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  • 1
    ISSN: 1573-8868
    Keywords: simulation ; conditional simulation ; fourier methods ; band-limited fractal ; variogram ; fast kriging
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Mathematics
    Notes: Abstract We evaluate the performance and statistical accuracy of the fast Fourier transform method for unconditional and conditional simulation. The method is applied under difficult but realistic circumstances of a large field (1001 by 1001 points) with abundant conditioning criteria and a band limited, anisotropic, fractal-based statistical characterization (the von Kármán model). The simple Fourier unconditional simulation is conducted by Fourier transform of the amplitude spectrum model, sampled on a discrete grid, multiplied by a random phase spectrum. Although computationally efficient, this method failed to adequately match the intended statistical model at small scales because of sinc-function convolution. Attempts to alleviate this problem through the “covariance” method (computing the amplitude spectrum by taking the square root of the discrete Fourier transform of the covariance function) created artifacts and spurious high wavenumber content. A modified Fourier method, consisting of pre-aliasing the wavenumber spectrum, satisfactorily remedies sinc smoothing. Conditional simulations using Fourier-based methods require several processing stages, including a smooth interpolation of the differential between conditioning data and an unconditional simulation. Although kriging is the ideal method for this step, it can take prohibitively long where the number of conditions is large. Here we develop a fast, approximate kriging methodology, consisting of coarse kriging followed by faster methods of interpolation. Though less accurate than full kriging, this fast kriging does not produce visually evident artifacts or adversely affect the a posteriori statistics of the Fourier conditional simulation.
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  • 2
    ISSN: 1573-8868
    Keywords: stratigraphy ; bathymetry ; conditional simulation ; spectral modeling ; coherence modeling
    Source: Springer Online Journal Archives 1860-2000
    Topics: Geosciences , Mathematics
    Notes: Abstract Stratigraphic modeling based on physical and geologic principles has been improved by more sophisticated process models and increased computer power. However, such efforts may reach a limit in their predictive power because of the stochastic, multiscaled nature of the physical processes involved. Building on techniques from the geostatistical literature, a conditional simulation method, dubbed “SimStrat,” has been developed to improve predictions of stratigraphic architecture from limited data. No physical processes are invoked. Rather, the prediction is based solely on geometric and statistical principals. The method takes as input sonar bathymetry, seismically defined stratigraphic horizons, and core-defined horizons. Each stratigraphic horizon is characterized using spectral modeling and coherence modeling for adjacent horizons. Predictions of subsurface horizons are improved where seafloor bathymetry conforms with the underlying strata. Conditional simulations can then be generated that conform to available data constraints and statistical characterization. Tests with synthetic data in one and two dimensions for differing spectral models confirm the reliability of the method.
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  • 3
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 1997. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 102, no. B7 (1997): 15447–15462, doi:10.1029/97JB00723.
    Description: The results of a multiscale spectral analysis of bathymetric data on the flank of the Mid-Atlantic Ridge are described. Data were collected during two cruises using Hydrosweep multibeam (tens of kilometers to ∼0.2 km scale range) and Mesotech scanning pencil-beam sonar attached to remotely operated vehicle Jason (∼1 km to ∼0.5 m scale range). These data are augmented by visual data which enabled us to identify bathymetric profiles which are over unsedimented or thinly sedimented crust. Our analysis, therefore, is focused primarily on statistical characterization of basement morphology. Work is concentrated at two sites: site B on ∼24 Ma crust in an outside-corner setting, and site D on ∼3 Ma crust in an inside-corner setting. At site B we find that an anisotropic, band-limited fractal model (i.e., the “von Kármán” model proposed for abyssal hill morphology by Goff and Jordan [1988]) is not sufficient to describe the full range of scales observed in this study. Our observations differ from this model in two ways: (1) strike and cross-strike (dip) spectral properties converge for wavelengths smaller than ∼300 m, and (2) in both strike and dip directions the fractal dimension changes at ∼10 m wavelength, from ∼1.27 at larger scales to ∼1.0 at smaller scales. The convergence of strike and dip spectral properties appears to be associated with destruction of ridge-parallel fault scarps by mass wasting, which develops canyon-like incisions that cross scarps at high angles. The change in fractal dimension at ∼10 m scale appears to be related to a minimum spacing of significant slope breaks associated with scarps which are created by faulting and mass wasting. At site D, although there is no significant abyssal hill anisotropy, the spectral properties at all scales are consistent with the von Kármán model. The fractal dimension at this site (∼1.15) is less than at site B. This difference may be reflect different morphology related to crustal formation at inside-corner versus outside-corner position or, more likely, differences in the degree of mass wasting. The smoothing of seafloor morphology by sediments is evident in Hydrosweep periodograms where, relative to basement roughness, spectral power decreases progressively with decreasing wavelength.
