Sonar Measurements in Ship Wakes Synchronous with Terrasar-x Overpasses

A pilot experiment was conducted by NSUOC and DLR in April-June 2008 in the Straits of Florida near Port Everglades, Florida in order to study the dynamics of far wakes of ships. In this experiment, a small boat with downward-looking sonar made “snake-like” sections through wakes of ships of opportunity during the TerraSAR-X overpasses. The ship identity and its parameters (length, speed, course, etc.) were identified utilizing AIS (Automated Identification Systems). The sonar responded to the clouds of micro-bubbles generated in the ship wake by the propulsion system and ship-hull turbulence. Different sonar frequencies (83 kHz, 200 kHz, and 455 kHz) corresponded to different bubble sizes and bubble-cloud turbulent inhomogeneities. The ship wake was traced in the sonar signal typically from 10 to 30 min after the ship passage. Preliminary analysis of the measurements suggests that the visibility of the wake in SAR is correlated with the presence of micro-bubbles in the wake. A possible explanation is as follows: the micro-bubbles scavenge surface-active materials in the upper layer of the ocean and bring them to the surface. These materials are then concentrated in narrow zones on the sea surface by convergent motions in the ship wake. The viscoelasticity effect of surface films leads to the suppression of gravity-capillary waves, which contrasts the ship wake on SAR images. Influence of wind-wave field on the ship wake as well as the effect of screening of the wind-wave field by the ship hull add another level of complexity to wake patterns observed in SAR.