USING A HELICOPTER-BORNE EM-INDUCTION SYSTEMTO VALIDATE RADARSAT SEA ICE SIGNATURES

I. K. Peterson1 , S.J. Prinsenberg1 and J. S. Holladay2

1 Bedford Institute of Oceanography, Dartmouth, N.S., Canada2 Vanguard Geophysics Inc., Toronto, Ontario, Canada

ABSTRACT

Field surveys over the past several winters in the Gulf of St. Lawrence and off Labrador usehelicopter-borne sensors to validate SAR ice signatures in RADARSAT imagery. Ice-plus-snow thickness profiles were collected using an electromagnetic (EM) induction systemtowed 15-25m above the ice surface by helicopter. Measurements from the laser altimetercontained in the EM system were high-pass filtered to derive ice surface topographyprofiles. In RADARSAT images, changes in SAR backscatter values are usually associatedwith changes in EM-measured ice thickness and laser-measured surface roughness. Forexample in the Gulf of St. Lawrence, large floes having low SAR backscatter are associatedwith uniform EM-measured ice thicknesses of 30-50 cm, while more variable icethicknesses are present in the surrounding areas having higher backscatter. EM-measuredice thicknesses representing both deformed and undeformed ice are 50% higher than icethicknesses obtained through augered ice holes in undeformed ice. In a SAR image of theLabrador shelf area, the inshore ice appears dark, with bright streaks visible southeast(downwind) of small coastal islands. These streaks correspond to ice rubble with EM-measured ice thicknesses of about 1-2 m and laser-measured ridge elevations up to 0.9 m.

1. INTRODUCTION

The principal use of RADARSAT data is for ice reconnaissance in support of marinenavigation in Canadian waters. While RADARSAT can provide some information onconcentrations of the various ice types and on ridging, it cannot provide absolute icethicknesses. Interpretation is often difficult, particularly in the near-range of ScanSARproducts, and in regimes of low-to-moderate ice concentrations and small floe sizes (Ramsayet al., 1998). In early March 1996, field experiments were conducted off eastern Canada inthe Gulf of St. Lawrence and off Labrador to validate SAR ice signatures in RADARSATimagery, and to demonstrate how data from a helicopter-borne electromagnetic (HEM)induction system can be used to help interpret the images.

2. DATA COLLECTION

Ice-plus-snow thickness profiles were collected over many tens of kilometers using an HEMsystem owned by the Canadian Coast Guard. It consists of a 4m-long cylindrical sensorpackage or “bird” towed 30m below the helicopter and 15-25m above the ice surface. Thesystem estimates the distance to the ice-seawater interface by means of the electromagneticinduction method, which uses frequencies in the 1-100 kHz range. The distance between the

bird and the surface of the ice (or snow if it is present), is measured with a laser altimeteralso contained in the bird. The difference between the two distances gives the ice-plus-snowthickness (Rossiter and Holladay, 1994).

The HEM system uses frequencies of 30 and 90 kHz, and a horizontal coplanar coilconfiguration which has a footprint of about 3.7 times the height of the bird (Kovacs et al.,1993), or 74 m for a flying height of 20 m; the footprint of the laser is a few centimeters. ATrimble TANS Vector GPS orientation sensor in the bird is used to correct for pitch and rollwith an accuracy of 0.1° (Holladay et al., 1997).

The laser altimeter is an IBEO PS-100E and the sampling rate is 28 Hz. Ice surfacetopography profiles were extracted from the laser data by removing the bird altitudevariation using the three-step procedure described by Hibler (1972).

Video imagery was collected using a downward-looking S-VHS video camera mountedin a pod on the outside of the helicopter. The imagery was annotated with position andtiming information using a video overlay unit.

The ice thickness surveys were conducted from February 27 to March 7 in the Gulf of St.Lawrence and from March 10 to March 12 off the Labrador coast (Moucha et al., 1998).Because of ice drift, the HEM data collected in the Gulf were repositioned to the acquisitiontimes of the imagery by tracking features near the end-points of the EM flight lines in theRADARSAT imagery and in SLAR imagery acquired near the time of the EM flights. OffLabrador, the HEM data were repositioned using drift estimates obtained from satellite-trackedice beacons deployed on the ice by helicopter.

