A direct helicopter EM sea ice thickness inversion, assessed with synthetic and field data

Accuracy and precision of helicopter electromagneticHEM sounding are the essential parameters for HEM seaicethickness profiling. For sea-ice thickness research, thequality of HEM ice thickness estimates must be better than10 cm to detect potential climatologic thickness changes.Weintroduce and assess a direct, 1D HEM data inversion algorithmfor estimating sea-ice thickness. For synthetic qualityassessment, an analytically determined HEM sea-ice thicknesssensitivity is used to derive precision and accuracy. Precisionis related directly to random, instrumental noise, althoughaccuracy is defined by systematic bias arising fromthe data processing algorithm. For the in-phase component ofthe HEM response, sensitivity increases with frequency andcoil spacing, but decreases with flying height. For small-scaleHEM instruments used in sea-ice thickness surveys, instrumentalnoise must not exceed 5 ppm to reach ice thicknessprecision of 10 cm at 15-m nominal flying height. Comparableprecision is yielded at 30-m height for conventional explorationHEM systems with bigger coil spacings. Accuracylosses caused by approximations made for the direct inversionare negligible for brackish water and remain better than10 cm for saline water. Synthetic precision and accuracy estimatesare verified with drill-hole validated field data fromEast Antarctica, where HEM-derived level-ice thicknessagrees with drilling results to within 4%, or 2 cm

The quantitative capabilities of HEM inversion for the sea ice case

Semi-empirical methods are routinely used for Helicopter Electromagnetic (HEM) sea ice thickness mapping. Although these methods yield sufficiently accurate thickness data, it is of interest to determine whether formal one-dimensional (1D) geophysical inversion could yield improved results. If both the thickness and the ice conductivity could be mapped, the results could be used to estimate glaciological parameters such as the age of the sea ice. Sea ice conductivity data could also be potentially used to estimate the strength of the ice sheet, which would be valuable information for planning of icebreaking operations. By investigating synthetic and field data we show that, in the case of level sea ice of thickness up to 2 m, the accuracy of our HEM system is not high enough to sense the small conductivity variations arising from the age of the ice. Sea ice conductivity has a stronger influence on the measured HEM responses in areas of thick, deformed sea ice (pressure ridges), where bulk conductivity is higher as a result of the large seawater-filled porosity. Synthetic three-dimensional HEM data generated for pressure ridge models has shown that the 1D interpretation methods conventionally used for interpretation of sea ice thickness overestimate the true bulk conductivity at 3D features

Empirical inversion of HEM data for sea ice thickness mapping

Since 2001, the Alfred Wegener Institute for Polar and Marine Research in Bremerhaven(AWI) has operated a purpose built, unique HEM system to map sea ice thickness in theArctic and Antarctic. To determine the sea ice thickness from the measured EM field we usean empirical curve fitting approach. The master curve is tuned for every flight leg usingmeasurements over open water between the ice floes in leads or polynyas. The level icethickness, which is a key value for sea ice science and climate research can be derived with anaccuracy of a few centimeters

In-situ measurements of the direct-current conductivity of Antarctic sea ice: implications for airborne electromagnetic sounding of sea-ice thickness

Airborne, ship-borne and surface low-frequency electromagnetic (EM) methods have become widely applied to measure sea-ice thickness. EM responses measured over sea ice depend mainly on the sea-water conductivity and on the height of the sensor above the sea-icesea-water interface, but may be sensitive to the sea-ice conductivity at high excitation frequencies. We have conducted in situ measurements of direct-current conductivity of sea ice using standard geophysical geoelectrical methods. Sea-ice thickness estimated from the geoelectrical sounding data was found to be consistently underestimated due to the pronounced vertical-to-horizontal conductivity anisotropy present in level sea ice. At five sites, it was possible to determine the approximate horizontal and vertical conductivities from the sounding data. The average horizontal conductivity was found to be 0.017 Sm1, and that in the vertical direction to be 912 times higher. EM measurements over level sea ice are sensitive only to the horizontal conductivity. Numerical modelling has shown that the assumption of zero sea-ice conductivity in interpretation of airborne EM data results in a negligible error in interpreted thickness for typical level Antarctic sea ice

