Displaying the Delta Curve

Delta curves are generated by comparing velocities derived through model building to well information. These delta curves can be used to build 3D delta models for anisotropic depth migration.

In general, delta is calculated at the wells using the checkshot interval velocities and an initial isotropic interval velocity field (Vnmo). The delta curves are automatically (smoothly) extrapolated to the "surface" defined as either the water bottom value for the well or a user-defined horizon.

Available in: Image Gather Processing module.

Generate a delta curve

  1. In the Synthetics window, open the View tab.
  2. Under the Tracks section, click the amber circle at Delta to display the Delta track in Synthetics.
  3. Open the Delta tab in the Synthetics window.
  4. Calculate Delta: Enables the delta curve derivation.

Note: Delta values are required to be in the range of -0.25 to 0.35.

Generate a delta curve
  1. Velocity Volume (V0): Velocity volume for delta derivation.
  2. Velocity Type: The type of velocity for the V0 volume.
  3. Velocity Units: The units of velocity for the V0 volume.
  4. Delta Volume: Optional input interval delta volume. If no volume is selected input delta will be 0 everywhere and the Vnmo used to derive delta will be identical to V0.
  5. Sample Interval (ms): The sample interval for the computed delta curve. Defaults to the sample interval of the supplied V0 velocity. If the velocity is not in time then defaults to sample interval of the input delta volume. If an input delta volume is not supplied or is not in time then defaults to 24 milliseconds.
  6. Layer: The delta calculation can be split between multiple layers. When set to "Single", delta parameters are for the entire curve. The delta curve can be split into multiple small calculation layers by selecting the "Multiple" option. The configuration parameters for these options are discussed below in detail.

Single Layer

  1. Extrapolation Top: Select Well Water Bottom or Horizon (and select the water bottom horizon). This parameter defines the depth above which the delta value will be extrapolated.
  2. Smoothing Window: The smoothing value for the delta curve. By default, this is set to 12.
  3. Blocking: Defines the blocking option to be used for the delta curve after smoothing. Available options include:
    • No Blocking: The delta curve will appear with the configured sample interval.
    • Regular Blocking: The resulting delta curve will appear with a coarser sample interval, as specified by the Block Size (ms) parameter. Note the value of this parameter should generally be a multiple of the sample interval.

Multiple Layer

The delta curve can be computed from a set of layers with a custom boundary and per-layer parameters. A layer can be added by clicking the blue + button.

  1. The Filter settings apply to all layers:
    • Delta Parameters: If checked the per-layer parameters may be configured. If unchecked a summary will be shown instead.
    • Marker Misties: If checked a marker may be selected any mistie in depth shown. If unchecked a summary will be displayed instead.

For each layer the following parameters are available:

  1. Layer Top: Defines the start of the layer. A layer extends to the start of the next layer, or to the bottom of the supplied V0 velocity. See Defining a Layer Top for details on how to configure.
    The layer top time in milliseconds for the currently selected well is displayed below. Two depths are also shown, with an arrow (→) between them. The first is the depth calculated using the supplied V0 velocity. The second is the depth calculated using the updated V0 velocity derived from the computed delta curve and therefore depending on any preceding layer settings.
    If the layers are not ordered (by their displayed top time), a warning is displayed at top. You can then auto sort these layers by clicking the   button.
  2. Well Marker: The well marker to compare for mistie.
  3. Marker: The marker depth value in the current display units.
  4. Mistie: The mistie between the top and marker depth values, in the current display units and as a percentage. The mistie is calculated as the horizon depth minus the marker depth. A positive mistie indicates the seismic event is deeper than the marker. The mistie is shown for both depth values, calculated using supplied and updated V0 velocity.
  5. Delta Source: Defines how the delta values in this layer are calculated. The supported options are:
    • Computing delta from Vp and Vnmo Please see the Computing delta from Vp and Vmo section for details.
    • Use supplied delta values Fill the layer with values from the supplied delta volume.
    • Interpolate/Extrapolate Interpolate the layer values based on the previous and next layer delta values.
    • Constant A constant delta for the layer.
    • Gradient Linearly interpolate between the given start and end delta values.
    • No value Outputs NaN as the delta value.
  6. Bottom Taper: Taper length to smoothly interpolate between the end of the current layer to the start of next one.

Defining a Layer Top

A layer top (or extrapolation top for a single layer configuration) can be defined using any of the following options:

  • Constant: A constant value in time or depth.
  • Horizon: The horizon intersection with the currently selected well.
  • Top of well log: The time of the first checkshot.
  • Bottom of well log: The time of the last checkshot.
  • Well water bottom: The water depth configured for the well.

Horizons may be scaled by a constant. A non-constant layer may be shifted by a constant value in time or depth. All time/depth conversion is performed using the supplied V0 velocity. The layer top is found by treating the layer configuration as a horizon and finding the intersection with the well. For example, given a depth horizon and a constant time shift, the horizon will first be converted to time, then shifted, then converted back to depth and intersected with the well. The depth at the intersection location is then converted back to time for the purposes of computing the updated delta curve, since all interval delta calculation is performed in the time domain.

Computing delta from Vp and Vnmo

A layer delta value is computed from the Vp and Vnmo as normal and then following post-processing is performed on those computed delta values to further refine the delta values:

  1. Scale: Scale the computed delta by given value.
  2. Blocking: Defines the blocking option to be used for the delta curve after smoothing. Available options include:
    • No Blocking: The delta curve will appear with the configured sample interval.
    • Regular Blocking: The resulting delta curve will appear with a coarser sample interval, as specified by the Block Size (ms) parameter. Note the value of this parameter should generally be a multiple of the sample interval.
    • Layer: When selected, the delta is the mean of layer delta values.
  3. Smoothing Window: The smoothing value for the delta curve in milliseconds. It is not displayed for the "Layer" blocking method.

Layer Templates

The multiple layers state can be saved as a template, and lets you easily define and apply per-well or per-layer delta computation parameters. When in the Multiple layer mode, a template can be created by clicking the Save icon. This then prompts for the template name, and once entered, a new template of that name is created and displayed in the Layer combo-box.

An already saved template can be removed by clicking the Trash icon next to its combo-box.

Note: These templates are currently a per-session object, so you can't directly load these in to other sessions. However, layer templates are stored under snapshots, so copying or opening a synthetic snapshot in another session will load these templates there.

Delta and Vertical Velocity V0

As you start calculating delta curves using the above parameters, Insight computes the vertical velocity field, V0, from the equation below:

The computed V0 velocity is displayed as a curve in the Vp track.

Click the Export Marker Misties button to export the marker misties in CSV format.