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Sidescan Sonar Imagery of the Winter Marginal Ice Zone Obtained from an AUV

P. W ADHAMS Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, and Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll, United Kingdom

J. P. WILKINSON Scottish Association for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll, United Kingdom

A. KALETZKY Department of Applied Mathematics and Theoretical Physics, University of Cambridge, Cambridge, United Kingdom

(Manuscript received 27 January 2003, in ®nal form 3 October 2003)

ABSTRACT The ®rst Arctic under-ice sidescan sonar imagery from an autonomous underwater vehicle (AUV) has been obtained in the winter marginal ice zone of the East Greenland Current at 73Њ00ЈN, 11Њ47ЈW, using a Maridan Martin 150 vehicle operated from R/V Lance. First-year, multiyear, brash, and frazil ice can be discriminated, with the underside of multiyear ice appearing smooth as compared to the rough underside detected by submarine- borne sidescan in the Arctic Basin, implying downstream bottom melt. The ice draft pro®le was obtained from the vertical part of the sidescan beam, and the probability density function of ice thickness was derived and found to agree well with upward sonar results from this region obtained in 1987 from a British submarine.

1. Introduction fully validated in comparative experiments; other satellite-based techniques using synthetic aperture radar Recent evidence of changes in the Arctic indicate that (SAR) or passive microwave involve inference from the sea ice cover is undergoing a signi®cant thinning other measured parameters. Airborne techniques (laser (Rothrock et al. 1999; Wadhams and Davis 2000) and altimetry for freeboard, electromagnetic sounding for retreat (e.g., Bjùrgo et al. 1997). General circulation models using greenhouse gas forcing predict that the thickness) are expensive for obtaining data over large Arctic ice cover will continue to diminish because of areas, while through-ice techniques (hole drilling, sur- global warming and may become seasonal by the 2080s. face sounding) are purely local. This leaves under-ice At the same time, it is possible that the current ice mapping as the most readily available and commonly shrinkage is a response to oceanic and atmospheric used technique, involving the use of upward-looking changes associated with the Arctic Oscillation (AO) sonar from moorings or from submarines. Once again, (Thompson and Wallace 1998), and that these will re- moorings offer data only at ®xed locations, even though verse when the AO itself changes phase. To resolve this these may be critical choke points (e.g., Fram Strait), question, it is essential to monitor the Arctic ice cover so only submarines have offered true synoptic ice-thick- on a seasonal and interannual basis. This is easy from ness mapping. The continued availability of submarines the point of view of extent, because of the availability (U.S. and British) is therefore essential to the task of of satellites, but dif®cult from the point of view of thick- monitoring Arctic ice thickness through the present pe- ness. riod of rapid change. Since the end of the Cold War, Current methods of ice thickness monitoring have however, the deployment of British submarines in the been reviewed by Wadhams (2000). Essentially, the only Arctic has become more sporadic, and the U.S. civilian direct satellite-borne technique is the radar altimeter, Scienti®c Ice Expeditions (SCICEX) program, which which measures freeboard and which has not yet been also produced many valuable data on Arctic sea ice from submarines, has been reduced in scope. Given the probable continued shortage of submarine Corresponding author address: Prof. Peter Wadhams, Scottish As- availability, the use of autonomous underwater vehicles sociation for Marine Science, Dunstaffnage Marine Laboratory, Oban, Argyll PA37 1QA, United Kingdom. (AUVs) under sea ice is clearly an option that needs to E-mail: [email protected] be pursued vigorously, as AUVs offer the only platform

