740 



EXPLORATION GEOPHYSICS 



layer 2, and layer 5 has the same general character as layer 3. Layer 6 (not shown) 

 is rock which has a porosity such that petroleum may be trapped within it. 



The shapes of beds 3 and 5 illustrate a geological condition usually realized : viz., 

 the deeper beds exhibit more closure or steeper dip. Such beds are said to diverge 

 from each other "off the structural feature." Thus, ivi is the interval "on the structure" 

 and K2 is the interval at a point "off the structure." 



The times of arrival of the rays traversing the various paths are shown on the 

 seismogram. (Figure 451.) Trace 23 is the trace produced by the oscillograph con- 

 nected to seismometer Si. Point 40 on trace 23 represents the instant of arrival at Si 

 of the ray traveling along OASi] point 41 represents the instant of arrival of the 

 ray traveling along path OCASi; point 42 represents the instant of arrival of the ray 

 traveling along path OEASi. 



Fig. 451. — Theoretical seismogram showing traces produced by oscillographs connected 

 with seismometers Si and S2 of Figure 450. (McCollum, U. S. Patent 2,118,441.) 



Point 20 on trace 25 represents the instant of explosion. 



Trace 24 is the trace of the oscillograph connected to seismometer S2. Point 50 rep- 

 resents the instant of arrival of the ray traveling along the path OBS2. Point 51 

 represents the instant of arrival of the ray traveling along the path ODBS2. Point 

 52 represents the instant of arrival of the ray traveling along the path OFBSz. 



The observed time intervals used for mapping the structure are ATi which is 

 the difference between points 41 and 51 on traces 23 and 24 and AT2 which is the 

 difference between points 42 and 52 on traces 23 and 24. It will be noted that these 

 time intervals are independent of time of origin represented at point 20. 



The normal value of (AT2 — AT1) for a given pair of reflecting horizons may 

 be obtained by averaging a large number of observed values of (AT2 — ATi) obtained 

 at random over the area being explored. 



In all portions of an area in which there is no angular divergence or in which 

 the geological formations are parallel, the value of (Ar2 — ATi) is a constant for all 

 seismograms for which the same size spreads were used. If the quantity (AT: — ATi) 

 is not constant, it is indicative of angular divergence. In particular, if the observed 

 (ATz — ATi) is greater than the normal value for the area, the direction in which 

 the more distant seismometer is positioned from the shot-point is the direction of 

 angular divergence from the shot-point. If the observed (AT2 — ATi) is less than 

 the normal value for the area, the direction of angular divergence is reversed. 



Evidently, it is possible to plot arrows with lengths proportional to the divergence 

 on maps and thus determine the differences in intervals. 



Moreover, it is possible to evaluate the angle of divergence a between layer 1 

 and layer 2 in terms of measurable quantities. To calculate a, it is convenient to use 

 the constructions shown in Figure 452. Xi equals the distance O^i and X2 equals the 

 distance QS2. Li corresponds to layer 3 and L2 to layer 5. Hi is the perpendicular 

 distance from to plane Li and H2 is the perpendicular distance from to plane L2. 

 <pi is the dip of layer Li and ^2 the dip of layer L2. 



h is the image of the shot-point in plane Li and h is the image of in the 

 plane L2. 



