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<< Click to Display Table of Contents >> Navigation: Velocity > Average velocity (time) scaling with T-D pairs (3D) |
This module calibrates a 3D average velocity (time-domain) volume to well time-depth (T-D) pairs. At each well location, the module computes the ratio between the average velocity derived from the T-D data and the average velocity read from the input seismic velocity volume. These per-well scale factors are then interpolated spatially across the 3D grid using radial basis function (RBF) interpolation, producing a smooth, spatially continuous scale factor volume. The input average velocity volume is multiplied by this scale factor volume to yield a well-calibrated average velocity output.
Use this module when you have a seismic-derived average velocity volume that needs to be tied to actual well control. The calibration honours the well data exactly at each well location and transitions smoothly between wells. A maximum allowable velocity change (as a percentage) prevents unrealistic corrections in areas away from well control. Both the calibrated velocity volume and the scale factor volume are written as SEG-Y outputs for further use in depth conversion or interval velocity computation.
The trace header collection describing the 3D geometry of the input average velocity volume. This defines the inline and crossline numbering scheme used to locate well positions within the seismic grid. The headers must cover the full inline/crossline extent of the input velocity volume.
The input 3D average velocity volume in SEG-Y format. Each trace must contain average velocity values (in m/s) as a function of two-way time. This volume is the uncalibrated velocity field that will be scaled to match the well T-D data.
The physical distance in metres between adjacent inline lines in the 3D seismic survey. This value is used to convert inline index differences into real-world distances for the spatial interpolation of scale factors between wells. The default value is 25 m. Set this to match the actual inline spacing of your survey grid. Incorrect spacing will distort the spatial weighting of the RBF interpolation, causing scale factors to be pulled too strongly toward nearer or more distant wells.
The physical distance in metres between adjacent crossline lines in the 3D seismic survey. Like the inline spacing, this is used to correctly scale spatial distances during RBF interpolation of scale factors across the crossline direction. The default value is 25 m. Verify this matches your survey geometry before running the module.
Path to a plain-text ASCII file that lists the well identifier and its 3D seismic grid position for each calibration well. Each row must contain three space-separated values: WellID InlineNumber CrosslineNumber. The WellID must exactly match the identifiers used in the Time-depth data file. Wells listed here that fall outside the spatial extent of the input velocity volume are automatically skipped with a warning. At least one valid well inside the survey area is required for the module to proceed.
Path to a plain-text ASCII file containing the time-depth (T-D) pairs for each well. Each row must contain three space-separated values: WellID Time Depth (or WellID Depth Time depending on the Column sorting parameter). Multiple rows for the same well define the complete T-D relationship at that well. Depth is always in metres. Time units and convention (one-way or two-way) are controlled by the Time unit and Time parameters respectively. Entries with WellIDs not present in the Well IDs and locations file are ignored.
Specifies whether the time values in the Time-depth data file are Two-way travel time (TWT) or One-way travel time. The default is Two-way. If your T-D file contains one-way times (for example, from a VSP check-shot table where times are listed as one-way), select One-way. The module will automatically multiply one-way times by two to convert them to TWT before computing average velocities, ensuring consistency with the seismic velocity volume which is always in TWT.
Controls the order of the time and depth columns in the Time-depth data file. Select Time - Depth (the default) if the file lists time in the second column and depth in the third column. Select Depth - Time if depth appears before time. This setting only affects column parsing and does not change the units or time convention.
Specifies the unit of the time values in the Time-depth data file. Select ms (milliseconds, the default) if your T-D file contains times in milliseconds, or s (seconds) if times are in seconds. The seismic velocity volume is always assumed to be sampled in milliseconds, so this conversion is critical for correct depth-to-time matching at well locations.
Sets the length of a vertical boxcar (moving average) smoothing window applied to the scale factor volume before it is multiplied into the velocity volume. The value is expressed in samples. The default is 5 samples. Enter 0 or 1 to disable smoothing. Values less than 2 are treated as no smoothing. Smoothing reduces high-frequency artefacts in the scale factor field that can arise from noisy or sparsely sampled T-D data, resulting in a geologically smoother calibrated velocity volume. Increase this value when the T-D data contains noise or when the well spacing is large relative to the expected velocity variation.
Limits the maximum correction that can be applied to the average velocity at any location. The scale factor at each sample is clamped so that the output velocity cannot deviate from the input by more than this percentage. For example, a value of 10 (the default) means the calibrated velocity is constrained to between 90% and 110% of the original seismic velocity at every location. Valid range is 1 to 99. This safeguard prevents runaway corrections in areas far from well control where the interpolated scale factor may be unreliable. Increase the limit only when the seismic velocity field is known to be significantly biased relative to the well data.
Path and file name for the output SEG-Y file that will contain the well-calibrated 3D average velocity volume. The output has the same geometry, sample interval, and number of samples as the input velocity volume, but with each velocity value multiplied by the spatially interpolated and optionally smoothed scale factor. Use this volume for improved depth conversion or as input to interval velocity computation. Accepted file extensions are .sgy and .segy.
Path and file name for the output SEG-Y file that will contain the 3D scale factor volume. Values in this volume represent the multiplicative correction applied at each time sample and location — a value of 1.0 means no correction, values above 1.0 indicate the seismic velocity was increased, and values below 1.0 indicate it was decreased. Inspecting this volume is recommended as a quality control step to confirm that the corrections are geologically reasonable and spatially consistent. Accepted file extensions are .sgy and .segy.