VelPAK Glossary

A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

Glossary

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A

AOI or Area of InterestThis is the region in space that the velocity model occupies. It is determined by the extents of the data loaded into the model.

Generally it is a good idea to create a grid of sea bottom, basement, or laterally ubiquitous event to act as a master or parent grid. You can then generate all other grids to match the X and Y extents & cell size of this grid. Note the cell sizes in VelPAK grids must be square; the X dimension must match the Y dimension, and all grids must have the cell size.

Note that with surface mode depth conversion, holes in a grid are propagated downwards through the volume, so it is important to precondition the grids correctly prior to depth conversion. See ‘Preparing Grid Data for the VelPAK Model’ in the ‘Introduction’ chapter under Help > Tutorials > VelPAK.

When creating a grid in the Surface module, the Grid fly out has a Range tab that features icons for automatically setting or computing the AOI. Be consistent with these options in the model.

Apparent VelocityAlso known as pseudo velocity. The velocity estimated by matching seismic event interpretations in time to formation tops from wells in depth.

Average VelocityThe ratio of the depth of an event to the vertical travel time of the seismic wavefront to that event. In VelPAK, this is referenced from the seismic datum. Note that as seismic times are typically measured in two way travel time, the average velocity to any given depth is given by:

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A B C D E F G H I J K L M N O P Q R S T U V W X Y Z E
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In the Velocity module, average velocities for wells can be plotted (after Layer definition has been performed). In this module average velocities can be computed from RMS stacking velocities and overlain with the well average velocities.

An average velocity volume can be generated using the Volume fly out of the Velocity module. The average velocity volume is most useful for depth conversion of surfaces not in the velocity model.

See also Seismic velocity.

B

Background VelocityThe regional trend of the velocity field.

BIN fileSee VelPAK Model File.

D

DatumsA discussion of datums is given in the whitepaper ‘VelPAK and Reference Datums’ available to download on the SMT website. Use the My Account link on www.seismicmicro.com to log in and access the whitepaper.

Vave 2.depth( ) time⁄=

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http://www.seismicmicro.com

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Depth conversionDepth conversion is usually the final objective of a study using the VelPAK module. In Surface Mode, grids are depth converted as you work down through the layers. In Profile Mode, the velocity characteristics of each layer must be defined prior to performing the depth conversion if each horizon interpretation.

Within KINGDOM, there are a variety of methods for accomplishing each of these tasks, as indicated in the following table.

Diagnostic modeThere is a useful option in the Velocity Volume Generation Fly out that allows you to generate an ASCII text file that details all of the parameters in the velocity model used for depth converting each layer. To use this option, set the Type parameter to Diagnostic. The output file will be that set in File (which will be written into the author’s project folder). Set the Diags flags to Yes or No to incorporate more or less information into the text file.

Note that the ASCII files generated can be huge in size so limit the velocity volume generation range to a few traces in the area of interest!

Dix interval velocityDix derived an equation to approximate the interval velocity for a layer from the RMS velocities at two surfaces:

where

Vint = Dix interval velocity

V1 = RMS velocity at the first surface

2d3dPAK TracePAK VelPAK Profile VelPAK Surface

Grids

Horizons

Faults

Seismic volumes

T-D chart

VintV2

2T2 V12 T1⋅–

T2 T1–---------------------------------------=

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V2 = RMS velocity at the second surface

T1 = Time of the first surface

T2 = Time of the second surface

Note that this equation makes the assumption that the surfaces are flat, parallel layers.

You can apply the Dix equation to RMS stacking velocities using the Velocity module Dix fly out. This computes both interval and average velocities for that layer. Note that as RMS velocities are typically several percent higher than the average velocity, the default percentages for the Dix equation are set to 92%.

See Dix, C.H. (1955), Seismic Velocities from Surface Measurements, Geophysics,20:68-86.

