Seismic velocity modeling – 3D grid based Objectives • Create a simple 3D grid • Create layers in the 3D grid • Sample the interval velocity data points into the 3D grid • Populate the 3D grid using Ordinary kriging • Sample well velocity into the 3D grid • Calibrate the interval velocity property – anisotropy factor • Calibrate the interval velocity property – anisotropy factor • Convert calibrated interval velocities to average velocities • Setup Velocity model – calibrated seismic velocities • Do Checkshot velocity modeling guided by seismic velocities Ordinary kriging with trend Collocated co-kriging - optional • Setup Velocity model – checkshots guided by seismic velocities • Quality control the velocity models
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Seismic velocity modeling – 3D grid basedObjectives
• Create a simple 3D grid
• Create layers in the 3D grid
• Sample the interval velocity data points into the 3D grid
• Populate the 3D grid using Ordinary kriging
• Sample well velocity into the 3D grid
• Calibrate the interval velocity property – anisotropy factor• Calibrate the interval velocity property – anisotropy factor
• Convert calibrated interval velocities to average velocities
• Setup Velocity model – calibrated seismic velocities
• Do Checkshot velocity modeling guided by seismic velocities
� Ordinary kriging with trend
� Collocated co-kriging - optional
• Setup Velocity model – checkshots guided by seismic velocities
• Quality control the velocity models
Load:
• Seismic DIX
Make seismic
velocity surfaces
corrected to well
velocities
Setup velocity
model
using velocity
surfaces
Seismic velocity workflows
• Seismic
velocities
• Checkshot
surveys
DIX
conversion
Set up 3D grid and
model seismic
velocities together
with checkshot
velocities
Setup velocity
model
using 3D grid
velocity property
Recommended workflow:
• Load SEGY or ASCII seismic Interval velocities into Petrel
• Sample the seismic interval velocities into a 3D Grid
Petrel Velocity modeling can use an average seismic velocity field property of the 3D Grid
Seismic velocities - 3D grid modeling
• Sample the well interval velocities into the 3D Grid
• Derive the anisotropy factor from the well data and the seismic data and apply it to
the velocity field
• Quality control the calibrated interval velocity field
• Convert the interval velocity field into an average velocity field
• Setup a velocity model using the calibrated average velocity property
It is recommended to model interval velocities instead of average velocities because they are easier to QC :
• Interval velocity is a petrophysical property
• Average velocity shows only smooth velocity variations
• It is difficult to locate the origin of velocity anomalies shown by the average velocity field
Interval velocity versus average velocity
Seismic interval velocities Average velocities, derived from
interval velocities
Choose a grid spacing of approximately the horizontal and vertical velocity data sampling
Reasons:
• The number of grid cells has a big influence on the performance of the chosen gridding algorithm
• A smaller horizontal grid spacing would make sense if local velocity variations should be captured.
However due to their nature seismic velocities generally can only provide a reliable regional velocity
trend; therefore strong local variations should be filtered
Setup of a 3D grid - consideration
trend; therefore strong local variations should be filtered
• A small vertical layering is not necessary because the velocity field varies smoothly with depth
Setup of a simple 3D grid
• Input Data: enter the constant surfaces for top (0 ms) and bottom of the velocity cube. Include the time
surfaces that are defined as velocity boundaries
• Geometry: use Automatic (from input data/boundary) and choose a grid increment of approximately the
velocity location distance
• Tartan grid: allows to create grids with non-uniform refinement
Setup of a 3D grid - Layering
• In general the type of layering is not of critical importance
• For Proportional layering the number of layers defines the layer thickness. Use trial & error to get a layer
thickness similar to the vertical velocity sampling spacing: display an Intersection in a 3D window and
measure the layer thickness. Then adjust the number of layers
Sampling of seismic velocities into 3D grid
Sampling done via the Scale up well logs
process under Property modeling
Sample checkshot interval velocity into 3D grid
Open the Scale up well logs process
Upscaled check shot velocities
and wells
Choose Point attributes and select the checkshot
survey of interest
Select the interval velocity attribute of this data set
Three suitable gridding algorithms:
Functional - Moving average - Kriging
Important parameters:
Functional, Moving average
- Use a vertical range that covers 2 or more velocity samples
- Use Inverse distance (squared) as point weighting
Kriging
Gridding of seismic velocities
Kriging
- Select Ordinary from the Expert tab. It is more suitable for velocity interpolation than Simple
because it uses a locally varying mean. Consequently trends in the data are better honored.
Note:
Kriging is an optimized algorithm introduced in Petrel 2008. It shows very good performance and offers
collocated co-kriging and kriging with trend.
‘Kriging by Gslib’ shows a low performance and should not be used any more!
‘Kriging Interpolation’ does not provide Ordinary kriging option and should no longer be used.