1. Seismic Refraction Interpretation
2 and 3 Layers Case

2. Refraction Seismology Definition
A method that maps geologic structure using the travel times of head waves.
Ground surface

3. Refraction Seismology Head Waves
Head waves are elastic waves that enter a high-velocity medium (refractor) near the critical angle and travel in the high-velocity medium nearly parallel to the refractor surface before returning to the surface of the Earth. S R
Layer 1
Velocity = V1 ic
Layer 2
Velocity = V2
V2 > V1

4. Refraction Seismology Objectives
The objective in refraction surveys is to measure the arrival times of head waves as a function of source-receiver distance so that the depth to and velocity of the refractors in which they traveled can be determined. X T

5. 2 Layer Case V2 > V1 Z x S R ic A B Layer 1 Velocity = V1 Layer 2
Refracted time from S to R is given by

6. 2 Layer Case

7. Straight line equation
2 Layer Case
Straight line equation
A is the slope
B is intercept with vertical axis at x = 0

8. Traveltime Curve
Refracted waves
Direct waves
Time (s)
Offset (m)

9. to is the intercept time
Depth to Refractor
to is the intercept time

10. 3 Layer Case Horizontal RefractorB D C 1 2 x
Layer 1
Velocity = V1 Z1
Layer 2
Velocity = V2 Z2
Layer 3
Velocity = V3
V3 > V2 > V1
Refracted time from S to R is given by

11. 3 Layer Case Horizontal Refractor

12. n Layer Case Horizontal Refractor

13. Definitions
Critical distance: Critical distance (xc) is the minimum horizontal distance from the shot point at which the first refracted pulse can be recorded.
Critical refraction has same travel time as reflection
Angle of reflection same as critical angle
Cross-over distance: Cross-over distance (xco) is the horizontal distance from the shot point where the direct wave reaches the receiver simultaneously with the refracted wave.
xco xc ic

14. Shot – Geophone Relation
Definitions
Shot – Geophone Relation
Forward shooting
Reverse shooting
Split shooting
Offset shooting

15. 2 Layer Case Dipping Refractor
Time (s)
Offset (m)
Reciprocal time
Slope=1/Vd
Slope=1/Vu
t1d
t1u
Slope=1/V1 S R  Zd  Zu A  B

16. 2 Layer Case Dipping Refractor
Downdip shooting
Updip shooting

17. 2 Layer Case Dipping Refractor
If  is small enough so that, cos  = 1 and sin  = , then
If  is very small then

18. Recording Instrument (Seismograph)
120 channels Bison from 1980’s
Up to 64 channels Stratavisor NZ from Geometrics
24 channels Geode from Geometrics

19. Receivers (Geophones)

20. Seismic Sources (Land)
Sledgehammers
Find trapped miners experiment
(AZ. USA)
Find sinkholes
(Utah, USA)

21. Seismic Sources (Land)
Weight drop (Nevada, USA)
Vibroseis
Explosive

22. Setup a Seismic Survey Seismograph Source cable Source point
Geophone cable
Geophones

23. Seismic Sources (Land)

24. Picking First Arrivals
Data example, Park City, UT. Profile # 1

25. Our First Field Test Why? Introduction to field work
Where? Here on campus
What? 2D profile
When?
48 Channel
4 shots (2 forward and 2 reverse)
Targets: find layer velocity, thickness and dip
Survey layout: two perpendicular receiver lines, 24 channel each
Number of stacks and geophone interval will be determined in the field

26. Summary Seismic refraction can be used to find layer velocity
Depth to refractor and its dipping can also be found
Layer velocity is equal to 1/slope
Layer thickness is found from the value of the intercept time
Layer dip can be found if the depth at two points is known