744 EXPLORATION GEOPHYSICS 



been mentioned that, generally, reflections are sorted into four classifica- 

 tions : A, B, C and D. Grade A may correspond to good, B to fair, C to 

 poor and D to questionable.f No general system or set of fundamentals 

 has been widely accepted by geophysicists as a basis for grading reflections. 

 Different bases of grading are occasionally used by the same individual, 

 depending upon whether the area gives consistently good or consistently 

 poor reflections. 



In a general way reflection grading seems to depend on the character 

 of the reflections obtained, together with the opinion and judgment of the 

 observer. For example one observer may classify a reflection as good in an 

 area difficult to prospect by seismic methods, while a second observer, 

 working in a less difficult area, would classify the same reflection as poor 

 according to his standard of grading. 



As long as grading is done on the basis of opinion and judgment, a 

 certain amount of confusion will exist. Fundamentally the purpose of 

 grading is to evaluate the trustworthiness of the data. Therefore it is 

 desirable to establish a system of grading which is based on the same 

 fundamentals in every case and is independent as far as possible from 

 personal opinion. 



Gabyl has suggested an outline for a system of grading which differs 

 from most procedures in that the evaluation of the data as a true reflection 

 is considered together with the accuracy of the computable dip. 



When the question arises as to whether a certain recorded wave group 

 is or is not a reflection, and an attempt is made to measure this probability, 

 the prospector might become frustrated in trying to separate the more 

 dependable from the less reliable data. To overcome this difficulty, it is 

 often possible by graphical analysis of the travel-time curves of the reflec- 

 tion to determine with certainty whether or not a recorded event is a true 

 reflection. The grading of correlations may be made on the basis of the 

 degree of certainty of the correlation and the agreement of reflection time 

 of the correctable events, without raising the question as to whether the 

 data are real or unreal, because true reflections may or may not correlate. 



As seismic work is extended into the marginal areas, the quality of the 

 reflections will decrease and visual interpretation will become increasingly 

 difficult. As these conditions become prevalent, more and more dependence 

 will be placed upon instrumental interpretation employing the various meth- 

 ods of wave analysis, frequency patterns, and selective filtering. 



Cross Section and Maps 



The computed data on dip and depth of inferred strata are generally 

 shown on cross sections or profiles which depict the dip attitude and depths 

 in the vertical plane of shot-point and spreads. One form of section is 

 shown in Figure 454, (the computed positions of interfaces giving reflec- 



t Another classification uses the letters G, F, P, VP; G denotes good reflections, F fair, P poor, 

 and VP very poor or doubtful. 



t Phil P. Gaby, "Grading System for Seismic Reflections and Correlations," Geophysics, Vol. 

 XII, No. 4, Oct., 1947, pp. 590-617. 



