Showing papers in "IEEE Journal of Oceanic Engineering in 1989"

Swath bathymetry: principles of operation and an analysis of errors

Abstract: The principles of swath bathymetry are described, and the main cause of depth error is identified as acoustic interference, particularly from the sea surface. An error analysis is presented which gives the relationship among depth errors, the signal-to-interference ratio, the grazing angle, receiver spacing, and area resolution. It permits a prediction of when its measurement of depth can meet the accuracies required for nautical charting. Ways of reducing multipath interference and of minimizing its effect when it does occur are discussed. Particularly important are area averaging, the use of widely spaced receivers with ambiguities resolved by the vernier technique, and phase tracking for avoiding bias problems. >

Microtopographical roughness of shallow-water continental shelves

Abstract: High-resolution (<1 cm) roughness height measurements were made of the seafloor at seven locations on continental-shelf sediments on water depths ranging from 18 to 50 m. Roughness profiles of the sediment-water interface were digitized primarily from stereo photogrammetric measurements of varying pathlengths and increments. The data show that the root-mean-square roughness height varies from 0.3 cm for flat, featureless bottoms to 2.3 cm for rippled bottoms. Slopes of the roughness power spectra were calculated to be -1.5 to near -3.0 and depended to a large extent on contributions in higher spatial frequencies due to coarse sediments. Correlation lengths of different bottom types were estimated by using the Weiner-Khintchine theorem and examining the low-frequency behavior of the roughness spectra derived from the longest roughness profiles.

Quantitative backscatter measurements with a long-range side-scan sonar

Abstract: It is shown that useful relative backscatter strengths can be calculated from GLORIA long-range side-scan sonar data using a simple acoustic model. The calculation was performed on GLORIA side-scan sonar data collected during 1987 in the southern Indian Ocean. GEOSECS hydrographic information was used to access the effects of refraction (ray bending and aspherical spreading signal losses). Sea Beam bathymetry was used to correct the effective insonified area and compute the grazing angle. A major difficulty in performing this calculation over the terrain chosen (mid-ocean ridge topography) was one of adjusting navigation so that small features in Sea Beam and GLORIA data matched. Preliminary results show a 10-dB falloff in backscatter strength with decreasing grazing angle (10 degrees -40 degrees ) at 6.5 kHz over what must presumably be a rough surface (extruded basalts and breccias). >

A statistical study of acoustic signals backscattered from the sea bottom

Abstract: Backscattering measurements were performed in shallow water on sand, gravel, and clay bottoms. The equipment included a parametric array that emitted pulses of differential frequencies (8 to 40 kHz) with a 3 degrees directivity. The ranges did not exceed 50 m. The grazing angles varied from 4 degrees to 90 degrees . The bottom backscattering strength does not depend on the emitted pulse type (frequency and length). If one fits a Lambert law to the variations of the backscattering strength versus the grazing angle, the value at the origin fluctuates between-15 and-22 dB without any clear effect from the different bottom types. Statistical tests show that under the experimental measurement conditions: (1) the alternative received signal does not generally follow a normal distribution; (2) among five classical distributions in sonar and radar that have been fitted to the detected-integrated signal (exponential, Weibull, chi-2, log-normal, Rice), the best-fitted law is the log-normal; (3) signals backscattered by separated areas of the same bottom can hardly be regarded as stationary and, even less, homogeneous; and (4) with an anisotropic bottom topography the statistical properties depend on the aspect under which this topography is seen. >

Sea-ice characterization measurements needed for testing of microwave remote sensing models

Abstract: The nature and accuracy of ice-characterization measurements needed to test two microwave backscattering models are clarified by examining the sensitivities of these models to variations in the geophysical parameters they require as input. First, the Bragg, or small perturbation, model for rough surface scattering, which appears appropriate for backscattering from new ice types at L-band, is considered. The sensitivities of this model to variations in the dielectric constant of the ice and to the power spectrum of surface roughness are examined. The dense-medium radiation-transfer model at X-band is considered for backscattering from air bubbles embedded in multilayer ice. The sensitivities of this model to air-bubble size, air-volume fraction, and dielectric loss in the ice are examined. Based on these sensitivities, quantitative characterization guidelines for model testing are discussed. >

Improved time-delay estimates of underwater acoustic signals using beamforming and prefiltering techniques

