Wednesday, January 11, 2017

Seismic Sherlock

When one works with students teaching seismic interpretation it can be a sinking feeling. All the basic elements get covered in the right order. Faults are picked, horizons are tracked, amplitude is extracted, attributes are applied. High tech, lots of workflow, parameters and button pushing. Then a question comes up and the student answer makes it clear something is missing. Perhaps after picking 20 faults the student has no clue of the structural style. Or noise is closely interpreted to have geologic meaning. Of course every mistaken concept is discussed and corrected, but it leaves the teaching like someone standing before a crashing surf having explained a few pebbles.

I've been thinking there may be a better way to jump start the learning process about seismic interpretation. No cubes, or N-level software applications, not even a colored pencil. Perhaps we need to think of a seismic section like a dead body in the morgue and the teacher is like one of those Victorian doctors who lectures to a classroom in the round as his assistant dissects the corpse, or  Sherlock explaining the geological implications and geophysical limitations of a seismic section...

Here is the patient (click to enlarge).
Data fade at the top of the section is due to shallow event muting and subsequent loss of CMP fold in the gather that has been summed to make each poststack trace we are interpreting. The mute cut is uneven, as evidenced by the irregular depth (that is, time) of the fade. Any interpretation in the mute zone is extremely hazardous, amplitudes are unreliable, structure is uncertain because missing offsets in the gathers make velocity analysis highly suspect. Only fools and lunatics interpret in the mute zone. Note the strong event at 250 ms on the left end of the line is undisturbed, no obvious lateral time or amplitude disruptions. At the same depth on the right side are lateral amplitude bursts with a 'sting of pearls' look. This is characteristic of spatial aliasing that arrises from poststack traces that are too far apart for the frequency and velocity at that location. The spatial aliasing effect in this area of the data is doubtless due to shallow (and therefore low) velocities, the effect largely subsiding by 400 ms.

Revisiting the strong 250 ms event, is shows as a peak-over-trough waveform, assuming black is positive amplitude and polarity is SEG normal. If it is further assumed that the seismic processor has done the job correctly and the wavelet is zero phase, it follows that the 250 ms event must be a seismic thin bed. A thin bed with zero phase wavelet, it is to be recalled, has the appearance of a 90 degree phase shifted wavelet, or a positive-over-negative as observed here. To quantify a bit with 10000 ft/s velocity and 55 Hz dominant frequency in this zone, the vertical resolution is about 45 ft, so the waveform of this event indicates it is less than 45 ft thick. Yet this event is very bright. With some basic investigation, one finds the surface geology is Pennsylvanian sandstone and shale, in short clastics. The possible lithologies that could give a strong reflection for a thin bed in an otherwise clastic section are anhydrite or carbonate (limestone or dolomite). Most likely a limestone, but well verification is needed.

To be continued...

Friday, December 2, 2016

Fall 2016 Semester Party

In keeping with my tradition of having a semester party for every class I teach, GEOS 4533 Petroleum Geophysics students gathered at Bear's Place in south Fayetteville for some refreshment and camaraderie.

Students, my wife Dolores (far right) and the professor.

The Bear himself telling a story in the kitchen. Great time thanks!