<p> The purpose of this thesis is to propose and develop a novel tool for measuring seafloor displacement using repeated sidescan sonar surveys, a data set that is collected alongside multibeam bathymetry. Chapter 1 presents the motivation for this study and introduces the work done in subsequent chapters. Chapter 2 is a brief overview of the design and underlying physics of the multibeam sonar, and presents a discussion on the uncertainties introduced by errors in the sound velocity profile. Chapter 3 presents the processing method for measuring seafloor displacement from sidescan sonar in detail, and tests this method by measuring synthetic displacements introduced into legacy multibeam data. Chapter 4 presents data collected in calibration surveys during the RR1605 and SR1704 cruises meant to test this method. We find displacements measurements with RMS uncertainties less than 0.5 m under optimal survey designs involving speeds less than 6 knots, kinematic post-processed ship navigation, and frequent measurements of the sound velocity profile. Chapter 5 demonstrates the improvement in ship positioning obtained when performing kinematic post-processing on the raw ship navigation data rather than relying on the real-time navigation solution.</p><p>