Abstract. Geophones mounted on floating ice sheets can potentially provide data for a robust method to monitor mechanical properties of the ice. The technique relies on the measurement and inversion of low-frequency elastic waves propagating in the ice. To test this hypothesis, several geophone systems were deployed on the Arctic ice as part of the Office of Naval Research Sea Ice Mechanics Initiative (SIMI), including a winter-over system of 20 triaxial geophones at the fall 1993 SIMI camp. An inversion technique is discussed here through references to the literature and by analysis of geophone data, specifically those data collected on "clean" first-year ice off Resolute Bay, Canada, and on highly irregular multiyear ice found at the fall SIMI camp. For undeformed first-year ice, the inversion technique gave an estimate of ice properties that agreed well with known values, although extensive work remains to determine the effects of anisotropy and inhomogeneity. The geophone data for the nonuniform ice at the SIMI camp were very complex and difficult to analyze. The inversion yielded only trends in what can, at best, be termed effective ice thickness and bending rigidity; it is uncertain how these relate to actual ice properties. The SIMI geophone system demonstrated the feasibility of performing autonomous measurements of the characteristics of propagating waves in the Arctic ice; with continued study, it seems evident that such systems can be used in clean first-year ice to monitor ice properties. However, extensive research is required to make this technique useful in multiyear or complex first-year ice. IntroductionModels for prediction of Arctic ice cover and global climate require long-term measurements of ice properties. These will be used as input data to drive models and as ground truth for remote sensing. The unmanned autonomous systems that will be required to make these measurements must be inexpensive and easy to deploy. Existing systems that meet these requirements make only point measurements. Examples are ice stress sensors, temperature sensors, and small, upward looking sonar systems used to measure ice thickness. Such point measurements can yield properties significantly different from those averaged over even a small area, especially in multiyear ice. Important properties such as ice modulus, porosity, and salinity have yet to be measured autonomously; such measurements are currently made by taking ice core samples, a labor intensive and costly procedure.The objective of the work reported here was to explore a proposed method to autonomously determine averaged mechanical properties of Arctic ice. The technique was the inversion of the equations of motion for low-frequency elastic waves in the ice, in order to calculate mechanical properties of the ice from measured characteristics of the traveling waves, such as wave speed. This inversion was similar to that typically performed for seismic signals in land-based geologic studies. The characteristics of the waves that propagate in the ice depen...

Abstract. A theory is presented that relates thermally induced fracturing of pack ice to under-ice noise level variations. It begins with the governing equations for the thermomechanics of pack ice. The thermomechanics relates thermally induced strain rates to the stresses within various vertical layers of the floe. In addition, paradigms are developed which specify the relative quantity of fracturing and stress relief in the floe as the tensile yield strength of the ice is exceeded. The thermomechanics is complemented by an acoustic propagation model that relates the number of fracture events at a given time to the acoustic levels at arbitrary frequency and depth below the ice. The acoustic theory assumes that each fracture acts as a simple monopole source, the fractures are evenly distributed horizontally, and the energy of each fracture propagates through the ice and the water column on the basis of the governing equations for elastic waves in a horizontally stratified medium. The results indicate that noise episodes resulting from fracturing occurring over the top 40 cm of a 160 cm thick floe will propagate over distances of up to 100 km. However, noise episodes associated with fracturing occurring in the lower 100 cm of the floe will only propagate over a range of -10 km. The thermomechanics and acoustic propagation theories were used to develop a numerical model for predicting under-ice noise levels for a given thermal forcing of floes within the arctic ice pack. The model was used to simulate stresses in a multiyear floe and under-ice noise levels at 500 Hz at 305 m below the floe. Model-predicted ice stresses and under-ice noise levels compare quite well to observed stresses and noise variations during the fall of 1988 in the eastern Arctic Ocean. The model predicts that most of the thermally induced, under-ice noise at 500 Hz was a result of fracturing occurring between 5 and 30 cm below the ice surface for a 1.6 m thick multiyear floe.

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A method for generating upwind sail force coefficients for arbitrary fore-and-aft rigs is presented, requiring a minimum amount of empirical data. Example calculations for an IACC boat showed that the resulting sail force model was quite realistic, and did not require any "tuning" of coefficients. Further developments of this technique are suggested, as well as the extension to off­wind sailing.