SEISMIC METHODS 701 



ground following a shot, it would generally be quite difficult to distinguish 

 with surety the reflected waves from the spurious energies recorded on the 

 seismogram. The visual identification of the reflection is made possible 

 by the method of simultaneously recording the motion of the ground over 

 a relatively large section or area. This is accomplished by using as many 

 as 12 to 48 separate detecting stations and recording the activation of all 

 the stations simultaneously on the same photographic record. In this 

 manner the statistical data on the emerging wave fronts are effectively 

 sampled at the instant they pass each of the detecting seismometers. The 

 refracted and surface waves which pass the seismometers are usually 

 identified by their speed through the field of receivers, which corresponds 

 to the seismic velocity of their respective media, whereas the reflected waves 

 may be identified by the very high apparent speed with which they move 

 through the field of receivers. In the case of relatively horizontal strata 

 this speed of the reflected wave usually exceeds any known seismic velocity 

 in the vicinity. For example, a reflected wave from a horizontal reflecting 

 strata would arrive throughout a horizontal area in the neighborhood 

 of the shot-point at virtually the same time. This would impart the same 

 movement of the ground almost instantaneously to all seismometers in the 

 field, and as a consequence all oscillograph trace records of the reflected 

 waves would be similar with respect to time, relative amplitude, phase 

 relationship and wave form. In other words, a similar event would be 

 recorded independently by all instrument traces on the seismogram. The 

 alignment of these similar wave forms across the seismogram is usually 

 referred to as the line-up of the reflection. The time difference as estimated 

 across the record is referred to as AT, or the "move-out" or "step-out" time. 



Therefore, in a study of records, similar wave forms which appear in 

 permissible "line-up" patterns will be identified visually as reflections. 

 Prominent reflections are selected, or marked at corresponding points or 

 phases* down the traces as shown in Figures 430 and 431. 



From a knowledge of the average velocity in the region under prospect 

 and the disposition of the seismometers with respect to the shot-point, 

 the interpreter of the field records can determine the range of difference 

 of times which may be expected for reflections from beds at any 

 depth and dip. The time-distance relation for reflection indicates the na- 

 ture of the patterns or so-called "line-ups" which are possible for reflections. 

 For a split spread, i.e., seismometers located in line with and on opposite 

 sides of the shot-point, the pattern or "line-up" is of the form indicated 

 in Figure 430; with increasing depth the "line-up" approaches a straight 

 line, i.e., curves drawn through troughs or peaks corresponding to the 

 same reflection on the various traces approximate straight lines. Actually, 

 it is seldom necessary to refer to velocity or spread length to recognize 



* The term "phase" is used here in the conventional sense of any particular time 

 in the history of a wave, rather than in the more limited meaning, p. 644, of the onset 

 of a new type of wave on earthquake seismograms. 



