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 enhance knowledge of seabed features such as slope, and marked variations. In this paper, we firstly consider characteristics of electro-acoustic transducers and study the resulting acoustic diffraction. Then, we introduce a new method aiming to extract seabed features 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 experimental validation.