Chap. 9] SEISMIC METHODS 551 



better confinement is obtained. Sometimes the reflection amplitude can 

 be increased, compared with the refraction amplitude, by firing simul- 

 taneously two charges one below the other. As the reflected energy 

 comes vertically from below, vertical type seismographs are used. How- 

 ever, this does not minimize refraction impulses, since they likewise come 

 in from an almost vertical direction, nor does it reduce the ground roll 

 (Rayleigh?) waves, since they too possess a strong vertical component. 

 However, if there is a difference in the frequency of undesirable impulses 

 and the frequency of reflected waves, the former may be eliminated or 

 reduced by selective response characteristics of the channels or portions 

 thereof (filters). The first high-speed impulses are generally reduced in 

 amplitude by automatic, semiautomatic, or manually operated volume 

 controls. Finally, a field technique combining suitable drilling depths, 

 charges, and shot distances is the most effective means of obtaining distinct 

 reflection records. 



Under favorable surface conditions there is virtually no limit to the 

 possibilities of the reflection method in sedimentary areas of low dip. The 

 range for which it is commonly used extends from 2000 to 10,000 feet; the 

 extremes are 300 feet and about 30,000 feet.^^ Depth penetration is not 

 controlled by the dimensions of the effective beds. Other advantages are 

 small charges, accuracy of depth determinations (0.2 to 0.5 per cent of 

 depth), completeness with which depth information can be obtained with- 

 out complicated calculations, freedom from terrain effects, and the fact 

 that depth data may be obtained for more than one layer with undimin- 

 ished accuracy. This is of importance to the geologist, since it makes 

 possible the determination of the displacement of the axes of folds with 

 an increase in depth, the mapping of variations in formation thicknesses, 

 and the determination of the existence and extent of unconformities. 



2. Instruments.'^'^ Three primary and two secondary devices are the 

 fundamental constituents of a reflection instrument: (a) an electrical 

 detector (phone), (6) an amplifier, (c) a recording device, (d) time-marking 

 mechanism, and (e) shot-instant transmission system. 



(a) Detector. The function of the detector is to convert the mechanical 

 ground vibrations into fluctuations of electrical current which are amplified 

 and reconverted into mechanical (rotational) motions of a recording gal- 

 vanometer. All types of electrical detectors are similar to microphones 

 in construction. The inductive type is comparable to the coil microphone, 



*^ See footnote, p. 451 



" References to the literature on this subject are given in C. A. Heiland, "Instru- 

 ment Problems in Reflection Seismology," A.I.M.E. Geophys. Pros., 411-454 

 (1934). This section gives only a general description of reflection instruments; 

 theory is discussed in section IV, 



