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Slide 1

Applied Geophysics – Seismology

Refraction

Reflection

MASW

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Generic Geotechnical Application

Pile driving for a bridge

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Subsurface:

Physical Property:

Source Data

Seismic Survey

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Elastic Properties of Materials

Different descriptions

Young’s modulus, Poisson’s ratio

Bulk modulus, shear modulus

Shear modulus, 1st Lame’ parameter

P-wave, S-wave velocities

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Relationship between Elastic Parameters

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Agile box

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Geotechnical Engineers: Vs

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Shear Modulus

depends upon Vs

Young’s Modulus strongly

controlled by Vs

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Elastic properties

GPG: elastic properties

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Subsurface:

Physical Property:

Source Data

Seismic Survey

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Slide 9

Seismic waves: think acoustic

Intuition – what do you already know?

Acoustic energy will travel what pathway(s)?

Think …. Sound .

Energy in the ground

How does it travel”?

Relevant readings:

see GPG on the web.

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Slide 10

Seismology Stresses, strains, and elastic moduli

“moduli” describe strain caused by stress.

Bulk and shear moduli are key.

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Slide 11

Elastic waves propagate in the earth.

Different types of waves: Body waves (P,S)

Surface waves

(Rayleigh, Love)

Velocities from three properties: ρ= density

k= compressibility (bulk modulus)

u= twistability (shear modulus)

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Student Participation

Seismic waves propagating along a line of

students

GPG: Basic principles

Waves

Ray paths

Attenuation

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Slide 13

Waves, rays and energy propagation

How does energy propagate?

Amplitude gets reduced further from the

source

Energy decays and the pulse

spreads out as it propagates

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Slide 14

2 body wave types – 2 surface waves

S-waves P-waves

Rayleigh waves Love waves

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Slide 15

Animation of waves in a layer over a halfspace

Slower over faster (most common)

V2 > V1

V1

V2 > V1

NOTICE relation between wavefronts and signal paths (arrows)

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Seismic Applet

Direct wave

Reflected wave

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Slide 17

Waves at interfaces:

Acoustic Impedance

Reflection coefficient

can be positive or negative

http://en.wikipedia.org/wiki/Reflection_coefficient

Negative: returning wave has reversed polarity

Positive: returning wave has same polarity

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Reflection and Transmission Coefficients

Impedance

Reflection coefficient:

Transmission coefficient

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Seismic Refraction

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Slide 20

Energy in the ground

How many possible pathways?

How many possible pathways to one surface point?

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Seismic Applet

Direct wave

Reflected wave

Refracted wave

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Analogy for refracted waves: Swimmer

Problem

You and a friend are swimming in the ocean. Your friend

gets into problems.

What is the fastest way to reach your friend?

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Analogy : Swimmer Problem

You and a friend are swimming in the ocean. Your friend

gets into problems.

What is the fastest way to reach your friend?

Swim directly or take the long route by land?

Fermat’s principle or the principle of least time: the path

taken for a ray of light to travel between two points is the

path that can be traversed in the least time.

Snells’ Law can be derived from Fermat’s principle.

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Slide 24

Snells law

Law of reflection: Θr = Θ1

“The angle of reflection equals the angle of incidence”

Snells law:

Low V to High V: refracted away from the normal

High V to Low V: refracted towards the normal

sin θ1

sin θ2

V1

V2=

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Slide 25

Animation of waves in a layer over a halfspace

Slower over faster (most common)

V2 > V1

V1

V2 > V1

NOTICE relation between wavefronts and signal paths (arrows)

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Slide 26

Snells law

What happens to energy at an interface:

ie where velocity changes

V1

V2 > V1

θ1 = θ1

θ2

sin θ1

sin θ2

V1

V2=

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Slide 27

What if θ2 = 90 … ?

V1

V2 > V1

θ1 = θ1

θ2

sin θ1

sin θ2

V1

V2=

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Slide 28

What if θ2 = 90 … ?

θ2 = 90, sin θ2 =1, so sinθ1 = V1/V2.

“Head waves” develop, sending energy back to the

surface.

sin θ1

sin θ2

V1

V2=

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Slide 29

Animation of waves in a layer over a halfspace

Slower over faster (most common)

V2 > V1

V1

V2 > V1

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Slide 30

What happens if signals are Faster in top

layer?

V1 > V2

V1

V2

> V2

Is refraction possible in this situation? Implications?

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Slide 31

Java applets on the Web

http://www.phy.ntnu.edu.tw/ntnujava/index.php?topic=16

Illustration of reflection and refracted wavefronts using Fresnel-Huygens principles.

http://staff.washington.edu/aganse/raydemo/RayDemo2.med.html

Ray paths in arbitrary 1D earth. Generate the velocity model and observe first arrivals

and curved ray paths. (Visualizing bending rays in linearly increasing velocities)

http://www.iris.edu/hq/programs/education_and_outreach/visualizations

Global Earthquakes recorded by US seismometer arrays. Learn about particle motions,

wave .

