The Q filter applies or removes corrections for dispersion and attenuation.
Because the subsurface is not perfectly elastic, higher frequencies are absorbed more rapidly and travel faster than lower frequencies. This results in frequency-dependent seismic attenuation (reduced amplitude of higher frequencies with travel time) and dispersion (a phase change in the embedded wavelet). Attenuation and dispersion can be quantified by seismic quality factor Q.
The filter is travel-time dependent. Remove NMO correction before applying the filter to gathers.
In deep water settings, a reference-time correction should be applied. This option requires values stored in the trace headers:
- GWDEP: water depth
- WEVEL: water velocity
Q Filter Direction
The filter operates in two models: forward modelling and address absorption.
Use Forward Q Modelling to:
- Evaluate Q effects. Compare different amounts of dispersion and attenuation by applying to spikes, a synthetic model or seismic.
- Remove previously applied Q compensation, if the previous Q compensation parameters are known.
Use Address absorption direction to:
- Fix or remove the modelled absorption and dispersion effects. I.e. apply Q compensation
Create a Q Filter Process
- In the Control Panel, open the Process tab.
- At the tab header, click the blue "+" icon and select New Process.
- Double-click or search for Q Filter.
- Type a name for the process and click OK.
Configuring the Q Filter
- Volume: data to be filtered.
- Amplitude only filtering: update frequency dependent amplitudes (address attenuation)
- Phase only filtering: update phase (address dispersion)
- Phase and Amplitude filtering: address both
- Forward Q Modelling: apply the modelled absorption and dispersion
- Address Absorption: fix or remove the modelled absorption and dispersion
Q model: Choose if the Q values model is a constant value or a temporally and/or spatially varying effective Q field.
- Constant Q value: Enter the constant Q value to use for all traces and times. This is used in the constant Q source case (see below for Q value guidelines).
Q Volume Type:
- Q is interpolated using input trace CMP X/Y: Q is interpolated using input trace CMP X/Y Each trace in the primary input volume is filtered using Q values interpolated from the Q volume. The Q volume can have different trace spacing and sample interval with this option.
- Single trace per gather: Each trace in a gather in the primary input volume is filtered using a single trace from the Q volume. The Q volume in this case should contain only a single trace per gather.
- Single trace: Each trace in the primary input volume is filtered against the single trace found in the Q volume. If the input volume is a 3D volume you must specify a 3D Q volume for the single trace. If the input volume is a 2D volume you can specify a 3D volume or a 2D volume for the single trace Q volume. If you specify a 3D volume, the Q volume must contain a single trace which will be applied to all the input traces. If you specify a 2D volume, each line in the 2D Q volume must hold a single trace that will be applied to the traces in the matching line of the input volume.
- Effective Q Volume: The volume of effective-Q data to use.
- Reference frequency: The chosen frequency to which all other frequencies will be corrected.
- Stabilization Type: Select the type of stabilisation to use to prevent excessive gain. Choices are a gain limit (in dB) and a tapered gain limit. The gain limit produces a flat frequency response once the limit has been hit. The tapered gain limit adds a high frequency taper to the gain limit to taper from the maximum allowed gain at the specified start frequency to no gain at the nyquist frequency. This parameter is not available in "Phase only" mode.
- Gain Stabilization Limit (db): A limitation on the amount of gain applied to the samples. This parameter is not available in "Phase only" mode.
- Frequency for start of gain taper (Hz): This is the frequency that the taper on the gain limit starts. The limit is tapered to have no gain at nyquist. This parameter is only available for the tapered gain limit.
Apply reference-time correction: Enables the time correction described below.
- Correction method: Select the moveout calculation method (only when reference time correction selected). The Xia method is described below. The Ray-trace method (default) uses 1D ray tracing to determine the effective Q model at each offset. Ray-tracing is the preferred method.
- AVO compliant: This enables an AVO compliant algorithm. This is not available for phase-only filtering or stack input. For amplitude-only filtering you will need to supply details of the previously/to be applied phase Q model when enabled.
- Input is moveout corrected: If the input is already moveout corrected (e.g after NMO or migration) then Q filter will remove this correction using the supplied velocity model, apply the Q filter, then re-apply moveout.
- Phase Q model: Choose the phase Q model. The AVO compliant mode requires knowledge of the Q model used to correct for phase. This is only visible if AVO compliant is selected and amplitude-only filtering is being used. This can be the same model as the amplitude correction model or you can enter the details of a different model.
- Velocity Volume: Select the velocity volume (only when reference time correction selected).
- Velocity Type: Select the velocity type (only when reference time correction selected).
- Velocity Units: Select the velocity units (only when reference time correction selected).
- Offset calculation: Select the offset calculation method (only when reference time correction selected).
Note: if input data starts above time 0, the filter is applied from time 0, and the samples above that left as they were.
Xia Reference-Time Correction
When enabled, the Q filter will be applied only below the sea floor, using the time-referencing corrections described in Ganyuan. Xia, Proper time-referencing for prestack Q estimation and compensation, EAGE 67th Conference & Exhibition Madrid, Spain, 13 - 16 June 2005.
It's equivalent to this workflow:
- Apply NMO correction with Vrms
- Apply reverse NMO with the modified velocity Vqrms
- Shift the volume up so that the zero-offset waterbottom is at TWT 0ms
- Apply Q filtering
- Reverse the shift
- Apply NMO with Vqrms
- Apply reverse NMO with Vrms
Q Value Guidelines
The following guidelines for Rock Q factor values are suggested by Sheriff and Geldart, 1995:
- Shale: 20-80
- Sandstone: 70-130
- Limestone,Dolomite: 60-200
- Granite: 100-500
- Gas Sand: 5-50
- Sedimentary rocks: 20-200
- Igneous rocks: 75-300
- Metamorphic rocks: 200-400
These headers are used only when "Apply reference-time correction" is enabled:
- GWDEP - Water depth in project length units at the midpoint (honours SCALEL) - in metric projects this value is in metres, in feet projects it is in feet.
- SCALEL - Depth scalar.
- WEVEL - Water velocity in project velocity units - in metric projects this will be in m/s, in feet projects this will be in ft/s (if not in the range 1450m/s-1550m/s, a default 1500 m/s is used and converted to the appropriate units).
- OFFSET – Used to compute offset.
- SX/SY/GX/GY – Used to compute offset.
After Inverse Q
In this example, we addressed the absorption and applied the filter to both phase and amplitude.