| Sea ice is a component of cryosphere in the earth climate system. It is very sensitive to climatic change. The thickness and change of sea ice has particularly significant effect on the coupling of atmosphere-sea-ice-ocean and directly determines the course and speed of energy and substance exchange between sea and air; the ice thickness is characterized by the sea ice dynamics and thermodynamics via various processes of drift, deformation, freezing and melting. Therefore, the research on physical process of sea ice needs precisely accurate thickness data of sea ice. The effective technology of detecting ice thickness includes satellite remote sensing, underwater sonar, upward looking radar, electromagnetic-induction and drill-hole. The most potential method of satellite remote sensing technology is to measure the height of freeboard firstly and then calculate the thickness of sea ice with the law of Archimedes. But the sea level derived from GPS geoid surface data may have some error, which consequently limits the accuracy of freeboard height measured with satellite; underwater sonar gets depth of draft and then converts it into thickness of sea ice. Its accuracy is limited by the position of underwater transducer and the effect brought by water temperature, tide and pressure change; Radar detection may easily have major error at the place where the sea ice is inhomogeneous; Drilling is the most immediate and reliable method, but it can't meet the need of large-scale observation due to its low efficiency. Based on the difference between the electrical properties of sea ice and sea water, Electromagnetic induction technology can determine the thickness of sea ice quickly without contact and has high precision and reliability. As it is flexible in use, it can be used to detect the thickness of sea ice directly from ice surface and by means of shipborne and airborne. The observations can directly correct and verify the observed data of satellite remote sensing and provide valuable information for calculation of sea ice numerical model at different scale in the research of climatic change. Due to its rapidness and flexibility, this technology has attracted attention in the world and been taken on a booming trend in recent years. It is considered as the observation technology with the biggest development potential.This paper focused on discussion on characteristics of detection of sea ice thickness with EM at Antarctic Neila Fjord and Bothnian Bay, in which the sea ice thickness profiles was given and sea ice thickness distribution was obtained.1. Instruments and theory: The electromagnetic inductive instrument has two terminals as the magnetic dipole antenna, and can be operated with coils either vertically or horizontally aligned, corresponding to the horizontal or vertical magnetic dipole (HDM or VDM) mode, respectively. It measures the quadrature component response of secondary magnetic field. The quadrature response is automatically transformed to an apparent conductivity in mS/m. EM measurements of sea ice thickness rely on the large contrast in electrical conductivity of sea ice and that of seawater, as the electrical conductivity of seawater is far greater than the sea ice, consequently the conductivity of the sea ice is negligible. Therefore, a quasi-static low-frequency EM field generated by the transmitter coil of the EM instrument will induce eddy currents mainly in the seawater below the ice. It, in turn, will result in a secondary field that is sensed by a receiver coil. The secondary-to-primary field ratio can be expressed in terms of an apparent conductivity.The apparent conductivity is a measure of integrated electrical conductivity of the halfspace underneath the instrument. Because the sea ice electrical conductivity is so small that it can be ignored, the value of apparent conductivity only depends on the distance from the instrument to the ice-water interface and the value of seawater electrical conductivity. If the seawater electrical conductivity is considered to be constant, the apparent conductivity is just the function of the distance. Theoretically, it is a negative exponential relation between apparent conductivity and the distance. Therefore, via accurate measurement of apparent conductivity, the distance from instrument to ice-water interface can be obtained by converting the apparent conductivity provided that seawater electrical conductivity is given.2. Work methods: In Antarctic, we carried out the ship borne measurements and the ground base measurements. The main equipments of ship borne measurements are EM31 and laser altimeter. Firstly, the EM instrument was fixed in a wooden frame to enable suspension below the bow crane and for mechanical protection; the laser was also mounted on the frame with a level. The system height above the water surface was approximately 4 m. At this height the shortest distance to the ship was 8 m. The ground-based measurements consist of EM31 observation and drilling. First of all, typical positions were selected in the area of level ice and deformed ice to complete measuring of 87 holes. EM31 is adopted at every hole position to observe the apparent conductivity. Through these measurements, the relation between sea ice thickness and apparent conductivity is developed. Finally, with the high efficiency advantage of EM31, 3 profiles at length from 400 m to 490 m respectively in different areas are completed.In Bothnian Bay, the ground based measurements and the airborne measurements were carried out. The progress of ground based measurements was similar to what was taken in Antarctic. The airborne sensor was a towed sensor (EM bird) to be suspended with a 20 m long cable fixed in an external load hook below the helicopter. The bird was flown at an altitude of 10 to 20 m above the ice surface. The bird has a laser altimeter for altitude control, whole distance readings are directly displayed to the pilot. The bird requires a power supply of 28 VDC and 16 A to be delivered from the helicopter.3. Forward modeling calculation: we used half space model and two layers model to study the forward response of EM31. Contrastive analysis of the model calculation and the data from the field shows there is deviation between the fitting curve based on observed data and the curve of theoretical model. There are many causes for the deviation and it can't be explained with any single factor. One of the main factors is the effect brought by the seawater electrical conductivity. The model curve is obtained through calculation based on the average value. The main advantage with EM is wide range detection with its high efficiency. The variation of seawater electrical conductivity in practice is hard to be avoided. In addition, the content of brine in ice is also an influencing factor of EM31 measuring result. But all these shows that the fitting curve obtained by combining apparent conductivity data and drill data from field observation meets the practical situation better.4. Observation results and analysis: In Antarctic, level ice accounts for the significant majority in Neila Fjord, and the mean ice thickness ranges from 0.8 m to 1.4 m. The maximum normal fitting value is at the ice thickness of 1.2 m; the results of ship borne measurements in Prydz bay is that the mean ice thickness ranges from 0.6 m to 1 m, and the maximum normal fitting value is 0.9 m thickness. In Bothnian bay, level ice thickness ranges from 0.4 m to 0.6 m, some ice ridges have thickness over 1 m. To analyze the error of sea ice thickness detected with EM quantitatively, we calculate the average relative error of sea ice thickness detected by EM31 relative to drill, and the average relative error is 5% and 12% respectively in Antarctic and Bothnian Bay.The results of quality analysis on sea ice thickness detected with EM show that the technology has very high accuracy, which meets the need for large-scale sea ice thickness detection. This article mainly discusses the application of EM technology in detection of sea ice thickness, the results obtained by this technology can be realized to provide basis for observation data of satellite remote sensing and provide valuable information for large-scale sea ice digital model in the research of climatic change. From the current research, there is still some error in sea ice thickness measurement with EM in the area with rich deformed ice. We believe that improving the ice thickness inversion calculation method will help to reduce the errors of detection under special ice conditions. This is one of our future research contents.
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