Chapter 4 



DIFFRACTED SEISMIC WAVES 



T. I. Oblocina 



INTRODUCTION 



In order to solv^e the problems which are encountered in seismic prospecting 

 over areas of complex geological structure it is essential to study the dynamic 

 properties of waves, since kinematic indications are often missing when 

 the wave pattern is of a complicated nature. 



When waves are diffracted their dynamic properties acquire fundamental 

 importance for determining the different types of waves, and so for a correct 

 geological interpretation of the data obtained in seismic prospecting when 

 tectonic disturbances or steeply dipping and tapering layers are present in 

 the cross-section. 



Diffraction is generally understood to mean either the curving of waves 

 round obstacles in their path or the scattering of waves on various irregularities 

 in the medium. 



Diffracting objects in seismic prospecting can be divided into three types: 

 diffracting edges, individual diffracting bodies and multiple diffracting 

 bodies. 



The first category includes the edges of extended interfaces. Examples 

 of structural features which contain diffracting edges are fault-type 

 dislocations, steeply dipping and tapering strata, the cleges of intrusions 

 and salt domes, and so forth. 



The second category includes more or less isometrically shaped bodies 

 which are small in comparison with the wave length: ore chutes, caverns 

 and kettle holes. We may note that ore chutes and kettle holes are not 

 objects of study by seismic prospecting methods; at present searches for 

 these are carried out by other geophysical methods. 



The third category includes a great number of bodies which are small 

 in comparison with the wave length: small inclusions of rocks which are 

 different in their lithological composition from the enclosing rocks, small 

 reflecting surfaces distributed at random in a fractured zone, small irregu- 

 larities in interfaces. Small diffracting bodies make the medium texturally 

 nonhomogeneous ("turbid"). In seismic prospecting this interferes with 



