Pierre Alais

Fourier formalism for describing nonlinear self-demodulation of a primary narrow ultrasonic beam

Presented here is the derivation of nonlinear interactions that occur within a primary narrow beam for which the temporal spectrum is continuous and narrow. This follows the bases of the Fourier formalism. Acoustics levels are presumed weak enough so that second‐order equations may be used. In the quasilinear case, the exact theoretical expression of the created parametric farfield, formed from a transient modulated primary signal, is established, by using weakly restrictive assumptions. The case of high primary levels is discussed. Some experimental results are presented.

Non intrusive measurements of the acoustic pressure and velocity fluctuations of fluids flowing in pipes

A nonintrusive flowmeter based on nonlinear acoustical interactions, has been developed. This tool measures simultaneously the pressure and velocity fluctuations of fluids flowing in pipes. A transmitter produces a CW ultrasonic beam which crosses the tube wall, propagates through the opposite side, and is collected by a receiving transducer. Both transducers are just put against the tube wall so that it is not necessary to drill holes in order to install the probes. The ultrasonic propagation across the tube is modelled using an exact matrix numerical method which allows one to specify the geometry of the device. The high frequency beam and the low frequency flow fluctuation interact nonlinearly so that the ultrasonic signal is phase modulated. The amplitude of the modulation (about a hundredth radian) is a linear combination of the pressure and velocity fluctuations. Specific electronics, based on a PLL technique, has been developed for demodulating the signal. We present measurements obtained after generating flow fluctuations in a cavity excited over a broad spectrum

A HIGH RESOLUTION BATHYMETRIC SIDESCAN SONAR USING DYNAMIC FOCUSING AND BEAM STEERING

Investigating the sea-floor may be done through several different means: — The simplest and oldest one is the sidescan sonar system which is made of two lateral transducers set on a fish towed by the surveying ship. These transducers deliver lateral beams whose shape is open in the vertical across-track plane and narrow in the axial direction, thus illuminating a narrow strip of the sea-floor. The reverberation information, properly decoded in distance from the echographic time of flight and the altitude of navigation, permits to obtain good images of the sea-floor when the local relief perturbations are well visualized due to the shadowing effect.

A high resolution sidescan sonar

A new sidescan sonar system has been conceived for the exploration of the continental shelf. High resolution imaging and bathymetry information are obtained by steering and focusing the ultrasonic beam with three arrays on each side of the vehicle, the erratic motions of which are compensated.<<ETX>>