Lowfrequency Wave Properties of Marine Sediments from Aircraft Noise

1
Low-frequency Wave Properties of Marine Sediments
from Aircraft Noise

• Eric M. Giddens
• Marine Physical Laboratory
• Scripps Institution of Oceanography

2

• Motivation
• Why study marine sediment wave-properties
• Why is there a need for in situ low-frequency
  measurements
• Goals
• Obtain wave properties of marine sediments
• Methods
• Aircraft as a sound source
• In situ Using single buried sensor
• Water column Using vertical array beamforming
• Work Completed
• Work Proposed
• Acknowledgments Works Cited

3
Why Study Sediment Acoustics?

• The sediment boundary greatly affects sound
  propagation in the water column
• The sediment wave properties greatly affect
  acoustic penetration into the sediment

4
c 1600 m/s
c 1800 m/s
5
Controversy in Sediment Acoustic Propagation
Models

• Biot model1 (1956) fits dispersion data, but not
  attenuation data
• Buckingham2 model fits attenuation data, but not
  dispersion
• Uncertainty in in situ measurements for both
  dispersion and attenuation at low-frequency3

6
Goals

• Obtain estimates of the following sediment
  properties using aircraft noise
• Sound Speed
• Dispersion/Attenuation
• Density/Porosity
• Use two methods
• In situ single buried hydrophone
• Water column vertical line array

7
Aircraft as a Sound Source

• Periodic sound signature from engine and
  propeller gives tonal set with usable frequency
  range from ? 80-1000 Hz4
• Inexpensive and highly mobile source of low
  frequency sound
• Motion of aircraft gives characteristic Doppler
  shift

8
In situ measurements

• Use a buried hydrophone to measure sound
  directly in the sediment
• Sound speed and dispersion from Doppler shift
• Attenuation from received levels as a function of
  path-length in the sediment

9
Sound Speed Using Doppler Shift5
Only good for a fast bottom!
10
Doppler Shift Received at a Sensor
11
Dispersion

• Determine sound speed for each harmonic
• Spans almost a decade of frequency
• Provides a point in the data set for the
  low-frequency region

12
Attenuation

• Comparison of received signal with two different
  path-lengths in sediment
• Direct overflight
• (maximum Doppler shifted region)
• Circular flight
• (center frequency)

13
Water Column Measurements

• Use coherent processing on a vertical line array
  to measure the Reflection coefficient as a
  function of grazing angle6, 7
• Sound speed and dispersion from critical angle
• Attenuation from curve below critical angle
• Density/porosity from normal incidence

14
Simple 2-layer Reflection Coefficient
-figure adapted from 8
15
In situ method

• Work Completed
• Development of theory5,12,13,14,15
• Series of 5 flying experiments verifying acoustic
  signals detectable in air/water/sediment
• Preliminary estimate of sound speed in
  air/water/sediment

16
Test Experiments

• Series of 5 experiments were conducted off the
  coast of La Jolla to test the feasibility of an
  aircraft as a source for underwater acoustics
  experiments
• Needed to determine the potential for received
  signals in the water and the sediment

17
(No Transcript)
18
Data from July 2, 2002

• Took temperature and pressure profile (Sea-Bird
  TP Profiler SBE 39)
• Microphone 1 m above the air/sea interface
• 7 Hydrophones spanning much of the 15 m water
  column
• Buried hydrophone

19
Some Preliminary Resultsfor Sediment Sound Speeds

• Used minimization technique with a cost function
  that maximized power along Doppler shift curve
• Started with microphone data get v, h, ca, t0
  and f0
• Proceeded to water column and sediment to get
  cw,cs

20
Application of Minimization Technique to Air Data

• Microphone data
• Predicts average sound speed in air (342.3 m/s)
  consistent with temperature conditions (343.5m/s)
• Showed a flight direction bias of about 5 m/s,
  consistent with a wind effect (verified in GPS
  data)
• Average aircraft velocity (54.5 m/s) in good
  agreement with average velocity from GPS data
  (54.8 m/s)

21
Water and Sediment Data

• Water data
• Acoustic data
• c 1529.5 m/s
• ? 23.4 m/s
• Sea-Bird data
• c 1512.4 m/s
• Sediment data
• Acoustic data
• c 1649 m/s
• ? 23.6 m/s

22
In situ method

• Work Proposed
• Improve Doppler fit analysis
• Obtain additional data with an emphasis on
  reduced noise
• Estimate dispersion/attenuation
• Simulated experiment using numerical model

23
Water column method

• Work completed
• Obtained experience in array signal processing
  and acoustic propagation models as well as
  Reflection coefficient measurements11
• Developed potential method for source
  cancellation with small number of snapshots10
• Developed method for deploying vertical array
  while minimizing tilt
• Obtained data set using array with assorted
  hydrophones

24
Normal-Incidence Reflection Coefficient
Measurements

• SAX 99 site
• Measured direct and reflected signals at 10,20
  30 kHz
• Presented at ASA Dec. 2001

25
Source Cancellation with Limited Snapshots
26
Water column method

• Work proposed
• Obtain additional data with an emphasis on
  reducing noise
• Simulated experiment using numerical model
• Use beamforming to obtain an estimate of the
  Reflection coefficient as a function of grazing
  angle
• Use critical angle to estimate sediment sound
  speed
• Use normal incidence to estimate density
• Use Reflection coefficient below grazing angle to
  get an estimate of attenuation
• Use critical angle as a function of frequency to
  estimate dispersion

