Description

This module removes the source and/or receiver ghost — the notch pattern in the frequency spectrum caused by the sea-surface reflection of a marine source or streamer close to the water surface — by working in the frequency-wavenumber (FK) domain. Given the source/receiver depth below the sea surface, the velocity of sound in water and the sea-surface reflection strength, it models the expected ghost notches and compensates for them iteratively, with safeguards to avoid excessively boosting noise near the notch frequencies. Use it on marine gathers that show a comb-like notch pattern in their amplitude spectrum caused by a shallow source or receiver.

Input data

Input gather

A gather (for example a shot or receiver gather) with a common exploration point and regular trace spacing, needed to build a valid FK transform. All traces should share consistent geometry so the wavenumber axis is meaningful.

Parameters

Delta offset

Trace spacing used to build the wavenumber axis of the FK transform. Leave it at the default value to let the module determine the spacing automatically from the gather's geometry; enter a positive value only if you need to override the detected spacing (for example on irregularly sampled data). The value actually used is reported back in "Calculated Delta offset".

Default: -1 m (automatic detection from geometry).

Source / Receiver water depth

Depth of the source and receiver below the sea surface, in metres. This depth controls the ghost delay time and therefore the spacing of the notches in the amplitude spectrum: shallower tow depths push the first notch to a higher frequency, deeper tow depths push it lower.

Default: 15. Minimum 0. Set it to the actual (or best estimate of) source/streamer tow depth for the survey; an incorrect depth will misplace the modelled notches and leave residual ghost energy.

V0

Velocity of sound in water used to compute the ghost travel-time delay from the water depth.

Default: 1500. Minimum 0. The typical seawater velocity of about 1500 m/s is usually appropriate; adjust only if local water conditions (salinity/temperature) are known to differ.

Reflection

Magnitude of the sea-surface reflection coefficient used in the ghost model, between 0 and 1. A value near 1 models a near-perfect reflector (typical of a calm sea surface); lower values model a weaker, less coherent ghost reflection (for example in rough sea conditions).

Default: 0.995. Range 0 to 1.

Advanced

min Tau

Lower bound of the ghost-delay (tau) search range considered by the deghosting algorithm.

Default: 0.0001 ms. Minimum 0.00000001. Rarely needs adjustment from its default.

max Tau

Upper bound of the ghost-delay (tau) search range considered by the deghosting algorithm.

Default: 0.002 ms. Minimum 0.00000001.

Max number Iteration

Maximum number of iterations the deghosting algorithm performs while refining its solution. More iterations can improve notch compensation but increase processing time.

Default: 20. Range 1 to 100.

treshold sigma

Threshold factor used internally to stabilize the deghosting solution and avoid over-fitting to noise near the spectral notches.

Default: 2. Minimum 0.

Max ratio of rpectral enlargement

Upper limit on how much the deghosting operator is allowed to boost weak spectral amplitudes near the notch frequencies. Because ghost notches drive the true signal amplitude toward zero, compensating for them exactly would infinitely amplify whatever noise is present there; this cap keeps the boost within a safe, practical ratio. Lower it if deghosting introduces visible high-frequency noise or ringing at the notch frequencies; raise it if notches are not being adequately filled back in.

Default: 6. Minimum 0.

Output data

Output gather

The deghosted gather, with the sea-surface ghost notch pattern compensated in the FK domain.

Calculated Delta offset

The trace spacing actually used to build the FK transform, reported for QC — especially useful when "Delta offset" was left at its automatic (-1) setting.

Matrix spectrum

The FK amplitude spectrum of the input gather before deghosting, for QC display — use it to visually identify the ghost notch pattern.

Matrix spectrum mute

The FK amplitude spectrum after deghosting, for QC display — compare it against the pre-deghost spectrum to confirm that the notches have been filled in without introducing excessive noise.

References

Posthumus, B. J., 1993, Deghosting using a twin streamer configuration: Geophysical Prospecting, 41, 267-286.