Seismic Events

Geophysics 629 1 1 Courtesy of ExxonMobil 1 Courtesy of ExxonMobil

R1 R2 R3 R4 R5

Θc

Shot Receiver Water Bottom Multiple

+100

-10

+10

RC = +.1

RC = +.1

RC = -1

-1

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Energy Source

Energy Source

Seismic Waves & Rays

• Seismic energy propagates out from the source as a

spherical wave

• Energy reflected at a boundary radiates up as if there was a

source at the reflection point (Huygen’s Principle)

• We can think of energy propagation in terms of wavefronts

or as raypaths

Energy Propagation as Waves

Energy Propagation as Rays

Geophysics 629 3 3 Courtesy of ExxonMobil

Zero Offset

• Offset is the lateral distance between a source and a receiver

• Our discussions thus far have been exclusively for zero offset, i.e., zero distance between the source and the receiver

• It is computationally simple to have the source and the receiver at the same location, but this is not how we operate in the real world

Shot Receiver


A Major Problem: Noise

• We acquire our data in a very noisy environment:

– On land: traffic, weather, equipment, etc.

– On sea: waves, weather, ship noise, etc.

• Noise is any signal picked up by the receivers that is not related to primary reflections from subsurface acoustic boundaries

• We can overcome random noise problems by getting multiple measurements for each subsurface point

• In theory, and in practice, when we add several measurements for the same subsurface point, the ‘geologic signal’ that we want adds constructively and the random noise that we do not want gets canceled - Fantastic!


Repeated Measurements

There are two ways to get 4 repeat measurements for the same subsurface point (the red box)

Use the same shot point

location and the same

receiver location 4 times

Use 4 different shot points

and 4 different receiver

locations, as shown


The Practical Solution

• We will talk more about seismic acquisition in lesson 8

• For now, suffice it to say that, in the field, it is much more practical and economical to use option two

• For the RED subsurface box, we get information from:

– Shot 1 into receiver 1 (S1 R1)




S4 S2 S3 S1 R1 R2 R3 R4


Trace Offset

The offset values are different for

these four measurements

S4 S2 S3 S1 R1 R2 R3 R4

0 500 meters

110 meters

220 meters

550 meters

880 meters


Water Bottom Reflection

Consider the water bottom reflection for 1 shot point and 5 receiver locations at sea level

0 1 km

V = 1500 m/s

The zero-offset time, from the shot into receiver 1, is:

(2 * 800 m) / 1500 m/s

which equals 1.067 s

D = 800 m

R1 R2 R3 R4 R5

The ‘bounce’ point


Time vs. Offset Plot

We will display seismic traces as a function of offset

0 m 200 m 800 m 600 m 400 m

1000

1100

1200

1300

1400

1500

1600

1067

Tim

e (

milliseco

nd

s)

Offset (meters)



Consider the shot into receiver 2

The ‘bounce’ point is midway between the shot and receiver

0 1 km

V = 1500 m/s

Travel distance down (T) is the hypotenuse of a right triangle

Dd = √ D2 + x2

Total travel distance = travel down + travel up = 2 Dd

Time = 2 Dd / V

Time = 1.099 seconds

D = 800 m

Shot R2 R3 R4 R5

x

D

Dd

200




0 m 200 m 800 m 600 m 400 m

1000

1100

1200

1300

1400

1500

1600

1067 1099

Tim

e (

milliseco

nd

s)

Offset (meters)



For the shot into receiver 3

0 1 km

V = 1500 m/s

D = 800 m

R1 R2 R3 R4 R5 200 200

Dd = √ 8002 + 2002

Time = 2 Dd /1500

Time = 1.193 s




0 m 200 m 800 m 600 m 400 m

1000

1100

1200

1300

1400

1500

1600

1067 1099

Tim

e (

milliseco

nd

s)

1193

Offset (meters)



Consider the shot into receivers 4 and 5

0 1 km

V = 1500 m/s

D = 800 m

R1 R2 R3 R4 R5 200

For R4

Time = 1.333 s

For R5

Time = 1.508 s

200 200 200




0 m 200 m 800 m 600 m 400 m

1000

1100

1200

1300

1400

1500

1600

1067 1099

Tim

e (

milliseco

nd

s)

1193

1333

1508

In Time-Offset space,

seismic reflections are

hyperbolic

Offset (meters)


Generalized Equation

Time = -2 * SQRT {(Depth)2 + (Offset/2)2 } / Velocity

We can generalize the formula to calculate the reflection

time for the depth of the first layer to:

Note that for the zero-offset case the term with offset is

zero and we get:

Time = -2 * SQRT {(Depth)2 } / Velocity or

Time = -2 * Depth/Velocity

Why the 2 and why the minus sign?


