Depointable parametric echosounder, and method for characterizing a portion of the sub-bottom of a subaquatic environment

A parametric echosounder comprising:

• a transmitting antenna comprising a plurality of transducers, the transmitting antenna having a two-arm cross shape, specific ones of said transducers being arranged in a line one after another along a first arm of the transmitting antenna, all other transducers of the transmitting antenna being arranged in a line one after another along a second arm of the transmitting antenna;
• at least one receiver;
• and
• a driving acquisition system configured to execute the following steps:
• a) driving said transducers so that the transducers transmit, in an underwater environment, a first acoustic wave having a first frequency and a second acoustic wave having a second frequency, the first wave and the second wave superposing each other along a superposition axis directed towards a point of a bottom of the underwater environment, each transducer being driven by a transmit signal, different transmit signals of the transducers having predetermined delays with respect to each other, and
• b) acquiring at least one echo signal, of frequency equal to a difference between said first frequency and said second frequency, picked up by said at least one receiver in response to the transmission made at step a),
• the driving and acquisition system being configured to execute all of steps a) and b) several times, by varying a part at least of said predetermined delays from one execution of step a) to a next execution of step a), to modify an orientation of said superposition axis from one execution of step a) to the next execution of step a), and
• c) determining, based on the at least one echo signal acquired during different executions of step b), a two-dimensional or three-dimensional image representative of a content of sub-bottom of the underwater a environment below each of said points of the bottom,
• wherein the driving and acquisition system is configured to, at step a):
• drive each of the transducers of a first group of said transducers by a first transmit signal varying over time at said first frequency, the transducers of the first group transmitting said first acoustic wave, the different first transmit signals having first delays, predetermined with respect to each other,
• drive each of the transducers of a second group of said transducers by a second transmit signal varying over time at said second frequency, the transducers of the second group transmitting said second acoustic wave, the second transmit signals having second delays, predetermined with respect to each other, and
• vary one or more of said first delays and said second delays from one execution of step a) to the next execution of step a), to modify the orientation of said superposition axis from one execution of step a) to the next execution of step a),
• wherein the transducers of the first group form the first arm of the transmitting antenna and the transducers of the second group form the second arm of the transmitting antenna,
• said first delays are such that the first acoustic wave propagates parallel to a first transmission plane, as a first beam having a narrow cross-section in a direction perpendicular to the first transmission plane and elongated parallel to the first transmission plane,
• said second delays being such that the second acoustic wave propagates parallel to a second transmission plane, as a second beam having a narrow cross-section along a direction perpendicular to the second transmission plane and elongated parallel to the second transmission plane,
• said superposition axis being located at an intersection between the first transmission plane and the second transmission plane, and
• wherein the driving and acquisition system is configured to, from one execution of step a) to the next execution of step a), one or more of:
• vary said first delays in order to pivot the first transmission plane about a first scanning axis perpendicular to the first arm of the transmitting antenna, and
• vary said second delays in order to pivot the second transmission plane about a second scanning axis perpendicular to the second arm of the transmitting antenna.