᭧ 2004 American Meteorological Society

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One of the key criteria of an AUV is its navigational accuracy. The navigation and positioning system used on the Martin 150 was the MARPOS system. The core is an inertial navigation system using a ring laser gyro, coupled to a Doppler velocity log with a Trimble dif- ferential global positioning for surface ®xes. In addition, the system is fed with information from a DigiQuartz pressure sensor to maintain accurate depth. The ex- pected horizontal error from this system is 0.1% of the track distance. However, accurate navigation of an AUV where bottom tracking is not possible, or where the surface being followed is moving (i.e., sea ice), is a major challenge. This was the case for these runs; however, the accuracy achieved was better than 25 m at all times during the submerged missions and typically 15 m when real and programmed trajectories are compared FIG. 1. Location map showing the Greenland Sea region, with (Maridan 2003, personal communication). Greenland to the west and Spitsbergen to the northeast, and the ice The scienti®c payload included a Tritech International edge from passive microwave data (north±south line). The geometry SeaKing 675-kHz sidescan sonar (with two transducers, of the two ice-pro®ling runs can be seen in the insert. located on the port and starboard sides of the AUV), a conductivity±temperature±depth sensor (CTD) and an that will be de®nitely available for scienti®cally con- acoustic Doppler current pro®ler (ADCP). The SeaKing trolled missions. The vehicles should be equipped with 675 has a ping rate of 5±7 Hz, a pulse length of 50± both upward-looking and sidescan sonar, to map both 200 ␮s, a 3-dB swath width of 50Њ, and an along-track the ice thickness and the bottom topography, the latter beamwidth of 0.7Њ. in order to detect evidence of bottom melt effects (Wad- The AUV was housed in an insulated 20-ft container hams 1997) or pressure ridge disintegration (Schramm located on the foredeck of the ship. The container had et al. 2000) that may be contributing to the observed access to heat and power and acted as the AUV storage ice loss. and maintenance facility during survey. The AUV was As a step in the development of such techniques, we deployed and recovered by a crane. The AUV can be have succeeded in obtaining sidescan sonar images from seen in Figs. 2a and 2b. During launching and buoyancy the underside of Arctic sea ice using an AUV, the ®rst trials the weather was uniformly bad with very high time that this has been done in the Arctic and also the winds, and super®cial damage to the vehicle was caused ®rst measurements done during a polar winter. The mea- during launch and recovery, necessitating repairs. On surements were carried out in February 2002 from the day of the successful data-gathering mission (27 R/V Lance (Norsk Polarinstitutt) using a Maridan Mar- February) the air temperature was Ϫ12.5ЊC, and wind tin 150 vehicle in the East Greenland pack ice at speed had moderated to 11 m sϪ1 from the north. 73Њ00ЈN, 11Њ47ЈW, as part of a research cruise to study The depth of the ®rst run was 20 m at a speed of 1.2 Greenland Sea convection under the European Union msϪ1. This depth was chosen to avoid the deepest ice (EU) CONVECTION program. In this paper we show in the region as estimated using the statistics of ice- some examples of the results obtained. thickness pro®les from earlier submarine missions (Wadhams 1992). After completing the mission data were sent back via a high-frequency radio link to the 2. The vehicle and the experiments ship. Via this link the AUV was reprogrammed for the The experiments were carried out within a mixture second run, eliminating the need for it to be recovered of ®rst-year and multiyear ice in the winter marginal between runs. A scan through the data revealed that ice ice zone (MIZ) of the East Greenland Current (Fig. 1), in the region was less than 5 m thick, and thus the second where the ice was broken into ¯oes by wave action but run was performed at a shallower depth of 10 m and where fresh ice was growing in the interstices between the same speed. Figure 1 shows the geometry of the ¯oes in the form of frazil and pancake ice (the types of two runs and the location of the operation: in run 1 the new ice that form in turbulent conditions). two parallel legs were 30 m apart while in run 2 they The AUV used for the ®eld campaign was the Mar- were 80 m apart. The total track length was 4.6 km. idan Martin 150. The Martin had a successful track record in inshore surveys but until this time had not been 3. Deriving ice draft from sidescan sonar used in the open ocean. Its speci®cations were as fol- measurements lows: length 4.5 m; beam, including hydroplanes, 2.0 m; height 0.6 m; dry weight 900 kg; and operational Under-ice sidescan sonar was ®rst used under ice in depth 150 m. the Arctic on a submarine in 1976 (Wadhams 1978) and

