cggveritas seismic overview

8/3/2019 CGGVeritas Seismic Overview

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Seismic Overview

The Purpose of Seismic

Slide 1 of 40

The main purpose of seismic

exploration is to render the most

accurate possible graphic representation

of specific portions of the Earth'ssubsurface geologic structure.

The images produced allow exploration

companies to accurately and cost-

effectively evaluate a promising target

(prospect) for its oil and gas yielding

potential.

December 1, 2011 | Copyright CGGVeritas

Page 1 of 1Seismic Overview

12/1/2011http://www.cggveritas.com/popup_page.aspx?cid=1-24-139


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Seismic Overview

Seismic Fundamentals

Slide 2 of 40

Seismic imaging is simple. But it takes

knowledge, experience and advanced

technology to do it right.

Acquisition of seismic data involves the

transmission of controlled acoustic

energy into the Earth, and recording the

energy that is reflected back from

geologic boundaries in the subsurface.

Information regarding the structure and

nature of the reflecting strata can be

derived from the two-way travel time,

and other attributes, of the returning

energy. Processing these reflections

produces a synthetic image of the

Earth's subsurface geologic structure.





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Seismic Overview

Acquiring Seismic Data at Sea

Slide 3 of 40

At sea, the procedure is essentially the

same except that our instruments are

continuously moving!

The seismic (energy) source is usually

an array of airguns towed behind the

survey vessel and just below the sea

surface. The airguns are fired at regular

intervals as the vessel moves along pre-

determined survey lines.

Energy reflected from beneath the

seafloor is detected by numerous

'hydrophones' contained inside long,

neutrally buoyant 'streamers' - often

almost 5 miles long - also towed behind

the vessel.




P 1 f 1S i i O i


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Seismic Overview

2D Seismic Data

Slide 4 of 40

Two types of seismic surveys are

available to the geophysicist: two-

dimensional (2D) surveys, or three-

dimensional (3D) surveys.

2D seismic data are displayed as a

single vertical plane or cross-section

sliced into the Earth beneath the

seismic line's location.

2D is generally used for regional

reconnaissance, or for detailed

exploration work where economics

may not support the greater cost of 3D .

. .




P 1 f 1S i i O i


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Seismic Overview

3D Seismic Data

Slide 5 of 40

3D seismic data are displayed as a three

-dimensional cube that may be sliced

into numerous planes or cross-sections.

More expensive than 2D data, 3D

produces spatially continuous results

which reduce uncertainty in areas of

structurally complex geology and/or

small stratigraphic targets.






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Seismic Overview

4D Seismic Data

Slide 6 of 40

Two or more 3D seismic surveys

acquired at different times can be

compared in order to search for

changes in the fluids within the rockformations.

This type of survey is known as 4D,

where elapsed TIME is the fourth

dimension of information.






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Seismic Overview

The Five Key Ingredients

Slide 7 of 40

There are five key ingredients to

acquiring useful seismic data:

1. Positioning / Surveying

2. Seismic Energy Source

3. Data Recording

4. Data Processing

5. Data Interpretation






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Seismic Overview

1: Positioning / Surveying

Slide 8 of 40

Accurate positioning is fundamental

and vital to acquiring seismic data.

We must know PRECISELY where all

our instruments are on the Earth's

surface.

Otherwise, however good the quality of

the recorded seismic data . . .






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Seismic Overview

Positioning / Surveying

Slide 9 of 40

. . . the data are worthless if we don't

know where they came from.


ge oSe s c Ove v ew




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Seismic Overview

1: Positioning / Surveying

Slide 10 of 40

In both marine (left) and land (right)

environments, energy source and

receiver layout patterns are pre-

planned, and their positions pre-determined, so that we can calculate

precisely where our recorded seismic

data originate.


g




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Seismic Overview

Positioning Technology

Slide 11 of 40

Today we are in the 'space age' of GPS

- the Global Positioning System -

which offers unprecedented accuracy.

GPS is a constellation of 24 satellites in

orbit about 20,200 kms above the

Earth. The satellites act as precise

reference points in space and transmit

radio signals that allow a GPS receiver

on Earth to triangulate its position to

within about 10 meters.

While 10-meter accuracy is adequate

for many purposes, for seismic we use

Differential GPS (DGPS) correction

techniques to bring our levels of

accuracy to between 2 meters and 30

centimeters!





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Seismic Overview

Positioning at Sea

Slide 12 of 40

At sea, positioning is more difficult

than on land because our vessel - and

all its towed equipment - is

continuously in motion.

Nevertheless, the precise locations of

the energy source(s) and the streamer(s)

MUST be known at all times.

In such a dynamic environment, real-

time positioning is extremely complexand highly computer-intensive.





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Seismic Overview

Positioning at Sea

Slide 13 of 40

We use an integrated combination of

multiple reference site DGPS, Relative

GPS, laser measurements of ranges and

angles, underwater acoustic ranging

and digital compasses along the

streamer(s).