27
Reduced Noise Using ITC Nested Vertical Array

• Ordered 11 element nested array from ITC
• Single cable should significantly reduce noise
  and facilitate deployment
• 4 nested sets of 5 elements
• 12 m aperture
• ?/2 spacing for 250, 500, 1000 2000 Hz
• Built-in tilt sensor and compass

28
Simulated Experiments

• Use a spectral method that incorporates source
  motion9
• Completed model that can handle
  air/water/sediment with arbitrary sound speeds
• Need to incorporate source motion

29
Summary

• Experiments have demonstrated that an aircraft is
  a viable low-frequency sound source for
  underwater/sediment acoustic experiments
• Have Developed a theory for in situ measurements
  of sediment sound speed and obtained preliminary
  results
• Plan to pursue additional sediment wave property
  estimates using in situ method
• Plan to pursue water column measurements using a
  vertical line array to estimate sediment wave
  properties
• Plan to compare results from the two methods

30
Acknowledgments
Thomas Hahn
Michael Buckingham
ARCS Foundation
Fernando Simonet
ONR
31
Works Cited

• 1. M. A. Biot, "Theory of propagation of elastic
  waves in a fluid-saturated porous solid I.
  Low-frequency range,," J. Acoust. Soc. Am. 28
  (2), 168-178 (1956).
• 2. M. J. Buckingham, "Wave propagation, stress
  relaxation, and grain-to-grain shearing in
  saturated, unconsolidated marine sediments," J.
  Acoust. Soc. Am. 108, 2796-2815 (2000).
• 3. K. L. Williams, D. R. Jackson, E.I. Thorsos,
  D. Tang, and S. G. Schock, Comparison of sound
  speed and attenuation measured in a sandy
  sediment to predictions based on the Biot theory
  of porous media, IEEE J. Ocean. Eng., vol. 27,
  pp. 413-428, July 2002
• 4. B. Magliozzi, D. B. Hanson, and R. K. Amiet,
  in Aeroacoustics of flight vehicles, edited by H.
  H. Hubbard (Acoustical Society of America,
  Hampton, Virginia, 1995), Vol. 1, pp. 391-447.
• 5. M. J. Buckingham, E. M. Giddens, F. Simonet,
  T. R. Hahn, Propeller noise from a light
  aircraft for low-frequency measurements of the
  speed of sound in a marine sediment, J. Comp.
  Acoustics, vol. 10, No. 4, In Press
• 6. C. H. Harrison, D. G. Simons, Geoacoustic
  inversions of ambient noise A simple method, J.
  Acoust. Soc. Am. 112 (4) 1377-1389 (2002)
• 7. M. A. Ainslie, Reflection and transmission
  coefficients for a layered fluid sediment
  overlying a uniform solid substrate, J. Acoust.
  Soc. Am. 99 (2), 893-902 (1996)
• 8. F. B. Jensen, W. A. Kuperman, M. B. Porter et
  al., Computational Ocean Acoustics, 2nd ed.
  (American Institute of Physics, New York, 1994).
• 9. H. Schmidt and W. A. Kuperman, "Spectral and
  modal representations of the Doppler-shifted
  field in ocean waveguides," J. Acoust. Soc. Am.
  96 (1), 386-395 (1994).
• 10. E. M. Giddens, P. Roux, W. A. Kuperman.
  Identifying individual sources in Matched-Field
  Processing by modifying the covariance matrix,
  to be presented at ASA conference, Dec. 2002.
• 11. E. M. Giddens, T. K. Berger, Iain Clark and
  M. J. Buckingham, Normal-incidence measurements
  of two sand-sediments in the Gulf of Mexico,
  presentation, ASA conference, Dec. 2001.
• 12. E. M. Giddens, F. Simonet, T. R. Hahn and M.
  J. Buckingham, Sound from a light aircraft for
  underwater acoustic inversions, to be presented
  at ASA conference, Dec. 2002.
• 13. M. J. Buckingham, E. M. Giddens, J. B. Pompa
  et al., "A light aircraft as a source of sound
  for performing geo-acoustic inversions of the sea
  bed," in Proceedings of the Sixth European
  Conference on Underwater Acoustics, edited by A.
  Stepnowski, (Gdansk University of Technology,
  Gdansk, 2002), pp. 465-470.
• 14. M. J. Buckingham, E. M. Giddens, J. B. Pompa
  et al., "An airborne sound source and an ocean
  receiver for remote sensing of the sea bed," in
  Proceedings of the Sixth Pan Ocean Remote Sensing
  Conference (PORSEC), edited by B. P. Pasaribu, R.
  Kaswadji, I. W. Nurjaya et al., (PORSEC 2002
  Secretariat, Bali, 2002) Vol. 1, pp. 203-207.
• 15. M. J. Buckingham, E. M. Giddens, F. Simonet
  et al., "Wave properties of sediments determined
  using the sound of a light aircraft," in
  Proceedings of the International Conference on
  Sonar-Sensors and Systems(ICONS-2002), edited by
  H. R. S. Sastry, (Cochin, 2002) Vol. in press.