?R and together these represent a transmission
loss TL -20log10R - 20?R
absorption
spherical spreading
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TL
Spreading
Oc679 Acoustical Oceanography
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Spreading
TL

• Spreading
• Due to divergence
• No loss of energy
• Sound spread over wide area
• Two types
• Spherical
• Short Range R lt 1000m
• TL (dB) 20 log R
• Cylindrical
• Long Range R gt 1000m
• TL (dB) 10 log R 30 dB

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now consider the reflected signal from a target
with reflection coefficient R12
p is the pressure at range R R12 is the target
reflection coefficient Srec is the receiver
sensitivity ?Pa/V V is the voltage o/p
measured at receiver
0
RS RL SL TL
TS
this is the voltage measured at the receiver
more commonly this would be in counts, which is
then converted to volts
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this is the physical relationship that we have
developed
this is how that relationship is represented
logarithmically terms in the SONAR EQUATION
represent logarithms
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lets take a step back now that we have seen how
the SONAR EQUATION is formed the physics is
straightforward as is the transformation to a
logarithmic equations more straightforward than
is implied by the large number of variations to
the SONAR EQUATION that appear these are
usually specific to the application and intended
for unambiguous use by operators well look at a
couple of variations
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Passive SONAR EQUATION passive sonars listen
only purpose detection, classification and
localization of an acoustic source in turn, a
particular source is embedded in a sea of
sources suppose a radiating object of source
level SL (decibels) is received at a hydrophone
at a lower signal level S due to transmission
loss TL (TL always gt 0) S SL TL where we
have already defined TL 20log10R 20?R to be
due to the product of absorption and spreading
(which appear additively in this logarithmic
representation) and we could represent the
signal to noise ratio at the hydrophone as SNR
SL TL N logarithmic representation of
this is the logarithmic representation of
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DI directivity index
SNR can be increased by beam-forming so that
sound does not spread spherically but is more
directional for an omnidirectional source I is
proportional to 4pR2 here it is constrained to
pr2 where r might be the piston diameter of a
cylindrical source Define DI 10log(intensity
of acoustic beam /intensity of omnidirectional
source) DI 10 log ((p/pr2)/(p/4pR2))

r
r
S
R 1 m
a/2
with R 1 DI 10 log (4/r2) and tan(a)
r/R r DI 10 log (4/tan2(a/2))
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SNR can be increased by beam-forming produced by
an array of transducers (perhaps in a single head
or maybe distributed geographically) the
directivity index DI represents this advantage
for a particular array so that SNR SL TL
N DI ideally, detection is possible when the
signal is sufficiently close and not disguised by
noise that is, when SNR gt 0 however, due to
the nature of the signal, interference, the sonar
operators training and alertness, etc
something more than 0 is necessary this extra
appears as a detection threshold, DT we now
write the SONAR EQUATION in terms of a signal
excess SE SE SL TL N DI DT this is
now the difference between the actual received
signal at the output of the beamformed array and
minimum signal required for detection
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if DT set to be too high, only targets with high
source levels are detected. Detection may be
difficult but the probability of a false alarm is
low as well. On the other hand, if DT is too low,
the probability of false alarms increases
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Looking at a single trace on an oscilloscope is a
little antiquated. A time history helps to see
whats going on.
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Active SONAR EQUATION now the transducer
transmits a signal that is reflected or scattered
from an object the modified signal is sensed at
the receiver (which may be the same as the
source, in monostatic mode) this modified signal
must be extracted from the background
interference which is not only the sonar noise
and ambient noise, but also the reverberation
generated by the original signal simple example
travel-time measurement of the echo to estimate
the distance to an object (such as a fathometer
which measures water depth by listening to the
echo of a ping off the sea floor) we can simply
say that the sound pressure level SPL at range R
is SPL SL - TL
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• three principal differences from passive case
• received signal level modified by target strength
  TS
• reverberation is the dominant interference
• transmission loss results from 2 paths
• transmitter to target target to receiver
• monostatic - transmission loss is 2TL
• bistatic - transmission loss is TL1 TL2
• Reverberation Level RL
• results primarily from scattering of the
  transmitted
• signal from things other than the target of
  interest
• boundary scattering may be due to waves, ice
  bottom features
• volume scattering may be due to zooplankton,
  fish, microstructure,

