78 



3-15 cps 

 h Sensitivity 



3-15 cps 



Low Sens! 



cps I 



fivify I 



20 - 200 cps I — 



500- 3000 cps 

 High Sensitivity 



500 - 3000 cps 

 Low Sensitivity 



30-3000 cps 

 Lmt Sensitivity 



Radio Receiver 



First 



Bottom 



Echo 



Sub 



Bottom 



Echo 



Second 



Bottom 



Echo 



^ r^yl\fW\A/^^'^^--^^^\[\J\j\f\/^-^'-^^ 





-H*?)^.jtt}»*»1— 



s,..or.»*.«-i»./fc 



Fig. 17. Oscillogram of Bottom Echo in San Nicolas Basin. 



Figure 12 illustrates a block diagram of the various elements used in re- 

 cording the output of one hydrophone channel. The principal record is made on 

 a 15-channel Miller oscillograph recording on photographic paper 6 inches wide. 

 A Brush pen oscillograph is also used to monitor the record. Two other hydro- 

 phones record the three frequency bands on the Miller oscillograph but are un- 

 monitored by the Brush oscillograph. Figures 13 and 14 are examples of rec- 

 ords made in deep water and shallow water respectively. 



The performance of seismic refraction equipnnent is best evaluated in 

 terms of the maximum range at which the beginning of the bottom refracted wave 

 can be determined with reasonable reliability. This range is quite variable be- 

 cause of the variability of the intensity of the refracted waves but even more be- 

 cause of the variability of noise levels. Figure 15, which shows a cumulative 

 distribution of the spectrum levels observed on the low frequency channel on the 

 1950 expedition to the Mid-Pacific. Each point represents the average of 20 to 

 30 measurements at a single station, so the variations are systematic changes 

 due to day-to-day changes of conditions, and represent a cross section of obser- 

 vations in the North Equatorial Pacific, where the trade winds commonly have a 

 strength of Beaufort 4. 



Maximum ranges obtained under these conditions are plotted as a function 

 of charge wei,ght in Figure 16. The straight line through the points has the equa- 

 tion A = 6.8W2, where A'is the range in kilometers and W is the weight of TNT in 

 lbs. 



Under proper conditions seismic reflection measurements can give useful 

 information about the subsurface. An example is shown in Figure 17 which 

 shows an oscillogram of a bottom echo obtained in San Nicholas Basin about 80 