Direct measurements of snow and ice thickness were obtained through augered ice holes,mostly at ice beacon deployment sites, CTD stations and along lines used for calibration andvalidation of the HEM system. Ice salinity samples were collected at depths from 0 to 30 cm.All on-ice measurements are presented in Moucha et al (1998).

3. RESULTS

A ScanSAR Narrow sub-image (64 by 64 km) acquired on March 6, 1996 (10:26 Z) showspack ice in the Gulf of St. Lawrence, west of the Magdalen Islands (Figure 1). Many largedark-toned floes can be seen, surrounded by lighter-toned areas and a few dark leads. In thefield, the large floes were observed to be very smooth with little snow cover, and weresurrounded by smaller ridged floes and leads containing young brash ice. EM-measured icethickness profiles collected on the same day (~17:00 Z) are superimposed on the image. Theprofiles measured over the dark-toned floes (segments B and C) are clearly less variable andcontain fewer high ice thicknesses than the profiles measured over the brighter areas in theSAR image (segments A and D); the high-pass laser altimeter data are also less variable overthe dark-toned floes. Although the higher backscatter in the areas surrounding the largeundeformed floes is in part due to ridging, it is mostly due to the young brash ice; ridgingusually occurs in regions of thinner ice.

4 m

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Figure 1. ScanSAR Narrow image of the southern Gulf of St. Lawrence from March 6, 1996(1026Z, 64 km by 64 km) with EM-measured ice thickness profiles superimposed; thevertical scale denotes 4 m thickness. © CSA/ASC, 1996

Ice thickness distributions for segments A, B, C and D are shown in Figure 2. Forsegment B, 65% of the observations are between 30 and 50 cm; a peak is also seen at thesethicknesses for segment C. For segments A and D however, the ice thickness distributionsappear to be bimodal, with peaks at both 30-60 cm and 70-90 cm, suggesting that theyrepresent a mixture of deformed and undeformed ice.

Ice-plus-snow thicknesses were measured directly through a total of 91 auger holes at14 measurement sites, mostly where ice beacons were deployed. The sites chosen were onundeformed first-year floes which were large (0.5-4 km) in order to ensure that the beaconslasted more than a few days. The ice thickness distribution from the auger holes (Figure 3a)shows that 60% of the observations were between 30 and 50 cm, and is similar to thedistribution of ice thicknesses from segment B (Figure 2) obtained over a dark-toned floe.From the ice thickness distribution for all EM measurements plotted in Figure 1, the ice

concentration is 0.94 assuming thicknesses less than 10 cm are open water (Figure 3b). Forthe remaining thicknesses, only 28% of the observations are between 30 and 50 cm. Themean and median ice thicknesses from the auger holes are 49 and 42 cm respectively, whilethe mean and median ice thicknesses from the Ice Probe are 72 and 62 cm. Thus assumingthe auger measurements are representative of undeformed ice, and the EM-measurementsrepresent both deformed and undeformed ice, the overall mean ice thickness is 50% higherthan that of undeformed ice.

Figure 2. Ice thickness distributions for line segments A, B, C and D shown in Figure 1.

Figure 3. Ice thickness distribution from (a) auger hole measurements (b) EM-measurementsof Flight 53, lines 10050-10080.

Figure 4 shows the relationship between the ice-plus-snow thickness data (Figure 4a) andlaser data using a 2 km profile from segment A in Figure 1. In Figure 4b, the laser altimeterdata sampled at 28 Hz are plotted after removing the bird altitude variation to show the ice

surface topography. The low-pass filtered laser altimeter data with a cut-off frequency of 2.5Hz are plotted in Figure 4c; these data are used to estimate the distance to the snow surfacein the determination of ice thickness. In Figure 4d, the ice surface topography has beenaveraged over 75 m, the footprint of the EM system for a bird height of 20 m.

