Utilizing a hull-mounted, multinarrow beam echosounder onboard RVPolarstern, we measured variation of acoustic backscatter withincidence angles at two different sites in the Southern Oceans (AgulhasPlateau and the Rijser Larsen Sea). We modeled the data, using acomposite roughness model, including water-sediment interfaceroughness and sediment volume roughness parameters. The modeleffectively uses the near normal incidence angle backscatter todetermine the seafloor interface roughness parameters employingHelmholtz-Kirchhoff theory. Beyond 20° incidence angles, anapplication of Rayleight-Rice theory is made by using a necessarysplicing technique (combining both of the theories at 20° incidenceangle). The estimated interface and volume roughness parameters arefound to be in accordance with the known area geology.