Fonseca 07 MGR Remote Estimation of Surficial Seafloor Properties Through Angular Range Analysis PDF
Fonseca 07 MGR Remote Estimation of Surficial Seafloor Properties Through Angular Range Analysis PDF
Fonseca 07 MGR Remote Estimation of Surficial Seafloor Properties Through Angular Range Analysis PDF
DOI 10.1007/s11001-007-9019-4
Received: 27 September 2005 / Accepted: 17 April 2007 / Published online: 19 June 2007
Springer Science+Business Media B.V. 2007
Introduction
The remote characterization of the seafloor by acoustic
methods has important practical applications in a broad
range of disciplines, including marine geologic, geotechnical, hydrographic, biological, fisheries and environmental
research (Hughes-Clarke et al. 1996). Examples of seafloor
acoustical and physical properties that we would hope to
estimate remotely are the grain size, acoustic impedance
(product of density and sound speed), acoustic attenuation
and the roughness of the near-surface sediments. Unfortunately, these properties are not normally measured directly
by remote sensing methods. Instead, we have to rely on
measurements of other properties (e.g., depth, acoustic
backscatter), and estimate the values of the desired seafloor
properties by means of theoretical or empirical models.
Multibeam sonars provide us with coincident measurements of depth and acoustic backscatter over a large swath
of the seafloor and thus offer a promising tool for seafloor
characterization.
The acoustic backscatter returned to a multibeam sonar
is the result of a complex interaction of the acoustic
wavefront with an often rough and inhomogeneous
seafloor. The wavefront from a typical multibeam sonar
system usually intersects the seafloor at an angle, and is
subject to scattering, which redistributes the incident
acoustic energy in multiple directions. The nature of the
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Model inversion
The direct inversion of acoustic backscatter for key physical properties is an ill-posed problem, in the sense that a
solution may not be unique or may not even exit. In order
overcome this limitation, we applied a constrained iterative
inversion of the model, imposing constraints based on well
established inter-relations for sediment physical properties
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in situ measurements. (b) Index of impedance (sediment bulk density sound speed ratio), draped over sun-illuminated bathymetry, color
coded with low index in blue and high index in red. (c) Roughness, rms
height in cm, color coded and draped over sun-illuminated bathymetry
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R2=0.88
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0.98 1.00 1.02 1.04 1.06 1.08 1.10 1.12 1.14 1.16 1.18 1.20
Fig. 5 Remotely estimated acoustic impedance versus in situ measurements of sound speed. The location of the in situ measurements are
shown in Fig. 4a. Note the very good linear correlations (R2 = 0.88)
Conclusions
The Angular Range Analysis of multibeam sonar data is a
promising technique for acoustic seafloor characterization.
This technique was successfully applied to the Simrad
EM3000 multibeam sonar data from Little Bay. The remotely
estimated impedance was compared to in situ measurements
of sound speed, indicating a strong correlation between these
two acoustic parameters. Additional field work is required to
include a larger sample of sediment types. The key to the
success of this approach is the collection of radiometrically
calibrated and geometrically corrected acoustic backscatter
data in conjunction with a well-defined model for the interaction of sound with the seafloor. As our understanding of the
interaction of sound with the seafloor improves, the proposed
technique, which is in principle independent of the underlying
model, can easily incorporate new modeling approaches.
Acknowledgement This research was supported by the Office of
Naval Research under the GEOCLUTTER program.
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References
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