Literally hundreds of complex

mathematical position calculations are

carried out every few seconds, enabling

the precise positions of the vessel, the

seismic source(s) and the individual

hydrophone groups in the streamer(s) to

be calculated in real-time as the vessel

continuously moves along.





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Seismic Overview

Energy Source

Slide 14 of 40

To gather seismic data, we must first

generate and transmit controlled

acoustic energy into the ground.

In the past, dynamite was the preferred

seismic energy source both on land and

at sea.

Dynamite is still used on land, usually

in areas of soft, unconsolidated or

weathered surface layers.

When buried and detonated in safely

plugged shotholes below the surface

layer, dynamite produces a sharp,

acoustically clean energy pulse.

However . . . . .





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Seismic Overview

Energy Source

Slide 15 of 40

. . . in urban and/or populous areas,

dynamite is obviously not practical!

There are several other energy source

technologies used for acquiring seismic

data, but the main one is 'vibroseis'.





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Seismic Overview

On Land: Vibroseis

Slide 16 of 40

Large servo-hydraulic vibrators on

vibroseis trucks are safer, faster, more

adaptable and more environmentally

friendly than dynamite, and can yield

equal (or sometimes better) data

quality.





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Seismic Overview

How Vibroseis Works

Slide 17 of 40

A vibroseis truck generates a controlled

vibratory force of up to 70,000 lbs

through a baseplate that is placed in

contact with the ground.





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Seismic Overview

At Sea: Airguns

Slide 18 of 40

In the marine environment, and

sometimes in swamp or marsh,

dynamite has been almost completely

replaced by airguns.

In an airgun . . . . .





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Seismic Overview

How Airguns Work

Slide 19 of 40

. . . high pressure air is stored in a firing

chamber and explosively released

through portholes by the action of a

sliding shuttle with pistons at each end.

Seismic energy is generated by the

rapid, explosive release of compressed

air through the airgun's ports . . . .





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Seismic Overview

How Airguns Are Deployed

Slide 20 of 40

. . . into the surrounding water. This

produces a primary energy pulse and an

oscillating bubble.

Typically, multiple airguns are towed

behind the vessel, several meters below

the sea surface in a pre-determined

combination, or 'array' of different

chamber volumes designed to generate

an optimally tuned energy output of

desirable sound frequencies.





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Seismic Overview

3. Data Recording

Slide 21 of 40

Some of the energy we send into the

ground, or water, is reflected back from

geologic boundaries in the sub-surface.

This reflected energy is detected by a

connected network of geophones (left)

planted in the ground, or by groups of

hydrophones contained inside the

neutrally buoyant seismic 'streamer(s)'

towed behind the vessel at sea (main

picture).

Similar to microphones, these devices

convert the reflected energy into

electrical energy which is transmitted

to . . . . .





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Seismic Overview

Central Recording System

Slide 22 of 40

. . . a central recording system, usually

housed in the instrument room (or

'doghouse') for recording as raw

seismic data, and for quality control

checks.

Quality control is vital, not just during

data recording, but at every stage of a

seismic project.





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Seismic Overview

Multiple Lines of Data at Once

Slide 23 of 40

At sea, several lines of seismic data can

be recorded simultaneously by towing

multiple source arrays and streamers.

Here, two source arrays and four

streamers allow eight lines of seismic

data (shown in yellow) to be recorded

at once.

It is generally much faster to acquire

seismic data at sea than on land.





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Seismic Overview

Health, Safety & Environment

Slide 24 of 40

Preservation of health, safety and the

environment (HSE) are of paramount

importance in conducting any seismic

operation.

By its nature, whether on land or at sea,

seismic work is not without risk.

However, through effective HSE

management, education, training and

planning, and by following HSE rulesthat reduce these risks to a minimum,

everyone can come home safely at the

end of every day.





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Seismic Overview

4: Data Processing

Slide 25 of 40

We must make sense of the recorded

seismic 'squiggles' to produce the truest

possible image of the Earth's sub-

surface geologic structure.

Reflected seismic response is a mixture

of our output pulse, the effect of the

Earth upon that pulse, and background

noise, all convolved together.

We must remove the output pulse andthe noise to leave just the 'Earth model'.

This is the role of seismic data

processing, which requires accuracy,

reliability, speed and . . . . .





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Seismic Overview

Data Processing

Slide 26 of 40

. . . substantial computing power. The

advanced mathematical algorithms and

complex geophysical processes applied

to 3D seismic data require enormous

computing resources.

Not to mention the massive volumes of

data involved.

For example, the amount of seismic

data recorded by CGGVeritas duringjust ONE medium-sized marine 3D

survey would fill more than 20,000

compact disks, forming a stack over

650 feet high!





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Seismic Overview

Data Processing: Deconvolution

Slide 27 of 40

Ideal seismic response would be a

single sharp reflection for each sub-

surface rock layer boundary. Actual

seismic response is less than ideal

because our output pulse is not

perfectly sharp and changes its shape

while passing through the Earth.

