70 



GEOPHYSICAL MEASUREMENTS* 

 Russell W. Raitt 



This report is restricted to the technique of seismic measurements 

 made by the author in the Pacific Ocean. It is hoped that other participants in 

 the symposium, having direct experience with seismic measurements in the 

 Atlantic Ocean, gravity and magnetic observations, and geothermal gradient 

 -measurements, will contribute their knowledge of techniques used in these im- 

 portant fields. 



The technique of seismic refraction measurements in the deep sea, 

 thousands of miles from land, is strongly conditioned by two principal aspects: 

 (1) all explosives must be carried from supply bases; (2) operating costs of 

 long range, deep sea ships are high. Refraction profiles must be long in order 

 to achieve penetration to the base of the earth's crust; they should be fired rap- 

 idly to avoid unnecessary loss of ship's time; the charges should be small to 

 attain maximum use of the limited explosives capacity. Efforts to achieve these 

 objectives have followed two lines of development: 



1. Improvement of detectability of bottom refracted waves by reducing 

 noise level, by filtering unwanted noise, by recording several hydro- 

 phones, and by proper choice of hydrophone depth. 



2. Development of a simple, rapid, and reliable method of firing under- 

 way by dropping TNT bombs fused with slow-burning fuse cut to fire 

 at desired depth. 



Reduction of "dangling" noise at frequencies of the order of 10 cps was 

 achieved by buoying hydrophones and the outer 50 foot of cable at neutral buoy- 

 ancy. As the weak first arrivals at long range arrive nearly vertically, the 

 optimum hydrophone depth is one quarter wave length of the dominant frequency 

 of the refracted wave. 



The Fourier spectrum of the explosive sound wave has a prominent peak 

 at the "bubble pulse" frequency and falls off rapidly at lower frequencies. Max- 

 imum low frequency intensity of bottom refracted waves occurs at a shot depth 

 of one quarter of the wave length of "bubble pulse" frequency. Optimum shot 

 depths and frequencies according to this doctrine vary from 80 ft. and 15 cps 

 fori lbs. TNT to 205 ft. and 6 cps for 50 lbs. TNT. 



Maximum range attained by this technique varies widely with sea condi- 

 tions and bottom propagation. Under average conditions prevailing in the region 

 of the northeast trade winds in the Pacific Ocean average maximum ranges are 

 represented by the equation: 2^ 



^ = 7W2 



* - Contribution No. 680 from the Scripps Institution of Oceanography. 



