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