D.L. Aleynik, V.V.Goncharov, Yu.A.Chepurin
ACOUSTIC TOMOGRAPHY OF INTHERMOCLINE
FROM MOVING VESSEL
P.P.Shirshov Institute of Oceanology of Russian Academy of Sciences
36 Nakhimovskiy Pr., Moscow, Russia, 117851
Tel: (7-095) 129 1936; Fax: (7-095) 124 5983
An experiment on acoustic tomography from moving ship was carried out in the Western Mediterranean Sea in 1994. Six moored transceivers were used in the framework of THETIS-2 experiment. These tranceivers were used as a wide-band sources. Our acoustic measurements carried out with a single hydrophone deployed from a drifting research vessel "Akademik S. Vavilov" (IORAS, Russia). The CTD survey was also made. An interthermocline eddy with a cold core was found during this experiment. In horizontal plane the eddy was close to the ellipse with axes of 25 and 40 nm. Maximum sound speed difference inside and outside the eddy was about 2 m/s. Acoustic signals from the transceivers were recorded at 16 points within and around the eddy to study the feasibility of reconstruction of such mesoscale inhomogeneities by acoustic tomography means. A technique of coherent signal processing used made it possible to resolve multiple eigenray arrivals from transceivers at each point of measurements. The main acoustic effect of eddy is a distortion of arrival patern in comparison with this pattern for the background media. We use linear tomography scheme to reconstruct these inhomogeneite in horizontal plane. Reconstructed value is a sound speed averaged trough depth. For this purpose we summed appropriate matrix coefficients. This procedure allow us to diminish sufficiently the number of unknown variables and make the inversion feasible. The traditional regularization methods were used. Numerical simulations showed the reliability of this approach. Our scheme is able to locate such inhomogeneite and estimate its power. These primary reconstructed data can be used in lens reconstruction in vertical planes. The real experimental data was also used in this inversion scheme. This procedure correctly locate our inhomogeneite and allow us to estimate difference of sound speed inside and outside the lens. To find the points of measurements which are most important for the reconstruction procedure we investigate the different sets of these points using a simple reconstruction method based on time delay between eigenrays unperturbed and perturbed by lens.