Ray Tracing and Survey Design

Transcript Ray Tracing and Survey Design

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

Simple Slope Model Introduction

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.

Aperture Study, All Azimuth Survey Introduction

Shot Pattern 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.

Receiver boundary for indicated shot

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.

Aperture Study: Shots Contributing Introduction

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

Hit Map, E to W Direction Introduction

Hit map for E-W The central part of the map, the attribute is striped. This is caused by the dip of the reflector.

Hit Map, Alternating Directions Introduction

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

SMA 20Hz, E to W Directions Introduction

Would this appear in migrated seismic data? Plot the migration amplitude using SMA The stripes persist.

SMA 20Hz, Alternating Directions Introduction

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.

Hit Map, E-W Survey, 6km Streamer Introduction

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.

Hit Map, N-S Survey, 6km Streamer Introduction

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. Dense set of receivers. 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

Shooting from target: Flower Plot Introduction

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 Offset

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

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 The SRME method estimates the multiple by using the surface bounce. The 2D SRME method can be used when Surface Bounce Streamer Points Array

Salt Top Multiple Introduction

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.

NORSAR-3D Work Flow

Surfaces Properties

Introduction

Model Components

 Generated internally  Imported surfaces – – Depth/time grids GoCAD trimeshes – GoCAD model  Imported properties – SEGY property field

Introduction

NORSAR-3D Work Flow

Surfaces Properties Survey Build Model 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

Surfaces Properties Survey Ray Code Build Model Depth Model Ray Trace

Introduction

Reflected Wavefront before Registration

3D Wavefront Construction

Direct wavefront

Introduction

Shot point and streamer array.

Salt Reflected wavefront returning to surface.

Rays for 1 Streamer

3D Wavefront Construction

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

Introduction

Introduction NORSAR-3D Work Flow

Surfaces Properties Survey Ray Code Map Illumination Build Model Make Seismogram Depth Model Ray Trace Export Attributes Events Toolkit Processing Map Seismogram ASCII Table User Format

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

Ray Tracing and Survey Design

Transcript Ray Tracing and Survey Design

Reflected Wavefront before Registration

Rays for 1 Streamer

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