Sandra C. Saldaña - IRIS · PDF fileSandra C. Saldaña. 2 Outline ... Seismic...

Fundamentals ofSeismic Acquisition &Processing

IRIS Intern Orientation2011

Sandra C. Saldaña

2

Outline

Concepts

Seismic Reflection Acquisition

Seismic Reflection Processing

3

Concepts

Basic Wave Theory

The Law Reflection & Snell’s Law (Refraction)

Theoretical Ray Path & the Earth Model

Types of Seismic Energy

Reflections

Refractions

Air Waves

Surface Waves

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Concepts: Basic Wave Theory

Halliday et al., 2002

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6



RayWave Front

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Slow Fast Fast Slow

Concepts: Law of Reflection & Snell’s Law

Keary et al., 2002

q1

q1

q2

V1<V2

V1

V2

q1

q1

q2

V1>V2

V1

V2

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Concepts: Theoretical Ray Path & Earth Model

Keary et al., 2002

Vsand=1800 m/s

Vshale=2500 m/s

Vsand=3000 m/s

Vcarbonate=4100 m/s

Vwater=1520 m/s20º

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Concepts: Types of Seismic Energy

Reflections

Air Wave

Surface Wave

Refractions

20º

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Seismic Reflection Acquisition

Definitions

Geophone

Common Midpoint (CMP)

Resolution

Fresnel Zone

Tuning Thickness

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Definitions: Geophone

http://www.iongeo.com/Land_Imaging/Geophones/http://www.google.com/imgres?imgurl=http://micromachine.stanford.edu/smssl/projects/Geophones/GeophoneColorSketch.jpeg&imgrefurl=http://micromachine.stanford.edu/smssl/projects/Geophones/&h=288&w=396&sz=40&tbnid=A3tBS206DhsgEM:&tbnh=90&tbnw=124&prev=/search%3Fq%3Dgeophone%26tbm%3Disch%26tbo%3Du&zoom=1&q=geophone&hl=en&usg=__w-mO0DRWgF5RtleHveY9SSHeoFY=&sa=X&ei=cnnWTcqfBsq2tge-2NSsBw&ved=0CCAQ9QEwBQ

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Definitions: Common Midpoint

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Movie

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CMP Location 100

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Resolution: Fresnel Zone

Determines Lateral Resolution

Keary et al., 2002

Source

Wave Fronts

Ray

Target

l

l/4

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Resolution: Tuning Thickness

Determines Vertical Resolution

Keary et al., 2002

Source

Wave Fronts

Ray

Target

l

l/4

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Seismic Reflection Processing

Frequency AnalysisFilter Noise

Apply Geometry

Sort by CMPLocation

Apply Mutes to EliminateRefractions & High Frequency

Surface Waves

Use Dix’s Equation and Vrms toConvert Time to Depth

Apply NMO to FlattenHyperbolas

Migration used to move interfacesto proper horizon in time domain

and collapse diffractions

Pick Mutes

Pick Tail MutesKill Surface/Air Waves

Pick Early MutesKill Refractions

Itera

tive

Velocity Semblance toCalculate NMO and Vrms

Brute StackQuick Look

Datum StaticsAccount for Elevation Changes

Pick Refractions(First Break Picks)

{ {

19


Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

Seismic Reflection Processing: Geometry

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Raw Shot Gathers – What You’ll See in the Field

We have to tell the computer where each trace is located on the ground...

TraceField File Number Channel Number

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Raw Data

Operation: ComputerStores a Location on theground for each trace

Where EachHammer Hit is onthe Ground

Where Each Channel(Geophone) is on theGround

Which FFID goes withWhich Source Location

Output File

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Shot Gathers Post Geometry Function

Offset Geophone’s Distance From the Hammer

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Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

Seismic Reflection Processing: Static Shifts

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Elevation Profile

630

632

634

636

638

640

642

644

646

648

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Channel/Geophone Number

Ele

vati

on

Ab

ove

Mean

Sea

Level(m

)

Fault Scarp

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Elevation Profile

630

632

634

636

638

640

642

644

646

648

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24


Ele

vati

on

Ab

ove

Mean

Sea

Level(m

)

Shot(Hammer Strike)

Geophones


Direct Arrivals (first breaks)travel along the surface

and are the first to arrive atthe Geophone

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Elevation Profile

630

632

634

636

638

640

642

644

646

648

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24


Ele

vati

on

Ab

ove

Mean

Sea

Level(m

)

Shot(Hammer Strike)

Geophones

Datum

Vp

velocity

distancetime =

tstatic

tstatic

Direct Arrivals (first breaks)travel along the surface

and are the first to arrive atthe Geophone

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Shot Gather First Break Picks

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Use First Breaks to Calculate tstatic forEach Receiver & Source

Operation: Apply tstatic (shift) to each trace

Operation: Sort Tracesby CMP Location

Create Output File

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Seismic Reflection Processing: Velocity Analysis


Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

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G2 S1

G4 S2

G6 S3

G8 S4

G10 S5

G12 S6

Common Midpoint

Path 1

Path 2

Path 3

Path 4

Path 5

Path 6

t0

Tim

e(m

s)

Offset (m)(Distance from Source toGeophone)

V1

V2V3

Path

1

Path

2

Path

3

Path

4

Path

5

Path

6

Stacking Velocity isdetermined by theamplitude height

VelocityV1 V2 V3

t0

A B

C

Kearey and Hill, 2002

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G2 S1

G4 S2

G6 S3

G8 S4

G10 S5

G12 S6

Common Midpoint

Path 1

Path 2

Path 3

Path 4

Path 5

Path 6

t0

Tim

e(m

s)

Offset (m)(Distance from Source toGeophone)

V1

V2V3

Path

1

Path

2

Path

3

Path

4

Path

5

Path

6Stacking Velocity isdetermined by theamplitude height

VelocityV1 V2 V3

t0

A B

C

Kearey and Hill, 2002

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Flattening Hyperbolas movie

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Think About the Geologic Setting:

What do we expect the seismic to look like?

What happens to the rock in the fault zone?

How does this affect the acoustic wave propagation?

How do you expect it to affect the seismic image?

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Top: Photo mosaic of the natural trench. Bottom: Interpretation of the trench. Notethe location of two strands of the California Wash Fault(provided by Bidgoli et al., 2003).

Fault Locations

Fault LocationsChannel 12

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Key: What Speed do We Expect for Sound in this Media?

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300 361 422 483 544 605 666 727 789 850

Stacking Velocity (m/s)

300 361 422 483 544 605 666 727 789 850

Stacking Velocity (m/s)

Note: Assumes laterally continuous Earth layers!

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From here it’s all about clean up….

Brute Stack

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Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

Seismic Reflection Processing: Frequency Analysis

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Fourier Transform Amplitude SpectrumThe Earth is a filter,where’s the frequencycontent?

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Brute Stack (Unfiltered) Stack (Filtered)

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Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

Seismic Reflection Processing: Muting

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Seismic Reflection Processing: Muting

Refractions

Reflections

Surface Waves

Shot Gather Mute Everything but the

reflections

Reflections are only a smallpart of the energy

Better at long offsets

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Seismic Reflection Processing: Migration


Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

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Seismic Reflection Processing: Migration

Explain Migration

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Seismic Reflection Processing: Depth Conversion


Apply Geometry

Sort by CMPLocation


Surface Waves





Pick Mutes



Itera

tive





{ {

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Sandra C. Saldaña

Geophysicist

[email protected]

(281) 872-2571

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