896 EXPLORATION GEOPHYSICS 



experimental conditions it was found that, for charges buried to or just 

 below sea level, as many as eight successive reflections were obtained with 

 but little intervening reflection energy. When air-shooting was employed, 

 it was found that not more than two reflections were obtained. 



To further illustrate how air-shooting may minimize multiple reflec- 

 tions, two seismograms are shown in Figure 564. These records were taken 

 in the Benedum Area, Upton County, Texas, A general examination of the 

 records shows a smaller number of more sharply defined energies on the air 

 shot than on the hole shot, but more particularly, the almost complete 

 absence of apparent reflections on the air charge record beyond the base- 

 ment reflection which occurs between 1.9 and 2.0 seconds. 



Note the reflection which occurs at 2.376 (Y) on the hole shot, but which 

 is absent on the air shot. If this is inspected to determine the possibility 

 of its being a first reverberation, the original reflection would have to be at 

 1.175 seconds and there is one at 1.176 (X). If it were a second reverbera- 

 tion, there would have to be a first reverberation at 1.580 and a reflection 

 at 0.778, and these do not exist. There is, therefore, a possibility that this 

 is a first reverberation from the reflection at 1.175. 



Now, examine the reflections at 2.160(3). For this to be a first rever- 

 beration there would have to be a reflection at 1.074, and none exists. For 

 it to be a second reverberation, there would have to be a first reverberation 

 at 1.462(2), and a reflection at .740(1), and both of these are definite possi- 

 bilities. The reflection is picked at .735 instead of .740, and a strong 

 energy and phase shift starts at approximately 1.462. Other examples are on 

 the record. 



Experience in seismic work indicates that reverberations or multiple 

 reflections occur commonly between the near surface and some early, good 

 reflecting interface. The most frequently observed reverberations appear to 

 arrive from igneous layers of basalt, etc., interbedded with the sediments. 

 Oftentimes between the near surface and a shallow basement, in interpret- 

 ing such a reverberation as a reflection, a shift is produced which is more 

 than twice as great as a real reflection, while the second reverberation pro- 

 duces a shift in excess of three times that which would be caused by the 

 actual dip of the horizon. Since the reverberations occurring between the 

 surface and some early reflecting horizon are in the region of lowest velocity, 

 the displacement of the reverberation on the record is appreciably in excess 

 of the two- or three- fold actual displacement of the reflecting horizon. 



The fact that the reflections shown on these records have an energy 

 apparently greater in the reverberation than in the initial reflection is not 

 surprising, since most of the random energy has disappeared by the time 

 that the reverberation arrives and the overall amplification or gain control 

 has been vastly increased. 



