Seismic energy is reflected (echoed) from interfaces as well as transmitted.
             Strength of reflection depends on
             Seismic speed contrast across interface
             Density contrast across interface
             Transmitted energy can be partially reflected by still-deeper interfaces
             Travel-time, strength of "echoes" from layers below source measured
             Travel-times give relative depth
             Strength gives property contrast
             Repeated at close spacings along profile (compare echo-sounder)
             Result is seismic section, which mimics layer distribution in subsurface
             Major tool in oil,gas exploration
             Hydrocarbons trapped in deformed beds (for example, folds, domes)
             Relative travel-time of reflections shows relative depths
             Structural interpretation possible up to several km below surface
             Seismic signal processing techniques focus on improving signal-to-noise
             ratio(SNR) and resolution of seismic data from pre-stack throught to
             post-stack A device used in surface seismic acquisition, both onshore and on the seabed offshore, that detects ground velocity produced by seismic waves and
             transforms the motion into electrical impulses. Geophones detect motion in only
             one direction. Conventional seismic surveys on land use one geophone per
             receiver location to detect motion in the vertical direction. Three mutually
             orthogonal geophones are typically used in combination to collect 3C seismic
             data. Hydrophones, unlike geophones, detect changes in pressure rather than
             motion. A magnet is connected to spike, and is
             surrounded by a coil of wires mounted on very sensit
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