SEISMIC METHODS 7AZ 



Similarly, elimination of H2 between Equations 93 and 94 leads to 



. _V2^TJ_ X2 + X-, . - 



sm <^2 - orr 7^ (97) 



Furthermore, for small angles sin 0i and sin ^2 are approximatelj' equal to ^i and 

 03 respectively. Hence, 



_ _ V^^Ti — 

 sm rf>2 — sm (pi — d>2 — 01 — a — 



T2-ViATi -r. + ^x / 1 1_^ /QQ^ 



.r2-;ri 2 \ViTx V^tJ ^"^^^ 



Equation 98 expresses a as a function of measurable quantities and is therefore the 

 relation sought. It is important to note that the values of AT2 and ATi have opposite 

 signs in Equation 98. Hence, errors due to the low velocity layer are substantially 

 avoided. 



Figure 453 is a map showing a structural divergence over an actual salt dome 

 in Louisiana. The figures on the dip arrows indicate the amount of divergence in feet 

 per mile in the direction of the arrow. The length of each arrow represents the 

 spacing of 50-foot contours at the position of the arrow. The divergence was observed 

 between beds located at approximately 3000 feet and 7000 feet ; that is, the divergence 

 was observed over an interval of approximately 4000 feet. It is evident from the 

 map that the crest of the structure coincides closely with the center of convergence. 



Grading of Reflections for Visual Correlations 



The evaluation of the quaHty of a recorded reflection involves the 

 follow^ing factors. (1) The consistency of wave form for every trace of 

 a record and the consistency of the difference in reflection time for different 

 phases of the wave : these are the primary factors by which reflections 

 are identified and by which accuracy of dip determination may be gauged. 



(2) The wave form or character is judged by the degree to which it 

 approaches the response of a seismometer to a single impulse of steep 

 wave front. The presence of all phases of a wave without interference 

 is some assurance that a reflected wave and not spurious energy is involved. 



(3) The relative amplitude of a reflection with respect to that of the 

 waves on neighboring portions of the record determines the prominence 

 of a reflection and is some gauge of its reliability. (4) To a certain degree, 

 all the traces during a reflection disturbance display essentially the funda- 

 mental oscillation of the reflection as a whole. That is, despite interference 

 of spurious energy, at least two complete cycles of the oscillation should be 

 more or less apparent on each trace in the recording of the reflection. 



Unfortunately all of these factors are not uniformly realized in reflec- 

 tion work. In some areas more are realized than in others, due to the 

 variations of complexities in the reaction of the ground to the propagation 

 of reflected waves. Experience indicates that all grades of conditions 

 exist, from those that give good reflection results to problem areas where 

 the standard reflection method completely fails to produce reliable data. 

 One such problem is exemplified in parts of the Edwards Limestone area 

 of Texas. 



The grading of reflections is a practical method of attaching an 

 empirical factor of merit or accuracy to the individual reflection. It has 



