Dictionary:Shadow zone

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{{#category_index:S|shadow zone}} Effects seen on seismic sections or in other ways that obscure or distort data from a particular region, usually affecting amplitudes (amplitude shadow) but sometimes effects of other kinds (such as a fault shadow or velocity shadow, q.v.). 1. A subsurface region in which there is little penetration of waves, usually because of the velocity distribution; a region that is not illuminated or only poorly illuminated. See Figure S-6. 2. A portion of the subsurface from which reflections are unobservable because raypaths do not reach the geophones on the surface. The overlying beds may have such dips and velocity contrasts that raypaths to or from reflectors within the shadow zone become refracted or undergo total reflection. A different configuration of sources and receivers may permit recording data from the region. 3. A region from which reflections are incoherent or exceptionally weak because of shallower complicated velocity distribution, such as beneath very rough (rugose) surfaces such as the boundaries of some allocthonous salt. Irregular gas disseminated throughout a gas chimney can have similar effects. Irregular raypath bending because of rapid changes in large velocity contrasts along faults may also destroy coherency. 4. A portion of the subsurface that does not evidence itself on refraction profiles, such as beds whose velocity is lower than that of an overlying refractor. See also blind zone, hidden layer, channel wave. 5. A region 100–140° from the epicenter of an earthquake in which there is no direct penetration of P-waves due to refraction at the mantle-core boundary. A similar shadow region for direct S-waves extends from about 100° to 180°. 6. A region of weakened amplitude, such as sometimes appears under a hydrocarbon accumulation.