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Volume 14 Number 4
  • E-ISSN: 1365-2478

Abstract

ABSTRACT

Refraction 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 given 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−n e−k1x law, where n is close to I and R 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 pseudoperiod larger than when no layer exists in the overburden. The intensity of the basement arrival decreases and the pseudoperiod increases with increasing laer thickness.

The pseudoperiod and the attenuation of refracted arrival 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 cbain is sufficiently broad. IN all cases layer arrivals show a character very different from basement arrivals.

RESUME

La refraction le long des bancs minces rapides et du socle moins rapide sous‐jacent, est etudiee sur modeles dans deux cas:

a) banc rapide reposant directement sur le socle;

b) banc rapide situe plus haut dans le recouvrement.

La period et l'attenuation des ondes rerractees sont donnees en fonction den l'epaisseur H du banc rapide. Les arrivees refractees le long des bancs rapides ne sont pas visibles sur une distance importante si l'espaisseur de ces bancs est inferieure a A/6 A designant la longueur d'onde longitudinale dans le milieu rapide. La pseudoperiode est proportionnelle a l'epaisseur H du banc. L'attenuation a grande distance suit une loi eb x−n e−k1x avec n voisin de l et k inversement proportionnel a H.

Les arrivees refractees le long du socle sont observables en presence de bancs minces rapides dans le recouvrement, mais avec des internsites plus faibles que dans le cas d'unrecouvrement homogene. La pseudoperiode des impulsions refractees du socle augmente avec l'epaisseur H du banc rapide. L'attenuation en fonction de la distance est superieure a l'attenuation observee pour un recouvrement homogene, et elle croit avec H.

La pseudo‐periode et l'attenuation des arrivees refractees le long des bancs rapides et des marqueurs sous‐jacents sont largement fonction des impedances acoustiques des milieux en presence.

Les arrivees d'un banc mince rapide et socle peuvent etre observees simultanement a condition que la chaine d'enregistrement ait un spectre en frequence suffisamment large. On constate dans tous les cas que le caractere des arrivees des bancs est tres different du caractere des arrivees du socle.

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2006-04-27
2024-04-27

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