704 EXPLORATION GEOPHYSICS 



based primarily on reflection time. This process corresponds to running 

 relative elevations on a continuous surface by following along that surface. 

 It has been stated that the recognition of a reflection on a seismogram is 

 based on the correlation of time, wave form, amplitude, etc., among the 

 individual traces of a recorded event throughout the field of seismometers 

 of a particular instrument set-up. By expanding the field of seismometers 

 using a method of overlapping these fields for adjacent stations, the 

 recognized correlation of events on one station may be extended to a 

 second station, then to a third, to a fourth, etc. Under favorable conditions 

 this expansion may be extended to any number of stations. In this manner 

 a single pulse from a persistent reflecting interface may be recognized 

 and its reflection time plotted over an extensive area. This method of corre- 

 lation is sometimes spoken of as continuous profiling. 



Since the reflection time is directly proportional to the distance traveled 

 by the reflected wave, these times may be converted into distances and 

 consequently a subsurface set of contours on a particular reflecting inter- 

 face may be drawn for an area covered by continuous pulse correlation. 



A modification of pulse correlation is sometimes used when a persisting 

 pulse cannot be carried throughout an area. Under this condition it is often 

 possible to make single pulse correlations over limited areas only, but usually 

 when one reflected pulse disappears from seismograms of adjacent sta- 

 tions, other correlatable pulses from different reflecting interfaces will be 

 found on the seismograms. By attempting correlations of the various 

 pulses, overlap of the discontinuous correlations often will be obtained. 

 When this has been accomplished, a theoretical surface may be inferred 

 which is more or less parallel to the aggregate of all the correlated inter- 

 faces of a particular zone. 



Extreme care must be used in inferring such theoretical surfaces, 

 making certain that these surfaces do not pass through known geologic 

 features such as unconformities, stratigraphic overlaps, faults, etc. 



Continuous pulse correlation is usually good in most of Oklahoma, West 

 Texas, and parts of Kansas and California. In these areas, the strata dip 

 very gently, seldom more than two or three degrees. Gently dipping strata 

 are not always a sufficient condition for reliable pulse correlation, so that 

 in some cases dip-shooting must be used. It is becoming common practice 

 to utilize both dip computations and correlations wherever possible. The 

 simultaneous use of dip and correlation methods provides essentially two 

 independent types of delineation of subterranean geologic structure. Com- 

 bination of these two methods usually gives more satisfactory results than 

 can be had by the use of either one or the other separately. 



■Spot Correlation. — Spot correlation is based on character, event se- 

 quence, and relative time of reflected events on the seismogram. This type 

 of correlation also corresponds to securing elevations on a particular sub- 

 terranean surface, but by a slightly different procedure than that used for 