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determined time for the sound to be re¯ected back before transmitting another pulse. It is important to note that sidescan, unlike multibeam, has only one receiver per ship side. Thus, declination must be inferred from target distance only, using the assumption that the target ®eld is essentially ¯at and horizontal. Although the raw swath from the sidescan appears to be a real representation of the target area, it is in fact distorted, with the greatest distortion occurring in the near range of the record. This is due to the geometry of the system, as the initial return to the sidescan transducer is approximately vertical and the return from far range is approaching horizontal. Producing a true two-dimensional image from sidescan can be complex; however, by assuming that the ®rst sonar return is from ice immediately above the AUV, the process can be simpli®ed. This assumption may not be valid in all cases, as it assumes that the ice is generally ¯at. It is conceivable that thicker ice, that is, a keel, may exist offtrack, yet with a travel time for the sonar pulse that is shorter and thus returns earlier than from ice that is closer to the vehicle's track. With this proviso, a sequence of processing techniques was applied to the raw sidescan data in order to obtain a cor- rected two-dimensional image. They were the removal of the water column, slant-range correction, and velocity correction. As there were only very small excursions in AUV pitch and roll throughout the missions, no cor- rections were made in this instance.

FIG. 2. (a) An AUV surfacing in the marginal ice zone of the East Greenland Current. All ice types were present from newly formed a. Removal of the water column ice to multiyear ¯oes. (b) Recovery of the AUV by starboard-side crane. A zodiac was lowered into the water to attach a safety line to The water column offset is the distance between the the AUV. AUV and the bottom of the ice. In order to produce an image swath of the underside of the ice this must be removed before the port and starboard sidescan sonar subsequently in 1987 (Wadhams 1988; Wadhams and swaths are married together. Sea ice is acoustically high- Martin 1990), in both cases using equipment ®tted by ly re¯ective and leaves a bright, well-de®ned image on the experimenter. However, sidescan sonar is a well- a sidescan record; thus, the interface between the water proven acoustic technique for mapping features on the sea¯oor (e.g. Blondel and Murton 1997; Medwin and and the bottom of the ice is generally very pronounced Clay 1998). It works (Seatronics 2002) by transmitting (Fig. 3). However, noise within the return signal can a fan-shaped beam of acoustic energy outward from port make the automatic detection of the interface dif®cult and starboard transducers so as to sweep the seabed in and in some cases can lead to an erroneous detection a line-scan fashion as the vehicle carrying the trans- of the ice±water interface. Noise in the sonar record ducers (submarine, ship, or tow®sh) advances. As the manifested itself as either a bright along-track streak at energy radiates outward a proportion is re¯ected from the start of the record, due to return signal from the objects intercepted at different distances from the sonar. sonar±water interface, or as random cross-track strips The intensity of the returning energy or echo is a func- throughout the record, possibly due to noise in the water tion of the shape and density of the objects encountered. column (e.g., ship's noise). The return signal from the Furthermore, a target with relief will result in an absence sonar±water interface only affected the ®rst few bins of sound, or shadows, behind the object, just as shining and thus was easily removed. Random noise (the hor- a beam of light at a low angle along the ground will izontal streaks) was removed by a Butterworth ®lter. An create shadows behind objects that it hits. This intensity intensity threshold was chosen to distinguish the water± variation manifests itself as tonal variations, with light ice interface and thus identify into which bin it fell. and dark portions representing strong and weak echoes, Figure 3b shows the ice±water interface line, displayed respectively. After each pulse the sonar waits for a pre- in red, superimposed over the raw data.

