GOCAD

Aditya Tiwari

Introduction

http://scholar.harvard.edu/files/plesch/files/gocadinahurry.pdf

Application

http://www.gocad.org/w4/images/gocad/Paradigm_University_Program2012.pdf

Interphase

File Format

http://tu-freiberg.de/fakult3/gy/mageo/lehre_3d/vorlesung/script_uebung.pdf

Features

http://scholar.harvard.edu/files/plesch/files/gocadinahurry.pdf

Features

Stores Co-ordinates and add scalar /vector properties. Input Data: Sampling points

with orientation, Geochemical or Petro physical properties or locations for terrain

modeling.

Point Sets: File - Import Objects - Horizon/Fault Interpretations – PointsSet – Column-based File ..

FeaturesRegion:PointsSet - Region – Create PointsSet - Region -

Initialize From Property RangeScript: Available on right click on Properties.

Allows to write small logical operations :

Example: Define Sw

Sw=1-sqrt (30/(resistivity*pow(porosity,2)));

Features

Curves:Curves are often used to import 2d pre-interpreted data like geological maps and proles, or to

digitize data, like seismic reflectors, in Gocad.

File - Load Objects "CurveDigi.mx“Object Panel - Objects - Curve - From Digitized Polyline

Edit:

Curve mode - Tools -Curve mode - Tools -NodeConnect:

Curve - Tools - Node –Bridge

Merge

Curve - New - From Curves - Curves

Features

Surfaces:Represent geological boundaries, like faults and bedding planes.Commonly they

are created by interpolating input data like Points Sets, Wells or Curves.

Surface -New From several curves -Name:cross section-ok-press Ctrl-key and keep pressed-pick the curve objects profil1 and profil 2 in the 3D camera -leave Ctrl-key

1.• New Surface with Direct Triangulation

�Surface - New - From Several Curves• nb of levels: how many triangle strips will be inserted between the copied nodes of the Curve Parts, • low value would result in very narrow triangles and rough surface

�Surface - Tools - Beautify - Beautify TrianglesSmoothens the surface

Features

2.• New Surface with Discrete Smooth Interpolation (DSI)

� DSI method allows to smoothly interpolating Properties while taking into accountConstraints

Surface - New - From Points Set Medium Plane� The new plane Surface consists of only two triangles; we need to discretize appropriately in order to fit data,

Surface - Tools - Split All Apply +Apply +Apply +OKSurface - Tools - Split All Apply +Apply +Apply +OK� Set the nodes of the All Curves as Control Points.

Surface - Constraints - Control Points - Set Control Points� Set a Constraint on the Border which fixes the Border nodes in the XY plane

Surface - Constraints- Constraints on Borders - Set On Straight Lines – All Borders� Interpolation

Surface - Interpolation - Geometry - On Entire Surface – Apply� Compute Thickness

Surface - Compute - Compute Vertical Thickness

Features

EXAMPLE: Import wells with a marker point sandstone_top and set it as an additional constraint

File - Load Objects� Well markers cannot be set as Control Points directly. Convert the Well markers to a Points Set.

3.• Adapting a Surface to Honor Well or Point Data

PointsSet - New From Well - Well MarkersSurface - Constraints - Control Points - Set Control PointsSurface - Interpolation - Geometry - On Entire Surface

Surface - Constraints - Control Nodes - Set - From Control Points (Select top of sandstone)

Surface - Tools - Beautify - Beautify Triangles for EquilateralitySurface - Tools – Node/Triangles

Modelling Fault

Steps:

� Data import and data preparation, image georeferencing, and data

integration with DSI

�From a set of wells, outcrop points with fault azimut/dip measurements, and a 2D

seismic section.

�Create two conjugate normal faults master Fault and northFault.

(Shown in Figure)(Shown in Figure)

�Import of Elevation Points and Outcrop Sampling Points

File - Import Objects - Horizon Interpretations - X Y Z

Velocity Model

Velocity Model�Prepare velocity model:

Smoothing and replacement of infill

velocities

Extend to expected seismic datum of survey

�Time convert velocity model using itself:

Shift top to 0

Calculate average velocity

Use linear interpolation at conversion

Prepare time converted velocity model:

http://scholar.harvard.edu/plesch/pages/depth-conversion-pdgm-gocad

�Prepare time converted velocity model:

Recalculate average velocity in time

converted velocity model

Requires replacement of any no-data values

with infill velocity

�Depth convert seismic survey:

Define sub-region in model;

Transfer average velocity to seismic survey as

additional property

Depth conversion.

Prepare velocity model:

• Smoothing

� Severely smooth an existing velocity model to avoid any ungeologic features being introduced by the depth conversion

Compute-Apply Median/Average Filter�Filter will replace the no-data with average values. But need to experiment with window size. Because if model is fast and small window is not selected average velocity increases. velocity increases.

