Ice Profiling Sonar Research Articles

Analysis of the relationship between level ice draft, ridge frequency and ridge keel draft for use in the probabilistic assessment of ice ridge loads on offshore structures

Level ice draft, ice ridge keel draft and ridge frequency are important variables in the probabilistic assessment of ice ridge loads on offshore structures. We use ice profiling sonar (IPS) measurements of ice draft from the Beaufort Sea to analyse the relationship between these three variables. We propose a probabilistic simulation technique of ridge keel drafts. Two examples of simulations are given. The first example simulates the weekly deepest ridges. The simulated distribution of the weekly deepest ridge keel draft agrees with the measured data. The second example simulates all ridges deeper than 5 m. This simulation results in overestimation of the ridge keel draft in the tail section of the distribution. For both simulations, with the relationships established in this paper, the only needed input is level ice draft. Future studies should investigate whether the relationships found in the Beaufort Sea are valid in other areas or if there is a possibility of scaling the correlations. If the correlations prove predictably scalable for other locations, it could be possible to estimate the ridge keel draft distribution and ridge frequency by knowing only the level ice draft (thickness) statistics. This study is our first endeavour in this direction.

Analyses of Peace River Shallow Water Ice Profiling Sonar data and their implications for the roles played by frazil ice and in situ anchor ice growth in a freezing river

Abstract. Peace River SWIPS (Shallow Water Ice Profiling Sonar) data were analyzed to quantify the roles of frazil ice and riverbed anchor ice grown in situ during the initial buildup of a seasonal ice cover. Data were derived through quasi-continuous monitoring of frazil parameters throughout the water column, providing direct and indirect measures of anchor ice volume and mass growth rates. Analyses utilized water level and air and water temperature information in conjunction with acoustic volume backscattering coefficient data to track and interpret spatial and temporal changes in riverbed and water column ice. Interest focused on four frazil intervals characterized by anomalously low levels of frazil content (relative to simulations with an anchor-ice-free river ice model) as distinguished by two strikingly different types of time dependences. A simple physical model was proposed to quantitatively account for discrepancies between measured and simulated results in terms of the pronounced dominance of anchor ice as an initial source of river ice volume and mass. The distinctive differences in temporally variable water column frazil content are attributed, in this model, to corresponding differences in the stabilities of riverbed anchor ice layers against detachment and buoyancy-driven movement to the river surface. In accord with earlier observations, the stability of in situ grown riverbed ice layers appears to be inversely proportional to cooling rates. The strength of the coupling between the two studied ice species was shown to be strong enough to detect changes in the anchor ice constituent from variations in water column frazil content.

Observational evidence of supercooling and frazil ice formation throughout the water column in a coastal polynya in the Sea of Okhotsk

This paper examines underwater frazil ice formation in a coastal polynya. In a coastal polynya, the seawater near the ocean surface can become supercooled due to intense atmospheric cooling. Supercooled water sinks deeper under turbulent conditions, resulting in frazil ice formation in the water column. For the winter of 2002–2003, mooring measurement with an Acoustic Doppler Current Profiler (ADCP), an Ice-Profiling Sonar (IPS) and a conductivity-temperature recorder was conducted off the east coast of Sakhalin in the Sea of Okhotsk with the water depth of 33 m. IPS-derived ice draft data showed several polynya periods lasting for 1–2 weeks, associated with strong offshore winds. During these polynya periods, the ADCP acoustic backscatter strength was enhanced throughout the water column, accompanying in-situ and/or potential supercooling. This is evidence of underwater frazil ice formation associated with supercooling. Our data indicate that ice divergence/convergence, caused by the combination of wind and tidal forcing, results in a diurnal cycle of underwater frazil ice formation. According to a heat budget analysis, the contribution of frazil ice production to the total ice production during the winter is estimated to be at least 60% of mass. Based on the mooring data, this study shows that frazil ice formation occurs throughout the water column in a polynya, resulting in effective ice production.