Deconvolution 'deconvolves' our output

pulse from the seismic response and

converts it into a cleaner, sharper, less

confusing pulse.

Can you determine the number of rock

layers here by examining the actual

seismic response (before

deconvolution)?





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Seismic Overview

Data Processing: Stacking

Slide 28 of 40

Seismic traces from the same reflecting

point are gathered together (CRP

gather) and summed, or 'stacked'.

The six seismic traces on the left are

from the same reflecting point. As the

traces are merged into one (right),

background noise cancels itself out

while the seismic signals add together,

producing a stronger signal-to-noise

ratio. (The output trace on the right is

shown here six times only to provide a

better comparison.)

The more of these seismic traces we

can stack together into one output trace,

the clearer the seismic image.





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Seismic Overview

Data Processing: Stacking (2)

Slide 29 of 40

This first image shows a seismic

section produced after the seismic

traces have been sorted, adjusted for

varying path lengths and signal

strength, and stacked.

Here, each trace is the summation of 48

individual 'shot' traces.

Note the water bottom 'multiple'

reflection (arrowed) -- a seismic 'echo'of the seafloor caused by energy

bouncing back-and-forth within the

water layer to produce a 'false'

reflection obscuring the real data.





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Seismic Overview

Data Processing

Slide 30 of 40

This second image shows the result of

suppressing the water bottom multiple.

The seismic image is enhanced by a

process that suppresses the multiple

without harming real reflections.





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Seismic Overview

Data Processing

Slide 31 of 40

This third image is further enhanced by

'focusing' energy for both flat and steep

reflectors.

Any missing traces are 'filled in' by

interpolation.





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Seismic Overview

Data Processing

Slide 32 of 40

This fourth image most closely

resembles the true sub-surface geology.

A process called 'migration' moves

reflected energy to its true sub-surface

position of origin.





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Seismic Overview

Data Processing Sequence

Slide 33 of 40

Animating the preceding four steps, we

can clearly see the gradual

enhancement in seismic image

achieved through data processing.

This example shows only four steps. In

data processing, there are many steps

required to arrive at the final seismic

image.

This particular project requiredapproximately 25 separate steps!





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Seismic Overview

Advanced Data Processing

Slide 34 of 40

More advanced processing techniques,

such as Prestack Depth Migration

(PSDM), can significantly improve

seismic imaging, especially in areas of

complex geology.

In this example from the Gulf of

Mexico, see how PSDM has improved

the imaging of a) a massive salt body,

and b) sedimentary layers beneath the

salt.

Processes such as PSDM take more

time, expertise and resources to apply,

but accurate 3D seismic images can

mean the difference between success or

an expensive dry hole.





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Seismic Overview

In-Field Data Processing

Slide 35 of 40

Our customers usually need the data

delivered as fast as possible!

In fact, today's industry demands for

ever-faster turnaround of seismicprojects necessitates that data now be

processed, at least to a preliminary

stage, in the field immediately after

recording.

This requires equipment and personnelin the field to be almost as sophisticated

as those onshore.





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Seismic Overview

5: Data Interpretation

Slide 36 of 40

We must interpret the seismic data to

understand the geology and assess the

likelihood of finding oil and gas

accumulations.

Geophysicists at CGGVeritas interpret

the processed seismic data and integrate

other geoscientific information to make

assessments of where oil and gas

reservoirs may be accumulated.





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Seismic Overview

Data Visualization

Slide 37 of 40

Powered by advanced supercomputer

power, rapid data loading, high-speed

networking and high-resolution

graphics, CGGVeritas visualization

centers provide the ability to display

and manipulate complex volumes of 3D

data in a collaborative, team

environment.

The result is . . . better interpretation . .

. of more data . . . in less time.





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Seismic Overview

Data Integration

Slide 38 of 40

CGGVeritas offers a broad range of

advanced interpretation services

including PSDM, seismic attribute

analysis, amplitude variation with

offset (AVO) analysis, and reservoir

characterization.

Our visualization centers enhance our

ability to integrate additional

geophysical and geologic data such as

well logs, and to visualize and rapidlymature prospects for testing.





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Seismic Overview

The End Product

Slide 39 of 40

The end product of all this work and

technology is a graphic 3D

representation of the Earth's sub-

surface geologic structure.

Based largely on this information,

exploration companies will decide

where (or if!) to drill for oil and gas.

This example (left) represents over 600

square kilometers of complex geology

down to a depth of more than 6,000

meters!





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Seismic Overview

CGGVeritas

Slide 40 of 40

We hope our on-line seismic 'guided

tour' has given you a basic idea of what

we do at CGGVeritas.

You now qualify as a 'Twenty-MinuteGeophysicist'!

If you'd like to learn more, please

contact the CGGVeritas office nearest

to you, or e-mail ourwebmaster.


12/1/2011http://www.cggveritas.com/popup page.aspx?cid=1-24-178

cggveritas seismic overview

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