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A Calibration Procedure for Ship-borne Em31 Sea Ice Thickness Measurements
| Content Provider | Semantic Scholar |
|---|---|
| Author | Reid, James B. Kimber, Bronwyn Worby, Anthony |
| Copyright Year | 2003 |
| Abstract | Calibration of ship-borne electromagnetic sea ice thickness data can be performed by making measurements at a range of heights over level sea ice of known thickness, and by comparing the observed data with the expected layered-earth response. Calibration corrections can be derived using least-squares inversion to minimise any error between the observed data and the theoretical response. These corrections can be applied to ship-borne sea ice thickness data during post-processing. Analysis of calibrated data indicates that ship-based vertical coplanar EM31 measurements can provide reliable estimates of the thickness of level sea ice up to around 2.7 m thick. INTRODUCTION Electromagnetic (EM) induction methods have become widely-used for estimation of sea ice thickness (Kovacs et al., 1987; Kovacs and Morey, 1991; Prinsenberg et al., 1996; Haas et al., 1997; Haas, 1998; Worby et al., 1999). Calibration of the measured EM responses is an important initial step in data acquisition and processing. For measurements made on the sea ice using handheld instruments, calibration can be performed in the field by comparison of EM-derived and drilled sea ice thicknesses, and suitable adjustment of the instrument settings (e.g., Worby et al., 1999). Calibration of ship-based EM data is more complicated, as the instrument is suspended from a crane boom (Figure 1), and cannot be directly accessed while measurements are being made. It is not possible to calibrate the EM instruments on the deck of the ship prior to deployment from the crane due to the effect of the ship’s hull on the measured responses. This paper describes a practical procedure for calibration of ship-borne EM sea ice thickness data acquired using a Geonics EM31 instrument. INSTRUMENTATION The Geonics EM31 is a single-frequency device operating at 9800 Hz. The instrument measures both the in-phase and quadrature of the secondary magnetic field, although only the quadrature component is used for practical sea ice thickness calculations. The measured quadrature (imaginary) component of the secondary magnetic field is automatically transformed into an apparent conductivity (σa) in milliSiemens/m, according to the equation (McNeill, 1980) 1 School of Earth Sciences, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia. 2 Antarctic CRC and Australian Antarctic Division, GPO Box 252-80, Hobart, TAS 7001, Australia. |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://web2.clarkson.edu/projects/iahrice/IAHR%202002/Volume%203/142.pdf |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
