Ice Canopy Research Articles

Ice canopy roughness inversion and Arctic propagation loss estimates via elastic perturbation theory

Low-frequency (f≪100 Hz) boundary interaction loss in the central Arctic has been observed to exceed the losses that may be predicted by free surface Kirchhoff or MSP scattering theories, 2-D boss scattering models, or large elastic feature scattering arguments. In addition, the measured frequency dependence of the scattering loss in the Arctic (∝f1.5) is lower than that which is predicted with either the boss or the large elastic feature scattering models. Here a self-consistent elastic perturbation theory is used to estimate coherent boundary interaction loss for the Arctic scenario. Using this theory it is found that the elasticity of the ice canopy can account for a substantial decrease in the local coherent reflection coefficient in comparison with a rough free surface interaction, while the frequency dependence of the loss remains proportional to f2. Using acoustics measurements obtained by a horizontal array under the Arctic ice, a ML inversion for the ice canopy roughness statistics is obtained using the perturbation theory, and these parameters are used to estimate the loss per interaction. This loss, when combined with a ray cycle distance taken from the relevant literature, yields an estimate of scattering propagation loss (dB/km), which closely matches the data in the low-frequency regime. When the canonical roughness parameters and cycle distances from the literature are used in the theory, the estimated loss is lower, but improvements are still obtained over the free surface results. [Work supported by ONR.]

High-frequency scattering in the Arctic

There currently exists no high-frequency model of propagation in the Arctic. This article describes a simple, fast model for computing high-frequency propagation in the Arctic. This model is a one-parameter model of scattering in the Arctic, where the free parameter of this model is the average rms slope of the small-scale surface roughness of the ice canopy. This model couples ray theory to a scattering strength formalism of scattering from a rough surface. This coupling is achieved by associating an effective area of ensonification with each ray trajectory from the source to the receiver. The average intensity at the receiver is obtained by performing an incoherent summation over all possible ray trajectories from the source to the receiver, including nonspecularly scattered rays. A comparison between model predictions and experimental measurements of forward scattering is made. The model predicts the overall shape and amplitude of the scattered signal within 5 dB depending upon the range of the source to the receiver.

High-frequency forward scattering from the ice canopy: Experiment and comparison with specular point theory simulations.

Acoustical forward scattering experiments were carried out in the spring of 1990 in the Beaufort Sea. Both 25- and 45-kHz tone bursts of 1- to 5-ms duration were used. A broad beamwidth four quadrant receiver and an omnidirectional source were employed in the experiment. The horizontal ranges between source and receiver for the results to be presented were 400–500 m. The receiver was 61 m below the arctic canopy and the sender depth was varied from 7 to 350 m. The canopy topography between the sender and receiver was profiled with an upward looking sonar. This allowed deterministic modeling of the forward scattered signal using specular point theory (SPT). Comparisons will be shown between individual experimental pings and SPT. The SPT based simulations reproduce the general characteristics of the experimental results in most cases. However, in certain geometries the surface topography is such that the approximations used in SPT are invalid. Examples of data/model comparisons for these geometries will be given. The physical reason for failure of SPT for these geometries will be shown to be due to the existence of random caustics. [Work supported by ONT with technical management by the Naval Coastal Systems Center (NCSC).]

Comparison of experimentally observed long-range horizontal under-ice coherence in the Arctic with elastic scattering perturbation theory predictions.

During the CEAREX ’89 experiment in the Arctic Ocean, long-range (≊200 km) coherent transmissions from an air gun positioned off the coast of Greenland were received on a horizontal array deployed from a drifting ice sheet located in the Fram Straits. These multichannel data have been analyzed for covariance on spatial scales of ≤1 km. Because in general the vectors connecting the receiver pairs are not parallel to the wave front vector of the propagating signal, these data contain information on both in-plane and out-of-plane scattering off the sea ice canopy. Assuming a modal (discrete) mean signal structure, the elastic scattering perturbation theory of Kuperman and Schmidt [J. Acoust. Soc. Am. 86, 1511–1522 (1989)] is used to estimate the structure of the scattered field. Comparisons are made between the observed coherence and that predicted by the theory for various isotropic or nonisotropic surface roughness power spectra, including power law spectra. [Work supported by ONR.]

