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.
- Forward Q Modelling: apply the modelled absorption and dispersion
- Address Absorption: fix or remove the modelled absorption and dispersion
- Amplitude only filtering: update frequency dependent amplitudes (address attenuation)
- Phase only filtering: update phase (address dispersion)
- Phase and Amplitude filtering: address both
Q value: The Q value to model
- Note: A single Q value produces good results when applied appropriately. In theory, this is an incorrect assumption.
- Window length(ms): perform analysis over this window
Dominant frequency: The dominant frequency of the seismic data.
- Review spectra to determine, e.g. 45-50 Hz.
- Not applicable when applying "Amplitude only" filtering
Gain stabilisation limit (db): limit the gain applied to the samples (2-100 dB)
- Not applicable when applying "Phase only filtering"
Apply reference-time correction: Enables a deep-water time correction
- This option requires water depth and velocities loaded into trace headers.
- See Reference-Time Correction
- Velocity Volume, Type, Units: Select the velocity model to use
Offset calculation: Source of offset values when applying the filter to gathers
- User the OFFSET header
- Calculate offset from SX/SY/GX/GY
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
These headers are used only when "Apply reference-time correction" is enabled:
- GWDEP - Water depth at the midpoint (honours SCALEL)
- SCALEL - Depth scalar
WEVEL - Water velocity in m/s
- 1500 m/s is used if this value is not in the range 1450-1550.
Before Inverse Q
After Inverse Q
In this example, we addressed the absorption and applied the filter to both phase and amplitude.
Amplitude Spectrum after Inverse Q
Use the Spectral Analyser tool after applying a Q Filter to compare the spectra.