46 GEOPHYSICAL METHODS IN OIL EXPLORATION [Chap. 4 



for reconnaissance, provided it is expected that basement rocks are some- 

 what conformable to the structure in the sedimentaries, or provided some 

 of the sedimentary beds are magnetic themselves. Second and third 

 choices, respectively, in structure exploration are reflection-seismic and 

 gravity methods. For detailed surveys, reflection work is first choice, 

 torsion balance and gravimeter second. 



The reflection method has had the most brilliant success in Oklahoma 

 for mapping the Viola and Hunton (Ordovician) limestone topography 

 and has been applied in most other petroliferous areas in the United States 

 and abroad. There are comparatively few prospective oil territories that 

 cannot be worked with reflection methods. 



For general structural reconnaissance, gravimeter surveys to locate re- 

 gional structural trends have been and are being conducted on a large 

 scale along the coastal belt of the Gulf and in the northern parts of Texas 

 and in the mid-continent. The torsion, balance has successfully located or 

 detailed general structure-type fields in Texas, Oklahoma, California, New 

 Mexico and other states. For the fault-type of structure, magnetic 

 methods are probably not recommendable for first place in reconnaissance 

 survejdng. Even if basement rocks are expected to be somewhat con- 

 formable to the oil-bearing series, they usually are too far away to furnish 

 satisfactory information regarding faults — that is, faults of such throw as 

 to be of importance in oil work. However, there are some faults which 

 give a magnetic expression without apparent association with igneous 

 rocks — the indication being directly above the fault plane. In the absence 

 of a definite explanation for this phenomenon, magnetic methods should 

 be relegated to second place for reconnaissance, while first place probably 

 would be given to reflection and possibly torsion-balance methods, for 

 both reconnaissance and detail. Applications of these two methods to the 

 location of faults have been numerous in oil areas of this country and 

 abroad. 



In cases where oil occurrence is associated with volcanic intrusions or 

 dikes, the magnetometer is first choice for reconnaissance and detail, and 

 the torsion balance is second. Examples are the intrusions at Monroe and 

 Richland, Louisiana; and Jackson, Mississippi; and the intrusive dikes in 

 the Tampico region of Mexico. 



There has been so much work done with geophysical methods in the 

 salt-dome areas of the Gulf coast that the choice of methods in salt dome 

 exploration is very definite, although the order of preference has undergone 

 some changes during the past ten years. For domes down to 5000 to 

 6000 feet, refraction-seismic work was first choice, but the torsion balance 

 and gravimeter are now more in favor, particularly for the deep domes and 

 for detailing their crests. Reflection-seismic methods are best adapted for 



