Applied Ocean Physics and Engineering (AOP&E)
http://hdl.handle.net/1912/10
2014-04-17T06:58:08Z
2014-04-17T06:58:08Z
Internal wave effects on acoustic propagation
Duda, Timothy F.
http://hdl.handle.net/1912/6550
2014-04-12T08:46:38Z
2005-06-01T00:00:00Z
Internal wave effects on acoustic propagation
Duda, Timothy F.
Internal gravity wave induced acoustic fluctuations are reviewed. It is well known
that internal waves cause temporal and spatial variability above and beyond that caused by
mesoscale and larger ocean heterogeneity. Increasingly detailed work over many decades
has shown that deep-ocean internal waves, which have often been parameterized using the
Garrett-Munk spectrum as a guide, are responsible for rapid acoustic field variability at all
propagation ranges. Numerous experiments have shown that various sections of wavefronts
from impulsive sources have fluctuation qualities well-described by theory, simulation, or
both. In contrast, fluctuations in shallow water experiments, although known to be consistent
with those expected from internal waves via theoretical and simulation arguments, are
incompletely described by theories for a number of reasons. These reasons include
nonstationary, inhomogeneous or anisotropic wave environments, unknown geoacoustic
properties, and rapidly changing background currents, all of which prevent detailed
comparison of observation and prediction. At this time, many different shallow-water
internal wave scenarios give rise to similar field fluctuations, within reasonable confidence
intervals for the predictions. This may simplify order-of magnitude fluctuation prediction,
while simultaneously making inversion and highly-detailed prediction problematic.
Underwater Acoustics Measurements (UAM) 1st International Conference “Underwater Acoustic Measurements: Technologies & Results,” 28 June- 1 July 2005, Heraklion, Crete, Greece,
2005-06-01T00:00:00Z
Acoustic field coherence in four-dimensionally variable shallow water environments : estimation using co-located horizontal and vertical line arrays
Duda, Timothy F.
Collis, Jon M.
http://hdl.handle.net/1912/6549
2014-04-12T08:46:39Z
2007-06-01T00:00:00Z
Acoustic field coherence in four-dimensionally variable shallow water environments : estimation using co-located horizontal and vertical line arrays
Duda, Timothy F.; Collis, Jon M.
The implementation of two- and three-dimensional acoustic receiver arrays is
challenging in the ocean environment. Fixed geometry and connectivity can only be built
and maintained at great expense. However, such ideal arrays can be very powerful for
signal detection, classification, and tracking, although many of the signal-processing
methods employed are subject to constraints of acoustic field temporal and spatial
coherence. Thus, understanding the processes at work determining coherence is essential
because system effectiveness may then be predictable from environmental parameter
input. To study acoustic fields and coherence over finite aperture, the research
community has recently taken steps to enable routine use of co-located horizontal and
vertical line arrays, typically arranged in the shape of the Roman letter L (or Greek ),
with the horizontal leg on the seafloor. This is a small subset of all possible geometries,
but it enables measurements of acoustic field coherence not possible with single line
arrays. Here, new L-array measurements made in the SW06 field program are used to
measure coherence and test coherence predictions via joint analysis of vertical and
horizontal line array receptions. Impulsive mode arrivals (including mode multipath)
from fixed sources will be estimated using the vertical array. Signals on the horizontal
array, which is neither broadside nor endfire, will be compared with signals synthesized
using the mode arrivals to estimate azimuthal decorrelation effects. Results can be
compared with coherence estimates from computational and theoretical studies.
Underwater Acoustic Measurements (UAM) 2nd International Conference & Exhibition on "Underwater Acoustic Measurements: Technologies & Results," 25-29 June 2007, Heraklion, Crete, Greece.
2007-06-01T00:00:00Z
Theory and observation of anisotropic and episodic internal wave effects on 100-400 Hz sound
Duda, Timothy F.
http://hdl.handle.net/1912/6548
2014-04-12T08:46:40Z
2011-06-01T00:00:00Z
Theory and observation of anisotropic and episodic internal wave effects on 100-400 Hz sound
Duda, Timothy F.
Propagation of sound through shallow-water internal waves of various types
is discussed. The anisotropy of the waves imparts an anisotropy to their effects on sound.
The internal waves are of two types: Long-wavelength internal tides and short-wavelength
high-frequency waves. On the continental shelf both types of waves tend to move
shoreward from deep water (i.e. have anisotropic motion and anisotropic correlation
scales). The internal tides are less predictable than the surface tides that generate them.
The short-wavelength nonlinear internal waves are also somewhat unpredictable, and
also have anisotropic correlation scales, having crests of tens of kilometres in length but
wavelengths of order 300 to 1000 m. Coupled-mode propagation dominates across-shelf
sound propagation, which in the direction of short internal wave correlation scale.
Refracted-mode propagation dominates along-shelf propagation. Data from two sea
exercises illustrate the character of the waves and their effects on sound.
Underwater Acoustic Measurements (UAM) 4th International Conference and Exhibition on "Underwater Acoustic Measurements: Technologies & Results." 20-24 June 2011, Kos, Greece
2011-06-01T00:00:00Z
Robust magnetotelluric inversion
Matsuno, Tetsuo
Chave, Alan D.
Jones, Alan G.
Muller, Mark R.
Evans, Rob L.
http://hdl.handle.net/1912/6534
2014-04-03T08:45:45Z
2014-01-02T00:00:00Z
Robust magnetotelluric inversion
Matsuno, Tetsuo; Chave, Alan D.; Jones, Alan G.; Muller, Mark R.; Evans, Rob L.
A robust magnetotelluric (MT) inversion algorithm has been developed on the basis of quantile-quantile (q-q) plotting with confidence band and statistical modelling of inversion residuals for the MT response function (apparent resistivity and phase). Once outliers in the inversion residuals are detected in the q-q plot with the confidence band and the statistical modelling with the Akaike information criterion, they are excluded from the inversion data set and a subsequent inversion is implemented with the culled data set. The exclusion of outliers and the subsequent inversion is repeated until the q-q plot is substantially linear within the confidence band, outliers predicted by the statistical modelling are unchanged from the prior inversion, and the misfit statistic is unchanged at a target level. The robust inversion algorithm was applied to synthetic data generated from a simple 2-D model and observational data from a 2-D transect in southern Africa. Outliers in the synthetic data, which come from extreme values added to the synthetic responses, produced spurious features in inversion models, but were detected by the robust algorithm and excluded to retrieve the true model. An application of the robust inversion algorithm to the field data demonstrates that the method is useful for data clean-up of outliers, which could include model as well as data inconsistency (for example, inability to fit a 2-D model to a 3-D data set), during inversion and for objectively obtaining a robust and optimal model. The present statistical method is available irrespective of the dimensionality of target structures (hence 2-D and 3-D structures) and of isotropy or anisotropy, and can operate as an external process to any inversion algorithm without modifications to the inversion program.
Author Posting. © The Author(s), 2014. This article is posted here by permission of The Royal Astronomical Society for personal use, not for redistribution. The definitive version was published in Geophysical Journal International 196 (2014): 1365-1374, doi:10.1093/gji/ggt484.
2014-01-02T00:00:00Z