854 EXPLORATION GEOPHYSICS 



method. First, the energy received by the channel is increased considerably. 

 Second, the ratio of useful to extraneous energy recorded by the group 

 generally is increased. The latter condition is realized because vertical 

 waves strike the seismometers on the surface fairly well in phase whereas 

 extraneous energy, such as the surface wave, will in general strike the 

 seismometers in different phases at any given instant ; that is, a dis- 

 crimination is created in favor of vertical waves by a group of seis- 

 mometers placed on a horizontal plane. Where reflected waves from 

 strongly dipping beds are involved, however, the discriminatory powers of 

 the seismometers may be a disadvantage unless the seismometers in each 

 group are closely spaced or the line of seismometers is oriented in the 

 direction of strike of the reflecting beds. Another disadvantage of 

 multiple detection is that the time of arrival recorded by a channel fed 

 by several seismometers often is not as sharp as that recorded by a single 

 seismometer because, in general, the times of arrival will differ slightly 

 for each seismometer, resulting in a "smoothing-out" of the wave 

 characteristics. 



It often has been found true that if records of seismometers placed 

 a short distance apart, 2 feet for example, are appreciably dissimilar, the 

 use of multiple detection is helpful; whereas, if they are similar, benefit 

 is seldom obtained because the separation of the seismometers is relatively 

 unimportant. 



Overlapping Seismometer Output 



Another proposed aid to the detection of reflections is an artifice 

 known by the various titles : "composite," "inter-locking," "diversity," 

 "overlapping," etc. recording, t In this type of recording, each seis- 

 mometer output feeds two or more traces of the record in some specified 

 manner. The seismic energy is thus overlapped across more than one 

 trace and a particular seismometer cannot be associated with a given 

 trace alone. The reason for using overlapping may be two-fold. First, 

 for a given total number of seismometers, it permits a greater number 

 of seismometers to operate in series and thereby increases the dis- 

 crimination in favor of reflected waves (provided that the reflecting beds 

 are not excessively steep). Second, reflections can be followed more 

 readily across a record, because the difference between adjacent traces 

 is less. 



The procedure of overlapping is dangerous when carried too far, as 

 will be evident from the following considerations. Spurious energy is 

 usually disclosed by dissimilarities in traces. Hence a diminution of 

 dissimilarities due to overlapping may yield a pattern or "line-up" which 

 is due to spurious energy rather than reflections. Any overlapping 

 at all, in fact, is condemned by some operators for this reason, while 

 others approve its use to a limited and definitely controlled degree. 



t Henry Salvatori, "Apparatus for Determining Subsurface Geological Formations," U. S 

 Patent 2,064,385, issued Dec. 15, 1936. 