Acoustic reflections from arctic ice at 15–300 kHz

Underwater acoustic systems operating in the Arctic are generally used to detect ice or objects in an ice background. In either case, knowledge of the reflections from ice, both undisturbed and ridged, is necessary. For the past few years, the authors have been measuring monostatic reflections near normal incidence from arctic ice to advance this knowledge. In 1988, reflections at 20–80 kHz were measured from the ends of four cylindrical blocks of flat arctic ice with diameters of 27–84 cm. The blocks were individually depressed below the surface so that their reflections were separable from those off the underside of the ice canopy. The transducer was moved horizontally beneath the block to measure the angular response pattern. Examination of the bottom of the block after it was removed showed that the surface was flat except for some roughness with a standard deviation less than 3 mm. Compared with solid ice, the skeletal layer reduced normal-incidence reflection by 8–11 dB. The results were consistent for all four blocks, confirming that the results were valid. Measurements of reflection from the underside of the ice canopy at 15–300 kHz gave similar results and extended the frequency range. Measurements of the physical properties of the ice were helpful in analyzing the nature of the reflection.

Observation and inversion of seismo-acoustic waves in a complex arctic ice environment

The propagation of low-frequency seismoacoustic waves in the arctic ice canopy is examined through the analysis of data collected during the 1987 PRUDEX ice camp. Study of the geophone time series generated by under-ice detonations reveals not only the expected longitudinal and flexural waves in the ice plate, but also unexpected horizontally polarized transverse (SH) waves. The travel paths of all three wave types are found to be refracted in the horizontal plane along a known ridge line locally separating the ice canopy into two distinct half-plates of thin first-year ice and thicker multi-year ice. The need to determine both the origin of the SH wave and location of each detonation from the received time series highlights the dramatic superiority of geophones over hydrophones in this aplication. Inversion of the geophone data for the ice sheet’s low-frequency elastic parameters is conducted initially by modeling the ice as a single homogeneous isotropic plate using SAFARI. A modified stationary phase approach is then used to extend SAFARI modeling to invert for the parameters of the two ice half-plates simultaneously. The estimated compressional/shear bulk wave speeds are comparable to previously obtained values; however, the compression/shear attenuation values are up to four times greater than earlier estimates.

Shot propagation from the Arctic Basin

Shot experiments provide a means of measuring dispersion and mode excitation. The Arctic Ocean, with its rough ice canopy, variable bottom geoacoustics, and bathymetry, contains many complex acoustic environments with interesting modal propagation characteristics. At three deep‐water sites, a series of explosive charges was detonated at three different depths. The signals were recorded with a nine‐element vertical array located on the continental shelf. Sonograms were used to determine the dispersive properties and depth‐dependent nature of the modes. Dispersive properties observed in the data were formed in the deep‐water environment and were only slightly modified by the shallow (500‐m) waveguide at the vertical array. The depth dependence of the observed mode shape is computed by mode fitting with theoretical shapes and applying weights to the complex summation of these modes. Shear properties play a dominant role in determining the mode shapes in this shallow‐water environment.

Measurements of under-ice broadband coherence

The Arctic ocean is a unique region acoustically, dominated by the under ice canopy. The ice varies seasonally in coverage, thickness, and roughness, causing varying degrees of acoustic spatial and temporal variability. In this paper, horizontal coherence, from explosive acoustic sources under the arctic ice pack, is examined in the frequency range 90–1400 Hz with a nominal sensor separation of 370 m. Magnitude-squared coherence is computed separately for the direct and ice-reflected propagation paths. Frequency averaging is used to compute the coherence estimate that fluctuates with horizontal range, sensor depth, and frequency. Frequency and depth dependence is associated with background coherence inside and below the acoustic half-channel determined by sound speed. The lowest frequency bands do not always give the highest coherence. Coherence degradation due to ice interaction may be as high as 60% for the higher frequency bands inside the half-channel.

On modeling high-frequency acoustic scatter from a basic Arctic under-ice experiment: The benchmark ice-block problem

High-frequency underwater acoustic propagation and scattering in the presence of the underside of the Arctic ice canopy have been extensively studied in the last decade. A few promising computer simulation models and some good experimental data have resulted from these studies. As the decade of the nineties begins, it is worthwhile to reflect back on the modeling achievements made in high-frequency Arctic acoustic modeling and to speculate on what is yet needed. While presenting a brief overview of the Arctic modeling achievements, this talk will concentrate primarily on the need for more benchmark modeling-experimental efforts. The ice-block reverberation experimental data taken by Francois and Garrison will be used as an illustration. Under-ice acoustic scattering models based on various approaches (Kirchhoff, edge diffraction, and finite element) will be used to analyze the ice-block data and comparisons of accuracy versus computation effort will be presented. The presentation will conclude with the suggestion that a need exists for more Arctic under-ice acoustic benchmark cases (analogous to the ASA benchmark wedge problem [L. B. Felsen, J. Acoust. Soc. Am. Suppl. 1 81, S39 (1987)]). [Work supported by ONR/NOARL.]

