Tim Carr - West Virginia University

Basic Principles

What to Geophysicists Do?

Seismic Acquisition

Seismic Processing

Seismic Interpretation

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Seismic Acquisition

geophones

recording truck

reflecting boundary

reflecting boundary

energy source

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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 & $$$$$

Acquire “shoots” seismic surveys

Seismic Acquisition - Offshore

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•

An air gun towed behind the survey ship transmits sound waves through the water column and into the subsurface

Changes in rock type or fluid content reflect the sound waves towards the surface

Receivers towed behind the vessel record how long it takes for the sound waves to return to the surface

Sound waves reflected by different boundaries arrive at different times.

The same principles apply to onshore acquisition

Seismic Acquisition - Offshore

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Vibroseis trucks

Seismic Acquisition - Onshore

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Onshore seismic acquisition requires an energy input from a vibroseis truck or other source. Geophones arrayed in a line behind the truck record the returning seismic signal.

Sub-horizontal beds

Unconformity

Dipping beds

Geophones (receivers)

Vibrator (source)

Seismic Acquisition - Onshore

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Seismic Acquisition - Geophones

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Laying out Geophones in Africa

Seismic Acquisition - Onshore

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Seismic Acquisition - Onshore

Laying out Geophones in Mountainous Areas 10

Seismic Processing

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Wiggle Trace to CDP Gather

Normal Move Out Correction

Stacking

What is a Reflector?

Understanding Seismic Data

Seismic Processing

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Listening Devices 0 s

An Explosion! 0 s Energy Source .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

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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

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Data for one point from different signals to different receivers

1. More time needed to reach distant receivers so the data look like a curve.

2. Correcting for normal move out restores the curve to a near horizontal display.

Sound receivers

R3 R2 R1

Change in lithology from mud to sand

so sound is reflected back to surface

CDP

CMP Sound sources

S1 S2 S3

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ExxonMobil

Field Record

(marine)

Data Processing

Stream

Subsurface ‘Image’

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ExxonMobil

Source Receivers

R1 R2 R3 R4 R5 S1

Direct Arrival

Reflections

2 W

ay T

rave

l T

ime

Offset (Distance)

R1 R2 R3 R4 R5

Direct Arrival

Reflection For 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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Sources Receivers

R1 R2 R3 R4 R5 S1 S2 S3 S4 S5

We sort the shot-receiver pairs so that

data from the same ‘bounce’ point

(e.g., at ‘A’) is captured

CMP = common mid point

For Point A

A

CMP Gather

Offset Distance

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The travel times differ since the path for a near offset trace is 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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CMP Gather

Offset Distance

Velocity Too Slow

Velocity Correct

Velocity Too Fast

Flat

Curves Down

Curves Up

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Next, take all the traces of energy for that one place and stack them on top of each other

First, gather energy data for one location and correct for delayed arrival (normal move out)

Finally, place stacks for adjacent locations side by side to produce a seismic line

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CMP Gather Moveout Corrected

Midpoint Gather

Stacked

Trace

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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Energy Source

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

0.4 s -

The reflection is displayed beneath the

source-receiver midpoint

Bounce

Point

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ExxonMobil

1

2 3 4 6 5

Where would the reflection lie?

90º

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ExxonMobil

1

2 3 4 6 5

Where would the reflection lie?

Compass

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ExxonMobil

1

2 3 4 6 5

Where would the reflection lie?

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ExxonMobil

1

2 3 4 6 5

The reflection is downdip and its dip is less than the interface

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ExxonMobil

Sweep Ellipse

S R Unmigrated energy on single trace...

...spread to all possible locations of origin

S R

Sweep Ellipse

S R

Sweep Ellipse

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ExxonMobil

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

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L 5 – Seismic Method Courtesy of ExxonMobil

Seismic Migration

Unmigrated Image

Migrated Image

Positioning Problems ‘Blur’

the Image

Migration Reduces Positioning

Problems, which Improves the

Image

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Unmigrated or Improper Migration

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Pre-Migration Stack

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Migrated Stack

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There are many reflectors on a seismic section. Major changes in properties usually produce strong, continuous reflectors as shown by the arrow.

A seismic reflector is a boundary between beds with different properties. There may be a change of lithology or fluid fill from Bed 1 to Bed 2. These property changes cause some sound waves to be reflected towards the surface.

Bed 1

Bed 2

lower velocity

higher velocity

energy

source

signal

receiver

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Seismic Data Requires Careful Acquisition

Processing

Interpretation

Power of Stacking and Migration Improves Signal to Noise

Improves Resolution

A Seismic Reflector is Boundary Between Beds of Different Properties Relates to Lithology – Density and Velocity

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Assignments Reading for this week

Stoker et al., 1997

Complete the Log Exercises by Wednesday (3/18) Archie

Correlation

Discuss Current Energy Events Read Today in Energy for Tuesday (3/17) at

http://www.eia.gov/

Be Prepared to Discuss in Class - Wednesday

Discussion Leader – Melissa Fahnestock

Quiz – Wednesday 3/18 1:00pm closes Friday 3/20 1:00pm Homework handed out on Wednesday (3/20) due Monday

(3/30)

No Class on Friday (3/20)