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Homework 1 Martin Hoecker-Martinez 18 Jan
2011
http//wart.coas.oregonstate.edu/Documents/for20o
thers/jim/150W.gif
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• So, there are essentially two types of background
  that may mask the signal that we wish to detect
  
• Noise background or Noise Level (NL). This is an
  essentially a steady state, isotropic (equal in
  all directions) sound which is generated by
  amongst other things wind, waves, biological
  activity and shipping. This is in addition to
  transducer system self-noise. (Wenz curves)
• 2) Reverberation background or reverberation
  level (RL). This is the slowly decaying portion
  of the back-scattered sound from one's own
  acoustic input. Excellent reflectors in the form
  of the sea surface and floor bound the ocean.
  Additionally, sound may be scattered by
  particulate matter (e.g. plankton) within the
  water column. You will have experienced
  reverberation for yourself. For example if you
  shout loudly in a cave you are likely to here a
  series of echoes reverberating due to sound
  reflections from the hard rock surfaces. These
  reverberations decay rapidly with time.
• Although both types of background are generally
  present simultaneously it is common for either
  one or the other to be dominant.

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we could define SONAR EQUATION for a monostatic
system as SE SL 2TL TS (RL N) DI
DT And for a bistatic system SE SL TL1
TS TL2 (RL N) DI DT
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Sound scattered by a body - RL in SONAR
EQUATION scattering is the consequence of the
combined processes of reflection, refraction and
diffraction at surfaces marked by inhomogeneities
in ?c - these may be external or internal to a
scattering volume ( internal inhomogeneities
important when considering scattering from fish,
for example ) net result of scattering is a
redistribution of sound pressure in space
changes in both direction and amplitude the sum
total of scattering contributions from all
scatterers is termed reverberation this is heard
as a long, slowly decaying quivering tonal blast
following the ping of an active sonar
system consideration usually begins by
considering scattering from spheres
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direct signal
explosive source at 250 m nearby receiver at 40
m bottom depth 2000 m
reverberation following explosive charge initial
surface reverb is sharp, followed by tail due to
multiple reflection scattering then volume
reverb in mid-water column (incl. deep scattering
layer) then bottom reflection, 2nd surface
reflection, and long tail of bottom reverb
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sounds in the sea or N in the SONAR
EQUATION natural physical sounds natural
biological sounds ships
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Wenz curves used to determine ambient noise
thick black line empirical minimum A - seismic
noise B ship noise (shallow water) C ship
noise (deep water) H hail W sea surface
sound at 5 wind speeds R1 drizzle (1 mm/h) 0.6
m/s wind over lake R2 drizzle, 2.6 m/s wind
over lake R3 heavy rain (15 mm/h) at sea R4
v. heavy rain (100 mm/h) at sea F thermal
noise (f1) - molecular
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on-axis source level spectra of cargo ship at 8
16 kts measured directly below ship this
represents the details of what we saw in the
compilation slides B propeller Blade rate F
diesel engine Firing rate G ships service
Generator rate
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propagation of coastal shipping noise into deep
sound channel
c(z)
this is due to c(z) profile over the shelf,
causing a progression of sound down the slope
until the axis of the deep sound channel is
reached after that, a reversal of refraction
occurs, and signal trapped in sound
channel coastal shipping noise can be propagated
long distances
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Homework Assignment 2 assigned 18 Jan
2010 due 27 Jan 2010 A yellow submarine is
conducting a passive search against blue
submarines. Yellow submarines have a sonar with
directivity index of 15 dB and detection
threshold 8 dB. Blue submarines have known source
level 140 dB. Environmental conditions yield an
isotropic noise level of 65 dB. You can assume an
absorption decay coefficient 0.02 dB/km. At what
range can the blue submarine be detected by the
yellow submarine? Show your answer graphically.
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Oc679 Acoustical Oceanography