A marine geophysical concept defining measurements and products used to characterize seafloor shape and subsurface structure. It governs acquisition, processing, and interpretation steps that convert instrument signals into interpretable maps and profiles. It does not establish material properties without calibration, integration with samples, and validation against independent observations. It materially affects understanding of tectonics, hazards, and sediment pathways by revealing geometry and stratigraphic patterns. The concept is generally stable, though instruments, processing methods, and resolution capabilities improve over time.