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of 1.5 m, each sonar does not see the same point on the underside of the ice; thus, the ice draft from each chan- nel is slightly different, and a 1.5-m zone of missing data exists in the middle.

c. Slant-range correction The slant range is de®ned as the straight-line distance from the sidescan transducer to the point of re¯ection on the target, assumed to be the hypotenuse of a triangle de®ned by the AUV, the water±ice interface directly above it and the re¯ection point. Through simple trig- onometry the ``real'' horizontal distance between bins can be calculated, making the assumption that the draft variations are small compared to the vehicle depth. Deep keels appear closer to the centerline than their real po- sition, so the full image is subject to local distortions.

d. Velocity correction The last stage in the production of a correct 2D image with a 1:1 aspect ratio was to correct for inconsistencies in the along-track velocity of the AUV. Navigational data from the AUV were used to calculate the distance traveled between successive pings of the sonar using the World Geodetic System 1984 (WGS 84) ellipsoid. FIG. 3. (a) Subset of raw data from port sidescan sonar. The dark With each ping of the sonar now associated with a dis- region, to the left of the picture, represents the water column above tance from the start of the run, a true visualization of the AUV. (b) Image shown in Fig. 1a but with the water±ice interface (red line) detected using the method described within the text. the underside of the ice could be obtained.

4. Results from sidescan b. Deriving ice draft We now show some examples of the imagery ob- In our case the intensity of the sonar returns was tained, along with corresponding draft pro®les of the delivered in bins, with each bin representing an increase portion of track that lies along the centerline of the in the slant range of 0.1 m. In order to obtain an ice image. draft pro®le from the raw sidescan sonar data the lo- Figure 4 is a 100-m-long sidescan section starting at cation of the water±ice interface must ®rst be correctly 700 m into the ®rst run of the AUV. It reveals a close- located. Once located, the bin that corresponds to the packed array of ¯oes, the majority with smooth fea- interface can be identi®ed, and the distance from the tureless bottoms. Since the almond-shaped ¯oe (40 m AUV to the ice bottom surface obtained. The depth of across) that crosses the centerline of the survey track is the AUV is known at all times from the time-stamped 2.3 m thick, it is therefore likely to be multiyear ice, as CTD log. However, the AUV's internal microprocessor ®rst-year ice at this latitude in the Greenland Sea is recorded the CTD and positional data at a slightly lower usually around 1±1.5 m thick. The smooth homogeneous frequency than the sidescan data. In order not to lose bottom of this and other ¯oes in the images shows that the high de®nition of the sidescan images, the CTD and considerable bottom melt has been going on, since in navigational data were interpolated to the same fre- the Arctic Basin itself multiyear ice has a very rugged quency as the sidescan, allowing each time stamp to underside morphology of bulges and depressions, which have navigational, oceanographic, and sidescan data as- enables it to be readily distinguished from smooth-bot- sociated with it. Finally, the ice draft was calculated by tomed ®rst-year ice on sidescan imagery (Wadhams subtracting the vertical distance from AUV to bottom 1988). Many of the other ¯oes in the image have con- of the ice from the AUV depth at the time. A constant siderable thickness, as shown by the width of the shad- was further subtracted from these measurements to al- ow zone on the side distant from the sidescan transducer, low for the difference between the vehicle location of yet are only a few meters in diameter, sometimes the CTD and the sidescan. This method was applied to 2±3 m. Such thick, but small, ice cakes are called brash each sonar, and thus ice draft measurements for both ice and are the product of the extreme storm conditions the port and starboard sides of the AUV were obtained. to which this part of the ice ®eld (and the R/V Lance) Owing to the lateral separation of the two sonar units had been subjected for the preceding 3 weeks, which

