SEISMIC METHODS 705 



continuous pulse correlation, which requires continuous following along 

 the surface in question. Spot reflection correlation may be compared to 

 obtaining elevations by means of an aneroid barometer, and the stations 

 may be relatively large distances apart. Each observation is independent 

 of any other, and the reflections from a certain interface or group of inter- 

 faces are recognized and correlated from station to station by wave form, 

 amplitude character, sequence of reflected wave events, and general magni- 

 tude of the time for reflection. There is an analogy between this type of 

 correlation and that accomplished by the paleontologist when he recognizes 

 a certain geologic horizon by the presence of a specific fossil fauna. 



In regions where competent strata exist over large areas (as, for 

 example, the Denver or Julesburg Basin, containing uniform geological 

 formations of wide areal extent such as the Dakota Sandstone, Madison 

 Limestone, etc.) reflections from certain interfaces seem to persist over 

 extensive areas, and spot correlation shooting is good. However, in areas 

 where relatively incompetent strata occur and to which the term lenticularity 

 is frequently applied by the geologist, reflections of definite character do 

 not seem to persist from interfaces extending over large areas, and spot 

 correlations become impossible. Considerable parts of California and the 

 Gulf Coast exhibit this condition. When such areas are explored by seismic 

 methods, recourse usually is made to reflection dip shooting or modified 

 pulse correlation work. 



Geological Correlation. — The correlation of reflections with definite 

 geological marker beds can best be made in conjunction with seismic 

 velocity surveys of bore holes. In this manner the reflections can be precisely 

 correlated with or relative to geological marker beds. It is quite common 

 practice for geophysicists to place the reflection record along the "time" 

 coordinate of the seismic velocity survey graph, with the well log along 

 the "depth" coordinate. The reflection times can thus be correlated by 

 inspection with the geologic well log. 



An observation often commented on by geophysicists and geologists 

 is that, while excellent correlatable reflections occur over relatively large 

 areas, these reflections are not found to represent any known or recogniz- 

 able geological horizon marker. For example, excellent reflections often 

 seem to originate from interfaces within what the geologist would describe 

 as a massive stratum of uniform character. During the early period of 

 reflection work this fact caused some suspicions among geologists as to 

 whether the reflection seismograph always mapped geologic structure. The 

 success of the reflection seismograph as a powerful tool for mapping buried 

 geologic structure is based on the condition that the interfaces of discon- 

 tinuity of the elastic properties of the strata coincide or are parallel to the 

 bedding of the geological formations. Practice has demonstrated this 

 assumption to be sufficiently accurate to give useful and trustworthy esti- 

 mates of geologic structure. However, it does not necessarily follow that 