Year-round observations of sea-ice drift and near-inertial internal waves in the Northwind Abyssal Plain, Arctic Ocean

In this study, intra-annual variation of near-inertial internal wave (NIW) in the Arctic Ocean is examined using year-round mooring in the Northwind Abyssal Plain. Our emphasis is on dynamical responses of NIW to local sea-ice variables such as concentration, draft, and drift. We obtained those using a coupling system of ice profiling sonar (IPS) and an acoustic Doppler current profiler (ADCP) deployed at the top of the mooring. According to the wavelet spectrum, the inertial oscillation of ice drift becomes considerably strong during periods of ice formation and decay. Results show that the NIW amplitude in the upper part of the water column responds more sensitively to the sea-ice inertial oscillation than to the mean component of ice drift heading to the northwest. We also conducted an experiment with a mixed-layer slab model using the IPS-ADCP measured ice speed to examine the NIW generation responding to the ice-to-ocean stress. Experiment results suggest that the mixed-layer inertial oscillation is amplified in the early time of ice formation, through the ice-water resonance process. It is then concluded that the mixed-layer inertial current driven by ice drift is the primary driver of the enhanced NIW generation.

Algorithm for Flat First-Year Ice Draft Using AMSR2 Data in the Arctic Ocean

AbstractFirst-year ice has replaced multiyear ice in the Northern Sea Route area since 2008. In this area, sea ice survival during summer substantially depends on first-year ice thickness at melt onset, and thus monitoring of first-year ice thickness in the freezing period is a key to forecasting sea ice distributions in the following summer. In this paper we introduce a new algorithm to estimate flat first-year ice draft using brightness temperature data measured by the Advanced Microwave Scanning Radiometer-2 (AMSR2). The algorithm uses a gradient ratio (GR) of 18- and 36-GHz vertically polarized brightness temperatures based on decreases in sea ice emissivity in higher AMSR2 frequency channels with thermodynamic growth associated with an increase in volume scattering. Such spectral characteristics of the emissivity are examined by comparing GR values with flat first-year ice draft extracted by mode values of in situ draft data measured by a moored ice profiling sonar. The accuracy of the daily draft estimated from GR values after applying proper noise filters is about 10 cm for a draft range of 0.4–1.2 m.

The effects of ice cover and oceanography on medium-frequency acoustic propagation on the Chukchi Shelf

The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.The Canada Basin Acoustic Propagation Experiment (CANAPE) was a year-long experiment exploring the changing nature of sound propagation and ambient noise in the Arctic ocean. As part of this experiment, medium-frequency signals at 0.7–1 kHz and 1–4 kHz were transmitted by two sources on the Chukchi Shelf. One of these sources was located in an area of 150 m of water depth, approximately 350 m from a directional receiver array and 50 km from an 8-element vertical line array in a water depth of about 125 m. Oceanographic sensors were located both on the arrays and on a set of moorings on the shelf, and an ice-profiling sonar was located between the arrays about 15 km from the source. In this talk, we will focus on using the measured environmental data and propagation modeling to characterize the variability observed in the short-range and long-range received acoustic signals over the course of CANAPE.

Sea-ice thickness in the coastal northeastern Chukchi Sea from moored ice-profiling sonar

ABSTRACTTime series ice-draft data were obtained from moored ice-profiling sonar (IPS), in the coastal northeastern Chukchi Sea during 2009/10. Time series data show seasonal growth of sea-ice draft, occasionally interrupted by coastal polynya. The sea-ice draft distribution indicates a slightly lower abundance of thick, deformed ice compared with the eastern Beaufort Sea. In January, a rapid increase in the abundance of thick ice coincided with a period of minimal drift indicating compaction again the coast and dynamical thickening. The overall mean draft and corresponding derived thickness are 1.27 and 1.38 m, respectively. The evolution of modal ice thickness observed can be explained mostly by thermodynamic growth. The derived ice thicknesses are used to estimate heat losses based on ERA-interim data. Heat losses from the raw, 1 s IPS data are ~50 and 100% greater than those calculated using IPS data averaged over spatial scales of ~20 and 100 km, respectively. This finding demonstrates the importance of subgrid-scale ice-thickness distribution for heat-loss calculation. The heat-loss estimate based on thin ice data derived from AMSR-E data corresponds well with that from the 1 s observed ice-thickness data, validating heat-loss estimates from the AMSR-E thin ice-thickness algorithm.