E

EventAn Event is either a horizon or grid, loaded into the velocity model, which delineates a change in velocity regime in the vertical domain. Typically these correspond to changes in rock lithology. Events correspond to a layer number. Therefore, the first event represents the bottom of the first geological layer in the model.

If using well data in the velocity model, formation tops must be associated to Events using the Layer definition module.

F

Fly out

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Fly outs are pop out menus, typically located down the right-hand side of each module window. The fly out content are specific to each module window. Fly outs can be pinned open by pressing the pin icon in the top right hand corner of the fly out:

Multiple fly outs can be pinned open at any one time—they will stack on the screen in order to maximize space for the main display window.

G

GridA grid is spatial representation of data that contains data samples in regular array.

It is strongly recommended that all grids used VelPAK have a square grid cell size and have the same extents as a master (or Parent grid). Typically the Parent grid will be Event 1, and should cover the whole area of interest. This typically may be water bottom, bottom of the weathered layer etc.

Using Surface Mode, grids will be depth converted from shallowest to deepest, so if a grid above the current event covers less of an area, only the lesser area can be depth converted.

Grids may be created in VelPAK from XYZ data points or by processing other grids (on the Process fly out of the Surface Module).

Grids are stored in the Model tree under the Surface > Event Name > Grid branch, and are stored by GridType.

See also AOI or Area of Interest, GridEvent, GridDesc and GridType.

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GridEventThe number of the grid event in the velocity model (0 is the surface, 1 is the shallowest event e.g. sea bed, increasing downwards). In dialogs, a grid number can be specified by number, or by using the nomenclature of Previous & Current. The Current surface is the one set in the Surface dialog box in the model tree window, Previous is the grid directly above.

For example, if the user wished to add the isopach of the current grid 3 to the depth of grid 2 to give the new depth grid for surface 3, and surface 3 is current, the parameter declaration would be valid as either of these options:

See also GridType and GridDesc.

GridDescPrefixed by a parameter number, GridDesc is a text field that the user can edit to apply a short description to a grid held in the model tree. The grid description is displayed in the model tree after the GridType label.

You can edit the description by clicking on the grid in the model tree and editing the Name field in the Properties table.

See also GridEvent and GridType.

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GridTypePrefixed by a number, GridType refers to the named slot in the model tree to read data from or write data to. There are a range of GridTypes predefined. You cannot add to the GridType list but there are generic ‘GeneralXX’ types you can use to store any data type in. You can add your own descriptive text to a GridType using the GridDesc field.

Note that it is perfectly safe to use the same input and output GridType in an operation. For example, to multiply a depth domain grid by 0.3048, you would use the Surface module Process fly out and set the parameters as follows:

The output depth grid will overwrite the input grid after it has performed the multiplication.

You can move data between GridTypes in a specified layer by right clicking on the source slot, selecting Copy, then right-clicking on the destination slot and selecting Paste. If you wish to do this in a workflow, you can use the node Surface.Process.Parameters and use the Math+ (Add) formula to add 0 to the source grid and store in the required output slot.

I

InterpretationHorizon and fault interpretation is imported into the VelPAK model, using either the TKS link or by ASCII import. This is generally only ever used in Profile Mode and must undergo the Snapping procedure before use.

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Snapped interpretation cannot be directly put back into the KINGDOM project database as the data may have been merged to produce pods, channels and multi-Z component interpretation. Instead it must be exported to ASCII files in segments for import back into the project database.

Interval VelocityThe ratio of the vertical thickness of a layer to the vertical travel time of the seismic wavefront through that layer. See also Seismic velocity and Dix interval velocity.

INDTIndeterminate or NULL value. These occur in grids where maybe a fault polygon exists, or grid cells are divided by a zero value, or the gridding algorithm doesn’t project data between sparse data points.

INDT values in grids can be replaced by a user specified constant (or values from another grid) using the Surface Module Process fly out; on the Parameters tab set the Formula to ‘SET INDTS TO GRID OR VALUE’, and specify the parameters as per usual.

