survey planning & illumination with norsar-3d. introduction overview this short presentation...

Survey Planning & Illumination with NORSAR-3D

Introduction

Overview

This short presentation gives some applications of NORSAR-3D ray tracing.

Survey aperture Fold and Amplitude, Planned v Modelled Survey offset and azimuth Migration aperture Effect of Overburden S.R.M.E. Aperture

Introduction

Simple Slope Model

A simple model was used to demonstrate the technique. The model contains a variable velocity field overlying a reflector with a cosine dip region in the center.

Introduction

Aperture Study, All Azimuth Survey

Shot Pattern

Receiver boundary for indicated shot

The survey used here is a “Wide Azimuth” survey. In the illustration, the closely spaced shot lines are shown, together with the receiver boundary square for the shot location marked with the cross. The receiver pattern moves as the shot location moves.

Introduction

Aperture Study: Area of Interest

Having ray traced the survey, the Hit Count attribute is plotted in the reflection point domain.

This shows where the reflection points are and the density.

The display shows the attribute after masking it to a 4km x 2km area of interest.

Introduction

Aperture Study: Shots Contributing

Down dip extension

An alternative domain for display is the shot domain.

This display shows the shots that contributed only to the area of interest on the target.

Clearly, the survey needs to be extended in the down-dip direction.

Introduction

Overview



Introduction

Hit Map, E to W Direction

Hit map for E-W

The central part of the map, the attribute is striped. This is caused by the dip of the reflector.

Introduction

Hit Map, Alternating Directions

This map was made using the same model and survey parameters as the previous map except that the sail line directions alternate.

This alters the hit count pattern

Introduction

SMA 20Hz, E to W Directions

Would this appear in migrated seismic data?

Plot the migration amplitude using SMA

The stripes persist.

Introduction

SMA 20Hz, Alternating Directions

This shows the Simulated Migration Amplitude llumination for the survey shot in alternating directions.

Introduction

Overview



Introduction

Illumination in SEG Salt Model

The SEG/EAGE salt model was used to illustrate this topic. The salt model consists of a salt body in a sedimentary velocity field.

A plane, horizontal target has been added.

Introduction

Hit Map, E-W Survey, 6km Streamer

This is the hit count attribute plotted for an east-west survey using a 6km streamer array.

Away from the salt, the reflection count is continuous.

Underneath the salt, the reflection count varies because of the focusing effects of the salt.

Under the southwest corner of the salt there is an illumination hole.

Introduction

Hit Map, N-S Survey, 6km Streamer

This is the hit count attribute plotted for a north-south survey using a 6km streamer array.

Under the southwest corner of the salt, the illumination hole persists but the details are different.

If one had a particular prospect location in mind, a choice might be made between the east-west and north-south surveys.

Introduction

Maximum Offset, 10km Streamer

For each image point, this attribute shows the maximum offset that contributed to that image point.

It confirms that for most of the target, 7km streamers are enough.

Only in a few places below the salt would a longer streamer contribute to the image.

Introduction

Shooting from target

In the previous SEG/EAGE salt model examples, non of the acquisition geometries filled the illumination hole.

An efficient alternative analysis for these local trouble spots is the flower plot display.

A shot is placed in the illumination hole at the target. One-way rays are propagated from the shot up to an array of receivers on the surface.

Shot point placed in the shadow zone

Dense set of receivers.

Introduction

Shooting from target: Flower Plot

Rays with equal departure inclination and opposite azimuth are paired and plotted

The figure shows that the east-west and north-south azimuths would not produce any ray pairs. However, the northwest-southeast direction produces ray pairs at long offsets.

Azimuth

OffsetBest acquisition direction

Introduction

Hit map for the new survey

Repeating the streamer survey using a northwest southeast sailing direction does indeed fill the illumination hole.

Complete illumination

N125

Introduction

Overview



Introduction

Max. CMP-CRP, 10km Streamer

Again using the SEG/EAGE salt model, the CMP-CRP distance attribute is plotted in the reflection point domain.

Away from the salt the CMP-CRP distance is effectively zero, indicating that small apertures are required for imaging reflections.

However, significant apertures are needed under the salt.

Introduction

Overview



Introduction

Total Illumination Amplitude

The illumination map here was made using the SEG/EAGE salt model and a streamer survey. All rays that reflected from the target were used, no matter what part of the model they had traveled through.

Introduction

Illumination Amplitude, No Salt

This illumination map was made in the same way as the previous one, except that any ray that had passed through the salt was rejected.

It shows the image that requires only sedimentary ray paths, and can be called the “high confidence” image, because it is not subject to uncertainties in the salt model.

Introduction

Overview



Introduction

Surface Bounce Points

Bounce Bounce

Surface Bounce Points

Streamer Array

The SRME method estimates the multiple by using the surface bounce. The 2D SRME method can be used when the surface bounce occurs within the streamer array. Ray tracing can be used to see if this is a valid assumption.

Introduction

Salt Top Multiple

If the surfaces generating the multiple are approximately horizontal, then the bounce points will occur within the streamer array.

This extreme example illustrates ray paths for the top salt multiple from a single shot.

Introduction

Salt Multiple, Surface Bounce Points

The surface bounce points for the top salt multiple from the single shot are depicted in red.

Clearly the bounce points are not contained within the streamer array, indicating that a 3D SRME technique would be necessary.

Introduction

NORSAR-3D Work Flow

SurfacesProperties

Introduction

Model Components

Generated internally Imported surfaces

– Depth/time grids– GoCAD trimeshes– GoCAD model

Imported properties– SEGY property field

Introduction

NORSAR-3D Work Flow

SurfacesProperties

Build Model

Survey

Depth Model

Introduction

Surveys

Simple Surveys Internally– Marine streamer– Ocean Bottom Sensor

Complex surveys import– P1/90– SPS– ASCII (for VSP)

Introduction

NORSAR-3D Work Flow

SurfacesProperties

Build Model

Survey

Depth Model

Ray Code

Ray Trace

Introduction

Reflected Wavefront before Registration

3D Wavefront Construction

Salt

Reflected wavefront

returning to surface.

Direct wavefront

Shot point and

streamer array.

Introduction

Rays for 1 Streamer

3D Wavefront Construction

This shows the result of the ray traced shot represented as the more conventional ray diagram.

Introduction

NORSAR-3D Work Flow

SurfacesProperties

Build Model

Survey

Depth Model

Events

Map Illumination

Make Seismogram

Export Attributes

Ray Code

Ray Trace

Map

Seismogram

ASCIITable

UserFormat

Toolkit Processing

Introduction

Summary

A variety of import formats for NORSAR-3D models and surveys.

Wavefront Construction is fast and robust Variety of survey analysis tools in NORSAR-

3D

Ask for a demonstration to see how NORSAR-3D can benefit your project

…Insight through modelling

survey planning & illumination with norsar-3d. introduction overview this short presentation...

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