8. EXPLORATION OF SUB-OCEANIC STRUCTURE BY THE USE OF 



SEISMIC SURFACE WAVES 



J. Oliver and J. Dorman 



1. Introduction 



The study of seismic surface waves generated by an earthquake or a large 

 nuclear explosion offers one of the two principal seismic methods for exploration 

 beneath the bottom of the sea. Surface-wave techniques, which utilize waves of 

 long ])eriods, i.e. greater than about 5 sec, determine average features of 

 structures of the order of tens or hundreds of kilometers in depth and hundreds 

 or thousands of kilometers in length. The other principal seismic method, the 

 seismic refraction or the travel-time technique, which is treated elsewhere in 

 this volume, provides relatively detailed information on earth structures of 

 the order of 10 km in depth and some tens of kilometers in length. The lengths 

 of waves used in the surface- wave method range from about 10 to about 

 2000 km. Resolution of structural features is surprisingly good in spite of the 

 great length of these waves, although local details are not as well resolved as in 

 the case of the seismic-refraction technique, which utilizes waves only fractions 

 of a kilometer in length. Data for surface-wave studies are abundant, at least 

 for certain areas, whereas seismic -refraction data are difficult and expensive to 

 obtain. 



Up to the present, most information obtained from surface waves has come 

 from analysis of their dispersion, i.e. the dej^endence of velocity on wave period. 

 Valuable data on dispersion may be obtained from the analysis of a single 

 seismogram and, since this dispersion is sensitive to and therefore diagnostic of 

 the layered structure of the earth, a mean structure along the path may be 

 deduced from such data. 



Surface-wave studies have established the relative uniformity of crustal 

 thickness in all major oceanic basins of the world, indicating that the mantle 

 rises to shallow depth in these regions. Average sediment thicknesses of the 

 order of 0.5 to 1.0 km and geographical variations in this thickness have been 

 determined by surface-wave methods. In the case of deeper structure, primarily 

 the velocity distribution in the upper few hundred kilometers of the sub- 

 oceanic mantle, surface-wave data are a most important source of information. 

 This is so, partly because the travel-time data based on both explosive and 

 earthquake sources are meager, and partly because of the inherent weakness of 

 the travel-time method for the study of structures in which the velocity 

 decreases appreciably with depth over certain depth ranges. Surface waves of 

 both the Love and Rayleigh tyjies show that the top of the low-velocity zone 

 of the upper mantle is shallower than it is beneath the continents. Our know- 

 ledge of earth structure based on surface-wave techniques can be markedly 

 supplemented in the future by analysis of existing data using conventional 

 techniques. In addition, there remain many unexplained or partially explained 

 features of surface-wave trains traversing oceanic paths, and these features, 

 once interpreted, will offer further sources of information on earth structure. 



[MS received July, 1960] 110 