Instantaneous VelocityThe velocity at any given time or depth.

IsochronA time thickness grid of a layer. After depth converting a layer, the Isochron grid for that layer can be displayed by double-clicking the Isochron grid in the Model Tree.

IsopachA thickness grid of a layer in the depth domain. Technically, the isopach grids generated by VelPAK are in fact depth domain isochore maps, as they measure vertical thickness. In the strictest terms, an isopach is the thickness of a layer measured normal to the bedding planes, but VelPAK uses the vernacular meaning of the term.

After depth converting a layer, the isopach grid for that layer can be displayed in the Surface module by double-clicking the Isopach grid in the Model Tree.

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K

KSee Velocity Gradient.

L

LayerLayer has two meanings in VelPAK1. The geological region between two surface events

2. The visible data on the Surface map and other displays. The visible layers may be toggled on and off using the Display fly out or the Layers Visible drop down

Layer cake depth conversionThis is the method that VelPAK uses to depth convert grids in the Surface module. The overburden is separated into velocity layers, the velocity change in that layer is modeled and then the isopach of the layer is computed. The isopach is added to the depth of the layer above to give the depth of this layer.

Layer definitionThis is the process of assigning the layer model from the layer event data to the well data. This is done by matching up formation tops that correspond to the layer events (grids) in the velocity model.

Layer definition is performed in the Layer module. Multiple formation tops can be assigned within one layer. The layer model can be seen in the Well module after the model has been applied. The layer definition in the wells is displayed to the left hand side of the well curve, the layer definition made by the event layers (grids) is displayed to the right hand side of the well curve. Ideally, the color blocks should match up either side of the curve.

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Model TreeAnalogous to the Project Tree displayed in the main application window, the model tree contains a list of data in the velocity model. However, the model tree differs in that it contains a sorted arrangement of Surfaces, ordered from the shallowest layer (the seismic reference datum) down to the deepest layer. Profile and well data is stored in alphabetical order in the model tree.

By default, the model tree only display data that is present in the model. To display Model Tree Slots unoccupied by data, click the ‘Show Unused’ icon in the model tree icon bar.

Model Tree Slots Each layer in the model will occupy a number in the range 1 to 31 (0 and 32 are reserved by the program). The number indicates increasing layer position (so Layer 1 is shallower than Layer 2, Layer 2 is shallower than Layer 3 etc). See also GridEvent.

Within each Surface, there are named slots in the model tree. These names are fixed, and represent the data store within them. For example, the ‘Time’ slot will hold time domain data for that layer, the Isochron slot will hold time thickness data for that layer etc. See also GridType.

Each Surface also can have a variety of data types: XYZ, Fault, Polygon, Grid and Interpretation. The XYZ, Grid and Interpretation data types have the same fixed name slots. For example, XYZ data for a surface may contains Time domain control points, Isochron time thickness control points for that layer.

When generating data in VelPAK, data to be stored in the model tree is specified by a fixed slot name. (in contrast to creating a grid, in the main application). By using the slots available there should be no confusion over data types created, and enables the construction of generic workflows using data held known variables in the model.

ModuleA VelPAK workspace activated by clicking on the Module name icon on the main VelPAK display. Modules may be detached from the main display by double-clicking on the module name tab after the module has been activated.

To reattach the detached module to the main display, double-click on the title bar of the module window.

The available modules are as follows:

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OptimizationIn VelPAK, optimization is a powerful & robust technique to determine the best parameters for a given velocity function using well velocity data. The objective function is set by the user in the Surface module Depth fly out (e.g. [Optimize] Linear V0 KZ, V= V0+Kz), the parameter space defined, the process is run and the optimal parameters displayed for automatic or user selection.

A number of pre-defined workflows are built into the Workflows fly out of the Workflow module, to make using this technique very simple.

