Velocity gradients in the oceanic crust

Abstract

Most marine crustal refraction lines have been interpreted in terms of a few relatively thick, constant velocity layers by using the well‐known slope‐intercept method. Recently, several researchers have used both arrival amplitudes and travel times to constrain candidate velocity‐depth functions. They discovered that the combined amplitude and travel time distribution required crustal velocity‐depth functions consisting of a series of velocity gradients and, occasionally, low‐velocity zones. The presence of velocity gradients in the ocean crust has important consequences for the propagation of low‐frequency sound in the ocean and for the geology of the oceanic crust. In this paper travel time inversion methods for determining crustal velocity gradients are discussed and illustrated using air gunsonobuoy refraction data. Careful picking and inversion of the travel time branches actually observed in the refraction data yield velocity‐depth functions similar to those from a combined amplitude and travel time analysis. [Work supported by ONR.]