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GG 450 April 31, 2008

GG 450 April 31, 2008. Reflection Interpretation 2. Seismic Interpreters are professionals – usually in the oil industry – that interpret reflection seismic data in terms of geology. They need to be knowledgeable in geomorphology, seismic processing, and sedimentary processes. .

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GG 450 April 31, 2008

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  1. GG 450April 31, 2008 Reflection Interpretation 2

  2. Seismic Interpreters are professionals – usually in the oil industry – that interpret reflection seismic data in terms of geology. They need to be knowledgeable in geomorphology, seismic processing, and sedimentary processes.

  3. This is what it’s all about in reflection seismology. Millions have been spent on data acquisition, more millions on processing, and now it’s up to you to interpret the results and exercise your geological insight to advise management on whether it’s worth the order-of-magnitude greater cost of drilling.

  4. Reflection profiles are full of “lines” across the profile. What criteria define which lines are significant? • Correlation: the same reflection is seen over significant distances, on crossing lines, and/or on other profiles. Reflections often keep the same character, or may be a member of a group of similar reflections, or a boundary between a region of many reflections and a region with no reflections.

  5. Amplitude: the reflection is quite noticeable over the surrounding “noise” of other lines. The SIGN of an arrival will change depending on the acoustic impedance between two layers (density*velocity). A Positive amplitude implies that the impedance increases on the other side of the boundary, and a negative amplitude implies that the impedance decreases. Recall the MATLab synthetic seismic wave? When the wave hit the fixed boundary, the reflection changed sign from the in-coming wave while the wave hitting the free end did not. • Coherence: the reflection is relatively smooth and doesn’t jump around much. • The human eye and brain are EXCELLENT detectors of patterns. We tend to see patterns on the slightest evidence without even knowing it. Example – how do you recognize your friends?

  6. Many “reflections” aren’t really reflections at all, but result from constructive interference of other energy. True reflections will “hold up” under changes in processing parameters, such as filtering. • How thin a layer can you resolve? Recall frequency=velocity/ wavelength. So that: • A 20 Hz signal traveling in 2 km/s sediment has a wavelength of 100 m. So- if you miss-identify a reflection by one wavelength, your depth will be off by 100 m!

  7. SEISMIC STRATIGRAPHY • Study of seismic reflection sections and their correlation to wells has greatly improved our understanding of sedimentary processes and the history of sea level change around the world. The Vail Hypothesis (*** look it up****), that sea level changes have occurred world wide, was based mainly on the correlation of reflection records and drilling. • Stratagraphic features are some of the most easy to determine on reflection profiles. Features such as unconformities, pinch-outs, onlap, offlap, etc. are usually easily recognized. Facies changes are more difficult, but an experienced interpreter will recognize the signs of reefal material and other commonly encountered deposits.

  8. Characteristics of faults on reflection profiles sharply terminated reflections – resuming across the fault Some faults may have a throw far less than half a wavelength but they will still be visible because of trace offsets across the fault:

  9. and because of diffractions: Watch out for distortion! Where is the fault plane? Red or Green line?

  10. deterioration of bedding beneath fault is common: flowerpot structures (common in strike-slip areas) This fault has almost no vertical throw but it bifurcates near the surface

  11. miss-ties: “Ties” are where a particular important reflector is traced around a box of profiles. The reflection on the last profile should be at the same depth as it is on the first profile where the two profiles cross: The heavy line represents a reflector that has been traced on four crossing profiles. The mis-tie in the heavier reflector (circle) is the discrepancy between where the reflector should be and where it is observed. It may be indicative of an undetected fault.

  12. Folding is nearly always the indication of a compressional environment, with some exceptions in areas where salt tectonics dominate. Folding is easily recognized on reflection records

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