Estimation of composite hydraulic resistance in ice‐covered alluvial streams

AbstractFormation, propagation, and recession of ice cover introduce a dynamic boundary layer to the top of rivers during northern winters. Ice cover affects water velocity magnitude and distribution, water level and consequently conveyance capacity of the river. In this research, total resistance, i.e., “composite resistance,” is studied for a 4 month period including stable ice cover, breakup, and open water stages in Lower Nelson River (LNR), northern Manitoba, Canada. Flow and ice characteristics such as water velocity and depth and ice thickness and condition were measured continuously using acoustic techniques. An Acoustic Doppler Current Profiler (ADCP) and Shallow Water Ice Profiling Sonar (SWIPS) were installed simultaneously on a bottom mount and deployed for this purpose. Total resistance to the flow and boundary roughness are estimated using measured bulk hydraulic parameters. A novel method is developed to calculate composite resistance directly from measured under ice velocity profiles. The results of this method are compared to the measured total resistance and to the calculated composite resistance using formulae available in literature. The new technique is demonstrated to compare favorably to measured total resistance and to outperform previously available methods.

Observations of supercooled water and frazil ice formation in an Arctic coastal polynya from moorings and satellite imagery

AbstractFormation of supercooled water and frazil ice was studied in the Chukchi Sea coastal polynya off Barrow, Alaska, USA, in winter 2009/10, using moored salinity/temperature sensors and Ice Profiling Sonar (IPS) data along with satellite data. Oceanographic data from two moorings revealed episodic events of potential supercooling at 30–40m depth, including the possibility of in situ supercooling, while the polynya was open. We identified frazil ice-like signals in the IPS data down to 5–15 m depth, associated with large heat loss and windy, turbulent conditions in an active polynya. This likely represents the first IPS observation of frazil ice in the marine environment. On the day of the maximum signal of frazil ice, spaceborne synthetic aperture radar shows streaks of high backscatter within the polynya, indicating active frazil ice formation just downwind of the mooring sites. In addition, the longer-term potential supercooling that persisted for 1–3 weeks occurred twice despite the absence of polynya activity at the mooring sites. These events occurred during periods dominated by the northeastward current. A series of coastal polynyas had formed southwest of the mooring sites prior to these events. Thus, the water masses with potential supercooling were likely advected from these polynyas.

Taking a look at both sides of the ice: comparison of ice thickness and drift speed as observed from moored, airborne and shore-based instruments near Barrow, Alaska

AbstractData from the Seasonal Ice Zone Observing Network (SIZONet) acquired near Barrow, Alaska, during the 2009/10 ice season allow novel comparisons between measurements of ice thickness and velocity. An airborne electromagnetic survey that passed over a moored Ice Profiling Sonar (IPS) provided coincident independent measurements of total ice and snow thickness and ice draft at a scale of 10 km. Once differences in sampling footprint size are accounted for, we reconcile the respective probability distributions and estimate the thickness of level sea ice at 1.48 ± 0.1 m, with a snow depth of 0.12 ± 0.07 m. We also complete what we believe is the first independent validation of radar-derived ice velocities by comparing measurements from a coastal radar with those from an under-ice acoustic Doppler current profiler (ADCP). After applying a median filter to reduce high-frequency scatter in the radar-derived data, we find good agreement with the ADCP bottom-tracked ice velocities. With increasing regulatory and operational needs for sea-ice data, including the number and thickness of pressure ridges, coordinated observing networks such as SIZONet can provide the means of reducing uncertainties inherent in individual datasets.

Multifrequency analyses of 2011–2012 Peace River SWIPS frazil backscattering data

A four-frequency SWIPS (Shallow Water Ice Profiling Sonar) instrument was deployed in the Peace River during the November 2011–May 2012 period near the Town of Peace River. The deployment site was previously used for deployments of one- and two-frequency SWIPS instruments. Acoustic volume backscattering coefficient and ice thickness data were collected at frequencies 125kHz, 235kHz, 455kHz and 774kHz. The data, acquired as a function of height in the water column, were processed to test particle size, concentration and acoustic frequency restrictions identified in a closely-related laboratory study of backscattering in pseudo-frazil suspensions (Marko & Topham, 2015) prior to applying optimal processing strategies on the 2011–2012 frazil data. Although one of the transducers (235kHz) displayed anomalous returns under certain environmental conditions, simultaneous measurements at 125kHz, 455kHz and 774kHz were found to yield consistent measures of particle concentration, size distribution and fractional volume with accuracies limited, primarily, only by the 1dB transducer calibration uncertainties. Fractional volumes were found to vary by less than an order of magnitude from interval to interval, with maximum values being approximately 10−4 (0.01%) except within, roughly, 0.75m of the river surface where interpretations were complicated by the frequent presence of surface ice. These results confirmed the effectiveness of multifrequency acoustic backscattering sonar (ABS) technology in applications to frazil monitoring problems and expanded its range of applicability beyond that conservatively estimated in the laboratory. The obtained data also provided an unprecedented basis for evaluating current river ice modeling assumptions. Comparisons with corresponding results from CRISSP model runs appear to be indicative of massive over-prediction of suspended frazil despite good model performance for simulating surface ice quantities. The origins of the possible discrepancies are discussed and accompanied by suggested options for improving model performance.

