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www.norsar.com Copyright © NORSAR 2005
Advanced Applications of NORSAR-3D Ray Modelling
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Introduction
What is NORSAR-3D
Software package for 3-dimensional seismic ray modelling.
Using the Wavefront construction method. It handles:
– General, layered subsurface models.– Multi-arrivals.– Anisotropy.
Takes advantage of Parallel computing – greatly reducing run times.
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Introduction
NORSAR-3D applications include
Survey planning– Land– Marine; including Multi, Wide and Rich Azimuth
Survey infill analysis; including onboard Survey QC Interpretation QC 4D seismic modelling Green’s functions for migration
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Introduction
Some Examples
Marine survey planning VSP close to salt Green’s functions for migration
The most common use currently is Marine survey planning. This example will quickly show a multi azimuth example using synthetic data.
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Introduction
Purpose
The purpose of the modelling is to guide survey planning by accurately simulating the illumination of the target.
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Introduction
Simplified salt model
In this case we are using a simplified salt structure. The horizon below the purple salt bodies is the target layer.
Our goal is to quantify some marine survey parameters:– Sailing direction – Cable length– Listening time
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Introduction
Simple, initial analysis: Flower plot
Illumination analysis for a single point on a reflector.
Checks all azimuths and offsets.
Azimuth
OffsetBest acquisition direction
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Introduction
Select reflection point on target
The location of the reflection point once more. Seen from
right above.
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Introduction
A hit map for this reflection point as a rose diagram.
In this example, there are no hits for small offsets. The best shooting direction is S/SW – N/NE, as this has the most offets contributing..
Output from the FlowerPlot
Azimuth
OffsetBest acquisition direction
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Introduction
Full survey North-South
Repeating for numerous points
The results indicate that there is no simple solution to the question of which azimuth to acquire the survey in.
Next step is to model a ‘real’ regular, marine survey. First we will try north-south acquisition direction, then we will try east-west and compare the two outputs.
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Introduction
This is a hit map for North-South survey.
White=no hits, blue=few, yellow/green=medium, red=many
Notice that the results indicate some illumination below the centre of some salt domes, but not below that flanks. This was not shown in the flower plots, as we located the flower plots directly beneath the salt and not below the flanks.
Hit map for full marine survey
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Introduction
Minimum travel time
Other maps:
There are numerous possible parameters which can be output to aid the survey design process once ray tracing has been performed.
In this case we can see the Minimum travel time. This is useful to determine minimum listening time. Largest values are found below the salt as expected.
Full survey North-South
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Introduction
Minimum offset
Full survey North-South
The minimum offset map indicates the required cable length.
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Introduction
Simulated Migration Amplitude
Full survey North-South
SMA models illumination amplitudes that correspond to depth migrated seismic data. By simulating the migration process locally, more realistic amplitudes are generated where both the seismic pulse and the Fresnel effect are included.
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Introduction
Flower plots and Hit map for East-West
Full survey East-West
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Introduction
Full survey North-South Full survey East-West
Comparing the Hit maps
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Introduction
North-Southplus
East-West
Multi Azimuth?
NORSAR-3D has been used extensively in the decision process leading up to some of the major multi azimuth surveys undertaken so far.
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Introduction
From the modelling
Select sailing direction – No direction is perfect for all targets– Select for most important targets– Use two directions?
Cable length Listening time
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Introduction
Examples
Marine survey planning VSP close to salt Green’s functions for migration
Within NORSAR-3D, sources and receivers can be placed anywhere within the model. This means that geometries such as OBS, Land (including complex topography) and VSP can be modelled in addition to the standard marine case.
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Introduction
Unknown salt flanks Top salt and main
reflectors known Shots on surface Receivers in existing well Task: Find good shot and
receiver positions to illuminate the salt flanks
Designing a robust VSP survey
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Introduction
Known structure
This example assumes that top salt, main reflectors, and the velocity structure are already known.
Here is a display of the know structure.
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Introduction
Unknown part - Is it like this?
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Introduction
or like this…
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Introduction
or…
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Introduction
or?
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Introduction
Shots on the surface, receivers in the well
We begin by using all shot and receiver positions.
The shots are on the surface, the receivers in the well.
The task is to find a good subset.
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Introduction
Visualizing the ray paths
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Introduction
Hit map - All receivers
On reflector From which shots? i.e. the shot domain
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Introduction
Hit map - All receivers
On reflector From which shots? i.e. the shot domain
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Introduction
Hit map - All receivers
On reflector From which shots? i.e. the shot domain
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Introduction
Hit map - All receivers
On reflector From which shots? i.e. the shot domain
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Introduction
Find the best receiver depths
The results so far have been for all shots and all receivers.
Now to find the best receiver depths. We divided the well into three parts: shallow, middle, and deep, illustrated by the thicker line down the well. Which receiver location provides the best illumination. Bearing in mind that it is unlikely that we can afford to populate the whole well with receivers.
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Introduction
Hit map - Shallow receivers
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Introduction
Hit map - Middle receivers
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Introduction
Hit map - Deep receivers
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Introduction
The best receiver position
By running the modelling for the two shapes shown, as well as the others we can quantatively show that the deepest receivers will provide the best illumination.
Next: Where – roughly – are the best shot positions?
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Introduction
Using the deepest receivers
So far we have been looking at this in the shot domain. i.e. using all shots and deep receivers.
Now we can split the shots up into near and far and see what effect that has on our illumination for the different salt flank geometries.
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Introduction
Shots near?
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Introduction
Or far?
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Introduction
Shots - All
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Introduction
Shots - Near
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Introduction
Shots - Far
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Introduction
From this analysis we can conclude that the far shots will provide the best illumination on the varying salt flank geometries.
Thus the recommended combination would be, the deepest receivers and the far offset shots.
Which is the best combination
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Introduction
What is the expected coverage
In addition the modelling also shows expected illumination as a function of flank shape – i.e. what parts of the flank we should
expect to see depending its shape.
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Introduction
Other useful information…
Maximum travel time
Minimum incidence angle
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Introduction
VSP illumination of salt flank
Conclusions The modelling indicates what parts of the salt
flank are illuminated, depending on the salt’s shape.
Shot and receiver positions are indicated. They are robust to the actual shape of the unknown structure.
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Introduction
With NORSAR-3D you can
Model complete surveys with ray-tracing by wavefront construction.
Determine and explain survey parameters.
Estimate how illumination depends on the shape of the unknown structure.
Make the survey geometry robust to structural uncertainties.
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Introduction
With NORSAR-3D you can
Model complete surveys with ray-tracing by wavefront construction.
Determine and explain survey parameters.
Estimate how illumination depends on the shape of the unknown structure.
Make the survey geometry robust to structural uncertainties.
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Introduction
With NORSAR-3D you can
Model complete surveys with ray-tracing by wavefront construction.
Determine and explain survey parameters.
Estimate how illumination depends on the shape of the unknown structure.
Make the survey geometry robust to structural uncertainties.
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Introduction
With NORSAR-3D you can
Model complete surveys with ray-tracing by wavefront construction.
Determine and explain survey parameters.
Estimate how illumination depends on the shape of the unknown structure.
Make the survey geometry robust to structural uncertainties.