I. GEOPHYSICAL EXPLORATION 



1. ELEMENTARY THEORY OF SEISMIC REFRACTION AND 

 REFLECTION MEASUREMENTS 



J. I. EWING 



1. Introduction 



Seismic techniques have been among the most useful in furthering our 

 knowledge of the structure and thickness of the earth's crust under the sea. 

 The techniques are based on the measurement of the propagation of sonic 

 energy, usually generated by the explosion of TNT or dynamite charges. The 

 sound energy is received at varying distances from the explosion site by hydro- 

 phones or geophones and recorded on oscillograph tapes with an accurate time 

 scale. Although useful information can sometimes be obtained from studying 

 the character of the transmitted energy, the basic measurement taken from 

 the oscillograms is the time of arrival of sound waves for which the path from 

 explosion to detector is known or can be ascertained. If the path (distance) and 

 the travel time are known, the speed of propagation can be computed. By 

 recording either a line of shots at a fixed receiving position or a single shot at 

 a line of receiving positions, a relationship of travel time versus distance is 

 established, from which the number and thickness of the sub-surface strata 

 and the speed of sound in each can be determined. Knowing the speed of sound 

 in a particular layer enables us to make at least an intelligent guess about the 

 type of material constituting the layer ; or if it is known, from drilling or other 

 means, that a certain layer is present at a particular dej^th beneath the surface, 

 this allows us to determine the continuation of the layer into areas where the 

 other information is unavailable. 



2. Reflected Waves 



If we know the speed of sound in various sub-surface strata and if the inter- 

 faces between the strata are good reflectors, it is simple to measure the thickness 

 of each by measuring the time required for a pulse of sonic energy to travel 

 through the layer and back by reflection. Standard echo-sounders measure 

 water depth in just this way, and some sounders under certain conditions will 

 measure the depth to sub-bottom interfaces within a few tens of meters of 

 the bottom. The limitations of this usage arise from the fact that relatively 

 high sonic frequencies are required to give good resolution of sea-floor topo- 

 graphy, and high frequencies, because of absorption, cannot penetrate far into 

 the unconsolidated sediments or rock layers. Low-frequency energy from 



[MS received July, 1960] 3 



*% 