Characteristics of sea ice in the Okhotsk coastal polynyas revealed by satellites, ice-profiling sonar and digital camera observations

AbstractThe characteristics of ice in the Okhotsk coastal polynyas are examined. A map of AMSR-E thin-ice thickness in the northwest shelf region shows that most of the coastal polynya area is covered by thin ice with a thickness of 0.1 m. the thickness increases sharply at the edge of the polynya. From comparisons with QuikSCAT backscatter, the thin-ice area corresponds well with the low-backscatter area during almost all the coastal polynya period. A comparison with ice thickness measured by an ice-profiling sonar in the coastal polynya region of northeastern Sakhalin shows a similar relationship, with the backscatter tending to be low when the ice is thin. the averaged backscatter of thin (≤0.1 m) ice is –19.6±2.8 dB (horizontal polarization). the backscatter of thin ice is considered to be mainly determined by the surface roughness because volume scattering of thin ice is low due to the dielectric properties associated with the high salinity. the results of this study suggest that the coastal polynya is covered with grease ice/nilas whose surface is almost mirror smooth. This is confirmed by a comparison with photographs taken at the coastal polynya region near Magadan, Russia. At the outer edge of the coastal polynya, a region of relatively high backscatter (>–16 dB) exists, probably because it has a rougher ice surface than the polynya area. This suggests that ice motion in this area may be convergent.

Sea-ice drift characteristics revealed by measurement of acoustic Doppler current profiler and ice-profiling sonar off Hokkaido in the Sea of Okhotsk

AbstractIn the southwestern part of the Sea of Okhotsk off Hokkaido, sea-ice drift characteristics are investigated using the ice and water velocities obtained from a moored upward-looking acoustic Doppler current profiler (ADCP) during the winters of 1999–2001. Using hourly-mean values of these data along with the wind data measured at a nearby coastal station, the wind factor and turning angle of the relative velocity between the ice and water velocities with respect to the wind are calculated assuming free drift under various conditions. Since the simultaneous sea-ice draft data are also available from a moored ice-profiling sonar (IPS), we examine the dependence of drift characteristics on ice thickness for the first time. As ice thickness increases and wind decreases, the wind factor decreases and the turning angle increases, as predicted by the theory of free drift. This study clearly shows the utility of the moored ADCP measurement for studying sea-ice drift, especially with the simultaneous IPS measurement for ice thickness, which cannot be obtained by other methods.

Sonar detection and measurements of ice in a freezing river I: Methods and data characteristics

The basic concepts and operating principles of SWIPS (Shallow Water Ice Profiling Sonar) measurements in freezing rivers are outlined and described with reference to ongoing BC Hydro ice- and flow monitoring programs in the Peace River. Emphasis is given to identifying the nature of the measured quantities and their connections to the parameters conventionally used to describe and model rivers and their ice contents. Difficulties in deployment and data recovery, mostly associated with the growth of anchor ice, are described in terms of the evolution of successful approaches and the pitfalls encountered along the way. Distinctions are made among results obtained from targets at or inside floating surface ice and from particulate targets, suspended in the water column. Example results are presented in each case and put in the context of seasonal changes in the monitored quantities of interest. The usefulness of lower frequency measurements for ice cover studies is highlighted. Limited results from simultaneous multifrequency measurements on suspended particulate (frazil) targets are reviewed and analyzed to show the likely applicability of Rayleigh Law assumptions for observations made prior to ice cover stabilization and in, at least, the lower water column. This applicability is combined with estimates of absolute return strengths to make rough assessments of the ranges of particle sizes and concentration encountered in the Peace River studies.

