Seafloor sediment acoustic returns are comprised of scattering from both the sediment-water interface and the sediment volume. At low-frequencies, volume scattering is often the dominant mechanism; however, direct measurements of this component have rarely been made, owing to the large beamwidths typically associated with low-frequencies. When beamwidths are large, the sediment interface and volume returns arrive at the same time, causing estimates of volume scattering to become biased by the interface scattering. End-Fire Synthetic Aperture Sonar (EF-SAS) can achieve narrower beamwidths by coherently combining multiple acoustic returns as a vertically oriented transmitter and/or receiver is moved towards the seafloor. A beampattern model developed for EF-SAS suggests EF-SAS processing can reduce the beamwidth of a sonar by a factor of five with an array length of 100 wavelengths. In this thesis, EF-SAS processing methods were developed and the resulting gains were experimentally verified through a motion-controlled EF-SAS field trial. Experimental tests indicate that EF-SAS gains can be accurately predicted using the EF-SAS beampattern model. Analysis of the data demonstrates the ability of EF-SAS to reduce interface scattering. EF-SAS can produce a variety of data products relevant to seafloor characterization, including: the angular response of interface and volume scattering, the sediment reflection coefficient, and the sediment attenuation coefficient. This thesis concludes with recommendations for the development of a specialized EF-SAS tool, and an analysis of its applications.