refraction intro

Seismic Refraction

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

ACTIVE STANDARD: D5777-00(2006) Standard Guide for Using the Seismic Refraction Method for

Subsurface Investigation $42.90 for PDF

• Developed by Subcommittee: D18.01See Related Work by this SubcommitteeAdoptions: 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.

Refraction Lay Out

Seismic Refraction

• Advantages– Simple layout– Low manpower requirements

– Limited Equipment Requirements– Rapid data reduction and analysis (computer

not needed)

– Easy interpretation

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)

Rippability versus seismic velocity. (Caterpillar.

Handbook of Ripping, 8 th Edition)

Direct Wave

0.00

10.00

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0 20 40 60 80

Meters

mil

isec

on

ds

0.00

Simple Plot

0

20

40

60

80

100

120

0 20 40 60 80 100

Distance (meters)

Tim

e (m

ilis

eco

nd

s)

Distance TimeD Time R

0 0 30

6 8 32

12 16 34

18 24 36

24 32 38

30 40 40

36 48 42

42 56 44

48 64 46

54 72 48

60 80 50

66 88 52

72 96 54

78 104 56

84 112 58

Direct and Refracted Waves

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0 20 40 60 80

meters

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isec

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Series1

Series2

Two Equations for Simple Refraction

These two equations should give the same answer. It should be less than half the crossover distance.

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)

Up DipDown Dip

Dipping Bed Equations

+

=

−

=

−−

−−

udc

ud

v

v

v

v

v

v

v

v

2

11

2

11

2

11

2

11

sinsin2

1

sinsin2

1

θ

γ