Navigation of autonomous underwater vehicles (A.U.V.) in very shallow waters implies acoustic detection. In single beam sonar systems, sound emitted by ultrasonic transducers is diffracted and secondary lobes appear. Considering the sea bottom's backscattering properties, secondary lobes can be used to work out the three-dimensional equation of the plane that represents the seabed. In this paper, we will first consider characteristics of electro-acoustic transducers with rectangular aperture and study the resulting acoustic diffraction. Then, we explain how to choose the best dimensions of the transducer and the related best orientation. Moreover, we introduce a methodaiming to extract the seabed 3D equation from the received acoustic echo. Thus single beam sonar systems can be used for bottom tracking purposes. We present the results of our simulations and our experimental device.