1. n. [Geophysics]

A layer or body of rock that cannot be detected by seismic refraction, typically because its velocity is lower than that of the overlying rocks; also known as a hidden layer.

See: refraction

2. n. [Geophysics]

A shadow zone, or a zone through which waves do not pass, or cannot be recorded, or in which reflections do not occur.

See: shadow zone