1 / 27

Reflection Seismic Method

Reflection Seismic Method. The hyperbolic form of the reflection travel-time: t 2 = (4h 2 + x 2 )/V 0 2 This form describes NMO (normal moveout). V 0 V 1  V 0. Recall ray paths in a 1-layer model. Direct ray: t(x) = x/V 1 Refracted ray: t(x) = t i + x/V 2

smoldenhauer
Download Presentation

Reflection Seismic Method

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. Reflection Seismic Method The hyperbolic form of the reflection travel-time: t2 = (4h2 + x2)/V02 This form describes NMO (normal moveout) V0 V1 V0

  2. Recall ray paths in a 1-layer model Direct ray: t(x) = x/V1 Refracted ray: t(x) = ti + x/V2 where ti = 2z(V22 – v12)1/2/V1V2

  3. Direct and Refraction ray travel times Direct ray: t(x) = x/V1 Refracted ray: t(x) = ti + x/V2 where ti = 2z(V22 – V12)1/2/V1V2

  4. A dipping layer – refraction paths

  5. Travel times – dipping reflector

  6. Diffractions masquerade as reflections

  7. Amplitudes of seismic waves Waves lose amplitude due to: Geometric spreading as waves move farther into volume: Amplitude = 1/(distance)2 Absorption of wave energy into heat: Amplitude ~ A0e-x Scattering -- reflections, refractions, diffractions from layer inhomogeneities and anisotropies

  8. Automatic gain control We compensate for the ever decreasing wave amplitude by preferentially amplifying the low amplitude signals... Logarithmic amplifiers... the signal records as the logarithm of its amplitude Gain-ranging amplifiers... the signal records according to the average signal amplitude during a short time window

  9. Seismic wave sources for reflection surveys Land surveys: Dynamite (geogel), hammer, thumper, dropping weights – shock sources are not easily controllable as to waveform Vibrational sources -- “Vibroseis” a controlled source

  10. Generator for shear waves Small explosive source

  11. Vibroseis system The Vibroseis “chirp” waveform

  12. The “geophone”

  13. Geophone response The geophone responds according to frequency of vibrations... The resonance: c = (m/k)1/2

  14. Geophone arrays... Sensitivity according to apparent wavelength

  15. Suppressing “ground roll” A geophone array is normally tuned to suppress “ground roll” -- the surface Rayleigh wave.

  16. Marine seismic surveys

  17. Marine source: Airgun

  18. The airgun pulse shape The gas bubble released by the airgun oscillates causing a repetition of the pulse

  19. Airgun arrays The pulse from 1 gun An array The pulse repetition is cancelled by interference

  20. Hydrophone, streamers and recording

  21. A marine seismic profile (i.e. 2d)

  22. Digital recording of seismic signals The electrical signals from the geophone or hydrophone are converted into a stream of “digital” values with a fixed interval between samples. Each “time series” of samples is recorded for subsequent data processing. A high-resolution marine seismic survey might accumulate several Gigabytes of data per minute.

  23. Aliasing by sampling To avoid aliasing, we must sample 2x per shortest period in data

  24. Anti-alias prefiltering A filter is designed to cut out the high frequencies (short periods) that would be aliased in sampling.

  25. S/N enhancement The signal-to-noise ratio (S/N) is increased by N1/2, where N is the number of data traces added.

  26. Noise reduction – CDP (CMP) We assemble many pairs of shot-detector spacings that reflect from a “common depth point”... we add the signals together to average away “noise”.

  27. Reflection seismic processes The “seismic section” is a metaphor of geological structure

More Related