www.norsar.com copyright © norsar 2005 advanced applications of norsar-3d ray modelling

www.norsar.com Copyright © NORSAR 2005

Advanced Applications of NORSAR-3D Ray Modelling

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.

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

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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Purpose

The purpose of the modelling is to guide survey planning by accurately simulating the illumination of the target.

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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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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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Select reflection point on target

The location of the reflection point once more. Seen from

right above.

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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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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.

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

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.


Introduction

Minimum offset


The minimum offset map indicates the required cable length.

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Simulated Migration Amplitude


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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Flower plots and Hit map for East-West

Full survey East-West

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Full survey North-South Full survey East-West

Comparing the Hit maps

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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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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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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.

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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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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Unknown part - Is it like this?

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or like this…

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or…

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or?

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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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Visualizing the ray paths

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Hit map - All receivers

On reflector From which shots? i.e. the shot domain

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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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Hit map - Shallow receivers

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Hit map - Middle receivers

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Hit map - Deep receivers

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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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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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Shots near?

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Or far?

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Shots - All

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Shots - Near

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Shots - Far

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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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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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Other useful information…

Maximum travel time

Minimum incidence angle

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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.

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.

www.norsar.com copyright © norsar 2005 advanced applications of norsar-3d ray modelling

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