CROSS VALIDATION OF IN SITU, AIRBORNE AND REMOTE SENSING DATA FROM EAST ANTARCTICA

Remote sensing of sea ice parameters plays a key role in polar research and climate change investigations. New sensors such as high resolution passive microwave scanners (AMSR-E) and visible/infrared radiometers (MODIS) provide new information from which, given appropriate algorithms, products including sea ice extent and concentration, snow thickness or ice temperature can be derived. These algorithms depend on approximations and assumptions, which have to be assessed in situ for quality control and/or to readjust the algorithms parameters. An Australian sea ice dedicated expedition in (austral) early spring 2003 to the East Antarctic marginal sea ice zone (RSV Aurora Australis, Voyage 1 - 2003/04) offered the opportunity for cross validation of diverse geophysical tools such as the AWIs helicopter-borne EM ice thickness profiler, as well as a helicopter-borne system containing a digital nadir looking camera combined with a thermal infrared radiometer, and remote sensing data from AMSR-E, SSM/I, MODIS, AVHRR, MISR, SAR, etc. satellite sensors. The airborne platforms could be precisely validated against ground truth data acquired on 13 ice stations and consequently could be used to validate remote sensing data.On two days during the expedition exceptionally good weather conditions with clear sky along several hundred kilometres provided a superb dataset. Flight tracks of altogether more than 500 km were profiled synoptically with the EM ice thickness platform and the aerial photography + IR radiometer system. Photography flights were carried out at 5000 feet altitude while EM bird was usually flown at around 100 ft allowing the EM operators to document the general ice conditions and take detailed geocoded digital pictures of the ice and snow conditions along the track, giving a ground truth dataset for the high altitude photos as well as structures found in satellite pictures.On both days near real time MODIS scenes containing the flight tracks were acquired and provide an excellent overview of the general ice conditions in the area. Along 109.3°E in the vicinity of the Australian Antarctic research station Casey, a 155 km long meridional flight transect from 65.75°S to 64.4°S passes a variety of different ice classes varying from a freshly refrozen polynya west of an iceberg grounding line along Peterson Bank to vast, snow - covered drifting floes at 65°S. Accounting for the different footprints and spatial resolutions of the systems, statistical properties are compared such as HEM derived ice thickness distribution with ice/snow surface temperature distributions from the helicopter IR radiometer and MODIS IR channel. Huge structures at the mentioned polynya extending for almost 10 km along the flight track, or the biggest drift floe measuring roughly 15 x 7 km can be described by the HEM thickness distribution computed from profile subsets with length equal to the highest spatial resolution for the AMSR-E (6x4 km @ 89 GHz) resulting in an average snow + ice thickness of 0.06 m with standard deviation 0.09 m and 3.49 m thickness (1.17 m SD) respectively, corresponding to a brightness of 2% and 80% in the visual MODIS channel. HEM thickness distributions from this 155 km long transect (including open water) were compared to AMSR-E ice concentrations

What can applied Geophysics do for Sea Ice Science?

Sea ice plays a key role in the earth climate system as it controls the fluxes between ocean and atmosphere and drives the global circulation due to its seasonal cycle of melting and freezing. Ice covered oceans also govern the earth's albedo and, therefore, the atmosphere's energy flux.Although satellites provide information on sea ice extent and seasonal variability, very little is known about the thickness of the sea ice and its long term thinning or thickening.Electromagnetic methods are perfectly suitable for sea ice thickness measurements as the ice represents a resistive layer covering a highly conductive ocean. For more than ten years, the Alfred Wegener Institute uses active frequency domain EM devices to assess the spatial and temporal evolution of Arctic and Antarctic sea ice. For that purpose Geonics' EM31 has been towed with sledges on ice surfaces and suspended from the ship's bow crane for continuos measurements while steaming through sea ice. Since 2001 a purpose built helicopter EM system is operating from ships and land stations delivering a unique sea ice thickness dataset in space and time.A ramac GPR system was used on sea ice in March and October 2003 in the Arctic and Antarctic respectively. These campaigns where one of the first successful adoptions of the GPR technique on sea ice