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FIG. 4. (top) A 100-m section of sidescan from mission 1 showing (a) a large multiyear ¯oe surrounded by a network of smaller ¯oes and brash fragments embedded in frazil ice. (bottom) The ice draft as seen directly above the port and starboard sidescan sensors. caused ¯oes to collide vigorously and break into a va- vailed on preceding days; visual observation revealed riety of shapes and sizes. Apart from the deep ¯oes and many ¯oes with brash fragments on the surface that had brash fragments, there is a light gray background with been rafted over the ¯oe by waves breaking against no apparent morphological contrast that surface obser- them. vations showed to be frazil ice, that is, a suspension of Figure 6 comes from the second run of the AUV and new ice crystals in water, growing rapidly in the cold shows a 110-m section of the sidescan imagery starting air but unable to form a continuous sheet on account of from 1330 m. This image shows heavy ice conditions the high turbulence. In the lee of the ¯oes there are also with little or no open water present. In general the AUV occasional black patches of open water. The black±white ran under more compact ice conditions during the sec- difference here, of course, is actually a difference be- ond run. The three angular multiyear ¯oes, each up to tween the high sonar backscatter of frazil ice (light) and 2.5 m thick, lying along the track are likely to have the low backscatter of open water (dark), as frazil ice originated from the same ¯oe that broke up because of has a higher acoustic backscatter. collisions or wave action. Their undersides have an un- Figure 5 is again from the ®rst run of the AUV and dulating structure but are basically smooth. Surrounding shows a more open part of the ice ®eld, with much open these ¯oes is a network of smaller ¯oes and brash frag- water in the upper section of the image. Within the open ments embedded in frazil ice. water are narrow Langmuir streaks of newly forming frazil ice. A thick ice ¯oe lying off the centerline near A 200-m section of sidescan imagery from the second the top is distinguished by its strong (white) nearside run can be seen in Fig. 7. This again shows very high re¯ections. Of interest is the ¯oe in the center at the concentrations of ice, with all ice types, from frazil to bottom, which has structure on its underside. Because multiyear ¯oes, being present. Toward the left-hand side it is offtrack we can only estimate its thickness from is a very thick ¯oe (around 5 m) casting a long shadow. the length of its farside shadow, which gives an estimate This ¯oe can easily be seen in the draft pro®le and is of 2 m, suggesting that it is a multiyear ¯oe. One pos- possibly rafted under another ¯oe or a thick multiyear sibility is that this is a ¯oe that has not been subjected ¯oe. On the far right are two other very thick ¯oes (lying to much melt and retains its characteristic ruggedness. just off centerline to left and right) with some structure A more likely possibility is that the protuberances on visible on the underside of the lower ¯oe, suggesting a the bottom are actually brash fragments that have been multiyear ¯oe where the bottom topography has not yet rafted under the ¯oe in the violent conditions that pre- been completely made smooth by the heat ¯ux asso-

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FIG. 5. (top) Section of sidescan from mission 1 showing a more open part of the ice ®eld, with much open water, and (a) narrow Langmuir frazil streaks in the upper section of the plot and (b) a ¯oe in the center at the bottom that has structure on its underside. (bottom) The ice draft as seen directly above the port and starboard sidescan sensors.

FIG. 6. (top) Section of sidescan from mission 2 showing a high concentration of all ice types: frazil, brash/pancake, and ¯oes. (a), (b), (c) Three angular multiyear ¯oes, each up to 2.5 m thick can be seen in the central region of the image. No open water is visible in the image. (bottom) The ice draft as seen directly above the port and starboard sidescan sensors.

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FIG. 7. (top) A 200-m section of sidescan from mission 2 showing (a) a very thick ¯oe (about 5 m) toward the left-hand side of image. Again the image is of high concentrations of ice (all types) with little or no open water present. (bottom) The ice draft as seen directly above the port and starboard sidescan sensors.

FIG. 8. (top) Section of sidescan image from run 2 showing both open water (top right) and closed pack conditions (bottom). (a) A ¯oe almost 3 m thick includes a structure adhered to the bottom. The shape and size of the protrusion on the bottom suggests that a brash fragment has been rafted under the larger ¯oe. Shown in the upper-right sector of the image are (b) Langmuir streaks of frazil ice surrounded by open water. (bottom) The ice draft as seen directly above the port and starboard sidescan sensors.

