Method for reducing travel-time data from multiple offset borehole seismic experiments to velocity-depth structure. Linear inversion. For a borehole receiver the velocity at the depth of the receiver can be obtained from the slope of the inflection point of the travel-time curve. Thus a direct measure of the uppermost velocity of a section can be obtained from the travel-time data of a shallow borehole receiver. The technique is applied to data from three borehole seismic experiments in oceanic crust: western Atlantic (crustal age approximately 110 m.y.), the Gulf of California (crustal age approximately 1 m.y.) and the Costa Rica Rift area (crustal age approximately 6 m.y.). All three experiments show relatively high upper crustal velocities (>4.0 km/s), suggesting that layer 2A is not present even in the very young crust. All sites had over 100 m of sediment thickness, and it is postulated that sediment thickness and sediment permeability, not merely age, govern the velocity of the upper crust by accelerating the cementation of fractures and cracks.--Modified journal abstract.