A comparison of long-range acoustic propagation at ultra-low (28 Hz) and very low (84 Hz) frequencies
Peter Worcester
Scripps Institution of Oceanography University of California
A short term transmission test, the Altemate Source Test (AST), was conducted during June-July 1996 to compare the long-range propagation of broadband transmissions at 28 Hz and 84 Hz (phase-locked coherent signals, each with a 10-Hz bandwidth). An HLF-6A acoustic source suspended from shipboard off of central California transmitted to two autonomous vertical line array (AVLA) receivers, at ranges of 3500 km and 5100 km, and to ten horizontal line array (HLA) receivers, at ranges from 150 km to about 5 Mm. The combination of temporal and spatial resolution makes it possible to isolate individual rays and, at the AVLA receivers, low order modes, in order to elucidate the basic scattering physics. The vertical line array data clearly shows that the 28-Hz data has a more stable arrival patterm than the 84-Hz data. This comparison holds for both the early ray-like arrivals and for late mode-like arrivals. Interpretation of the arrivai pattem via a "turning point filter" has allowed a unified framework for comparison of both regions. The horizontal line array data obtained on a receiver located below the sound-channel axis approximately 700 km west of the source gives rms travel-time fluctuations for one resolved "ray-like" arrival of 7.8 ms at 28 Hz and 10.2 ms at 84 Hz, in the range of expected values. The predicted ray arrivals turn above the receiver, however, and so the associated propagation is not fully understood. The scintillation index is approximately 0.11 at 28 Hz and 0.65 at 84 Hz, indicating much more stable amplitudes at the lower frequency. Travel-time spread is near zero for both frequencies.
Dynamics of post-convection in the Gulf of Lion
Fabienne Gaillard
IFREMER
During the Thetis 1 experiment, a period of 35 days following convection has been monitored by a large number of measurements: CTD profiles, fixed point time series of temperature and current, subsurface floats and reciprocal tomography. The time evolution of the temperature and stream function during this period has been reconstructed, based on a Kalman filter procedure. The analysis demonstrates the complementarity of the datasets and provides a tool for diagnosing the dynamical regime as well as measuring the advective heat transport.