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Analysis of first arrivals

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Slide 33

A B

A B

Seismic survey

offset

…

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Slide 34

The energy recorded: First Arrivals

What do we record?

Instrument records ground motion at each geophone for

some time following source.

Data we interpret is first arrival vs. distance from shot.

A “T-X” plot or first arrival time vs

distance between source & geophones

We usually plot

like this.

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Slide 35

Raw data

Direct arrivals

The T-X plot

* x x x x x x x x x x x x x x x x x x x x x x x

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Slide 36

Raw data

First refractions

The T-X plot

* x x x x x x x x x x x x x x x x x x x x x x x

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Slide 37

Raw data

Second refractions

The T-X plot

* x x x x x x x x x x x x x x x x x x x x x x x

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Slide 38

Analysis in the simplest possible earth model

We need to describe travel times, using relations

that yield depths & velocities.

Notes section 6 explains how these relations arise.

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Slide 39

Relations for things we want

Needed:

Relations between times, distances and velocites

Direct arrivals:

Easy: 1/v is slope.

* x x x x x x x x x x x x x x x x x x x x x x x

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Slide 40

Relations for things we want

Refractions – geometry must be accounted for:

Relations between times, distances, angles and velocites.

Relate x, z, θ

Relate v1, v2, θ (Snell’s law)

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Intercept time method (ITM)

What’s ultimately wanted from a seismic refraction survey?

Velocities

From slopes

Depth

From intercept time

ti

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Slide 42

The analysis procedure

To learn about the earth once a data set has been

obtained:

Plot data

Pick first arrivals

Build T-X

Identify “layers”

Estimate velocities from slopes

Estimate depth (from intercepts)

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Slide 43

Measurements - Sources

For small scale work, energy can be small.

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Slide 44

Physical Property: seismic velocity

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Seismic Survey

Sources: anything that causes earth particles

to move:

Natural: (earthquakes)

Man-made: explosives, “hammers”, shock waves

Receivers: measure

Displacement

Velocity

Acceleration

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Natural Sources: Earthquakes

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EOSC 350 2010

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Thumper truck: Vibroseis

Group of sources

Single Source

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Borehole explosion

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Sources in the ocean

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Single SourceMultiple Sources

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Slide 50

Measurements - Sources

For small scale work, energy can be small.

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Slide 51

Shotgun blast

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Slide 52

Sledge hammer

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Slide 53

Sledge hammer

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Slide 54

Detectors and Recorders

Geophones and Seismographs

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Slide 55

Set up the geophone array

Geophone

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Piezoelectricity: Lab 1

Some crystals accumulate charge in

response to mechanical stress

Electricity can create pressure.

Pressure results in electricity.

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Accelerometers: iPhone

Measure gravitational acceleration

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iPhone examples

iPhone app: iSeismometer

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Slide 60

Seismic refraction – T-X analysis

Goals

Begin interpretation of seismic refraction data

using T-X (travel time – offset) slope analysis

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Slide 61

Seismic survey again

offset

…

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Slide 62

Readings

Seismic refraction: GPG 3.e.7 - 3.e8

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Slide 63

Other topics

Low Velocity Zones

Refraction for multiple layers.

Dipping Layers

More complicated interfaces and approaches. Plus-Minus method Generalized Reciprocal Method Ray Tracing

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Low Velocity Zone

Snell’s law for decreasing velocity

http://staff.washington.edu/aganse/raydemo/RayDemo2.med.html

Ray tracing for non-layered velocity models (Illustration of Snell’s law)

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Low Velocity Zones

V2 < v1

No refracted arrival from the top of the second layer

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Hidden Layer

Layers that are too thin may not be seen

Arrival from layer 3

beats that from layer 2

V3>v2>v1

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Refraction 3-layers:

Snell’s law rules

Second refractions

The T-X plot

* x x x x x x x x x x x x x x x x x x x x x x x

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Snell’s Law for multiple layers

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Intercept time method (ITM)

What’s ultimately wanted from a seismic refraction survey?

Velocities

From slopes

Depth

From intercept time

ti

This formula works only for

horizontal layers.

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EOSC 350 ‘06 Slide 70

Dipping layers: interpretation requires two

shots

Resulting T-X plot

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Recall: dipping layers

ud

ud

V

V

V

V

V

V

V

V

2

11

2

11

2

11

2

11

sinsin2

1

sinsin2

1

X

Shot pointX

Shot point

Reciprocal time

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Depth estimates “Slant” depths can be obtained through the intercept

times

True depths can be estimated using dip-angle (see GPG

3.e.6)

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Irregular Layers

What happens when the boundary can no

longer be approximated with a plane?

Plus-Minus Methods

GRM (Generalized Reciprocal Methods)

Ray tracing

Other sophisticated procedures.

