The elastic properties of a sedimentary layer overlying young oceanic crust are investigated using field measurements and theoretical modeling. Based on a previously established dynamical theory of porous media, we further develop a theoretical model of velocity-porosity relationships for marine sediments. This physical model is in principle valid for the full porosity range from 0% to 100% and has many advantages over other empirical or semiempirical models. A new, critical parameter is defined as the flexibility factor of a formation that characterizes the softness or \"uncompactedness\" of the formation. It is found to be closely related to the sediment thickness and age. Nine sites were studied along a transect extending more than 100 km eastward from the Juan de Fuca Ridge, drilled during Ocean Drilling Program Leg 168 in 1996. The values of this flexibility factor decrease from 15 to 7 as the age of the sediment/basement contact along this transect increases from 0.9 to 3.6 Ma. Integrating independent core, log, and seismic measurements using the proposed velocity-porosity model, we are able to provide high-resolution velocity-depth and porosity-depth profiles at all nine sites, although in situ logs were recorded at only one location. These derived velocity profiles help to obtain a more accurate time-depth model and to constrain a two-dimensional depth model of the sediment/basement contact, which is important for hydrological modeling and borehole observatory installations. The derived porosity-depth profiles are also useful to construct two-dimensional and three-dimensional models of other physical properties such as permeability and thermal conductivity. Synthetic seismograms generated with these data agree well with field seismic data at all the sites along this transect, including good matches in traveltime, amplitude, and waveform. Using the log data at one site to correct for elastic rebound effects, continuous and discrete laboratory measurements of bulk density provide more reliable information than laboratory velocity measurements to construct synthetics for shallow depths. This process is repeated for the continuous density records (GRAPE density) at the other eight sites. Using generalized nonlinear inversion of parameterized seismic waveforms and the physical velocity-porosity model, iterative comparisons of field and synthetic seismic data are performed to estimate the flexibility factor in the model and to obtain optimal velocity-depth profiles. The proposed physical model is found to be suitable for the marine sediments at these sites and may be valid in other similar sedimentary environments.