Sonar detection and measurement of ice in a freezing river II: Observations and results on frazil ice

Results from 2004–2008 BC Hydro SWIPS (Shallow Water Ice Profiling Sonar) monitoring programs on the Peace River are presented and analyzed with respect to their content of information on the properties and behaviour of frazil ice suspended in water column. Strong distinctions were made in analyses applied to data collected, respectively, prior to and following local stabilization of the seasonal ice cover. This choice is shown to reflect both the behaviour and origins of the frazil ice present in these two intervals. Pre-freeze-up frazil tends to coat submerged surfaces, grows rapidly while in suspension and is only rarely and episodically present. It shows time dependences quite different from those associated with frazil detected prior to freeze-up. Frazil under a stationary ice cover is ubiquitous, does not adhere to other materials and appears to closely associated with the ice cover undersurface. The return intensity profiles observed in pre-freeze-up frazil events increased roughly linearly with height in the water column. Profile comparisons with earlier simulation results allowed inferences on particle size distributions and their controlling processes. The bulk of data analysis efforts was devoted to clarifying the time dependences of the more abundant post-stabilization SWIPS intensity data and its underlying connections to changes in the major river, atmosphere and ice cover environmental parameters. The obtained results showed unlagged or weakly negatively-lagged, positive correlations between such intensities and both river water levels and air temperatures (or solar radiation input) on, at least, diurnal and shorter time scales. The water level parameter was the stronger of these two influences and was believed to be representative of a physical link to river flow speed. Lower frequency connections to the physical state of the ice cover were also apparent. Interpretations of these results were offered in terms of a local equilibrium between the suspended frazil population and a dynamic slush layer at the bottom of the ice cover. Initial steps were taken toward developing and testing analysis tools potentially capable of supporting combined analysis and modelling of this equilibrium. A simple interpretative approach was applied to data acquired at a single acoustic frequency based upon combined use of a Rouse Law-based Inverted sediment model of uniformly-sized spherical frazil particles and volume backscattering coefficients measured in post-freeze-up intervals. Results suggested consistency with the basic form of return intensity profiles at most but not all times. For suitable measurement periods, the approach yielded reasonable estimates of the major particle size and concentration parameters. The deficiencies of this approach are discussed and compared with expectations and results from recently successful simultaneous measurements at two acoustic frequencies. Priorities are identified for validation, and refinement of single and multifrequency approaches along with integrated study of the lower ice cover as part of a broader program to understand frazil ice growth in freezing rivers.

Advances in upward looking sonar technology for studying the processes of change in Arctic Ocean ice climate

A major impetus for scientific studies of climate change in the Arctic Ocean has beenthe reduction in the areal extent and thickness of its sea ice cover. An extendedmeasurement record of the horizontal dimensions of this ice cover is available for thefull Arctic Ocean Basin based upon a record compiled from more than 30 years ofrelatively continuous satellite based measurements. Unfortunately, data accumulationsfor the ice cover’s vertical dimension, ie, sea ice thickness, tend to be limited to data setswith durations no longer than 15 years, reflecting underlying greater measurementdifficulties. Moreover, the longest duration ice thickness data collection efforts have beenconfined only to two specific portions of the Basin, namely, Fram Strait and the Canadiansector of the Beaufort Sea. Elsewhere, the available data sets are either of notablyshorter duration or non-existent.Upward-looking sonar (ULS) has been and continues to be the primary source ofdata with volumes and accuracy sufficient for meaningfully monitoring ice thickness.Originally deployed from polar-traversing submarines during the Cold War, the limitedamounts and accessibility of the collected data stimulated development of purpose-builtsea-floor moored ULS instrumentation which, beginning in the late 1980s, began tosupply the bulk of newly acquired ice draft and ice under-surface topography data.Technological advances have subsequently led to new generations of ULS instrumentsincluding ice-profiling sonar (IPS), incorporating much expanded on-board data storagecapacities (69 Mbytes to 8 Gbytes) and powerful real-time firmware which now allowunprecedented temporal (ping rates of up to 1Hz) and horizontal resolution of icetopography. These instruments operate autonomously during one year or longer deployments,returning draft data on time and spatial scales of 1s and 1m or better, respectively,which are essential to understandings of mechanical and thermodynamical aspectsof sea ice processes. Such processes govern ocean-atmosphere exchanges in polarwaters, thereby determining ice extent and thickness parameters. The larger data storage capacities of the newest instruments also allow collection of additional informationassociated with acoustic returns from different levels in the upper water column and thelower ice cover. Such data have potential for improving understandings of ice processesoccurring during the initial freeze-up and early consolidation phases of sea ice growth onthe basis of acoustic backscatter from frazil, grease, shuga and nilas ice forms. WithInternational Polar Year programmes now well underway, ice profiling instruments,sharing a common technology, are and/or will be deployed in unprecedented numbersfrom both fixed subsurface moorings and drifting buoys. This deployment commitmentholds great promise for delivery of data with both temporal and spatial detail and arealcoverage sufficient to strongly upgrade present capabilities for monitoring and modellingice cover change.

