Understanding how sedimentary basins evolve is an exciting, but difficult task. Seismic reflection data do a marvelous job of imaging the internal structure of sedimentary basins. There is no better technique. However, we often want more than just a detailed image of the basin. Of particular interest are the physical properties within the basin and below it, within the crystalline crust. For example, seismic P- and S-wave velocities are closely tied to lithology, and hence crustal type (oceanic versus continental). Consequently, seismic reflection studies are complemented by seismic velocity studies. Such velocity information is very effective in determining basin geometry, and can be used to determine Poisson’s ratio, an important parameter for determining the kind of source rocks, crustal type, as well issues related to the Law of the Seas convention.

One good way to obtain reliable deep seismic velocities is to shoot wide-offset seismic refraction profiles. Actually, this technique is a classical one that all students encounter in their first course about the Earth’s crust. Indeed, seismic refraction profiles formed the basis for the early exploration for hydrocarbon resources. Today, high-resolution seismic refraction profiles are recorded on land and at sea using a wide variety of techniques.

In this talk I will present a global synthesis of what we have learned from deep crustal seismic velocity profiles, from the Gulf Coast to the Bay of Bengal and beyond. A very extensive literature search has been undertaken to track down as many of the seismic refraction publications as possible. The resulting database contains over 10,000 one-dimensional P-wave and S-wave profiles, making it the largest such catalog in existence. These data, especially when examined as complete crustal cross sections, reveal a number of surprising results, such as the discovery of highly thinned continental crust extending 100s of km offshore, serpentinized peridotite (rather than basalt) right at the sea floor, and evidence for deep, highly metamorphosed sediments that mimic crystalline basement rocks, thus confounding traditional basin interpretations. Indeed, many of these results challenge conventional views of “how the Earth works”. That’s just what makes this investigation so exciting!

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