4d seismic survey

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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

    REPRINT

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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