Abstract: Passive sonar systems that localize broadband sources of acoustic energy estimate the difference in arrival times (or time delays) of an acoustic wavefront at spatially separated hydrophones, The output amplitudes from a given pair of hydrophones are cross-correlated, and an estimate of the time delay is given by the time lag that maximizes the cross correlation function. Often the time-delay estimates are corrupted by the presence of noise. By replacing each of the omnidirectional hydrophones with an array of hydrophones, and then cross-correlating the beamformed outputs of the arrays, the author shows that the effect of noise on the time-delay estimation process is reduced greatly. Both conventional and adaptive beamforming methods are implemented in the frequency domain and the advantages of array beamforming (prior to cross-correlation) are highlighted using both simulated and real noise-field data. Further improvement in the performance of the broadband cross-correlation processor occurs when various prefiltering algorithms are invoked. >

Segmentation of topographic profiles of the seafloor based on a self-affine model

Abstract: Profiles of seafloor topography can be statistically described as a self-affine stochastic time series characterized by only a few parameters. Two characteristic parameters are used: a correlation parameter (the spectral exponent beta , which is related to the fractal dimension D) and an amplitude parameter (an index of dispersion of the first differences S). This characterization provides a simple quantification of the intuitive notion of the roughness of topography. The quantification of roughness is important for the effective classification or areas of the seafloor where topography is dominated by different processes (e.g., sedimentation, volcanism, faulting). The procedure used can be subdivided into statistically homogeneous segments; statistical parameters are measured on each segment; and the segments are grouped into classes with similar statistical parameters. >

Review and status of remote sensing of sea ice

Abstract: The status of obtaining geophysical observations through the interpretation of satellite data over sea ice is discussed. It is pointed out that the community working in this area has grown in size and sophistication over the last decade, that the connection between microscopic properties of ice and its microwave behavior is now being understood, and that a good deal of accurate satellite-derived information on sea ice can now be obtained. Areas of ongoing, as well as needed, work are outlined, especially in the understanding of first-year and old-ice microwave properties, and it is pointed out that the efficient advance of remote sensing will require more active participation of scientists focused on in situ studies. >

Sharkbite on the SL submarine lightwave cable system: history, causes and resolution

Abstract: The nature of the sharkbite phenomenon is presented. The physical nature of the faults and their location along the OPTICAN-1 route are described. The morphology of the shark teeth recovered, and the identification and nature of the sharks believed responsible are discussed. A hypothesis as to the cause of the attacks, based on the findings of previous shark-sensory research, is detailed. The methods and results of an experimental program aimed at verifying this hypothesis are summarized. The results of development programs aimed at resolving this problem are presented. Two avenues were pursued: the first was the development of a mechanically protected SL deep-water, repeated lightwave trunk cable for use in areas of high sharkbite risk; the second was the development of a methodology to assess the sharkbite risk in different areas of the world's oceans. The resultant algorithm, for use in specifying the amount of protected cable to be deployed in an SL system is detailed. >

Stochastic modeling of seafloor morphology; Resolution of topographic parameters by sea beam data

Abstract: The authors explore the resolving power of an inversion algorithm which estimates five parameters of the seafloor covariance function from a single swath of multibeam echosounding data. The resolving power is evaluated as a function of the swath length, the orientation of ship track with respect to topographic grain, and the response width of the sounding system. The analysis is conducted by inverting sets of synthetic data with known statistics. The mean and standard deviation of the inverted parameters can be directly compared with the input parameters and the standard errors output from the inversion. Experiments show that resolution of the covariance parameters is strongly dependent on the number of characteristic lengths which are sampled. Root-mean-square seafloor height can be estimated to within approximately 15%, and anisotropic orientation to within approximately 5% (for a strong lineation), using track lengths as short as three characteristic lengths. >

Seafloor profiling by a wideband sonar: simulation, frequency-response optimization, and results of a brief sea test

Abstract: An ahead-looking probe of some kind, optical or acoustic, is critical when one is attempting seafloor exploration from a mobile platform. A single-frequency, split aperture sonar system can be used for this purpose, but a wideband monopulse sonar offers many advantages. It computes a running estimate of the vertical directional cosine of the source of the echo, and can thus reveal the positions of multiple wave scatterers as long as their echoes can still be time resolved. Theoretical studies of its performance have been made previously, but were directly applicable only to extremely simple seafloor geometries. A new time-domain digital simulation that largely circumvents this limitation has been developed. The simulation also provides a means for testing the theory and optimizing system parameters. The reverberation model does not account for some features of acoustic backscattering such as diffraction, but it is believed to be adequate for the investigation of most signal processing aspects of the sonar system. The theory of the simulation is developed and several examples are presented and discussed. In addition, some preliminary results are presented from a sea test that used the air-sea interface as a surrogate seafloor. >