� Remove very high velocities with no data values

• Replace Infill Velocity

� For offshore surveys the seismic velocity of water is often used (1480 m/s).

� For Onshore the elevation where travel time in the processed seismic is set to 0ms is considered below ground surface.

Refer : http://scholar.harvard.edu/plesch/pages/depth-conversion-pdgm-gocad for details on infill velocity in case seismic data includes data above ground level.

Prepare velocity model:

• Truncate/Extend to Seismic Datum

�Top of the velocity model should be at the same elevation as the seismic datum of the seismic survey to be depth converted.

�GOCAD depth conversion function offers arbitrary seismic datum but is inconsistent.

�Well tops can help to estimate seismic datum if their position is known in �Well tops can help to estimate seismic datum if their position is known in time and depth

� If the ground level in the seismic data can be identified, its elevation can be looked up from a DEM

�Convenient to use New-Extracted from Voxet to crop the velocity

model such that its top is identical to the seismic datum

Time Convert Velocity Model

• Shift Top to 0

� Use Tools-Resize Voxet with Vectors and provide a new Z

value for the origin which will then shift the voxet to proper amount.

• Calculate Average Velocity

Convert Velocity Type: In One DomainConvert Velocity Type: In One Domain� Input Velocity is the smoothed and filled interval velocity of the

model

� Input Velocity Type : Depth Interval

�Unit is m/s (one way time)

�Output velocity : Named vave,

�Output Velocity Type : Depth Average

�Visually quality check: Property should be smooth and increase in value downwards (Generally)

Time Convert Velocity Model• Convert model in time domain.

Time-Depth Conversion-Convert Seismic Cube�Voxet Seismic is the prepared velocity model voxet

� Seismic Properties : Original interval velocity smoothed and filled velocity and the calculated average velocity (vave).

� Pick Depth to Time Conversion�Voxet Velocity Cube : Pick the prepared velocity model voxet

�Average Velocity is v , and associated Velocity Unit m/s is (two way �Average Velocity is vave , and associated Velocity Unit m/s is (two way time because desired output should be in two way time)

�Output Voxet Name can be time VelMod

�Starting Z should be 0 and the Ending Z as high as the deepest level of the seismic survey to be depth convert in milliseconds

�Number of Depth/Time Steps : sufficiently high to capture the velocity model structure

�Choose linear interpolation as Interpolation Method, not wavelet interpolation which is used for seismic data and not velocity data.

� Seismic Reference Datum needs to be set as Constant and at 0.

Prepare Time Converted Velocity

Model

• Replace no-data values

�Applied to the time converted model when the process is

not able to map corresponding depth data

if (v_int_smooth_filled < 0){v_int_smooth_filled = XXXX;}

• Recalculate Average Velocity

� The result (v_ave_t) should be very similar to the average

velocity calculated in the depth domain (v_ave) but

without no-data values

Depth Convert Seismic Survey

• Define Sub-Region in Model

� Critically speed up the transfer of the average velocity property to lines in the seismic survey.

� Define a closed curve in survey.

Region-create inside closed polygons• Transfer Average Velocity to Survey• Transfer Average Velocity to Survey

Property-Transfer From Nearby Grid Node or CellPoint Property Client :2d survey to be depth converted

Region :everywherePoint Property Server : Time converted velocity model

From Region : Region defined in the previous step (Sub-Region)Property : Average velocity

Depth Convert Seismic Survey

• Depth convert with a seismic datum of 0m

Time-Depth Conversion-2D Survey Conversion2D Survey Object : survey to be depth converted

Seismic Properties: data properties (seismic by default)

Average Velocity is the average velocity transferred from the model

Starting Z : 0 and Ending Z : is max depth.Starting Z : 0 and Ending Z : is max depth.

Interpolation Method : Wavelet interpolation

Seismic reference datum : Constant at 0.

• Shift depth converted survey to the actual seismic datum:2D Survey Menu-Tools-Move

Objects : Depth converted survey

Translate: Amount to be shifted as the Z component for the Translation

(- Survey Depth)

References (Web Links)

• http://paulbourke.net/dataformats/gocad/gocad.pdf

• http://tu-freiberg.de/fakult3/gy/mageo/lehre_3d/vorlesung/script_uebung.pdf

• http://scholar.harvard.edu/plesch/pages/depth-conversion-pdgm-gocad

• http://www.geol.agh.edu.pl/~dwornik/mtex/Gocad_Tutorial.pdf

• http://www.gocad.org/w4/index.php/gocad/presentation

• http://scholar.harvard.edu/files/plesch/files/gocadinahurry.pdf

Other Useful Links:

http://sepwww.stanford.edu/public/docs/sep80/bob/paper_html/

• http://www.pdgm.com/getattachment/Resources/Brochures/SKUA-Structure-and-

Stratigraphy/SKUA-Stratigraphy.pdf.aspx