SECT. 1] 



SEISMIC REFRACTION AND REFLECTION MEASUREMENTS 



Since we are dealing with seismic measurements in the sea, here and in 

 future discussions l^ayer 1 is the water layer. The direct wave in Layer 1, D, 

 and the refracted waves in Layers 2 and 3, Gi and G2, determine straight Una 

 segments, and the reflected waves determine h3rperbolic curves. There is a 

 critical distance at which a refracted wave from either of the deeper layers can 

 be returned to the surface. This distance is def)endent on the velocity contrast 

 between the layer in question and Layer 1, and on the velocity and thickness 



Time 



."^• 



Fig. 3. Ray diagram and time-distance graph for reflected and refracted waves in a 

 three-layer model, Ci < (^2 < C^- 



of the layers between. Notice that the refraction line G for any layer is tangent 

 to the reflection curve associated with the top of that layer. The point of 

 tangency occurs at critical distance. In addition to the above relationship, 

 i.e. Gji is tangent to R„, notice also that R^ is asymptotic to G^-i. Since both 

 refracted and reflected waves are recorded in the normal method of operation, 

 these relationships are of great importance in the analysis of seismic data, 

 imposing, as they do, strict conditions upon the identification and interpreta- 

 tion of arrivals. Fig. 4 shows sample oscillograph records with various reflected 

 and refracted arrivals indicated. Notice that the waterborne waves, D and Ri, 

 contain energy in a wide range of frequencies, whereas the refracted and 

 reflected ground waves contain only low-frequency energy. In general it is not 

 possible to identify the path of a ground wave arrival by its appearance on one 

 record. Such identification is made principally from the relationship of the 

 arrival to others on the time-distance graph. 



For simplicity of discussion we have been considering structures in which 

 all of the interfaces are horizontal. In such cases the inverse slope of each 

 refraction line Gi, G2, G3. . .Gn gives the true seismic velocity in that layer. If 



