ZERO-PHASE CONVERSION in practice

Contents

• Introduction
• Definition of Terms and Assumptions
• Methods of Zero-Phase Conversion

Introduction

Seismic data is often converted to zero-phase to improve resolution and aid interpretation.

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Definition of Terms and Assumptions

To a certain extent this depends of the method used, but most methods assume the input (usually) migrated data are minimum phase.

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Types of Zero-Phase Conversion

Several methods of zero-phase conversion are commonly applied.

1. The commonest method is the so-called statistical approach. Here a window of the input data is defined around the target region. The average autocorrelation of the window is taken and used to define a minimum phase and zero-phase wavelet which have the same amplitude spectrum as the input data. An operator is then constructed which converts the minimum phase wavelet to the zero-phase wavelet and this operator is then applied to the seismic data. Several windows can be extracted and results compared to synthetic seismograms for accuracy. This is the simplest method of zero-phase conversion, often improves resolution and well ties and is a good benchmark test. For an exploration survey this can be very effective. Additionally this method can be applied by most contractors without additional costs or time delays.
2. A simple phase rotation may be applied to improve well ties. For a variety of little understood reasons a modern 3D seismic dataset is often closer to zero-phase than minimum phase so this method often works well within error.
3. Conversion of a wavelet extracted around the seabed. Shell UK currently use this method in conjunction with inverse Q filtering. The method can be highly diagnostic for deep-water data or that acquired with short near-trace offsets in which the seabed wavelet is uncontaminated from refractions.
4. Modelling of source signature. This method was used by Shell for many years. A source signature was modelled to pass through the various processing stage, the resulting output converted to zero-phase and the operator applied to the seismic data. This method could produce unpredictable results.
5. Extraction of wavelet from seismic using a well log to determine the phase. This type of process can be performed with the Geoquest software, in Hampson-Russell Strata and in LogM.

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Post-Stack Applications

The majority of zero-phase conversion is carried out following migration although some people prefer zero-phase data for enhanced resolution during velocity picking.

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