484 SEISMIC METHODS [Chap. 9 



been used or suggested. Fessenden proposed the use of sonic transmit- 

 ters, as applied in submarine signaling, for the location of ore bodies. 

 Some conmiercial companies and scientific institutions have experimented 

 occasionally with weights. Hubert,^' using weights of 20, 50, and 117 kg 

 dropped from heights of 1 to 1 1 m, could detect reflections from a number 

 of beds down to 5 km in depth by means of a Wiechert seismograph with 

 a magnification of 2 million located at a distance of 125 m from the weight 

 tower. He found that (1) the travel times of seismic impulses generated 

 by falling weights were independent of the masses and of the elevation 

 from which they were dropped, (2) seismograms obtained with different 

 masses and different heights could be correlated phase by phase; (3) the 

 observed amplitudes were proportional to the square root of the height 

 and to the weights of the masses used, hence, the amplitudes were propor- 

 tional to the square root of the fall energy. Experiments made in this 

 country with falling weights have shown that a 200-pound lead weight 

 dropped from about 20 feet can be detected with a Schweydar seismograph 

 under favorable circumstances up to about 300 feet. 



The energy liberated by falling weights is much less than that from a 

 * dynamite explosion. To release the same energy produced by a confined 

 buried charge of about 500 pounds of dynamite, an iron ball nine feet in 

 diameter and weighing 75 tons would have to be dropped from a height 

 of one mile. Nevertheless, weights have possibilities in reflection work. 

 Unbalanced flywheel machines (vibrators) have been used for testing the 

 dynamic response of buildings and surface formations. Details are given 

 in Chapter 12. 



With the exception of vibration tests of buildings and surface formations, 

 dynamite is used in virtually all commercial seismic exploration. Com- 

 mercial dynamites fall into two group 



(A) Straight dynamites (dynamites proper). These contain nitroglyc- 

 erine, in an amount equal to the grade-strength marking, and various 

 absorptive materials. 



{B) Gelatins. Some of the nitroglycerine is replaced by nitrocotton, 

 forming a gelatin. 



(a) Blasting gelatin: 91 per cent nitroglycerine, 8 per cent nitrocotton, 

 1 per cent chalk. 



(b) Straight gelatins: These are blasting gelatins, diluted with pulp and 

 sodium nitrate. The Du Pont Hi- Velocity Seismic Gelatin and the Atlas 

 and Hercules low-freezing gelatins are in this group. 



(c) Ammonia (or special) gelatins. These are equal in strength to the 

 straight gelatins, but ammonium nitrate replaces a portion of the nitro- 



"F. Hubert, Zeit. Geophysik, 1(6), 197 (1924-1925). 