P

PodsPods are discrete geological units whose velocity can be modeled and are used exclusively in Profile Mode. Typically they are introduced during or after the main layers of the profile have been defined. This event number must be classed as a Pod using the Pod tab on the Snap fly out in profile mode.

Typical uses for pods are to model channels, shallow surface anomalies, gas seepage between layers, and even lacoliths.

Module Primary Function

Surface Map display, depth conversion of grids, grid math

Profile Horizon display, depth conversion of geology that cannot be represented by grids

Layer Association of well formation tops with grids

Well Display & QC of well data, generation of residual errors, basic log/top editing

Velocity Display and Dix conversion of stacking velocities, survey definition & generation

Curve Graphing methods to derive velocity function parameters

Optimise Powerful tool for derivation of velocity function parameters

Fault Display of Allan diagrams

3d Basic 3d display of grid surfaces

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Profile ModeIf the geology of the project is complex and cannot be represented by a grid, Profile mode can be used. Typical scenarios are for regions with reverse faulting, salt diapirs and channel systems.

A profile is a single vertical section. This could be a 2d line, inline or crossline from a 3d survey, or a random (or arbitrary) line across multiple surveys.

Profile mode works by depth converting the data loaded into each profile. Typically this is horizon data brought into the model from the interpretation project. The horizon data must undergo the snapping process to produce a 2d sealed earth model, prior to modeling the velocity changes in each layer of the profile.

Unlike Surface mode, the velocity function for each layer must be defined prior to depth converting the profile.

Note the Profile mode can use data (e.g. interval velocity grids) to depth convert the horizons. In addition to layers, profiles can also incorporate sealed units called pods, which may or may not intersect with the layers in the profile.

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Figure 1 Profile incorporating a salt diapir and a pod

Property GridProperty grids are user interface elements that are tables of parameters and information, typically seen in the Fly out. They are not to be confused with the time or depth grid data for example (although there is a property grid, under the Model Tree, that contains information on the grid selected in the Model Tree).

Pseudo VelocitySee Apparent Velocity.

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RMS Velocity, VrmsDefined by:

Where Vik is the interval velocity for layer number k.

VRMS is the root mean square velocity. This is not synonymous with Stacking Velocity, though they can converge in value at zero offset.

S

Seismic velocityThe rate at which a seismic wavefront travels through the medium. Seismic velocity is typically anisotropic within a medium. In flat-bedded sediments, typically the horizontal velocity is higher than the vertical velocity.

With sedimentary rocks, usually the velocity increases with depth within a uniform layer due to burial compaction and diagenetic effects. Hence there is often a velocity gradient within a layer.

Typical velocities are for brine, for sandstone, for shale, and for granite.

Velocities may be categorized as average velocity, interval velocity or instantaneous velocity.

Vrms2 Vik2 Tk×( ) T0⁄k 1=

n

∑=

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Stacking VelocityStacking velocity is the value used to stack seismic traces, typically derived by semblance or coherency methods in seismic processing packages. Typically hyperbolic moveout with offset is assumed in the derivation of stacking velocities.

See also RMS Velocity, Vrms.

Surface ModeWhen the geological layers can be represented by a series of grids, it is best to use VelPAK to model the layer velocities and then depth convert each grid in turn (see Layer cake depth conversion). This situation lends itself well to the use of workflows.

When working with entirely grids, there is no advantage to be gained by bringing horizon data into the model.

When the geological layers cannot be represented by surface grids, Profile mode can be used to model vertical sections. Surface mode is much less labor intensive to work in than Profile mode.

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XITT
Table of Typical Estimated velocities

TKS LinkThis is the link between the velocity model, and the main KINGDOM project database. Data can be loaded into the velocity model from the database using File > Open TKS… and stored from the model into the database using File > Save TKS…

Once data has been loaded into the model, it can be edited, manipulated, or deleted without changing the data in the KINGDOM database.

See also VelPAK Model File.

