The encouraging results from automatic stations led to direct experiments in January 1981 by a party comprising the three authors, working from HMS Endurance. In situ studies of three icebergs included measurements of surface strain in three directions, tilt in two directions, vertical heave and rotation, and concurrent measurements of the sea state by Waverider buoy (Reference Kristensen, Orheim and WadhamsKristensen and others 1981). All icebergs were bending at the surface, and heaved and rolled in response to the long component of the incident waves (10 to 15 s and longer). Typical values were: 5 × 10−8 strain, 0.1 m heave, and ±1 × 10−4 rad tilt. Resonant responses with larger amplitudes and at longer periods were observed both in heave and tilt. The iceberg acts essentially as a low-pass filter on the sea state, but with different response periods to different types of motion.
Measurements of iceberg heading by the automatic stations and by in situ measurements show that the icebergs readily rotate in the horizontal plane, and that the magnitude and direction of rotation is sensitive to changes in wind conditions.
Fig.1. Data obtained from iceberg 2 (dimensions 1100 × 500 m, samples 11 January 1981 at 58°55'S, 26°06'W) and iceberg 3 (1300 × 1200 x 900 m triangle, 14 January 1981, 60°01'S, 43°15'W). (a) Concurrent heave and strain records (berg 2), showing an outbreak of resonant bobbing at 40 s period. Sawtooth on strain record is thermal drift and rezeroing. (b) Tilt records (berg 3); upper record is parallel with long axis, lower record is at right angles (not concurrent). (c) Power spectra of strain and tilt (berg 2) compared with the spectra of ambient wave energy, showing responses at 15 and 40s period.
