Seismic refraction is a geophysical method that uses seismic waves to determine subsurface layer properties like depth to bedrock or water tables. It works by measuring the refraction of seismic waves through subsurface layers with different velocities. Key uses include mapping bedrock depth, detecting faults or voids, and estimating material properties related to excavatability. The method has advantages of simple layout and analysis but is limited in resolving detailed subsurface structures. Interpretation involves analyzing refraction arrival times to calculate layer velocities and depths.

1. Seismic Refraction

2. Some uses of seismic refraction
• Mapping bedrock topography
• Determining the depth of gravel, sand or clay
deposits
• Delineating perched water tables
• Determining the depth to the water table
• Detecting subsurface caverns
• Estimating rippability
• Detecting shallow faults and fracture zones
• Detecting large boulders

3. ACTIVE STANDARD: D5777-00(2006) Standard Guide
for Using the Seismic Refraction Method for
Subsurface Investigation
$42.90 for PDF
• Developed by Subcommittee: D18.01
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Book of Standards Volume: 04.09
• 1. Scope
• This guide covers the equipment, field procedures,
and interpretation methods for the assessment of
subsurface conditions using the seismic refraction
method.
• Seismic refraction measurements as described in
this guide are applicable in mapping subsurface
conditions for various uses including geologic,
geotechnical, hydrologic, environmental, mineral
exploration, petroleum exploration, and
archaeological investigations.
• The seismic refraction method is used to map
geologic conditions including depth to bedrock, or
to water table, stratigraphy, lithology, structure, and
fractures or all of these.
• The calculated seismic wave velocity is related to
mechanical material properties. Therefore,
characterization of the material (type of rock,
degree of weathering, and rippability) is made on
the basis of seismic velocity and other geologic
information.

4. Refraction Lay Out

5. Seismic Refraction
• Advantages
– Simple layout
– Low manpower requirements
– Limited Equipment Requirements
– Rapid data reduction and analysis (computer
not needed)
– Easy interpretation

6. Seismic Refraction
• Disadvantages
– Relatively large energy input required
– Relatively long layout (10 times depth)
– Limited number of model layers
– Limited velocity differences
– Limited interface geometry (assume smooth)

7. Rippability versus seismic velocity. (Caterpillar.
Handbook of Ripping, 8 th Edition)

9. Direct Wave
60.00
50.00
miliseconds
40.00
30.00 0.00
20.00
10.00
0.00
0 20 40 60 80
Meters

10. Simple Plot
Distance TimeD Time R 120
0 0 30
100
Time (miliseconds)
6 8 32
80
12 16 34
18 24 36 60
24 32 38 40
30 40 40
20
36 48 42
0
42 56 44
0 20 40 60 80 100
48 64 46
Distance (meters)
54 72 48
60 80 50
66 88 52
72 96 54
78 104 56
84 112 58

11. Direct and Refracted Waves
60.00
50.00
miliseconds
40.00
Series1
30.00
Series2
20.00
10.00
0.00
0 20 40 60 80
meters

13. Two Equations for Simple
Refraction
These two equations should give the same answer.
It should be less than half the crossover distance.

14. Steps in solving for depth
• 1 Determine the velocities of both layers
in meters per second or feet per second
• 2 Determine the crossover distance
• 3 Determine the time intercept for V2
• 4 Determine the depth to layer 2 using
• Both refraction equations (internal check)

18. Down Dip Up Dip

21. Dipping Bed Equations
1  −1  v1  −1  v1 
γ = sin v  − sin 
 v 

2  2d   2u 
1  −1  v1  −1  v1 
θ c = sin v  + sin 
 v 

2  2d   2u 