Noise characteristics of an electromagnetic sea-ice thickness sounder on a fixed wing aircraft |
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| Authors: | Lasse Rabenstein Stefan HendricksJohn Lobach Christian Haas |
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| Institution: | a Alfred Wegener Institute for Polar and Marine Research, Bremerhaven, Germanyb Ferra Dynamics Inc., Mississauga, Ontario, Canadac Department of Earth Sciences, University of Alberta, Edmonton, Alberta, Canadad Institute of Geophysics, Swiss Federal Institute of Technology, Zurich, Switzerland |
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| Abstract: | In this paper, the noise sources of an airborne electromagnetic frequency domain instrument used to measure sea-ice thickness are studied. The antennas are mounted on the wings of an aircraft. The paper presents real data examples showing that strong noise limited the accuracy of the thickness measurement to ± 0.5 m in the best case. Even drift correction and frequency ?ltering did not reduce the noise to a level necessary for sea ice thickness measurements with an accuracy of 0.1 m. We show results of 3D ?nite element modeling of the coupling between transmitter and receiver coils and the aircraft, which indicate that wing ?exure is the primary cause of the strong noise. Wing de?ection angles below 5° relative to the fuselage are large enough to cause changes higher than the wanted signal from the seawater under the ice. Wing ?exure noise can be divided into an inductive and geometric contribution, both of the same order. Most of the wing ?exure signal appears on the inphase component only, hence the quadrature component should be taken for sea ice thickness retrievals when wing ?exure is present even when the inphase produces a larger ocean signal. Results also show that pitch and roll movements of the aircraft and electromagnetic coupling between seawater and aircraft can contribute signi?cantly to the total noise. For ?ight heights of 30 m over the ocean these effects can change the signal by about 10% or more. For highly quantitative measurements like sea-ice thickness all these effects must be taken into account. We conclude that a ?xed wing electromagnetic instrument for the purpose of measurements in a centimeter scale must include instrumentation to measure the relative position of the antenna coils with an accuracy of 1/10 mm. Furthermore the antenna separation distance should be as large as possible in order to increase the measured ratio of secondary to primary magnetic field strength. |
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| Keywords: | Sea-ice thickness Airborne electromagnetics 3D modeling Noise sources |
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