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PGS has recently devoted

significant resources to the

improvement of 4D technology. A

strong effort has been made to establish

a thorough, consistent product

including leading technology and tools

as well as the right expertise. PGS has

defined four Ds that characterize our

products and strengths with respect to a

4D project:

Detailed planning

Dense acquisition

Distinguished processing

Dedicated QC

This newsletter focuses on Dense

Acquisition and Distinguished

Processing.

When we design a seismic survey,

we would ideally like to have:

Dense sampling to avoid aliasing and

get optimum resolution.

High fold to decrease random noise

and get clear data.

Multiple azimuths to improve

imaging and map and control

anisotropy.

Efficient acquisition to reduce costs.

Dense sampling implies the

sampling interval (in space and time) is

so small that aliasing is avoided in all

relevant processing domains. It also

means we get a sufficient number of

samples or traces within each bin.

Proper sampling and proper sub-surfacecoverage is the best way to avoid

harming the data during processing.

The Ds in 4D Seismic from PGSPart 2: Dense Acquisition & Distinguished Processing

A Publication of PGS Geophysical March 2002Vol. 2 No. 2

PGS has introduced HD3D

technology for onshore, seafloor

and streamer surveys. This

technology is well suited for 4D

seismic in all these environments

because:

Dense acquisition gives

adequate sampling of thewavefield, higher resolution and

better velocity analyses.

Dense acquisition gives higher

fold, reduced random noise and

increased repeatability.

Dense acquisition may also give

better azimuthal coverage,

which enables better matchingof two surveys due to:

- Flexibility in selection of

input gathers of monitor

surveys.

- Opportunity to remove

multidirectional noise.

- Better imaging.

The efficient, dense

acquisition facilitated by HD3D

makes this a very appropriate

technology for acquisition of 4D

data.

Dense Aqcuisit ion

Overview

Continued on next pageFigure 1: Comparison of conventional 3D survey with 166400 traces per km 2 (lower part) and

HD3D survey with 576000 traces per km2 (upper part) - Courtesy BP.

The Advantages of Dense

HD3DTM for 4D Seismic

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PGS has introduced HD3D

technology for onshore, seafloor and

streamer surveys. Bin sizes in the range

of 10m x 10m can be efficiently

acquired using this technology.

An example of improvement in

data quality achieved by HD3D

streamer seismic is shown in Figure 1.

HD3D technology enables the

small bins/high resolution and high

fold/high S/N we need for detailed

reservoir studies. The higher trace

density resulting from HD3D will allow

for improved repeatability because

processing artifacts originating in

transforms or data interpolation are

reduced. The dense acquisition

improves the velocity analyses which

normally have a strong impact on data

quality. HD3D surveys are therefore

ideal for base line purposes for a 4D

project.

We frequently face the problem

that the monitor and baseline surveys

do not have the same survey

configurations and we

consequently have to

compare two surveys

with varying source-

receiver azimuths and

different sampling. For

streamer surveys, the

efficiency and

flexibility of HD3D

makes it feasible to

acquire swaths that are

overlapping, which

again makes available a

larger range of azimuths

for all source - receiver

configurations.

If we are acquiring

a monitor survey,

HD3D technology will

normally give more hits

per bin than the base

line survey. The surplus

of data makes it

possible to achieve a

better match between the data that are

used in each bin from the monitor and

base line surveys respectively. Hence, a

monitor survey acquired with HD3D

allows us to pick the traces that have

optimum match to the base survey's

offset and azimuth. In

addition, the level of

non-repeatable noise

will be reduced through

use of directional noise

suppression. Many

noise suppression

processes suffer

because the data are

sampled only in one

direction.

Again, HD3D can

allow superior

directional noise suppression when a

wider range of azimuths have been

acquired.

Distinguished Processing

High-quality 4D processing

requires an integrated processing

sequence that is specifically focused on

both structural fidelity and primary

amplitude preservation. The 4D

friendly processing flow must be highly

repeatable between base and monitor

surveys. Among the many vital

components of a 4D friendly processing

flow are pre-stack full or partial

migration, coherent and incoherent

noise removal, and consistent

amplitude recovery. As is clear from the

processing flow in Figure 2, it is crucial

to do QC after each step!

TechLink March 2002 Page 2

Continued from Page 1

The Advantages of Dense

HD3DTM for 4D Seismic

Figure 2: QC of the processing flow.

Figure 3: (a) Comparison of AVO response for Radon

demultiple using an aliased and an un-aliased Radon

transform. (b) Improvement in amplitude using the well-

parameterised high-resolution transform. The solid line

indicates the true AVO trend of a primary event. The long-

dashed line shows the AVO response of the event after

standard (least-squares) Radon demultiple. The short-

dashed line shows the AVO response after demultiple using a

high-resolution Radon transform.