Ice Interaction with the Arctic Shelf and Coast: Constraints On and Implication for Petroleum Development: ABSTRACT

Along the Arctic coast of Alaska sea ice affects structures placed on and under the sea bed and influences the erosion and dispersal patterns of sedimentary particulates and pollutants. Most directly, ice gouges the sea floor to depths of a few meters, with return periods on the inner shelf of tens of years and, in places, annually, primarily during freezeup and breakup. These sea-floor ice-gouge patterns provide information on the motion and characteristics of the local ice canopy. The design and alignment of pipelines and coast structures should consider that ice gouging extends to the coast and onto the beaches; however, ice gouging is less apparent on beaches owing to lower gouge intensities and vigorous wave reworking. In the long term, gouging contributes to high rates of coastal and shelf erosion. Protruding ice keels extending downward from the sea-ice canopy divert and focus currents causing increased sea-floor scour to depths of a few meters below the sea floor. Sea-ice freezing during the commonly turbulent fall storms causes large volumes of sediment and pollutants to be sorted, resuspended, and, ultimately, incorporated into the ice canopy. Most entrained material is re-released to the nearshore the following open-water season however, some material maymore » be transported offshore to become part of the Arctic pack. During freezeup and breakup (4-5 months/yr) when these processes are most active, trafficability in the Arctic nearshore is nil owing to the instability of the ice canopy, and so our comprehension of this processes is limited.« less

Measurements of under ice broadband coherence

The Arctic is a unique region acoustically, dominated by the under ice canopy. The ice varies seasonally in coverage, thickness, and roughness causing varying degrees of acoustic spatial and temporal variability. Horizontal coherence from explosive acoustic sources at a depth of 300 m under a relatively smooth Arctic ice pack is examined in the frequency range 100–1500 Hz over source‐receiver ranges from 3–50 km. Coherence estimation is a difficult problem and is not defined theoretically for transient broadband signals. Mean‐square‐coherence is computed using frequency band averaging and is computed separately for the direct and ice‐reflected propagation paths. Coherence is found to fluctuate significantly with source‐receiver range, and acoustic frequency for both acoustic propagation paths. The highest values of coherence do not always occur at the lowest frequency bands. Inside the acoustic half‐channel, above 300 m, coherence is less for the ice‐reflected arrival. Changes in coherence due to under ice reflection are smallest for the 100‐ to 200‐Hz octave band, indicating less coherence loss due to under ice scattering, but the change is sometimes as high as 50%. [Work supported by the Office of Naval Technology.]

Sound propagation under the Arctic ice canopy

Sound propagation under the Arctic ice canopy

Vertical spatial coherence of scattering from the Arctic ice canopy: Comparison of theory with experiment

The results of experiments to measure the vertical spatial coherence of the high‐frequency acoustic field scattered from the Arctic ice canopy are presented. The dependence of the coherence on measurement geometry is found to be in agreement with theoretical predictions based on the method of physical optics. In addition, analyses to isolate the effects of under‐ice roughness on the coherence yield results in accord with theoretical predictions.

Shallow water propagation under the Arctic ice canopy

Propagation in a shallow water waveguide, covered by a layer of “ice,” is being studied in a 10‐m‐long laboratory waveguide that models both the scale and the physical properties of the Arctic. Both smooth ice and rough ice effects are investigated. Comparisons are made with predictions of the Naval Research Laboratory computer model, KRAKEN. Although the ice canopy thickness is only a small fraction of a wavelength, the sound field is distinguishable from the usual Pekeris waveguide behavior by evidence of the presence of solid‐borne modes in the ice. The observed effects are related to the stimulation of ice head waves, which has been reported previously for deep water regions [J. Acoust. Soc. Am. 83, 1794 (1988) and J. Acoust. Soc. Am. Suppl. 1 82, S31 (1987)]. [Work supported by the office of Naval Research.]