Toward an automated system for a correctly registered bathymetric chart

Abstract: A scheme has been developed for automated bathymetric registration of multiple overlapping swaths of data collected by a ship equipped with a multibeam echo-sounder device. Because each swath of data overlaps with several others, registration is performed both at local and global levels. The primitives used for local matching are contours of constant depth which are extracted from the data and are represented as a modified chain code. The main heuristic guiding the search for matching contours of equal depth is their proximity to the middle of the apparent (unregistered) overlapping region. The degree to which two contours match is determined by the correlation of their respective chain codes and the geometrical proximity of their nodes. All best matches are considered tentative until their geometrical implications are evaluated and a consistent majority has emerged. To do global matching. a cost function is constructed and minimized. >

Using topographic slope distributions to infer seafloor patterns

Abstract: The authors present a method for obtaining parameters for Sea Beam swath bathymetry which describe the characteristics of abyssal-hill topography. The basic quantity used in the analysis is the distribution of topographic slopes in a region. A convenient means for parameterizing these slopes is through unit vectors that are normal to small patches of the seafloor; the normal vectors are decomposed into azimuthal and dip components. It is found that the azimuthal distribution of the vectors provides a first-order indication of the dominant elongation directions in the topography, including multiple lineations when they are present. It is shown that the slope statistics are relatively independent of long-wavelength depth variations and are robust even if large anomalous features such as seamounts and fracture zones are included in the section of bathymetry analyzed. >

Ocean acoustic tomography: estimating the acoustic travel time with phase

Abstract: Continuous acoustic transmission (133 Hz, 60-ms resolution) between a bottom-mounted source near Oahu, Hawaii, and a bottom-mounted receiver at 4000-km range near the coast of northern California was recorded to learn how to measure precisely the travel time so that basin-scale fluctuations in the Pacific can be detected. Daily incoherent averages of some of the multipaths exhibited stability during this period. The standard deviation of the travel time of the resolved peaks in the daily incoherent averages is about 30 ms. An acoustic method, based on cross-correlation, is derived to estimate the change in the average acoustic phase (travel time) to a precision of about 0.018 cycles (135 mu s) every 2 min. Travel-time estimates based on the cross-correlator reduce the aberrations due to internal waves by about 19 dB in comparison with CW transmissions. The new travel-time estimator is applied to the measurements to examine some of the fluctuations of the Pacific. >

Measurement of towed array position, shape, and attitude

Abstract: A practical method is described for the three-dimensional determination of the position, shape, and attitude of a hydrophone array towed from a surface vessel. It provides successive snapshots of the array configuration with a maximum rate of about three per minute. The method is intended as an alternative to the use of fixed test ranges and provides results suitable for validating computer models of array motion. It uses the travel-time differences of impulsive waves measured across the array. The waves are generated by two explosive charges dropped from consorts. Results of a typical experiment are presented as an illustrative example. The array position relative to the tow ship is obtained to within an accuracy of a few metres. >

GEOSAT altimeter sea-ice mapping

Abstract: The efforts to use GEOSAT data to refine an ice index that is applicable to widely varying ice conditions are detailed. The sea-ice mapping requirements, the present US Navy ice-index operational utilization, and ongoing and future work that promises to provide additional sea-ice measurement capabilities are discussed. Possibilities include discrimination among water, land, ice, combination water/ice, and water/land, as well as distinguishing various ice concentrations and possibly ice types. Coincident airborne passive microwave and synthetic-aperture-radar (SAR) data have been collected to test several methods which appear to be promising. >

The Autonomous Ocean Profiler: a current-driven oceanographic sensor platform

Abstract: The development and initial field test results of the Autonomous Ocean Profiler (AOP) are described. The profiler uses a hydrodynamic lift device to fly the instrument package up and down the water column along a taut vertical cable. Because the local currents drive the platform's vertical motion, power requirements are low, and therefore long, unattached deployments are possible. By using ARGOS or GOES satellite retrieval networks, the system can supply near-real-time data. The system provides profile data at very high vertical resolution in contrast to conventional buoys, which gather data only at fixed sensor depths. Because only a single set of sensors is required to cover the vertical range desired, the system is low cost and, for many applications, expendable. The initial deployment configuration is as an Arctic drifting buoy. >

