Abstract
Abstract. Air-coupled flexural waves (ACFWs) appear as wave trains of constant frequency that arrive in advance of the direct air wave from an impulsive source travelling over a floating ice sheet. The frequency of these waves varies with the flexural stiffness of the ice sheet, which is controlled by a combination of thickness and elastic properties. We develop a theoretical framework to understand these waves, utilizing modern numerical and Fourier methods to give a simpler and more accessible description than the pioneering yet unwieldy analytical efforts of the 1950s. Our favoured dynamical model can be understood in terms of linear filter theory and is closely related to models used to describe the flexural waves produced by moving vehicles on floating plates. We find that air-coupled flexural waves are a real and measurable component of the total wave field of floating ice sheets excited by impulsive sources, and we present a simple closed-form estimator for the ice thickness based on observable properties of the air-coupled flexural waves. Our study is focused on first-year sea ice of ∼ 20–80 cm thickness in Van Mijenfjorden, Svalbard, that was investigated through active source seismic experiments over four field campaigns in 2013, 2016, 2017 and 2018. The air-coupled flexural wave for the sea ice system considered in this study occurs at a constant frequency thickness product of ∼ 48 Hz m. Our field data include ice ranging from ∼ 20–80 cm thickness with corresponding air-coupled flexural frequencies from 240 Hz for the thinnest ice to 60 Hz for the thickest ice. While air-coupled flexural waves for thick sea ice have received little attention, the readily audible, higher frequencies associated with thin ice on freshwater lakes and rivers are well known to the ice-skating community and have been reported in popular media. The results of this study and further examples from lake ice suggest the possibility of non-contact estimation of ice thickness using simple, inexpensive microphones located above the ice sheet or along the shoreline. While we have demonstrated the use of air-coupled flexural waves for ice thickness monitoring using an active source acquisition scheme, naturally forming cracks in the ice are also shown as a potential impulsive source that could allow passive recording of air-coupled flexural waves.
Highlights
The term “air-coupled flexural wave” (ACFW) was coined by Press et al (1951) to describe wave trains of constant frequency, varying with the flexural stiffness of a floating ice sheet, that arrive in advance of the pressure waves produced by an explosive source
It is beneficial to consider the solution to the dynamical equation (Eq 1) in order to demonstrate that the key physics involved in the propagation of air-coupled flexural waves are represented by the model
Air-coupled flexural waves were found to be a robust feature of first-year sea ice excited by explosive sources over a range of ice thicknesses up to ∼ 80 cm
Summary
The term “air-coupled flexural wave” (ACFW) was coined by Press et al (1951) to describe wave trains of constant frequency, varying with the flexural stiffness of a floating ice sheet, that arrive in advance of the pressure waves produced by an explosive source. Several studies have used the dispersion of ice flexural waves (IFWs) to estimate ice elastic parameters; of these we highlight in particular the early work of Ewing and Crary (1934) and the study of Yang and Yates (1995) that advanced the use of transform methods in this field. Passive seismic studies of flexural wave dispersion are emerging as an effective means to continuously monitor sea ice properties using array-based wave field trans-
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