A Real Shot Record – Marine Case

Tim

e (

mil

liseco

nd

s)

Offset (feet)

Seafloor Reflection

Other

Reflections

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Other Seismic Events

18 Courtesy of ExxonMobil

• Seismic reflections are the types of seismic events that we need to image the layered subsurface

• You will be using seismic reflections to map subsurface faults and stratigraphic surfaces

• Unfortunately, our seismic records contain other types of events

• We will briefly discuss 4 other types of seismic events:

Direct waves

Refractions

Diffractions

Multiples

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Direct Waves


• A direct wave is a P-wave that travels near the surface (ground or water) directly from the source to the receiver

• On a shot record, it appears as a straight line

• The slope of the line is dictated by the average velocity of the near surface

V = 1500 m/s

D = 800 m

R1 R2 R3 R4 R5 200 200 200 200

0 1 km


Direct Waves

Receiver Distance Time Time

1 0 0/1500 0.000

2 200 200/1500 -0.133

3 400 400/1500 -0.267

4 600 600/1500 -0.400

5 800 800/1500 -0.533

-0.600

-0.500

-0.400

-0.300

-0.200

-0.100

0.000

0 200 400 600 800 1000

Offset (meters)

Tim

e (

se

co

nd

s)

Slope = Δx/Δy

= (600 – 0)/(0.4 – 0)

= 600/0.4

= 1500 m/s

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Example: Direct Wave

C. Liner, 2004

Direct Arrival

WB Reflection

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Refraction or Head Wave

• When velocity increases across a boundary, another type of seismic event is possible, called a refraction or a head wave

• Refractions occur when the angle of incidence exceeds a ‘critical’ angle (Θc)

R1 R2 R3 R4 R5 250 250 250 250

1500 m

2500 m/s

5000 m/s

Θc

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Some Equations

Critical Angle: Θc = sin-1 (Vabove/Vbelow)

Critical Distance: xc = 2z / √ (Vbelow-Vabove) - 1

Θc = 30° xc = 3000 m

750 m

2500 m/s

5000 m/s

Θc

xc

2000 m/s

1500 m/s

1500 m

750 m

2500 m/s

5000 m/s

Θc

xc

2000 m/s

1500 m/s

1500 m

Θc = 48.6° xc = 4500 m

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Refraction

C. Liner, 2004

WB Reflection

Refraction

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Diffractions

• Diffractions are generated by an abrupt change in subsurface impedance

• Using light as an analogy, it is like having a mirrored ball in the middle of a dance floor

Shot Record

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Diffractions


• We don’t have many ‘buried balls,’ but we do have abrupt changes in subsurface impedance

• Where there is a large impedance discontinuity, diffractions will be generated

Generates

Diffractions

Generates

Diffractions

Stratigraphic Cut

Offs at Faults

Patch Reef and

Edge of Salt Body

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Primary Reflections & Multiples


• A primary reflection is one whose path goes directly down to the reflector and back to the receiver – only one reflection point

• A multiple is any event which has experienced more than one reflection in the subsurface

• There are two types of multiples: free surface multiples and internal multiples

Shot Receiver

Primary Reflection

Water Bottom Multiple

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Free Surface

• By the free surface, we mean the air/ground boundary or the air/ocean boundary

• The RC at the air/ground boundary or the air/ocean boundary is close to -1

• This means acoustic waves traveling up to the free surface will be reflected, the reflected energy will be almost 100% of the incident (up-going) energy, but the polarity will be reversed (RC ~ -1)

• This fact leads to free surface multiples

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Water Bottom Multiple


• Let’s assume:

– The source sends out a minimum phase pulse of 100 units that has a lead PEAK

– The water bottom RC is +0.10

– Reflected energy hits the air/sea interface where the RC = -1

Shot Receiver Water Bottom Multiple

+100

-10

+10

RC = +.1

RC = +.1

RC = -1

-1 + = Peak/Trough

- = Trough/Peak

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Example of Water Bottom Multiple

Water

Bottom

WB

Multiple

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Other Free Surface Multiples

• One is related to the source, another is related to the receiver

• They can occur for both land and marine surveys

• They occur if the source and receiver are not exactly at the surface

• They are referred to as ‘ghosts’

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Source Ghost

Shot Receiver

Source Ghost

Primary

Source

Ghost

+100

-10

+10

RC = +.1

RC = -1

RC = -1

5 to 10 m

Time delay ~ (6m)/(1500m/s) ~ 4 ms

+ = Peak/Trough

- = Trough/Peak

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Receiver Ghost

Shot Receiver

Receiver Ghost

Primary

Receiver

Ghost

+100

-10

+10

RC = +.1

RC = -1

RC = -1

5 to 10 m

Time delay ~ (6m)/(1500m/s) ~ 4 ms

+ = Peak/Trough

- = Trough/Peak

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Source Ghost

Polarity Reversal

Receiver Ghost

Polarity Reversal

Direct

Direct + So. Ghost + Re. Ghost

Double Ghost

Direct

Source Ghost

Direct + Source Ghost

Direct

Direct

Ghosts

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Internal Multiples

• The seismic wave can get ‘rattle around’ within some of the stratigraphic layers

• When the wave travels more than once within a layer, it is called an internal multiple or, more commonly, peg leg multiples

Path Duplicated

within this layer

3 Reflection

Points

Peg Leg Multiple

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Summary

Types of Seismic Events

Events

Diffraction

Refraction (Head Wave)

Reflection

Direct

Multiples

Primary

Internal

Free Surface

W Bottom

Ghosts

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Seismic Events

Documents

Transcript of Seismic Events