0.7. seismic processing

7/30/2019 0.7. Seismic Processing

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Synthetic Seismogram School

7

Seismic Processing


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Introduction

If youre not achieving a good well tie it must be either your ...

Well data

This is something that you can check in OpenWorks.

Seismic Data

This takes a bit more of an investigation.

Many times it is the result of noise!


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Propagation

The earth is much more complex then our simple RC model. As the diagram

illustrates, various earth effects are captured in recording. Processing acts to suppress

Most, but not all, of these effects. Those that remain can distort the final trace.


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Convolution

The mathematical representation for Convolution is given by:

Seismict = n

s=0 Wavelets.RC t-s

With noise the equation becomes:

Seismict = n

s=0 RCs. Wavelet t-s + Noise

It is the noise term that can seriously degrade the matchbetween your synthetic seismogram (generally noise-

free) and the seismic signal.


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Signal to Noise

Gathers with varying signal to noise. The gather on the left is pure signal. The signal-

to-noise is varied to the right from 3 to 0.2. Having gathers with good signal-to-noise iscritical to seismic studies. Gathers give valuable insight into the quality of amplitude

information in stacks and angle stacks.

Decreasing Signal to Noise


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Coherent and Random

Coherent Noise includes:

Shot-generated noise such as ground roll

Propeller noise

Diffractions Cable Vibration

etc.

Random Noise includes:

Culture such as traffic

Weather such as rain or wind or lightning

Wildlife

etc.

What Are the Types of Noise?


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Pre-Stack Noise Example

Processed Raw

RMS 31 743

Diffracted Multiple

Multiples

Random

The gather on the right has some serious noise

issues. The gather on the left is at the samelocation as the right, after processing for noise

suppression.


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Seismic Processing:Pre-Stack Example

For a good seismic-to-synthetic ties, noise must be suppressed as much

as possible! The benefit of suppressing noise is obvious below!

Decreased noise with increased processing

CDP Gathers


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Seismic ProcessingPost-Stack Example

A post-stack example of signal-to-noise enhancement. Obviously the section on

the right is superior to that on the left. The improved data will provide a superior

well tie.

Increased post-stack processing


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Compare gathers near well with synthetics.(enhance S/N if necessary)

=

-

Final Contractor

Gather

After DSC P + PS

Synthetic

Excellent Tie

Comparison of gather and synthetic

Synthetic Corridor

Stack

Stack from Final Contractor CDP gathers

Stack after FK/Radon and DSC

Example from Norway

Note the improved synthetic-to-seismic

tie after suppression of noise.


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The Effects of OverburdenAnisotropy and NMO on AVO

With greater offsets, the effects of anisotropy increase. In the example below, NMO was used to move out the cdps.

The anisotropic gather illustrates that it was insufficient and that higher-order moveout is required. If the amplitude is

measured as a function of constant time (typical among contractors), two very different profiles are obtained. In addition,

the phase change with offset is not identified on the anisotropic gather. Processed gathers should be reviewed for the

effects of anisotropy or inappropriate moveout. Stacking the gathers would give a very different result.

Soft

Hard

Soft

Impedance

- +

Isotropic Anisotropic

=0.3

=0.05

=0.3 =0.05

= 0

= 0

Input

Wavelet

Offset Wider Offset

Amplitude Profiles as a function of offset

0.05

0


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Flat

Spot

Fourth order move out

FS2

Interval

Example from Angola, Block 14, at the FS-2 well location: Inline 4806, Xline 3100. Mutes created using

straight-ray approximation of 15 and 35 degree angle-of-incidence are displayed.

15 35

The Effects of Fourth-Order Moveouton the Stacked Section


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Flat

Spot


FS2

Interval

Example from Angola, Block 14 at the FS-2 well location: Inline 4806, Xline 3100. Gathers muted to

include fourth-order moveout using a straight-ray approxim at ion for the angles. The fourth-order

moveout is the result of anisotropy.

Angle Muted Gathers


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Flat

Spot


FS2

Interval

Example from Angola, Block 14 FS-2 at the FS-2 Well location: Inline 4806, Xline 3100. Gathers

muted to remove fourth-order moveout usin g a bent-ray estim ate for th e angles.

Angle Muted Gathers


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Example from Angola, Block 14 FS-2. Note the area within the white box and compare to that on the

following slide. This product is superior for use in well ties.

Flat Spot

Frequency Spectrum of Stack

Stack of Gathers without Fourth-Order Moveout


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Example from Angola, Block 14 FS-2. Although the events look higher in amplitude, they are also

lower in frequency. The effect of the fourth-order moveout is a smearing of the events. Although the

image may look stronger, it is an inferior product for seismic-to-well ties.

Flat Spot

Frequency Spectrum

Stack of Gathers with Fourth Order Moveout


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Wavelet Extraction

Angle mutes generated with curved rays from November 2002 velocity model.

0-15 (Near) 25-35 (Far) 40-60 (VFO)

Comparison of the extracted wavelets and angle-of-incidence-limited gathers at

the FS-2 well location. The near and the far offset gathers, when stacked, willretain the embedded wavelet shape (zero phase). Because of residual moveout

on the very-far offsets, the embedded wavelet is distorted.

Zone

of

Extraction

Gathers from G. Delorme

Far

Near

Vfar


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Summary

Seismic data need to be properly conditioned for a goodwavelet tie.

Coherent and random noise should be suppressed.

Stack and pre-stack data should be carefully reviewed.

Understand the l im i tat ions im posed by the seismic data!

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