Seismic Surveys 



85 



that derived from natural ones. Detonation 

 of many tons of explosive at Corona, about 

 72 km east of Long Beach, in 1949 and again 

 in 1951 provided opportunities to measure 

 the velocities of earth waves at various dis- 

 tances and thus at various depths below the 

 surface. By these measurements Gutenberg 

 (1952) found that the bottom of the crust 

 (Mohorovicic discontinuity) lay at a depth 

 of about 40 km. Velocities of the longitudi- 

 nal waves were 8.2 km/sec below the crust 

 and 6.5 km/sec at a depth of 5 km, cor- 

 responding closely to velocities obtained 

 from earthquakes. For comparison, a sim- 

 ilar large explosion at Beecher's Bay of Santa 

 Rosa Island in 1950 showed that the 

 Mohorovicic discontinuity in this part of the 

 continental borderland is as shallow as 20 

 km and that velocities above and below it 

 are similar to those at Corona (Tatel and 

 Tuve, 1955). 



Most seismic surveying utilizes much 

 smaller detonations than those discussed 

 above and for marine work are of two types, 

 reflection shooting and refraction shooting. 

 Reflection work is the simpler and consists 

 of finding the lapsed time between shot, re- 

 flection from the bottom and layers within 

 the bottom, and reception at the hydrophone 

 (Figs. 73, 74). Seismic surveying by oil and 



service companies to locate and characterize 

 structures in shallow water is almost exclu- 

 sively reflection work. An array of hydro- 

 phones strung out from a receiving ship re- 

 ceives sound impulses that travel diff'erent 

 paths from the shot point to the reflection 

 surfaces and thence to the hydrophones. 

 Many hundreds of reflection records have 

 also been made in the basins by Raitt (1952) 

 because the work can be done quickly and 

 by a single ship. However, for proper in- 

 terpretation the measurements require data 

 on sound velocities in the different layers of 

 the bottom. These data are supplied only 

 in part by acoustic studies of bottom sam- 

 ples (Shumway, 1956); mostly, therefore, the 

 work must be coordinated with refraction 

 shooting. 



Refraction work in the ocean requires 

 two ships, one to be the receiver and the 

 other to drop explosives at intervals as it 

 moves toward and then past and away from 

 the receiving ship. For areas as small as 

 the basins the maximum possible distance 

 between the ships is so short that sound 

 waves cannot be received from great depth. 

 However, using this method Raitt (1949) 

 and Shor and Raitt (1956, 1958) reported 

 that from the few points obtainable the 

 Mohorovicic discontinuity rises from about 



Figure 72. Generalized Bouguer gravity anomaly map of southern California region (milligals). Data for mainland 

 are from Duerksen's (1949) pendulum stations, those for islands are from Brisbin's (1957) stations, and those for the 

 sea floor are from submarines (Harrison, Brown, and Spiess, 1957). 



