FRIDAY MORNING, 20 MAY 2005 REGENCY E, 8:30 A.M. TO 12:00 ...

FRIDAY MORNING, 20 MAY 2005
GEORGIA B, 8:30 TO 11:45 A.M.
Session 5aUW
Underwater Acoustics: Scattering and Reverberation
Purnima Ratilal, Chair
Northeastern Univ., Electrical and Computer Engineering, 409 Dana Research Ctr., Boston, MA 02115-5000
Contributed Papers
8:30
5aUW1. Identification of strong scatterer locations on the Malta
plateau. Mark K. Prior NATO Undersea Res. Ctr., Viale San
Bartolomeo 400, 19138, La Spezia, Italy, prior@saclantc.nato.int
Reverberation data gathered in the Malta Plateau region of the Mediterranean
Sea using a Low-Frequency Active Sonar LFAS and a cardioid
receiver array are presented. A processing strategy involving normalization,
geo-referencing and averaging over pings is described and is shown
to highlight the locations of strong scatterers and to provide a first-order
estimate of their strength. A second process of calculating the ratio of local
standard deviation and mean values is shown to reveal scatterer locations
and to provide an indication of whether the probability density function of
the reverberation is well-described by the Rayleigh distribution. The characteristics
of the observed scatterers are revealed by comparison with independent
sidescan, echo sounder and boomer data. Areas of high reverberation
are identified as being associated with wrecks, oil production
platforms, rock outcrops and variations in local sediment properties.
8:45
5aUW2. Range-dependent 3D scattering and reverberation in the
continental shelf environment from biology, geology and
oceanography. Purnima Ratilal Northeastern Univ., 409 Dana Res.
Ctr., Boston, MA 02115, Sunwoong Lee, Yisan Lai, Tianrun Chen,
Deanelle Symonds MIT, Cambridge, MA 02139, Ninos Donabed
Northeastern Univ., Boston, MA 02115, and Nicholas Makris MIT,
Cambridge, MA 02139
Several unified scattering and reverberation models were developed in
support of the ONR Acoustic Clutter Program. They include a rangedependent
model based on the parabolic equation that can be used to
efficiently model scattering from a random spatial distribution of random
targets that obey the sonar equation in the waveguide and a similar but
range-independent waveguide model based on normal modes for scattering
from extended objects. Both these models are bistatic and fully 3D,
and the latter model also accounts for modal coupling between propagation
and scattering caused by extended objects in the waveguide. These
models are applied to examine both coherent and diffuse scattering measured
after beamforming and match-filtering on an array from schools of
fish, plankton, volume inhomogeneities in the sea bottom, roughness on
the seafloor, extended seafloor and sub-bottom features such as river channels
and reflective strata, and internal and surface waves. We provide a
review of the dominant sources of clutter as well as background reverberation
for long range active sonar based on comparison of model predictions
with measured data from the Acoustic Clutter Experiments of 2001 and
2003.
9:00
5aUW3. Dominant source of background reverberation during the
Acoustic Clutter 2003 experiment. Ninos Donabed, Purnima Ratilal
Northeastern Univ., 409 Dana Res. Ctr., Boston, MA 02115, and
Nicholas Makris MIT, Cambridge, MA 02139
Reverberation data from the Acoustic Clutter 2003 experiment on the
New Jersey Continental shelf are analyzed to provide estimates of the
background scattering strength over wide areas of the environment as a
function of time. We corrected the acoustic data in the 390–440 Hz range
for two-way transmission loss modeled using the range-dependent parabolic
equation, the spatially varying beampattern of the receiving array,
and source level to obtain estimates of the scattering strength per unit area
of the environment. Data from regions of space devoid of strong clutter
were statistically analyzed to provide a temporal and spatial characterization
of the background scattering strength. They were then compared to
scattering strength levels deduced from modeling from a variety of scatterers
including schools of fish, the sea bottom, internal waves and surface
waves. Analysis indicates that the dominant sources of background reverberation
can be either low density fish schools or the sea bottom.
9:15
5aUW4. Higher moment estimation for shallow water reverberation.
Kevin LePage Naval Res. Lab., Code 7144, 4555 Overlook Ave. SW,
Washington, DC 20375
Shallow water reverberation is characterized by clutter which causes
false alarms in sonar systems. In previous work we have estimated the
non-Rayleighness of shallow water reverberation time series predictions
obtained by the R-SNAP model. Here we extend the R-SNAP model to the
closed form estimation of the second moment of reverberation intensity,
allowing the estimation of pdf fits for the two parameter K distribution as
a function of system and channel characteristics and time after shot. This
work is intended to theoretically guide the selection of clutter-robust system
characteristics. Work supported by ONR.
9:30
5aUW5. Statistical characterization of sonar-like clutter observed on
the STRATAFORM during the 2003 Acoustic Clutter Experiment in
the 400–1500 Hz region. John Preston and Douglas Abraham Appl.
Res. Lab., The Penn State Univ., State College, PA 16804
In 2003 ONR sponsored the Acoustic Clutter Experiment to study
shallow water scattering and clutter in the STRATAFORM area off New
Jersey. Sources were bistatic coherent pulses from a vertical array. The
receiver was the Five Octave Research Array used horizontally. The
STRATAFORM is known to have benign surface morphology but contains
many buried river channels and other sub-surface horizons. MIT researchers
have shown fish to be a primary source of the observed clutter and
reverberation. The K-distribution’s shape and scale parameters have been
shown to be useful in describing non-Rayleigh behavior. Statistical characterization
is presented as a function of location. The ‘‘bandwidth’’ effect
is shown where the shape parameter first decreases inversely proportional
to bandwidth but then increases in a trend back toward the Rayleigh distribution
at higher bandwidths. The shape parameter estimates are well fit
by an elongated patch model of Abraham and Lyons. Differences between
the 2003 and 2001 data taken in the same area are discussed. It is believed
that the sound speed profiles in 2003 lead to more bottom interaction than
in 2001 producing a larger time spread in the multipaths leading to observed
differences. Work supported by ONR Code 32, Grant N00014-05-
1-0156.
5a FRI. AM
2611 J. Acoust. Soc. Am., Vol. 117, No. 4, Pt. 2, April 2005 149th Meeting: Acoustical Society of America 2611