Theory

What Is Phase in Seismic Data Processing?

Phase describes the timing relationship between different frequency components of the seismic wavelet. Phase tells us the shape of the seismic wavelet and how the wiggles line up in time. Amplitude tells us how strong the wavelet and Phase tells us when/where is the wavelet is strong.

Changing phase changes the shape of the trace without changing its frequency content.

Common Phase Types in Seismic

Zero-phase

• Main peak is centered exactly on the reflector.
• Symmetric wavelet.
• Preferred for interpretation.

Minimum-phase

• Physically realistic for earth wavelets.
• Wavelet energy starts early.
• Used in processing before deconvolution.

Mixed-phase

• A combination of phase types due to:

Why Phase Matters?

Even if amplitude is correct, phase errors make seismic difficult to interpret.

Phase affects:

• Reflector alignment
• Wavelet shape
• Tuning effects
• AVO analysis
• Inversion accuracy
• Horizon picking accuracy

What Is Phase Comparison?

Phase comparison is the process of comparing the phase of two seismic datasets to check whether they are aligned or not.

Example comparisons:

• Migrated vs unmigrated
• Raw vs filtered
• Vintage 1 vs Vintage 2
• Stack vs angle stacks
• Seismic vs well synthetic
• Time-lapse (4D) seismic

Phase comparison determines:

• Do wavelets line up correctly?
• Is one dataset shifted or rotated relative to the other?
• Do they have the same seismic character?

How Does Phase Comparison Work?

Phase comparison is done using cross-correlation, phase spectra, or wavelet estimation.

Cross-Correlation (Most Common)

How it works:

1. Choose two traces you want to compare.
2. Calculate cross-correlation.
3. Find:

If correlation peak = 0 time lag → phases are aligned. If correlation is low → phases differ significantly.

Phase Spectrum Comparison

Use FFT to compute:

• Amplitude spectrum
• Phase spectrum

Plot phase vs frequency for both datasets.

We look for:

• Constant phase shift → simple rotation
• Frequency-dependent shift → mixed phase distortion

Wavelet Extraction

Extract wavelets from both datasets:

• Compare their shapes
• Compare their peak location
• Compare symmetry

This tells you zero-phase vs minimum-phase differences.

Phase Rotation & Matching

If one dataset has different phase:

• Apply a phase rotation (e.g., +30°, –20°)
• Recompare
• Repeat until wavelets match

This is standard in:

• Time-lapse (4D)
• Merging vintages
• Matching seismic to well synthetic

Why Do We Do Phase Comparison?

• To ensure two volumes can be merged
• To check if processing changed the wavelet
• To match seismic to synthetics
• To prepare for impedance inversion
• To ensure consistency across angle stacks
• To detect 4D changes in reservoir monitoring

Input data

Input DataItem

Input gather

connect/reference to output gather of the input seismic data.

Parameters

Vertical window

specify the time window over which the phase is measured. The time window slides down as per the user defined value.

Horizontal window

specify the number of traces considered for phase comparison of the trace at a given time.

Settings

Auto-connection

By default, TRUE(Checked).It will automatically connects to the next module. To avoid auto-connect, the user should uncheck this option.

Bad data values option { Fix, Notify, Continue }

This is applicable whenever there is a bad value or NaN (Not a Number) in the data. By default, Notify. While testing, it is good to opt as Notify option. Once we understand the root cause of it,

the user can either choose the option Fix or Continue. In this way, the job won't stop/fail during the production.

Notify

It will notify the issue if there are any bad values or NaN. This will halt the workflow execution.

Fix

It will fix the bad values and continue executing the workflow.

Continue

This option will continue the execution of the workflow however if there are any bad values or NaN, it won't fix it.

Number of threads

One less than total no of nodes/threads to execute a job in multi-thread mode. Limit number of threads on main machine.

Skip

By default, FALSE(Unchecked). This option helps to bypass the module from the workflow.

Output data

Output DataItem

Output gather

generates the output gather as a vista item as well as the user can save this gather as an output gather.

Gather 2

Information

There is no information available for this module so the user can ignore it.

Examples

Actions

There are no action items available for this module.

Video lesson

YouTube video lesson, click here to open [VIDEO IN PROCESS...]

Related articles

Yilmaz. O., 1987, Seismic data processing: Society of Exploration Geophysicist

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Description

Input data

Input DataItem

Input gather

Parameters

Vertical window

Horizontal window