. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 86 RAITT [OHAP. 6 as a first arrival are: Layer 2, 7 km to 12 km; Layer 3, 12 km to 36 km; Layer 4, >36 km. In actual practice the time-distance plots differ from the idealized model of Fig. 1 for various reasons : 1. Layer velocities and thicknesses differ somewhat from the assumed values. 2. Layering is not horizontal. 3. The velocity within one layer may vary. 4. A layer may be prevented from observation as a fii'st arrival by prior arrivals from another layer. 9 III I I 1 I Z' ' ' ' 1 1 1 1 o V) E - y .^-^ »• - ^^^
. The Earth beneath the sea : History. Ocean bottom; Marine geophysics. 86 RAITT [OHAP. 6 as a first arrival are: Layer 2, 7 km to 12 km; Layer 3, 12 km to 36 km; Layer 4, >36 km. In actual practice the time-distance plots differ from the idealized model of Fig. 1 for various reasons : 1. Layer velocities and thicknesses differ somewhat from the assumed values. 2. Layering is not horizontal. 3. The velocity within one layer may vary. 4. A layer may be prevented from observation as a fii'st arrival by prior arrivals from another layer. 9 III I I 1 I Z' ' ' ' 1 1 1 1 o V) E - y .^-^ »• - ^^^ - "oi > o ^^ <^ - S 1 1 1 1 1 1 1 1 ! 1 1 1 1 1 1 1 5 10 15 Direct wafer wave travel time (sec) 20 Fig. 2. Travel-time plot for Challenger Station CR9 near the Hawaiian Islands. Layer 2 velocity = km/sec; Layer 3 velocity = km/sec. (After Gaskell, Hill and Swallow, 1958.) ^ 10 0) E. 5 10 Direct water wave travel time (sec) Fig. 3. Travel-time plot of Challenger Station 32 in the Atlantic near the British Isles. Layer 2 velocity = km/sec; Layer 3 velocity = km/sec. (After Gaskell, Hill and Swallow, 1958.) Nevertheless, widely spaced deep-sea seismic-refraction stations usually have a strong similarity to the model. Studies at widely separated stations in the Atlantic and Pacific Oceans have shown the presence of all of the foin- layers. In the Indian Ocean the few observations of Gaskell, Hill and Swallow (1958) on Challenger did not reach Layer 4, but Layers 1, 2 and 3 were observed. These seismic-refraction studies are not yet as widely distributed over the world oceans as might be desired. They tend to be concentrated in areas of good weather, or in regions close to the operating bases of the investigating. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Hill, M. N.
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Keywords: ., bookcentury1900, bookcollectionbiodivers, booksubjectoceanbottom