Non‐Kirchhoff elastic wave scattering from rough interfaces

An earlier developed solution technique for seismoacoustic scattering by stochastically rough interfaces [W. A. Kuperman and H. Schmidt, J. Acoust. Soc. Am. 79, 1767 (1986)] has been modified to treat the scattering without applying the Kirchoff approximation. This is accomplished by including the rotation of the boundary conditions in the perturbation formulation and by numerically evaluating the scattering integrals, allowing the roughness correlation lengths to be finite. It is demonstrated that the effect of the scattering on the mean (coherent) field can be accounted for in a self‐consistent manner by solving the unperturbed problem with modified boundary conditions. As was the case for the Kirchhoff approximation, the non‐Kirchhoff formulation has been implemented in the seismoacoustic SAFARI code [H. Schmidt and F. B. Jensen, J. Acoust. Soc. Am. 77, 813 (1985)], allowing for simultaneous treatment of multiple rough interfaces. The model is applied to analyze the effect of scattering from a rough shallow water sea bed as well as from an Artic ice canopy. By comparing these results to the Kirchhoff results for the same problems, it is demonstrated that the non‐Kirchhoff theory predicts significantly higher losses, in particular for acoustic wavelengths of the same order of magnitude as the roughness correlation length. [Work supported by NRL.]

Physical optics theory of scattering from the ice canopy : McDaniel, S.T., 1987. J. acoust. Soc. Am., 82(6):2060–2067. Appl. Res. Lab., Pennsylvania State Univ., University Park, PA 16804, USA

Physical optics theory of scattering from the ice canopy : McDaniel, S.T., 1987. J. acoust. Soc. Am., 82(6):2060–2067. Appl. Res. Lab., Pennsylvania State Univ., University Park, PA 16804, USA

Anchor ice, seabed freezing, and sediment dynamics in shallow Arctic Seas

Diving investigations confirm previous circumstantial evidence of seafloor freezing and anchor ice accretion during freeze‐up storms in the Alaskan Beaufort Sea. These related bottom types were found to be continuous from shore to 2‐m depth and spotty to 4.5‐m depth. Spotty anchor ice occurred as pillow‐shaped crystal aggregates on buried slabs of frozen sand surrounded by unfrozen sand. Considerations of required conditions for ice bonding and anchor ice growth allows regional extrapolation and suggests the possibility of anchor ice growth out to 20‐m depth, the estimated maximum depth of supercooling during fall storms. Anchor ice and seabed freezing apparently do not develop during a calm freeze‐up. Because of the abrupt growth of anchor ice during a freezing storm and its release soon after formation of a surface ice cover, this ice type has not been documented before. The concretelike nature of frozen bottom, where present, should prohibit sediment transport by any conceivable wave or current regime during the freezing storm. But elsewhere, particularly where the bonded crust is broken by grounded ice, anchor ice lifts coarse material off the bottom and incorporates it into the ice canopy, thereby leading to significant ice rafting of shallow shelf sediment and likely sediment loss to the deep sea.

Physical optics theory of scattering from the ice canopy

The method of physical optics is applied to treat high-frequency acoustic scatter from Arctic ice. The development takes into account that the under-ice surface is not statistically homogeneous, but consists of two types of ice: undeformed first year and multiyear ice, and very rough ice due to pressure ridge formation. Expressions are obtained both for the mean scattering strength and the spatial coherence of the scattered field. Measured ice statistics are used to evaluate the expressions derived for the spatial coherence and to estimate the variability of the mean backscattering strength.

Energy partitioning for very‐low‐frequency, long‐range acoustic propagation in the Arctic

In long‐range acoustic propagation in the Arctic, very‐low‐frequency signals interact with both the ice canopy and the bottom, both of which introduce loss. Signals from a sequence of long‐range (340‐km) shots detonated at several depths were recorded on a multichannel array and resolved according to their horizontal phase velocity and frequency components to separate the loss effects of the these two boundaries. Source levels and geometric losses based upon a determined sound‐speed model were subtracted to arrive at an absorptive/ scattered loss measure for each resolved path and frequency. These reduced data were used to estimate the partitioning of energy according to phase velocity and frequency from the ice canopy and the seafloor. At frequencies below 20 Hz, bottom interacting signals account for more than 50% of the received energy observed at the array on almost all the shots.

Elements of Arctic surface scatter: Part III, the head wave

Laboratory scale model studies have the virtue of revealing the detailed behavior of the elements of scatter from a complex ice field. Previous model studies have demonstrated the quasidipole radiation pattern of low-frequency scatter from an open lead. It has also been shown that leads and ridges backscatter principally from a one wavelength extent near the least-time intercept of the radiation from a point source. At this time, evidence is presented that mode conversion generates a compressional wave that propagates parallel to the surface of the ice. This mode is driven, and will radiate into the water, at the critical angle. The reradiated sound, an example of a head wave, causes a significant precursor to appear before the specular reflection from the ice canopy. In agreement with theory, the measured head wave amplitude is proportional to k−1 x−1/2 × L−3/2 where k is the wavenumber in the water, x is the horizontal range, and L is the path length in the ice. [Work supported by the Office of Naval Research.]