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Lecture 5Lecture 5

Mitchum et al., 1977b

AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use.

F W Schroeder ‘04

L 5 – Seismic Method Courtesy of ExxonMobil

Basic Exploration Workflow

To D/PDrop

Prospect

Drill Wildcats

ConfirmationWell

Identify Opportunities

ProcessSeismic Data

CapturePrime Areas

InterpretSeismic Data

AcquireSeismic Data

Success

Success

Failure

UneconomicEconomicAnalysis

AssessProspects

F W Schroeder ‘04

L 5 – Seismic Method Courtesy of ExxonMobil

The Seismic Method

Listening Devices 0 s

An Explosion! 0 sEnergySource .1 s.2 s.3 s

Some Energy is Reflected

Most Energy is Transmitted

.4 s.4 s .5 s

Some Energy is Reflected

Most Energy is Transmitted

.6 s.7 s.8 s.8 s

F W Schroeder ‘04

L 5 – Seismic Method Courtesy of ExxonMobil

Raw Seismic Data

Device#1

Device#2

0.0

0.3

0.4

0.5

0.6

0.7

0.8

0.1

0.2

For the explosion we just considered ...

Listening device #1 records a reflection starting at 0.4 seconds

Listening device #2 records a reflection starting at 0.8 seconds

To Image the Subsurface, We Use Many Shots (explosions)and Many Receivers (listening devices)

Arranged in Lines either on Land or Offshore

Tim

e

F W Schroeder ‘04

L 5 – Seismic Method Courtesy of ExxonMobil

Seismic Acquisition

• A 3D survey is designed based on:– Imaging Objectives: image area, target depth, dips,

velocity, size/thickness of bodies to be imaged, etc. – Survey Parameters: survey area, fold, offsets,

sampling, shooting direction, etc.– Balance between Data Quality & $$$$$

Land OperationsVibrators Generate a Disturbance

Geophones Detect Motion

Marine OperationsAir Guns Generate a Disturbance

Hydrophones Detect Pressure

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L 5 – Seismic Method Courtesy of ExxonMobil

Raw Data - Marine

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L 5 – Seismic Method Courtesy of ExxonMobil

Seismic Processing

Field Record(marine)

Data ProcessingStream

Subsurface ‘Image’

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L 5 – Seismic Method Courtesy of ExxonMobil

Shot Gather

Source Receivers

R1 R2 R3 R4 R5S1

Direct Arrival

Reflections

2 W

ay

Tra

ve

l T

ime

Offset (Distance)

R1 R2 R3 R4 R5

Direct Arrival

ReflectionFor each shot, reflections

are recorded in 5 receivers

There are 5 ‘bounce’ points along interface 3

1

2

3

For Shot 1

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L 5 – Seismic Method Courtesy of ExxonMobil

Common Midpoint Gather

Sources Receivers

R1 R2 R3 R4 R5S1S2S3S4S5

We sort the shot-receiver pairs so that data from the same ‘bounce’ point (e.g., at ‘A ’) is

capturedCMP = common mid point

For Point A

A

CMP Gather

Offset Distance

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L 5 – Seismic Method Courtesy of ExxonMobil

CMP Gather

The travel times differ since the path for a near offset traceis less than the path for a far offset trace

With the correct velocity, we can correct for the difference in travel time for each trace.

The curvature of this hyperbola is a function of the average velocity down to the depth of the reflection

CMP Gather

Offset Distance

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L 5 – Seismic Method Courtesy of ExxonMobil

CMP Gather

Offset Distance

With Correct Velocity, Gather is Flat

Velocity Too Slow

Velocity Correct

Velocity Too Fast

Flat

CurvesDown

CurvesUp

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L 5 – Seismic Method Courtesy of ExxonMobil

A Stacked Trace

CMP Gather Moveout CorrectedMidpoint Gather

StackedTrace

Offset Distance

We stack several offset

traces (# traces =

fold)

The geologic ‘signal’ will be

additive

The random ‘noise’ will tend

to cancel

Stacking greatly

improves S/N(signal-to-noise)

10 Fold

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L 5 – Seismic Method Courtesy of ExxonMobil

Positioning Problems

EnergySource

The seismic ray hits an inclined surface at 90º and reflects back

0.2 s down

0.2 s up

0.4 s -

The reflection is displayed beneath the

source-receiver midpoint

BouncePoint

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L 5 – Seismic Method Courtesy of ExxonMobil

Time for an Exercise

1

2 3 4 65

Where would the reflection lie?

90º

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L 5 – Seismic Method Courtesy of ExxonMobil

Time for an Exercise

1

2 3 4 65

Where would the reflection lie?

Compass

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L 5 – Seismic Method Courtesy of ExxonMobil

Time for an Exercise

1

2 3 4 65

Where would the reflection lie?

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Exercise Answer

1

2 3 4 65

The reflection is downdip and itsdip is less than the interface

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Migration – Correcting for Location

Sweep Ellipse

S RUnmigrated energy on single trace...

...spread to all possible locations of origin

S R

Sweep Ellipse

S R

Sweep Ellipse

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L 5 – Seismic Method Courtesy of ExxonMobil

Migration – Power of Correlation

Two reflections on unmigrated data After spreading to all possible locations

Reflections are not positioned in the subsurface correctly

since they have dip

Constructive interference occurs where the reflections are properly

positioned

Destructive interference dominates where the reflections are NOT

properly positioned

F W Schroeder ‘04

L 5 – Seismic Method Courtesy of ExxonMobil

Seismic Migration

Unmigrated ImageUnmigrated ImageUnmigrated ImageUnmigrated Image

Migrated ImageMigrated ImageMigrated ImageMigrated Image

Positioning Problems ‘Blur’ the

Image

Migration ReducesPositioning

Problems, which Improves the

Image

F W Schroeder ‘04

L 5 – Seismic Method Courtesy of ExxonMobil

Seismic Interpretation

Determine the local geology from the subsurface images• Map faults and other structural features

• Map unconformities and other major stratal surfaces

• Interpret depositional environments

• Infer lithofacies from reflection patterns & velocities

• Predict ages of stratal units

• Examine elements of the HC systems

Mitchum et al., 1977

AAPG©1977 reprinted with permission of the AAPG whose permission is required for further use.