700 EXPLORATION GEOPHYSICS 



strata vibration, sustained vibration at the shot-point, reverberations and 

 disturbances (not attributable to the origin of the initial impulse). All of 

 these impulses actuate the seismometer and the problem becomes one of rec- 

 ognizing the true reflection from the complexity of impulses with which it 

 is associated. These complications arise from the intricate natures of the 

 subterranean wave-transmitting media and of the initiation of the impulse. 

 The separation of the reflection from this background of the recorded 

 disturbances is accomplished by a study of relative time and amplitude. 

 The property of relative time is accentuated by the arrangement of detect- 

 ing seismometers, and the property of amplitude is accentuated by the 

 overall design and vibratory frequency response of the recording equipment. 



When the subsurface under investigation consists of more than two 

 strata, the penetration of the rays to the deeper strata must be considered. 

 At each interface the radiating wave is divested of energy, a part of which 

 appears as a reflected longitudinal wave and is detected by the seismometer 

 on the ground surface. The seismometer is therefore activated by a series 

 of reflected waves from successively deeper interfaces.* 



It might be expected that superposition of other energies (surface 

 waves, direct waves, transverse waves, refracted waves, sustained strata 

 vibration, reverberations and disturbances not attributable to the shot) 

 on the reflected energy would outweigh the effect of the reflected wave, 

 thereby preventing reliable recording of the reflections. To the dismay of 

 the seismologist, this situation is sometimes approached in practice. How- 

 ever the non-useful energy produced by the shot can be avoided to a large 

 degree by present-day instrumental and field techniques. Interfering energy 

 from sources other than the shot-point is generally maintained below a 

 "noise-level" (the background of the record), so that confusion with 

 reflected energy from this source occurs only in the case of a disturbance 

 of unexpectedly large amplitude. This factor is eliminated conclusively 

 by comparison of two or more seismograms secured from the same shot- 

 point. In field practice it has been found that seismic records are usually 

 reproducible in close detail as far as energy from the shot is concerned, 

 because successive shots under essentially similar conditions in the shot- 

 hole will yield substantially similar seismograms. 



The presence of rough surface topography is frequently a source of 

 error in seismic mapping. Widessf describes various aspects of the prob- 

 lem and indicates methods of solution for different topographic effects. 



If a sinsfle seismometer were utilized to determine movement of the 



* Secondary reflections may occur and show up on the record. Secondary re- 

 flections may be due to waves which have undergone multiple reflection before striking 

 the seismometers. Generally, multiple reflection, or reverberation, between two strata 

 is very seldom detectable because the energy imparted to a seismometer by a wave 

 which has undergone multiple reflection is small relative to the other energy which 

 the seismometer is receiving at the same time. Another class of secondary reflections, 

 however, is sometimes noticeable : namely, that corresponding to reflection of direct 

 waves from the base of the low velocity layer or from the ground surface. 



t M. B. Widess, "Effect of Surface Topography on Seismic Mapping," Geophysics, Vol. XI, No. 

 3, July, 1946, pp. 362-372. 