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FIG. 10. (a) Probability density functions of ice draft (in 0.1-m bins) along the centerline for combined port and starboard channels FIG. 9. Probability density functions of ice draft along the center- between 700 and 800 m along run 2. Individual ¯oes of different lines of (a) runs 1 and (b) 2 for combined port and starboard channels ages correspond to different peaks. (b) Probability density functions are shown in black. Also shown, in gray, are draft distributions ob- of ice draft (in 0.1-m bins) along the centerline for combined port tained in May 1987 from HMS Superb from the latitude range 72Њ± and starboard channels between 1700 and 1800 m along run 2. The 74ЊN. in¯uence of the rafted ice block surrounded by thinner ®rst-year ¯oes (0.5±1.8 m) is clearly visible. ciated with the high ice±water velocities in this turbulent environment. are taken over the whole of each run (1700 m for run Figure 8 shows a 100-m section from run 2. Of in- 1, 2900 m for run 2). There are some differences be- terest is the ¯oe in the center of the image, which has tween the PDFs, but on the whole there is good agree- structure on its underside. This ¯oe is almost 3 m thick ment between both missions. The large peak at 0±0.5 where it crosses the centerline, so it is clearly a multiyear m corresponds predominantly to the open water and ¯oe; however, when including the structure adhered to frazil ice that lay between the ¯oes, while the rest of the bottom of the ¯oe it has a draft of almost 6 m the PDFs show a general tailing off toward the larger (estimated from the shadow that it casts). The shape and ice drafts. The modal draft for the melting polar ice size of the protrusion on the bottom suggests that a brash ¯oes encountered is 1±1.5 m, and the maximum draft fragment has been rafted under the larger ¯oe. In the is 5 m, corresponding to a ¯oe in Fig. 7. The mean upper-right sector of the image we see Langmuir streaks drafts (including open water) are 0.75 and 0.87 m for of frazil ice surrounded by open water. On closer ex- each run, respectively. The closest comparable data from amination, however, there is a slight pattern to the water the winter months comprised an upward sonar pro®le that indicates low concentrations of frazil at the surface collected in May 1987 by HMS Superb (Wadhams 1992) of the water; that is, frazil ice is just beginning to form from the East Greenland Current; the results for 72Њ± in this region. 74ЊN (220 km of data), also shown in Fig. 9, agree remarkably well. The exception is that the deepest ice observed in 1987 was 7 m (72Њ±73ЊN) and 8 m (73Њ± 5. Results for ice draft 74ЊN), but this may simply be a statistical artifact of Ice draft pro®les, of which Figs. 4±8 show examples, the short record length in 2002. were used to generate probability density functions Probability density functions of shorter stretches of (PDFs) of draft. Figure 9 shows the probability density data, in 0.1-m bins, show the impact of individual ice of ice draft in 0.5-m bins for each of the two runs, with ¯oes. Figure 10a shows the PDF from the combined data from port and starboard sensors combined. The data port and starboard sensors at 700±800 m along run 2,

