1. Seismic Refraction Exercise for a Hydrogeology Course
Devin Castendyk
State University of New York,
College at Oneonta

2. Basic hydrogeologic questions
How deep is the water table?
What is the stratigraphy below a site?

3. Traditional solution:
Drill several boreholes & install wells:
Expensive ($500-$2000 per well)
Labor Intensive
Time
If site is contaminated:
Disposal of contaminated core
Clean equipment

4. What students need to know
What are seismic waves?
Useful property: Waves travel at different velocities
True of different phases (e.g. solid, liquid, and gas)
True of different Earth materials:
Dry soil (vadose zone) ~ 1000 ft/sec
Wet soil (phreatic zone) ~ 5000 ft/sec
Shale = ft/sec
Sandstone = ,000 ft/sec
Granite > 17,000 ft/sec
If we can measure the velocity of seismic waves versus depth, we can define the water table and infer the stratigraphy at depth without intrusive methods.

5. Objective: Depth Velocity Interpretation 0-10 ft 1000 ft/sec 10-20 ft
Unsaturated Soil
Water Table
10-20 ft
5000 ft/sec
Saturated Soil
> 20 ft
12,000 ft/sec
Bedrock

6. How to measure the composition of a 1-layer, homogeneous site
Place a geophone (mini-seismometer) at a known distance from a seismic source
Generate a seismic wave
Measure the time it takes the P-Wave to move from the source to the geophone
Calculate and interpret the velocity:

7. How to address 2-layered site using seismic refraction
Step 1:
Use multiple geophones (an array) typically spaced equal distances apart (e.g. 10 feet)
Step 2:
Generate a seismic wave
Interpret results

8. Set up the geophone array

10. “Bison Digital Instantaneous Floating Point Signal Stacking Seismograph”

11. Make an artificial seismic wave
THUD!

12. Borehole explosion

13. Thumper truck

14. Shotgun blast

15. Sledge hammer

16. Sledge hammer

17. How seismic waves refract
Time 1:
Direct wave travels in all directions from source
Vibrations from Layer 1 arrive at closest geophone (1) 1 2 3 4 5 6 7 8 9 10
Soil
Bedrock

18. How seismic waves refract
Time 2:
Wave front reaches layer boundary
Vibrations from Layer 1 recorded by next closest geophone (2) 1 2 3 4 5 6 7 8 9 10
Soil
Bedrock

19. How seismic waves refract
Time 3:
Wave front travels faster through Layer 2 (bedrock) than Layer 1 (soil).
This causes the interface between the layers to vibrate in advance of the wave front in Layer 1 (seismic refraction).
Waves generated by the interface travel back to surface. 1 2 3 4 5 6 7 8 9 10
Soil
Refracted Wave
Bedrock

20. How seismic waves refract
Time 4:
Refracted waves reach geophones ahead of direct waves from source. 1 2 3 4 5 6 7 8 9 10
Soil
Refracted Wave
Bedrock

21. Raw data Seismic Refraction, SUNY Oneonta, May 2006
Geophone Number
P-Wave Arrival Time (milliseconds)

22. Data analysis Step 1: Pick P-Wave arrival times (first deviation)
Graph arrival time versus distance

23. Pit P-wave arrival times Seismic Refraction, SUNY Oneonta, May 2006
Geophone Number
P-Wave Arrival Time (milliseconds)

24. Make a data table Distance from Source Arrival Time (Feet)
(Milliseconds) 10 9 20
15.5 30 18 40 50
22.5 60 23 70 80
25.5 90
26.5
100 26
110 27
120
28.5

26. P-wave velocity calculations
Step 3:
Connect points corresponding to the same velocity
Step 4:
Calculate the velocity represented by each line

27. v2=5000 ft/sec
v1=1000 ft/sec
v3=12,000 ft/sec

28. Depth of boundary between Layer 1 and Layer 2
Step 5:
Calculate the depth of the interface between Layer 1 and Layer 2.
The depth (z1) from the surface to the first interface is calculated using the following equation:
Where Xc1 is the distance to the first intersecting velocity lines. This represents the distance from the source where the direct wave and the refracted wave arrive at the same time.

29. v2=5000 ft/sec
v1=1000 ft/sec
v3=12,000 ft/sec
Xc = 20 feet

30. Depth of boundary between Layer 2 and Layer 3
Step 6:
Calculate the depth of the interface between Layer 2 and Layer 3.
The depth (z2) from the surface to the first interface is calculated using the following equation:
Where Xc2 is the distance to the first intersecting velocity lines.

31. v2=5000 ft/sec
v1=1000 ft/sec
v3=12,000 ft/sec
Xc2 = 50 feet
Xc = 20 feet

32. Final interpretation of stratigraphic column
Depth
Velocity
Interpretation
0-10 ft
1000 ft/sec
Unsaturated Soil
Water Table
10-20 ft
5000 ft/sec
Saturated Soil
> 20 ft
12,000 ft/sec
Limestone

33. Conclusion
Seismic refraction is a useful tool in hydrologic investigations:
Identifies stratigraphy
Identifies the depth to water table
Non-intrusive: No contaminated soil to dispose of or equipment to clean
Inexpensive and time saving compared to borehole drilling
IT’S FUN!