P. E. Gretener
Velocity reversals within the earth constitute anomalous conditions which deserve detailed scrutiny. Physically, such layers commonly are characterized by low density, low shear strength, and high energy absorption besides low seismic velocity. Because of their low velocity, they may act as waveguides and where thickness is variable they tend to distort all seismic events originating at greater depth. Geologically, such layers may be the source beds for diapirs and act as structural decollements.
The deepest known low-velocity layer is the outer core, anticipated by Wiechert in 1897 and defined by Gutenberg in 1912. In the early 1950s, Gutenberg further postulated a low-velocity layer in the upper mantle and one or more such layers in the continental crust. The significance of the former was only fully appreciated by earth scientists with the advent of plate tectonics. It is the decollement that separates the plates from the earth's interior. In 1966, Landisman and Muller provided further evidence for intracrustal low-velocity layers in central Europe. The importance of these features remains to be assessed. They may well play a decisive role in preventing basement structures, such as observed in Wyoming, from affecting the base of the crust. Within the sedimentary sequence, the velocity inversions associated with overpressured shales have become common knowledge within the las decade. On a smaller scale, coal seams are known to exhibit extremely low velocities.
With limited thickness, such low-velocity layers act as waveguides. It is this property that led to the early discovery of the low-velocity layer at the base of the lithosphere. On a different scale, guided-channel waves have been used in coal exploration to test seams for continuity.
Because of their low shear strength and low density, low-velocity layers are potentially capable of diapiric rise. The phenomenon is well known for overpressured shales and holds to a lesser degree for coal beds. For the same reason, such horizons also act as structural decouplers. Best known and most significant for hydrocarbon exploration is the flattening of growth faults into zones of overpressured shales.
AAPG Search and Discovery Article #90961©1978 AAPG Annual Convention and Exhibition, Oklahoma City, Oklahoma