Subsurface Methods as Applied in Geophysics 1101 



Correlation shooting with distances of 2,000 to more than 5,000 feet 

 between shot points may be practiced in those areas in which certain 

 horizons yield a similar and distinctive character of reflection that may 

 be readily identified by the interpreter. No effort is made to determine 

 the dip, as the areas in which this technique is applicable are those of 

 low relief. By picking corresponding phases of reflections that possess 

 character and abnormal amplitude, it becomes possible to trace and map 

 reflecting horizons over wide areas. Extreme care must be taken to insure 

 that the proper "legs" or phases of a reflection are picked from record to 

 record. (See fig. 580.) Gaby "^^ has described a number of the criteria by 

 which reflections may be correlated between records. Computations are 

 generally simple, as they may be based on a straight-line ray path. 

 Depths may be plotted directly on the contour map, which obviates the 

 necessity for the cross section. 



Continuous shooting is indicated in regions where correlations become 

 somewhat uncertain or maximum detail is required. Since only short in- 

 tervals exist between subsurface control points, reflections may be corre- 

 lated with greater certainty. By using interlocking or overlapping con- 

 tinuous spreads, the correlations become almost mechanical. The con- 

 tinuous methods are commonly used in regions of steep or variable dip as 

 well as in those areas where persistent reflecting horizons are absent. 

 The more rigorous computation methods involving the curved ray are 

 generally chosen in order to present dip and depth data as accurately 

 as possible. The computed dips and depths of strata are shown on cross 

 sections. True dips are only indicated when the line of traverse is per- 

 pendicular to the strike. In those regions where persistent reflecting hor- 

 izons are absent, subsurface relief may be expressed by use of the phantom 

 horizon. At an arbitrary point on the cross section, a traverse or phantom 

 is drawn by averaging (paralleling) the dips for a reasonable distance 

 on either side of the phantom horizon. By planning the survey so that 

 frequent ties are obtained to previous work, errors that have accumulated 

 in the loop may be adjusted out. Large portions of the Gulf Coast and 

 parts of California have been surveyed by this technique. 



Seismic Applications 



The application of the seismograph to exploration problems may best 

 be illustrated by reviewing the discovery history of several oil fields. 

 The reader is referred to the volume entitled Geophysical Case Histories '^^ 

 for a complete discussion. 



Cameron Meadows Dome, Cameron Parish, Louisiana — The first geo- 

 physical work was done in the area of the Cameron Meadows dome in 

 Cameron Parish, Louisiana, in 1926, using mechanical seismographs. Al- 

 though the Seismos Company report noted abnormal conditions, the data 



'' Gaby, P. P., Grading System for Seismic Reflections and Correlations : Geophysics, vol. 12, no. 4, 

 pp. 590-617, Oct. 1947. 



'* Geophysical Case Histories, vol. 1, 671 pp., Tulsa, Soc. Exploration Geophysicists, 1948. 