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Page 3 A Publication of PGS Geophysical

4D Optimized

Amplitude Recovery

It is important to recover the

amplitudes from the data in a consistent

and accurate manner to obtain a good

4D signal. Three of the most important

considerations in this process are: (i)

source/receiver directivity, (ii)

geometrical spreading and (iii) the

effects of Q. The source and receiver

arrays used in typical marine surveys

produce responses that depend on the

emission/emergence and azimuth

angles of each source-reflector-receiver

ray-path. These effects are time and

offset dependent for a given source-

streamer combination and should be

corrected for at an early stage in the

processing sequence.

Absorption results in a progressive

loss of high frequency signal as a

function of travel-time. Dispersion

results in progressive wavelet delay as a

function of travel-time. The main effect

is that the imbedded wavelet becomes

non-stationary and this, in turn, distorts

the true amplitude (and thus true 4D)

effect. A robust approach is to apply

scalars (that are functions of Q, time,

velocity and offset) to try and recover

peak amplitude. Computation of these

scalars is based on dominant frequency

and bandwidth of the data at various

travel-times.

Multiple Removal

In general, removing coherent

noise and multiple energy from the

seismic dataset is one of the most vital

processing steps. Of the many

demultiple techniques available, two

stand out as being generally good at

preserving pre-stack primary

amplitudes. Surface related multiple

elimination (SRME, Figure 4) shows

very good primary amplitude

preservation as long as care is taken

during the design of the adaptive

subtraction component of the process.

However, the most commonly used

demultiple algorithm, in cases where

pre-stack primary amplitude

preservation is critical, is Radon

demultiple.

When Radon demultiple is used, it

is vital that the Radon transform not

generates alsiasing. Figure 3(a) shows a

comparison of AVO behaviour for a

synthetic event after Radon demultiple

with both an aliased and unaliased

Radon transform.

The unaliased Radon transform

improves the amplitude preservation at

both far, and in particular, near offsets

(Figure 3(b)). Hence, a well-

parameterised high-resolution

transform is necessary for 4D

processing.

Figure 4: Seismic section before (left) and after application of SRME (right).

Continued on next page

PGS has developed a series of

algorithms specially suited for 4D

processing: Consistent amplitude recovery

compensating for

source/receiver directivity,

geometrical spreading, and

absorption.

High-fidelity DMO.

Multiple removal by SRME or

Radon demultiple using high-resolution transforms.

Pre-stack full or partial

migration using true-amplitude

pre-stack time migration that

takes into account velocity

variations as a function of

depth.

As a result, PGS has a total

processing flow that comprises 4D

friendly-processing.

Dist inguished

Processing Overview

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TechLink March 2002 Page 4

Although pre-stack depth/time

migration is rapidly becoming a

conventional tool, it is noticeable that

DMO still retains a useful role as a

preprocessing step rather than as a

partial imaging step. DMO is often a

critical step to regularize the input data

set to different offset planes.

The main objective of such a

processing sequence is to obtain an

optimal migration velocity field that

can also be used to develop a model for

pre-stack depth migration or a full time-

migration before stack. It is obvious

that any DMO implementation used in

such a processing sequence must be

able to preserve amplitudes at all

offsets.

PGS has implemented a HiFi DMOthat makes an efficient practical

application of near offset amplitude-

preserved processing possible. Figure 5

shows two CMP DMO gathers of real

marine seismic data with 16 streamers

before and after the application of HiFi

DMO. A significant improvement of

DMO on near offsets has been

achieved.

For a full imaging solution, true-

amplitude pre-stack time migration

(TA-PSTM) is available. The

conventional 3D Kirchhoff pre-stack

time migration (KPSTM) assumes

straight raypaths and constant velocity

media for the traveltime calculations.

TA-PSTM uses an amplitude

preserving KPSTM algorithm suitable

for a media where velocity can vary as

a function of depth.

The migration results for model

and raw data demonstrate that this

curved ray migration technique

produces better images and migration

velocities than those obtained using the

straight ray KPSTM. Moreover, the

amplitude preservation makes it ideal

for use in 4D and AVO processing.

For Updates on PGS Technological Advances, visitwww.pgs.com

More TechLinks atwww.pgs.com/techlink

C O N T A C T

PGS Geophysical

London

Tel: 44-1932-260001

Fax: 44-1932-266465

Oslo

Tel: 47-67-52-6400

Fax: 47-67-52-6464

Houston

Tel: 1-281-509-8000

Fax: 1-281-509-8500

Singapore

Tel: 65-6735-6411

Fax: 65-6735-6413

2002 Petroleum Geo-Services. All Rights Reserved

Figure 5: DMO gather: Left panel - conventional DMO is applied. Right panel - HiFi DMO is applied to the maximum offset 1200 m. In

particular, compare the amplitudes on the nearest offset traces (arrowed).

Pre-Stack Imaging - High Fidelity DMO and TA-PSTM