VV0

Material Density (gram/cm3) Vp (feet/second) Vs (feet/second)

Sandstone 2.65 18,500 11,500

Limestone 2.73 21,000 11,000

Dolomite 2.84 23,000 13,000

Quartz (beta) 2.53 21,770 13,240

Quartz (alpha) 2.65 19,870 13,620

Calcite 2.71 20,530 10,640

Halite 2.16 14,850 8,580

Sea water (20% NaCl) 1.14 5,290 n/a

Fresh water 1.00 4,590 n/a

Oil 0.83 4,200 n/a

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See Velocity Intercept.

VelPAK Model FileUse File > Save or File > Save As… to save the current velocity model and all the data therein, to a single file with the .bin suffix. The default location for this file is the author folder in the KINGDOM project path.

The model file can be opened using File > Open.

Saving data to a model file does not cause any data to be changed in the KINGDOM project database.

Model files can be transferred by copying the file (they often compress well with WinZip) and opened in any KINGDOM project, though obviously if you use the TKS link to store data to the KINGDOM project database, ensure the project you are saving to is compatible.

Velocity GradientThe rate of change of seismic velocity within a layer, often denoted by the symbol K. This typically represents the increase of velocity with depth in a homogenous layer due to compaction, and may be referred to as the compaction gradient.

The value of K for a layer may be derived using VelPAK’s Curve and Optimise modules if well velocity data is present. The latter is recommended, especially if only limited well data is available.

It is important to note that the value of K is unique to the velocity function being used. For example, for the Slotnick equation K may typically be 0.000258 but for the Houston equation K may be 0.00924 for the same rock mineralogy. Please refer to the VelPAK on-line Help section ‘Velocity Function Parameter Ranges’ for more information.

See also Velocity Intercept and Instantaneous Velocity.

Velocity InterceptThe instantaneous velocity at the seismic datum, typically denoted by V0 or V0 (V-nought).

It is related to the instantaneous velocity by the commonly used model function:

Vz = V0 + k. Z

where

Vz = instantaneous velocity at depth Z

V0 = instantaneous velocity at datum

K = velocity gradient

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Z = depth

See also Velocity Gradient and Instantaneous Velocity.

Velocity ModelA set of geological surfaces, methods and parameters are used to create a velocity model. Typically this will be in Surface Mode, where grids are used to define the geological structure, but this may also be in Profile Mode, where horizon interpretations and fault sticks are used to define the structure for any given vertical section. Methods are defined for each layer in the structural model (e.g. V = Vo+ Kz), along with any associated parameters for that method.

Depth conversion of surfaces takes place in a top-down fashion, but to generate a complete velocity volume, methods and parameters for every layer in the model need to be defined.

Velocity Volume Generation (VVG)A velocity volume may only be generated once the velocity model has been completed. It is generated using the Volume fly out on the Velocity module.

Note that in current versions of VelPAK, any values below the last event are assigned a value of zero, so in order to generate a volume with no zero values below the deepest time of the last event, it is recommended to introduce an event at or below the last time of the velocity volume required, and assign it a constant interval velocity.

See also Diagnostic mode.

W

Well VelocityThe velocity recorded from well data. VelPAK can use either a sonic log (which is automatically converted from slowness to velocity on import into the model) or a Time-Depth chart for each well. If using a sonic log, ideally it should be calibrated to check shot data. If this data is not available, a crude calibration to a time constant (or structural grid) may be made in the Well Module.

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X

XYZCollections of values representing data in space. Each surface has a number of predefined XYZ slots associated with it; these match the GridTypes. All XYZ data must exist within the predefined slots.

By default, horizon data imported through the TKS Link as XYZ data will be stored in the Time slot in the Model Tree. You can move this data to another slot by right-clicking in the Time slot, selecting Copy, then right-clicking on the destination slot and selecting Paste.

To generate grids in VelPAK, the input data must be XYZ data. Therefore, to regrid an existing grid, the grid needs to be converted to XYZ points first. This is done on the XYZ fly out of the surface module.

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