Diffraction phenomena around salt masses
Madrigal Ugalde, Luis
Clark, H. C.
Master of Arts
The problem was to determine the use of diffracted events as an interpretive tool in reflection seismograms. The phenomenon was studied in relation to salt masses because this type of plastic intrusion is associated with different structural shapes capable of causing diffraction. Those features are (a) the border of the crest in an anticlinal shaped body, (b) the intersection between the flank and the horizontal bottom of a syncline on the salt body, (c) relatively small anticline-like irregularities over the salt mass, and (d) tapered layers ending over the salt masses as wedges. The solution was obtained by a two-dimensional model technique. The laboratory seismograms showed that diffraction occurs in the presence of curved interfaces when the radius of curvature does not exceed a certain proportionality with the wavelength of the elastic wave; that there is a difference in amplitude decay and move-out between a reflection and a diffraction event; that the epicenter of diffraction is located by the minimum time coordinate of the corresponding hyperbolic curve; that shadow zones predicted by ray theory are "illuminated" by diffraction events; that on transition from diffracted to reflected waves the amplitude of the former increases; that the amplitude of interference waves reaches a maximum as the reflected and diffracted wave fronts reach a constructive interference and that the maximum shifts away from the focus of diffraction when the distance between the source and the same focus increases.