Diffraction by a hard truncated wedge and a strip

Abstract: A method of formulating the scattered acoustical field for certain types of hard bodies through the use of well-known exact solutions for the simple semi-infinite wedge and/or corner is presented. The method yields a representation of the total sound field for all frequencies and satisfying all boundary conditions. Relevant hard-wedge solutions for harmonic line sources and for plane waves are reviewed. Such solutions, which are rigorous, represent the total sound field as a superposition of directional line sources and sinks at the wedge vertex. A method of representing multiple scatter from combinations of wedges, allowing the explicit solution of the case of a truncated wedge, is introduced. This method uses the classic self-consistent algorithm for multiple scatter, together with a rigorous representation of vertex-diffracted fields. >

Comments on 'Theory and application of calibration techniques for an NDBC directional wave measurements buoy' by K.E. Steele, et al.: nonlinear effects

Abstract: For original paper see ibid, vol OE-10, no4, p382-96 (1985) The authors of the above mentioned paper present an extensive set of linear calibration techniques that are applied to National Data Buoy Center wave-buoy sensor spectral output before calculating and disseminating directional wave spectra The commentators identify and estimate the nonlinear effects that produce biases still present in the output, due both to wave nonlinearities themselves and to constraints on the buoy and mooring system to the driving forces Simple models show that these nonlinearities can produce spectral energy biases of 5-15% at and above the spectral peak frequency, and even greater errors below it NDBC presently records wave data from vertically stabilized and fixed accelerometers and slope sensors Calculations show that these sensors all incur bias due to wave nonlinearities: this is greater for vertically stabilized accelerometers and least for slope sensors Effects of the resulting inconsistencies between the different sensors are most pronounced below the spectral peak where the nonlinear terms dominate; these effects are illustrated with measured data >

Code-fed omnidirectional arrays

Abstract: The synthesis of linear and planar arrays of desired omnidirectional intensity patterns is considered. A novel approach that utilizes the relation between the array intensity pattern and the correlation function of the used feeding sequence, or code, is adopted. The basic theory of such code-fed arrays is reviewed, and it is shown that almost omnidirectional patterns result when codes with sharp autocorrelation functions are used as the feeding sequences. Examples of omnidirectional linear and planar arrays fed with Barker codes, Kuttruff-Quadt trial-and-error two-dimensional binary codes, and nonbinary Huffman-type codes are presented. The results have direct application in underwater communication systems, public address systems, and acoustical imaging systems. >

Trials and experimental results of the ECHOS XD multibeam echo sounder

Abstract: Results on the performance of the ECHOS XD multibeam echo sounder are presented. These tests were made during the sea trials of R/V Akademik Vavilov and R/V Akademik Ioffe in the Atlantic Ocean, the Mediterranean Sea, and the Baltic Sea. Since these ships are new, the author also gives a short description of them and their scientific equipment. The history which led to the development of the ECHOS XD system is reviewed. Finally, the author presents the recent research and development on multibeam echo sounders being done a Hollming Ltd. Electronics (Rauma, Finland). >

Stress and strain instruments developed for field measurements of ice

Abstract: The instruments developed and operational experience gained in the measurement of strain rates and stresses in floating sea ice are described to indicate the total global loads occurring naturally against an artificial island or similar structure. A delta strainmeter and a thin-disk stress sensor were developed to service particular measurement needs for which suitable equipment was not available. The loading conditions studied have included slow-moving winter first-year ice at Tarsiut, Mukluk, and Kaubvik Islands in the Beaufort Sea, freely moving summer multiyear ice at Hans Island, and a pack-ice mixture at Katie's Floeberg. The instruments have also been deployed on a surging glacier on Spitzbergen in Norway. >

Feasibility of interferometric swath bathymetry using GLORIA, a long-range sidescan

Abstract: The feasibility of adding an interferometric swath bathymetric system to GLORIA, a 6.6 kHz long-range sidescan sonar, is discussed. The size of GLORIA's low-frequency transducer arrays and towfish precludes significant modifications, but even without such changes bathymetric errors could be several tens of metres over a usable swath somewhat smaller than the normal GLORIA swath. A swath bathymetry based on GLORIA will have random errors depending strongly on wind speed, water depth, and swath width. Within the range of these parameters, root-mean-square bathymetry errors in the range of 1-100 m can be expected. >

Modification and deployment techniques for hand-deployed Arctic long-life sonobuoys