Characteristics of ice in the St. Lawrence River

The main objective of this paper is to describe the characteristics of brash ice in the St. Lawrence River downstream of Montréal over a period of three winters. We used two instruments deployed in the St. Lawrence River navigation channel through Lake St. Pierre to measure ice parameters: an acoustic Doppler current profiler (ADCP) and an ice-profiling sonar (IPS). This paper discusses the capacities of these instruments to quantify ice characteristics and to predict the risk of ice congestion. It was found that wind velocity and air temperature play major roles in the variation in ice parameters and, consequently, in the occurrence of ice congestion in the navigation channel through Lake St. Pierre. Comparison of the IPS and ADCP data showed good agreement and demonstrated that these two instruments can be very effective for certain ice applications.Key words: ice characteristics, ice congestion, ADCP, IPS, fuzzy logic.

Deriving sea-ice thickness and ice types in the Sea of Okhotsk using dual-frequency airborne SAR (Pi-SAR) data

AbstractDual-frequency, multi-polarization airborne synthetic aperture radar (Pi-SAR; developed by the Communications Research Laboratory and National Space Development Agency of Japan) observations of the seasonal sea-ice region off the Okhotsk coast of Hokkaido, Japan, were carried out in February 1999 using X- and L-band radar frequencies with a resolution of 1.5 and 3.0 m. In conjunction with the SAR observations, the sea-ice thickness (draft) and velocity were measured by a moored Ice Profiling Sonar (IPS) and an Acoustic Doppler Current Profiler (ADCP). Tracks of the sea ice passing over the IPS were estimated from the time series of the ADCP ice-velocity and -direction data. Along these tracks, the SAR backscattering coefficient profiles were compared with the IPS ice-draft profiles. The results showed that the L-band SAR backs cattering profiles correlated well with the IPS ice-draft data, particularly in the thicker part (a few meters thick) of the rim of first-year ice, which had a large backscattering coefficient. Although the X-band SAR backscattering profiles did not correlate well with the IPS data, thin ice (<10 cm thick) showed a large backscattering coefficient. The L-band SAR and IPS data did not distinguish thin ice from open water.

Sound Scattering from Sea Ice: Aspects Relevant to Ice-Draft Profiling by Sonar

Abstract Accurate discrimination between thin ice and open water using sonar is an important practical concern for the calibration of ice-draft observations and for the use of ice-profiling sonar in climate-related studies of sea ice. To guide improvement of the surface-detection algorithm used by this instrument, a narrowbeam 400-kHz sonar was used to record over 1 million surface echoes from pack ice in the Beaufort Sea during the winter of 1995–96. Large fluctuations in the amplitude of successive echoes were observed for all types of targets. The probability density of fluctuations had lognormal form, indicative of multiple-scattered incoherent returns for all targets, including calm ice-free water, uniform growing first-year ice, and ridge keels. Median values of the scattering coefficient ranged from −6.8 dB for calm water to −30 dB for thick level ice and ridge keels. These low values indicate that the surfaces of these targets appear very smooth to narrowbeam sonar at this frequency. Despite this,...

Measurements of the Underside Topography of Sea Ice by Moored Subsea Sonar

Abstract A practical technology based on moored subsea instrumentation has been developed to measure the draft of polar pack ice. The technology exploits the complementary capabilities of an ice-profiling sonar designed and built for the application and of a commercially available acoustic Doppler sonar. The former instrument observes the zenithal range of sea ice passing through its single narrow sonar beam, while the latter observes the radial motion of the ice along its four inclined beams. The sequence of ranges obtained by the ice-profiling sonar is combined with supplementary observations of hydrostatic pressure to yield a sequence of ice draft versus time; the sequence of Doppler speeds provides ice velocity that can be integrated to obtain displacement; by combining the draft and displacement sequences the profile of draft versus position is obtained. The foremost practical problem in calibration is establishing the temporal variation in the zero-draft reference. The technology is well suited to u...