Unauthenticated | Downloaded 09/30/21 02:32 PM UTC 1470 JOURNAL OF ATMOSPHERIC AND OCEANIC TECHNOLOGY VOLUME 21 where the preferred drafts are clearly separated into 0.7± or overlapping grid of imaging tracks. The main draw- 0.8, 1.5±1.6, and 2.8±3.0 m, respectively. These drafts back of the present AUV is lack of range. correspond to individual ¯oes of different ages and high- light the mixture of ice types found within the MIZ. Acknowledgments. We are grateful to Bo Krogh, the Figure 10b shows the PDF, again from the combination Maridan senior surveyor on board, for the deployment of the port and starboard sensors, at 1700±1800 m along of the vehicle. The work was supported by the European run 2. It is evident that the majority of ice is ®rst-year Union under Contract EVK2-2000-00058 for the CON- ¯oes, due to its shallow draft; however, the in¯uence of VECTION program. the 5-m ice block (seen in Fig. 7) is clearly seen. REFERENCES Bjùrgo, E., O. M. Johannessen, and M. W. Miles, 1997: Analysis of 6. Conclusions merged SMMR±SSMI time series of Arctic and Antarctic sea ice parameters 1978±1995. Geophys. Res. Lett., 24, 413±416. We have demonstrated that it is possible to perform Blondel, P., and B. J. Murton, 1997: Handbook of Sea¯oor Sonar high quality AUV surveys in the extreme polar envi- Imagery. Wiley, 314 pp. ronment in winter and obtain data that are comparable Medwin, H., and C. S. Clay, 1998: Fundamentals of Acoustical to those taken by manned submarines. It is particularly Oceanography. Academic Press, 712 pp. Rothrock, D. A., Y. Yu, and G. A. Maykut, 1999: Thinning of the reassuring that we were able to visualize all forms of Arctic sea-ice cover. Geophys. Res. Lett., 26, 3469±3472. sea ice from frazil, never seen in a sidescan image be- Schramm, J. L., G. M. Flato, and J. C. Curry, 2000: Toward the fore, to multiyear ice. modeling of enhanced basal melting in ridge keels. J. Geophys. Future AUVs for ice-bottom imaging should clearly Res., 105 (C6), 14 081±14 092. Seatronics, cited 2002: Tritech ROV Sidescan Sonar. Seatronics da- be equipped with multibeam sonar for complete quan- tasheet. [Available online at http://seatronics-group.com/ titative swath mapping of ice draft, or at the very least geophys/ztriside.htm.] a combination of simple sidescan and narrowbeam up- Thompson, D. W. J., and J. M. Wallace, 1998: The Arctic Oscillation ward-looking sonar. signature in the wintertime geopotential height and temperature ®elds. Geophys. Res. Lett., 25, 1297±1300. It is hoped that this successful ice-pro®ling mission Wadhams, P., 1978: Sidescan sonar imagery of sea ice in the Arctic will be a precursor to larger-scale missions with longer- Ocean. Can. J. Remote Sens., 4, 161±173. range AUVs that will develop into a major monitoring ÐÐ, 1988: The underside of Arctic sea ice imaged by sidescan sonar. effort for Arctic sea ice changes. If ice velocity is known Nature, 333 (6169), 161±164. ÐÐ, 1992: Sea ice thickness distribution in the Greenland Sea and (e.g., from drifters or successive satellite images), ice Eurasian Basin, May 1987. J. Geophys. Res., 102 (C4), 5331± draft distribution itself, as measured by the AUV, gives 5348. mass ¯uxes. Also, ice thickness results from future AUV ÐÐ, 1997: Variability of Arctic sea ice thicknessÐStatistical sig- runs can be used to validate freeboard estimates from ni®cance and its relationship to heat ¯ux. Operational Ocean- ography. The Challenge for European Co-operation, J. H. Stel the new altimeters (laser and radar) aboard IceSat and et al., Eds., Oceanography Series, Vol. 62, Elsevier, 368±384. Cryosat, in order to allow mean ice thickness to be ÐÐ, 2000: Ice in the Ocean. Taylor and Francis, 351 pp. estimated throughout the Arctic. ÐÐ, and N. R. Davis, 2000: Further evidence for ice thinning in The AUV has many advantages, notably the high res- the Arctic Ocean. Geophys. Res. Lett., 27, 3973±3975. ÐÐ, and S. Martin, 1990: Processes determining the bottom topog- olution that is possible by sailing close to the ice bottom, raphy of multiyear Arctic sea ice. Sea Ice Properties and Pro- a bene®t that manned submarines cannot enjoy for safety cesses Monogr. 90-1, U.S. Army Cold Regions Research and reasons, and the possibility of a closely controlled tight Engineering Laboratory, 136±141.

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