Oceanic upper crustal structure from variable angle seismic reflection -- refraction profiles

• Published in: Geophysical Journal International
• Main Author: White, Robert S.
• Format: Text
• Language: English
• Published: Oxford University Press 1979
• Subjects: Articles; North Atlantic
• Online Access: http://gji.oxfordjournals.org/cgi/content/short/57/3/683; https://doi.org/10.1111/j.1365-246X.1979.tb06785.x

Summary. The travel times of reflected and refracted arrivals observed on variable angle seismic profiles can be inverted to produce a first approximation to the variation of seismic velocity with depth. A method is discussed for deducing the interval velocity above a reflector by finding the best match between the observed variable angle reflection travel times and those derived by ray tracing through a series of trial models. Further constraints can be placed on the rate of change of velocity with depth by consideration of the amplitudes of both reflections and refractions. Synthetic seismograms calculated by the reflectivity method demonstrate that different kinds of velocity structure in the oceanic volcanic basement layer produce characteristically different variations of amplitude with range. In particular, the exact nature of the shear wave velocity transition at the top of the basement has a marked effect on the compressional wave reflection amplitudes. Pre-critical reflection amplitudes are seen to be generally more sensitive to the velocity structure at the sediment-basement interface than are post-critical reflections, whilst head wave amplitudes are strongly dependent on the velocity gradient in the upper part of the basement. The second part of the paper presents an analysis of variable angle seismic profiles obtained over the Madeira abyssal plain in the North Atlantic, using the travel time and amplitude interpretation techniques discussed earlier. It is concluded that the velocity structure of this area of typical oceanic crust comprises a stack of distinct layers within each of which the velocity increases smoothly with depth. An upper basement layer 600 to 700 m thick is a remnant of an originally lower velocity seismic layer 2A which is found on younger crust. Beneath this upper layer the compressional wave velocity increases with a gradient of less than 0.7/s to the base of seismic layer 2. The velocity gradient within the underlying seismic layer 3 is constrained by refraction amplitudes as ...