REFRACTION LE LONG DES BANCS MINCES RAPIDES ET EFFET D'ECRAN POUR LES MARQUEURS PROFONDS *

Abstract

ABSTRACTRefraction along thin high velocity layers and along basement is investigated in two cases.a) high velocity layer just on the basement.b) high velocity layer higher above.Period and attenuation of refracted waves are givers as a function of the layer thickness H. Refracted arrivals along thin high velocity layers are visible at significant distances if the layer thickness is not smaller than A/6, where A is the longitudinal wavelength in high velocity medium. The pseudoperiod is proportional to the layer thickness H. The attenuation at large distance follows an x‐ne‐k1x law, where n is close to I and k1 is inversely proportional to H.Refracted arrivals along the basement are observable even in the case of thin high velocity layers situated in the overburden; their intensity is smaller and their pseudo‐period larger than when no layer exists in the overburden. The intensity of the basement arrival decreases and the pseudoperiod increases with increasing layer thickness.The pseudoperiod and the attenuation of refracted arrivals along high velocity layers and along the basement are also highly dependent on acoustic contrasts.Both arrivals from a high velocity layer and from the basement can be recorded simultaneously, provided the frequency spectrum of the seismic chain is sufficiently broad. In all cases layer arrivals show a character very different from basement arrivals.