Abstract: Specific methods and equipment used to modify, power, and hand-deploy AN/SSQ-57A sonobuoys in the Arctic are discussed. The methods and suggestions can be easily extended for use with other types of sonobuoys. The modified sonobuoys transmit continuously for up to 30 days from a remote unmanned site to a manned base camp over a range of 20 km. Sample acoustic data from the ALPIS 87 ice station are presented. >

Acoustic scattering from composite wind-wave surfaces in 'bubble-free' regimes

Abstract: The problem of underwater acoustic scattering from truly composite wind-wave surfaces under zero-gradient conditions ( Delta c=0) is examined. Here the dominant small-scale component is postulated to be a soliton surface ensemble, produced by the nonlinear wind-wave interactions and associated with the wind-drift surface layer riding on the underlying, mostly large-scale gravity-capillary component of the composite surface. A general bistatic analysis, based on the Kirchhoff approximation, is presented, which includes arbitrary geometries, beam patterns, and general signals. Both low-frequency O(0.2-1 kHz) and high-frequency O(>or approximately=3 kHz) signals are considered, and far-field (Fraunhofer) geometries are assumed. Surface Doppler, including Doppler spread and the modulation effects of the large-scale component, are examined. Both forward-scatter and backscatter regimes are considered in the determination of the scattered field and received wave intensities, scattering cross-sections, and coherency measures of surface scatter. Particular attention is given to the high-frequency cases, with small grazing angles, moderate-to-strong mean surface winds, and essentially bubble-free regimes. Recent empirical data appropriate to these conditions are included, which support the soliton conjecture and illustrate the general results. Both coherent and incoherent scattering are examined, along with relevant surface Doppler data. >

Radar backscattering from artificially grown sea ice

Abstract: Fine-resolution X-band backscatter measurements were made at the US Army Cold Region Research and Engineering Laboratory in Jan. 1987. Backscatter data were collected from unmodified smooth ice and snow-covered ice and from ice from which the snow had been removed (slightly roughened ice). The results indicate that vertically polarized returns were consistently higher than horizontally polarized echoes from both the slightly roughened and snow-covered saline ice. A 6.5-cm-thick dry snow layer altered the sigma /sup o/ of the original smooth-surfaced saline ice only slightly, but introduced a noticeable volume scattering component. It is shown that although substantial agreement exists between the bare first-year ice measurements and commonly used surface-scatter model predictions, a complete model of first-year ice must include a volume-scatter contribution. >

The Polar Floats Program

Abstract: The Polar Floats Program, an adaptation of midlatitude SOFAR float technology to the polar regions, is used to monitor subsurface circulation patterns over large spatial and temporal scales as well as to track mesoscale features in near realtime The program is comprised of three major components that are now under development: an 80-Hz float transducer, American and French sea-ice-deployable listening stations using ARGOS data telemetry (ARS and SOFARGOS, respectively), and the software needed for real-time tracking The adaptation of SOFAR technology to the planar regime demands modifications of the existing design from both environmental and acoustical standpoints The authors describe the development and use of SOFAR technology within a relatively small sector of the north polar region that encompasses several thousand kilometers around Fram Strait (located between Greenland and Spitsbergen) >

Broadside resonance scattering from elastic spheroids

Abstract: The authors study the scattering of monochromatic plane acoustic waves incident at 90 degrees angles relative to the axis of symmetry (i.e. broadside or beam aspect) on solid elastic spheroids. In this analysis, the aspect ratios of the spheroids vary in the range 2 >

On underwater sound scattering by surface waves

Abstract: An angular-scattering approach is outlines that provides insight into some of the elementary acoustics involved in the perturbation theory. In the past, the comparison of predictions with experiment has been based mainly on propagation-loss measurements in surface ducts. Here the absorption is subtracted from total attenuation and the remainder is ascribed to scatter loss. >

The uniqueness of the Practical Salinity Scale (1978): testing the scale with natural seawaters

Abstract: The scale provides a precisely defined, unambiguous method that is universally applicable to all waters for determining the practical salinity from measurements of conductivity, temperature, and pressure. In the terminology of standards, it is highly desirable to have a scale that is also unique. The authors questioned whether measurements performed on a particular water sample at different temperatures would lead to the same value of practical salinity. They have investigated this problem by examining the behavior of natural seawaters from several oceans and conclude that the PSS provides a unique scale for salinity measurements for these waters within the bounds +or-0.001 in salinity. >