COHERENCE ATTRIBUTES

COHERENCE ATTRIBUTES

1. DEFINITION

   • Measurement of similarity among the seismic traces/ wavelets
     • How?
       • Calculate the localized waveform similarities in both in-line and cross-line directions.
   • COHERENCE VALUE
     • Highly coherent waveforms : laterally continuous lithologies
     • Low coherent waveforms : changes in waveform -> fault and fractures in the sediment.

2. WHEN AND WHY NEED TO USE COHERENCE?

   • For interpreting faults that run perpendicular to strike.
   • For identifying stratigraphic features with close scrutiny (small scale)
     • E.g: river channels, mud flows and submarine canyons
     • Hard to trace features in small scale
   • For interpreting faults that run parallel to strike
     • Difficult to interpret fault as faults run parallel to strike, the fault lineaments become superimposed on bedding lineaments.

3. ADVANTAGES

   • Easy visualization of geological features.
     • E.g : Faults, Channels, Reef Boundaries, Edge of Channels
   • Unbiased view of the features in the seismic volume.
   • Extracting more meaningful detail, improve accuracy and time effectiveness.
   • Sharp detection of reef, channel boundaries and deltaic sediments
   • No interpretation is required for viewing them

4. HOW TO VIEW COHERENCE ATTRIBUTES?

   • Cross - section
   • Horizon slice/map
   • Time slice map

5. METHODS TO MEASURE COHERENCE

   • Cross-correlation based Coherence
     • Simplest method. Sensitive only to waveform. Operates on 3 neighboring traces
     • How to measure?
       • Value =1 = high coherent = identical waveform
   • Semblance based Coherence
     • How to measure?
       • Value = 1 = high coherent = identical waveform
     • Semblance produced is the ratio of the energy of the energy of 5 average traces (c) to the energy of the five traces (a)
   • Variance based Coherence
     • A popular method., commonly used.
     • Almost identical to semblance based
     • How to measure?
       • Variance= 1- semblance estimate coherence
       • Coherence Value= 0 = identical/similar waveform
       • Coherence Value > 0.0 = identical traces but different amplitude

6. WHEN THIS ATTRIBUTE CAN BE APPLIED DURING THE INTERPRETATION?

   • Before the interpretation
   • Anytime during the interpretation process if we encounter difficulties in viewing in true amplitude.

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