Abstract: A processing circuit is connected to the output of a mosaic which consists of n first electrodes on a first principal surface of a TGS plate and 2p second electrodes on the second principal surface thereof. The circuit is constructed so that the output connections of the second electrodes are grouped in p resultant groups which are connected, via p paths which each comprise an amplifier and a variable delay line, to the inputs of a first adder. The output of the first adder is connected, via a network of (q-1) delay lines having a fixed delay time, to the inputs of q parallel variable delay lines which are connected in parallel and which are connected to the inputs of a second adder. The delay times thus realized are controlled so that a first ultrasonic lens and a second ultrasonic lens are successively formed.