SECT. 1] 



SEISMIC REFRACTION AND REFLECTION MEASUREMENTS 



the layers are dipping, the inverse slopes give apparent velocities which are too 

 high if the shots are up-dip from the receiving position or too low if the shots 

 are down-dip. In order to determine the existence of dips, and their direction 

 and magnitude, it is necessary to shoot end-to-end profiles or reversed profiles. 

 The former consists of two lines of shots usually fired in opposite directions 

 away from a common receiving position. The latter consists of a line of shots 

 received at both ends of the line. Certain conditions are imposed upon the time- 

 distance data by the end-to-end arrangement and others are imposed by the 

 reversed one ; hence it is most advantageous to combine the two and shoot 

 end-to-end reversed profiles. Fig. 5 shows time-distance graphs of two such 

 profiles and the structure section for a three-layer case with dipping beds. 



Layer I 



Fig. 5. Time-distance graphs for end-to-end reversed profiles across dipping beds, 

 showing relationship of i-everse points and intercepts. Only refracted waves are con- 

 sidered here. 



The conditions imposed by the end-to-end arrangement are that the intercepts 

 (squares) of the lines in Profile 1 must be the same as those in Profile 2 at 

 point B. The conditions imposed by a reversed profile are that the reverse 

 points (circles) for each layer be the same at each end of the profile. This latter 

 results from the reciprocity theorem, which states that, at any distance, the 

 shot and receiving positions may be interchanged without affecting the travel 

 time. These conditions, both for end-to-end and reversed profiles, provide a 

 valuable check on the interpretation of travel-time data. If two or more end-to- 

 end reversed profiles are shot and the data can be shown to satisfy these condi- 

 tions, the results may be viewed with considerable confidence. In addition to 

 these, it is also frequently possible to provide a further test of the data and of 

 the interpretation with the conditions imposed by the required relationship 

 of the refraction lines and reflection curves, as discussed earlier. The importance 

 of these various means of cross-checking the results is obvious when one bears 

 in mind that we are dealing with geological layering and not with models. 