It can be seen that the high frequency variations in the EM-measured ice thickness data(Figure 4a) are directly due to the laser altimeter data filtered at 2.5Hz (Figure 4c). Theremaining variations in ice thickness are of lower frequency but higher amplitude; they arehighly correlated with, but appear to be 5 to 10 times larger than the ice surface topographyaveraged over 75 m (Figure 1d). Assuming isostasy, the ratio of the total thickness to thefreeboard is ρw/(ρw - ρm), where ρw is the seawater density and ρm is the mean ice-plus-snowdensity. If ρw is 1026 kg/m3, and ρm is 910 kg/m3, then the ratio is 9. The observed ratio maybe less due to the porosity of first-year ice keels which would result in an underestimate ofthe keel depth by the EM system, or due to snow cover which would reduce ρm.

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Figure 4. (a) A 2 km sample of EM-measured ice thickness data from the Gulf of St.Lawrence, March 6, 1996, flight 53, line 10050. (b) The ice surface topography, extractedfrom the laser altimeter data sampled at 28 Hz by removing the bird altitude variation. (c)Low-pass filtered laser altimeter data with a cut-off frequency of 2.5 Hz (d) Laser altimeterdata averaged over 75 m.

EM-measured ice thickness data collected off the Labrador coast on March 10 and 11,1996 are superimposed on the ScanSAR Wide image acquired on March 10 in Figure 5, aftercorrecting for ice drift using ice beacon data. The landfast ice in the bays generally has

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Figure 5. RADARSAT ScanSAR Wide image of the Labrador shelf for March 10, 1996(21:36Z) with EM-measured ice thickness data collected on March 10-11 superimposed.© CSA/ASC, 1996.

uniform thicknesses of about 0.6 to 1.0 m, in agreement with drill-hole measurements, andcorresponds to dull-toned areas on the SAR image. The ice which has been compressedagainst the coast is rough with thicknesses of up to about 2 m and corresponds to brighter-toned areas in the image.

During the survey on March 10, the thicker pack ice had been blown about 30 kmoffshore. The inner edge of the main ice pack is visible as a bright band in the SAR image(Figure 5), where ice thicknesses up to 1.3 m were measured. Offshore in the main ice pack,ice thicknesses were generally about 30 cm thick. For flight 57, line 10030 (marked “A” inFigure 5) the mean, median and maximum ice-plus-snow thicknesses were 34 cm, 27 cmand 160 cm. For line 10040 (marked “B” in Figure 5) over the large dark-toned floe, themean and median ice-plus-snow thickness are 30 and 29 cm. In comparison, median ice-plus-snow thicknesses measured through augered ice holes at the ice beacon deployment

sites (crosses to the right of “A” in Figure 5) were 38, 44, 53 and 70 cm. The surfacemeasurements may be higher than the mean EM measurements because of a tendency to putbeacons on thicker floes to ensure they would last more than a few days.

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Figure 6. (a) Laser-measured ice surface topography and (b) EM-measured ice thicknessfrom flight 57, line 10010 over Labrador shelf on March 10, 1996.

Inshore of the main ice pack, EM-measured ice thicknesses are generally less than 20 cmand correspond to dark-toned areas in the SAR image. However several thin bright lines arevisible in the SAR image extending downwind (southeast) of coastal islands and correspondto lines of rubble where ice thicknesses of up to about 1 m were measured. The ice thicknessand laser data collected along flight 57, line 10010 (marked “C” in Figure 5) are plotted inFigure 6. Ice thicknesses were up to about 1.5 m, with laser-measured ridges of about 0.9 m.

An enlargement of the SAR image for the region near Porcupine Bay (marked “D” inFigure 5) is shown in Figure 7. Throughout most of the bay, the ice thicknesses are fairlyconstant at about 1 m. However near the mouth of the bay (marked “A” in Figure 7), the icethickness decreases sharply to less than 20 cm, corresponding to a sharp increase inbackscatter in the SAR image. At a validation site north of Huntington Island (marked “E” inFigure 5), EM-measured and surface-measured ice thicknesses also decreased from about100 cm in darker-toned landfast ice to 20 cm in a bright-toned area at the edge of thelandfast ice. The snow on the thinner ice along the line was about 4 cm thick and very wet.The high backscatter for the thinner ice may have been due the high snow salinity (43-62ppt), or the wetness of the snow which may have increased the surface roughness.