    Description: This work was supported under ONR grants N00014-94-1-0197 and N00014-96-1-0462 (J.A.G.) and N00014-90-J-1621 and N00014-94-1-0466 (B.E.T.).
    Repository Name: Woods Hole Open Access Server
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  • 4
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 1997. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 102, no. B5 (1997): 10203–10223, doi:10.1029/96JB03896.
    Description: We conducted a detailed geological-geophysical survey of the west flank of the Mid-Atlantic Ridge between 25°25′N and 27°10′N and from the ridge axis out to 29 Ma crust, acquiring Hydrosweep multibeam bathymetry, HAWAII MR1 sidescan-sonar imagery, gravity, magnetics, and single-channel seismic reflection profiles. The survey covered all or part of nine spreading segments bounded by mostly nontransform, right-stepping discontinuities which are subparallel to flow lines but which migrated independently of one another. Some discontinuities alternated between small right- and left-stepping offsets or exhibited zero offset for up to 3–4 m.y. Despite these changes, the spreading segments have been long-lived and extend 20 m.y. or more across isochrons. A large shift (∼9°) in relative plate motion about 24–22 Ma caused significant changes in segmentation pattern. The nature of this plate-boundary response, together with the persistence of segments through periods of zero offset at their bounding discontinuities, suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt. Within segments, there are distinct differences in seafloor depth, morphology, residual mantle Bouguer gravity anomaly, and apparent crustal thickness between inside-corner and outside-corner crust. This demands fundamentally asymmetric crustal accretion and extension across the ridge axis, which we attribute to low-angle, detachment faulting near segment ends. Cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal-thickness changes of at least 1–2 km every 2–3 m.y. These are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension, controlled by retention and quasiperiodic release of melt from the upwelling mantle. Detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes, creating crustal domes analogous to “turtleback” metamorphic core complexes that are formed by low-angle, detachment faulting in subaerial extensional environments.
    Description: This research was supported by ONR grants N00014-90-J-1621 and N00014-94-1-0466 and NSF grant OCE-9300708.
    Repository Name: Woods Hole Open Access Server
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  • 5
    Publication Date: 2017-01-04
    Description: © The Author(s), 2015. This is the author's version of the work. It is posted here by permission of Elsevier for personal use, not for redistribution.. The definitive version was published in Continental Shelf Research 98 (2015): 13-25, doi:10.1016/j.csr.2015.03.001.
