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Early refraction studies of the crust used widely spaced observing points. Consequently, secondary arrivals, including possible reflections, could not be identified. This required the use of first arrivals, which limited the definition of the method.
Under the VELA UNIFORM project for the improvement of seismology, techniques and instrumentation have been developed closely paralleling those used in the study of sedimentary columns: continuous profiles, close spacing of observation points, high recording speed, use of magnetic tape recording, etc. These techniques have been applied widely in the United States, less widely in Canada; analogous techniques are well advanced in the Soviet Union.
The crust in continents varies from a thickness of about 20 km. in some coastal areas to as much as 50 km. in high plateau areas. Over most continental areas of moderate elevation the average thickness is 35 km. The number of layers in the crust is open to question. It is generally accepted that there are two layers, an upper granitic one and a lower basaltic one, but these are clearly defined only in some high plateau areas. In most regions a model involving a functional increase of velocity with depth fits the data as well as any system of discrete layers.
Most recent studies show no simple relationship between crustal thickness, surface elevation, and gravity values. Isostatic compensation may be accomplished through variation in the thickness, or the mean density of the crust, or by mass redistribution in the mantle.
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