The EM data from 7 overflights at the validation site are plotted in Figure 8, along withice thickness measurements made through holes augered through the ice (from Moucha etal., 1998). The EM data show good repeatability, with measurements from the variousoverflights within about 20 cm of each other. The mean snow-plus-ice thickness from theEM data was 78.4 cm, within 2 cm of the mean from the auger hole data (76.5 cm). Alsoshown (with a vertical offset of 0.5 m), are the auger hole measurements averaged over 45 m(the EM footprint for a height of 12 m) using a boxcar filter (dashed line), and a filterderived by integrating the surfacial current density profile (Kovacs et al, 1993) across theflight path (dotted line). There is good agreement over most of the line, particularly at theboundary between the thicker first year ice and the young ice where ice thickness variabilityacross the flight path is probably minimal. However, discrepancies within the thicker icesuch as at 60 m are probably due to lateral heterogeneity.

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Figure 7. Enlargement of the RADARSAT image in Figure 5. The mouth of Porcupine Bayis denoted by “A”. © CSA/ASC, 1996

4. CONCLUSIONS

Ice-plus-snow thickness profiles collected with an HEM system show that changes in SARbackscatter are usually associated with changes in ice thickness distributions. In the Gulf ofSt. Lawrence, dark-toned floes are associated with low ice thickness variability and representsmooth undeformed ice, while the surrounding brighter areas are associated with morevariable ice thickness. Off Labrador, bright streaks downwind of coastal islands correspondto lines of rubble where higher EM-measured ice thicknesses and laser-measured roughnessare observed. However in an area of landfast ice, higher SAR backscatter is associated withlower ice thicknesses. Thus while RADARSAT imagery can provide information on thespatial extent of areas having similar ice thickness distributions, the HEM system canprovide quantitative information on the ice thicknesses in these areas.

Figure 8. Snow-plus-ice thickness measurements at CART#1 line from HEM system (solidlines) and auger holes (crosses); (from Moucha et al., 1998). The two upper lines have beenoffset by 0.5 m and denote the auger hole measurements averaged over the footprint of theHEM system using a boxcar filter (dashed line) and a filter derived from the surfacial currentdensity profile (dotted line).

5. ACKNOWLEDGEMENTS

This work was funded in part by the Federal Panel on Energy Research and Development. Wewish to thank the many participants of this project from the Canadian Coast Guard, CanadaCentre for Remote Sensing, Canadian Ice Service and Fisheries and Oceans Canada.

6. REFERENCES

Hibler, W.D. 1972. Removal of Aircraft Altitude Variation from Laser Profiles of the Arctic Pack. Journal of Geophysical Research, 77: 7190-7195.

Holladay, J.S., Lo, B. and Prinsenberg, S.J. 1997. Bird orientation effects in quantitativeairborne electromagnetic interpretation. Oceans ’97 Conference Proceedings, Halifax,Nova Scotia, Canada, pp. 1114-1116.

F56L030 F58L020 L030 L040 L060 L070 L080 S+I Thickness (m)

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Position along Line (m)

Labrador CART#1 Ground Truth Line Results: F56L030 - F58L080

Kovacs, A., Holladay J.S. and Bergeron C.J. 1993. Footprint Size of a Helicopter-BorneElectromagnetic Induction Sounding System versus Antenna Altitude. CRREL Report93-12, 13 p.

Moucha, R.Z., J.S. Holladay, and S.J. Prinsenberg. 1998. Airborne Electromagnetic Sea IceSounder Measurements of RADARSAT Validation Project 1996. Can. Contract. Rep.Hydrogr. Ocean Sci. 51:vii+349 pp.

Ramsay, B., Manore, M., Weir, L., Wilson, K., Bradley, D. 1998. Use of RADARSAT Datain the Canadian Ice Service. Canadian Journal of Remote Sensing 24: 36-42.

Rossiter, J.R. and Holladay, J.S. 1994. Ice-Thickness Measurement. In: Remote Sensing ofSea Ice and Icebergs, John Wiley and Sons, New York, pp.141-176.