    Description: We investigate the impact of superstorm Sandy on the lower shoreface and inner shelf offshore the barrier island system of Fire Island, NY using before-and-after surveys involving swath bathymetry, backscatter and CHIRP acoustic reflection data. As sea level rises over the long term, the shoreface and inner shelf are eroded as barrier islands migrate landward; large storms like Sandy are thought to be a primary driver of this largely evolutionary process. The “before” data were collected in 2011 by the U.S. Geological Survey as part of a long-term investigation of the Fire Island barrier system. The “after” data were collected in January, 2013, ~two months after the storm. Surprisingly, no widespread erosional event was observed. Rather, the primary impact of Sandy on the shoreface and inner shelf was to force migration of major bedforms (sand ridges and sorted bedforms) 10’s of meters WSW alongshore, decreasing in migration distance with increasing water depth. Although greater in rate, this migratory behavior is no different than observations made over the 15-year span prior to the 2011 survey. Stratigraphic observations of buried, offshore-thinning fluvial channels indicate that long-term erosion of older sediments is focused in water depths ranging from the base of the shoreface (~13-16 m) to ~21 m on the inner shelf, which is coincident with the range of depth over which sand ridges and sorted bedforms migrated in response to Sandy. We hypothesize that bedform migration regulates erosion over these water depths and controls the formation of a widely observed transgressive ravinement; focusing erosion of older material occurs at the base of the stoss (upcurrent) flank of the bedforms. Secondary storm impacts include the formation of ephemeral hummocky bedforms and the deposition of a mud event layer.
    Description: This work was funded primarily by a rapid response grant from the Jackson School of Geosciences, The University of Texas/Austin
    Keywords: Superstorm Sandy ; Sand ridges ; Sorted bedforms ; Shoreface ; Inner shelf ; Ravinement
    Repository Name: Woods Hole Open Access Server
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © Oceanography Society, 2007. This article is posted here by permission of Oceanography Society for personal use, not for redistribution. The definitive version was published in Oceanography 20, 4 (2007): 156-167.
    Description: Since the end of the Cold War, the US Navy has had an increasing interest in continental shelves and slopes as operational areas. To work in such areas requires a good understanding of ocean acoustics, coastal physical oceanography, and, in the modern era, autonomous underwater vehicle (AUV) operations.
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 1995. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 100, no. B11 (1995): 22509–22522, doi:10.1029/95JB02510.
    Description: A recent cruise to the Office of Naval Research Atlantic Natural Laboratory obtained ∼100% Hydrosweep bathymetrie coverage, 〉200% Hawaii MRl (HMRl) side scan coverage, gravity and magnetics over an area spanning three ridge segments along axis (∼25°25′N to ∼27°10′N), and crustal ages from 0 to 26–30 Ma (∼400 km) on the west flank of the Mid-Atlantic Ridge. This data set represents a first opportunity for an extensive regional analysis of abyssal hill morphology created at a slow spreading ridge. The primary purpose of this work is to investigate the relationship between abyssal hill morphology and the properties of the ridge crest at which they were formed. We apply the method of Goff and Jordan [1988] for the estimation of two-dimensional statistical properties of abyssal hill morphology from the gridded Hydrosweep bathymetry. Important abyssal hill parameters derived from this analysis include root-mean-square (rms) height, characteristic width, and plan view aspect ratio. The analysis is partitioned into two substudies: (1) analysis of near-axis (〈 7 Ma) abyssal hills for each of the three segments and (2) analysis of temporal variations (∼2–29 Ma) in abyssal hill morphology along the run of the south segment. The results of this analysis are compared and correlated with analysis of the gravity data and preliminary determination of faulting characteristics based on HMRl side scan data. Principal results of this study are: (1) Abyssal hill morphology within the study region is strongly influenced by the inside-outside corner geometry of the mid-ocean ridge segments; abyssal hills originating at inside corners have larger rms height and characteristic width and smaller plan view aspect ratio than those originating at outside corners. (2) The residual mantle Bouguer gravity anomaly is positively correlated with intersegment and along-flow-line variations in rms height and characteristic width, and it is negatively correlated with plan view aspect ratio. From this result, we infer that lower-relief, narrower, and more elongated abyssal hills are produced when the crust being generated is thicker. (3) Intersegment variations in near-axis rms height negatively correlate with average fault density as determined from analysis of HMRl side scan imagery.
    Description: This research was supported by the Office of Naval Research under grants N00014-92-J-1214, N00014-94-I-0197, N0014-90-J-1621, and N0014-94-1-0466. G.E.J. was supported by ONR AASERT grant N00014-93-I-1153, and additional support to J.L. was provided by NSF grant OCE93-00708.
