64 Subsurface Geologic Methods 



tions may be accurately extended over appreciable distances without undue 

 concern. 



Methods of correlation vary according to stratigraphic and struc- 

 tural complexity (whether general or detailed results are desired, whether 

 the problem is of surface or subsurface variety, or both) the time and ex- 

 penditure allotted to the assignment, the quality of the personnel, and 

 the policy of the management. 



Purposes of Correlation 



The correlation of rocks is the foundation upon which the history 

 of the earth can be deciphered. Correlations result in (1) constructing 

 composite geologic sections; (2) deciphering surface and subsurface con- 

 ditions; (3) coordinating surface and subsurface sequences; (4) evaluat- 

 ing contemporaneous and noncontemporaneous rocks; (5) interpreting 

 geologic history; (6) identifying and evaluating unconformities; (7) in- 

 terpreting environmental conditions and variations; (8) evaluating iso- 

 pachous and lithofacies data; (9) identifying outliers; (10) exploring 

 and developing natural resources; and (11) making recommendations for 

 well locations, casing points, testing, and abandonment of wells. 



Methods of Correlation 



Numerous methods of varying complexity and simplicity are em- 

 ployed in correlating sedimentary strata. Techniques applied in subsur- 

 face work may diverge widely from those applied in surface investigations. 

 The more commonly used correlation procedures are as follows: 



1. Tracing formations or key beds from one locale to another. Strat- 

 igraphic relationships of subjacent and superjacent strata should be care- 

 fully analyzed when this method is used. 



2. The establishment of control lithologic and paleontologic se- 

 quences. 



3. The application of air photographs supplemented by ground in- 

 vestigation. Frequently structural and stratigraphic trends that are incon- 

 spicuous or unobservable on the ground may be observed on air photo- 

 graphs. Photographs may often be used to fill in isolated areas between 

 well-controlled stratigraphic sections. 



4. The establishment of erosional surfaces. 



5. The use of paleoclimatic data. These have been found adaptable 

 to correlation work in parts of the geologic column. 



6. The application of other techniques, which include chemical, in- 

 soluble-residue, specific gravity, stain, detrital-mineralogy, screen, spec- 

 trographic, paleontologic, porosity and permeability, settling-rate, and 

 X-ray analysis. 



7. The use of well-logging methods, such as electric, radioactive, 

 thermal, caliper, mud, and drill-time. These methods have greatly as- 

 sisted in establishing more reliable correlations in the subsurface. 