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Reading for Team Exercise

Near-surface SH-wave surveys in unconsolidated, alluvial

sediments (on website)

This is a case history for a landfill in Norman, Oklahoma

Pay attention to:

7-Step Process

Understand data plots

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Typical data plots

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Reflection Seismology

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EOSC 350 2010 Slide 77

Applied Geophysics – Reflection Seismology

Seismic refraction was effective for finding interfaces and

velocities (so long velocity increased with depth) Only first

arrivals used.

What if we wanted to image something much more

complicated?

Impedance contrasts gives

reflections

Generate an image made from

normal incidence seismograms

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A seismic image with geologic information

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Slide 79

Seismic Reflection: Smaller scale

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Main uses for reflection seismology

Hydrocarbon exploration!!!

General earth structure:

Whole earth

Continental scale (lithoprobe)

Regional scales

Local structure: geotechnical and

environmental work

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Slide 81

What are seismic images

Each trace is an ideal reflection seismogram of a wave

that travels vertically downward, reflects off of

boundaries, and arrives back at the surface along the

same vertical path.

Plotting many such traces yields an image in which

structure can be observed.

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Slide 82

Two Questions?

What precisely is the ideal seismogram?

How do collect and process seismic data

to make these ideal traces?

Data from a single shot is shown.

Seismic survey consists of many shots.

Need to convert into a suite of ideal

seismograms.

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Ideal Seismogram

GPG: see explanation

Impedance

Reflection coefficient:

Transmission coefficient

Slide 83

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Slide 84

Reflection Coefficients

Acoustic Impedance

Reflection coefficient

can be positive or negative

http://en.wikipedia.org/wiki/Reflection_coefficient

Negative: returning wave has reversed polarity

Positive: returning wave has same polarity

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Ideal seismogram

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Slide 86

Reflection seismic acquistion

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Slide 87

Basics of Reflection Seismic Surveying

Collect common shot

gathers

Reorder to collect common midpoint (CMP)

gathers

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Common Short Gather (SGG) to Common Midpoint

Gathers (CMP) to Zero Offset trace

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“fold”: how many times a subsurface

point is sampled by pull-along arrays.

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N number of geophones

n move up number

N=8

n=2

fold=4

Maximum reflections for a

single subsurface point = 4

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Slide 90

Some Common Shot Gathers

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Slide 91

CMP gathers:

Fit hyperbola to find stacking velocity (also referred

to as RMS velocity)

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Slide 92

Basics of Reflection Seismic Surveying

Reorder to collect common midpoint gathers

For horizontal layers Reflections on CMP obey

Arrival is hyperbola

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NMO Correction

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Slide 94

Basics of Reflection Seismic Surveying

For each reflection on CMP gather find a best

velocity that describes the “moveout”. That is

find the appropriate velocity v in

Correct for moveout. (Flatten the arrival times)

Stack (to enhance signal to noise)

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Slide 95

Basics of Reflection Seismic Surveying

Stack (to enhance signal to noise). The number of

traces in the stack is called the “fold”

Result is one seismic trace for each CMP gather.

Put all traces together. Interpret as vertical wave

propagating down and reflecting upwards

Interpret geologically. (Polarity is important)

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Slide 96

Summary of CMP processing

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Slide 97

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Slide 98

Some Common Shot Gathers

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Slide 99

CMP Gathers: Correct for Normal Moveout:

NMO

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Slide 100

Processed CMP Section; Converted to depth

Conversion of time to depth requires an average velocity.

This is obtained from the stacking velocity used to perform

the NMO correction

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Vertical resolution

Slide 101

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Geotechnical applications for reflection

seismology

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MASW

Multichannel Analysis of Surface Waves

Slide 104

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Geotechnical Engineers: Vs

Slide 105

Shear Modulus

depends upon Vs

Young’s Modulus strongly

controlled by Vs

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MASW technique (www.masw.com)

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Obtaining a 2D profile

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Case History for MASW:

(Hodgson et al.)

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Dublin: Case History for MASW

Setup: Dublin underlain by Dublin boulder clay (DBC).

Hard lodgement till with high stiffness and low

permeability. Large cobbles and boulders above DBC.

Properties: elastic parameters, stiffness is related to

shear velocity

Survey: Seismic. MASW

Data Acquisition: Roll-along survey. Land streamer 24

plated-coupled 4.5Hz geophones. Tractor-mounted

weight drop with shots every 6m.

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Dublin: MASW

Analysis: Dispersion curve analysis of surface waves.

Invert to obtain Vs. Generate a 2D profile.

Interpretation: Low Vs (200-700 m/s) associated with

glacial till (0-10m thickness. High velocity material

(Vs>900 m/s) at depth

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MASW

Dublin

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Dublin: MASW

Interpretation: Low Vs (200-700 m/s) associated with

glacial till (0-10m thickness. High velocity material

(Vs>900 m/s) at depth

Synthesize: Borehole verified the high velocity material

as limestone bedrock. The depth is variable.

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Next week

Monday: Thanksgiving No Lecture/Lab

Wednesday: Guest: Mike Maxwell Golder Assoc

Friday: TBL (Seismology)

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End Seismology

Slide 115

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