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  • 8
    Publication Date: 2017-01-05
    Description: Author Posting. © The Author(s), 2007. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Continental Shelf Research 28 (2008): 614-633, doi:10.1016/j.csr.2007.11.011.
    Description: We present a methodology for statistical analysis of randomly-located marine sediment point data, and apply it to the U.S. continental shelf portions of usSEABED mean grain size records. The usSEABED database, like many modern, large environmental datasets, is heterogeneous and interdisciplinary. We statistically test the database as a source of mean grain size data, and from it provide a first examination of regional seafloor sediment variability across the entire US continental shelf. Data derived from laboratory analyses (“extracted”) and from word-based descriptions (“parsed”) are treated separately, and they are compared statistically and deterministically. Data records are selected for spatial analysis by their location within sample regions: polygonal areas defined in ArcGIS chosen by geography, water depth, and data sufficiency. We derive isotropic, binned semivariograms from the data, and invert these for estimates of noise variance, field variance, and decorrelation distance. The highly erratic nature of the semivariograms is a result both of the random locations of the data and of the high level of data uncertainty (noise). This decorrelates the data covariance matrix for the inversion, and largely prevents robust estimation of the fractal dimension. Our comparison of the extracted and parsed mean grain size data demonstrates important differences between the two. In particular, extracted measurements generally produce finer mean grain sizes, lower noise variance, and lower field variance than parsed values. Such relationships can be used to derive a regionallydependent conversion factor between the two. Our analysis of sample regions on the U.S. continental shelf revealed considerable geographic variability in the estimated statistical parameters of field variance and decorrelation distance. Some regional relationships are evident, and overall there is a tendency for field variance to be higher where the average mean grain size is finer grained. Surprisingly, parsed and extracted noise magnitudes correlate with each other, which may indicate that some portion of the data variability that we identify as “noise” is caused by real grain size variability at very short scales. Our analyses demonstrate that by applying a bias-correction proxy, usSEABED data can be used to generate reliable interpolated maps of regional mean grain size and sediment character.
    Description: The authors thank the Office of Naval Research for support under grants N00014-05-1-0079 (JAG) and N00014-05-1-0080 (CJJ), and the USGS/Coastal and Marine Geology Program (SJW).
    Keywords: Grain size ; Continental shelf ; Database ; Semivariogram ; Statistical analysis ; Kriging
    Repository Name: Woods Hole Open Access Server
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  • 9
    Publication Date: 2018-06-04
    Description: Author Posting. © Acoustical Society of America, 2018. This article is posted here by permission of Acoustical Society of America for personal use, not for redistribution. The definitive version was published in Journal of the Acoustical Society of America 143 (2018): EL405-EL411, doi: 10.1121/1.5039769.
    Description: This paper presents single receiver geoacoustic inversion of a combustive sound source signal, recorded during the 2017 Seabed Characterization Experiment on the New England Mud Patch, in an area where water depth is around 70 m. There are two important features in this study. First, it is shown that high-order modes can be resolved and estimated using warping (up to mode number 18 over the frequency band 20–440 Hz). However, it is not possible to determine mode numbers from the data, so that classical inversion methods that require mode identification cannot be applied. To solve this issue, an inversion algorithm that jointly estimates geoacoustic properties and identifies mode number is proposed. It is successfully applied on a range-dependent track, and provides a reliable range-average estimation of geoacoustic properties of the mud layer, an important feature of the seabed on the experimental area.
    Description: This research was supported by the Office of Naval Research and the Office of Naval Research Global.
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  • 10
    Electronic Resource
    Electronic Resource
    [s.l.] : Macmillan Magazines Ltd.
    Nature 392 (1998), S. 224-227 
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Abyssal hills cover most of the Earth's sea floor. They are, in fact, the most common morphological feature on the Earth's surface, yet they are among the least understood. These elongate hills are created at mid-ocean-ridge spreading centres by faults which offset the volcanically created sea ...
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