Building an accurate digital terrain model (DTM) of the seabed is a key issue for various military and civilian hydrographers applications. In the past decades, the emergence of autonomous underwater vehicles (AUV) offers new methodologies to collect the bathymetric data used in the estimation of the DTM. In our study, we use the DAURADE AUV platform which is capable of acquiring bathymetry with two acoustic sensors: A multibeam echo sounder (MBES) and an interferometric sidescan sonar (ISSS). The two sensors (MBES and ISSS) are synchronized to operate concurrently. In fact, the final DTM can be improved by performing a fusion of the data; the two systems acquire the bathymetry with different resolutions, geometries and error models; these parameters are introduced in the fusion process to improve the estimation of the DTM and to increase its accuracy. The aim of this paper is to describe the fusion method and discuss our simulated results. First, the modeling of two acoustic sensors (MBES and ISSS) will be briefly described. The input data sets are simulated by applying the sensor models on simplified seabed models. The use of seabed models provides ground truth and, therefore, allows for quantifying the accuracy of the fusion process.