1 / 13

Medium effects on waves Geometrical spreading

Medium effects on waves Geometrical spreading. Wave spreads over a larger surface as it travels through the medium. For a spherical wave, the wave energy falls off as the square of the distance. Its effect is to weaken the later arrivals in the seismic section.

tola
Download Presentation

Medium effects on waves Geometrical spreading

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Medium effects on wavesGeometrical spreading • Wave spreads over a larger surface as it travels through the medium. • For a spherical wave, the wave energy falls off as the square of the distance. • Its effect is to weaken the later arrivals in the seismic section. • We correct this effect by multiplying the amplitude by the distance (if known) or traveltime.

  2. Medium effects on wavesAbsorption • A part of wave energy is dissipated into the earth as heat. • Wave energy falls off exponentially with distance. • In seismic exploration, its effect is usually very small compared to that of geometrical spreading. • It is usually neglected.

  3. Medium effects on wavesGeometrical spreading versus absorption S r A: absorption GS: geometrical spreading S: source position r: distance from source h: absorption coefficient GSa 1/r2 Aa exp(-h r) h = 10-5 - 10-3 m-1

  4. Medium effects on wavesGeometrical spreading versus absorption GS A Amplitude

  5. Medium effects on wavesReflection/refraction • When a wave encounters an interface between two layers, part of its energy is reflected back. • The other part is refracted (transmitted) into the other medium. • Snell’s law governs reflection and refraction angles.

  6. Sr Pr Pi fr qi qr a1, b1 a2, b2 qt ft Pt St Sinqi Sinqr Sinqt Sinfr Sinft a1 a1 a2 b1 b2 Medium effects on wavesSnell’s law

  7. Medium effects on wavesDiffraction • occurs when wave encounters sharp discontinuities in the medium • important in defining faults • generally considered as noise in seismic sections • seismic migration usually corrects for this effect

  8. X Earth model Z X Seismic Section T Medium effects on wavesDiffraction

  9. Sr Pr Pi fr qi qr a1, b1 a2, b2 qt ft Pt St Sinqi Sinqr Sinqt Sinfr Sinft a1 a1 a2 b1 b2 Reflection coefficients

  10. r - r V V 2 2 1 1 RC = r + r V V 2 2 1 1 Reflection coefficientsi ≈ 0 r1: density in incident medium r2: density in refraction medium V1: seismic velocity in incident medium V2: seismic velocity in refraction medium

  11. Reflection coefficientsi ≤ 15 • very slight deviation from the normal-incidence case • For most seismic exploration purposes, i ≤ 15 is a good assumption. • Therefore, normal-incidence RC is used in general.

  12. Reflection coefficientsi > 15 • high deviation from the normal-incidence case • Therefore, normal-incidence is NOT a good assumption. • Full Zoeppritz equations have to be used. • Zoeppritz equations are very complicated algebraically. • If i ≤ 30, approximations of Zoeppritz equations are used. • Studies involving amplitude variation with offset (AVO) use these approximations.

  13. Reflection coefficientsMagnitude • Rock-rock RC < |0.3| • Rock-soil RC ~ |0.7| • Rock-water RC ~ |0.7| • Rock-air RC ~ |1.0| • Water-deep-sea sediments RC ~ |0.3| • Water-air RC ~ |1.0|

More Related