Author: Mike Cline, T/X Resources
It’s pretty common knowledge that sunlight is composed of many different frequencies (colors) , and we perceive the color of an object by the color(s) that is/are reflected from the object—that is, an object is red because all of the other colors are absorbed (or filtered out) by the object.
We also know that seismic data contains many different frequencies, usually within a range from about 6-120 Hz (Hertz, or cycles per second). Have you ever considered that seismic data can be similarly filtered to reveal its “colors”? I know that this is probably pretty basic stuff for most of you, but bear with me for a minute so that I can better illustrate how you can eliminate most of the seismic frequencies, to reveal hidden features. Some might call it the “poor man’s” spectral decomposition.
See the larger Adobe Reader pdf file (three pages).
Have you ever seen an Anaglyph? That’s one of those blurry, blue-and-red pictures that requires you to wear those silly-looking, cardboard glasses to view it properly. After you put on the glasses, a 3D picture appears. Here are a few from the Department of Geosciences, at North Dakota State University. You should break out your silly glasses, if you have a pair, for some pretty cool pictures on their site.
The glasses, with one red lens, and one blue lens (usually cheap plastic) were prominent in the 1950’s 3D movies, and work by filtering the red colors in one eye, and the blue colors in the other eye, allowing you to see two juxta- posed, but slightly different, pictures at once. If you alternately close either eye while looking through the glasses, you will see the different images. Fortunately, our brains subconsiously merge them together into a coherent picture.
This is the general idea for band-limited seismic interpretation. We filter out certain frequencies (colors), to reveal the remaining information that was buried in the original data. In the example above, are three versions of the same seismic line: (1) the final migration, (2) the migration with spectral whitening, and (3) the spectral whitening with a band-limited filter. The version with the band-limited filter in the example is actually ”high-pass” filtered—that is, the lower frequencies below about 30 Hz have been filtered out, leaving the higher frequencies up to about 60 Hz. The data is a little “ringy” (reverberating reflections) because the original data was filtered, so I wasn’t able to let in as much high frequencies as I would have wanted.
The areas highlighted within the blue ellipses are a few of the features that were revealed by filtering the lower frequencies out of the original seismic data. You can actually interpret, and map the band-limited data for improved resolution. However, since the frequency bandwidth of seismic data is time (or depth)-dependent (higher frequencies are filtered with increasing times, or depths), the amount of filtering that you select will change the character of the data. You will want to annotate any horizons that interpret on the filtered data with the filter settings, so that you can keep track of what you used.
You will need to do some filter testing, to decide what are the best filter settings. Also, when you determine the optimum frequency range within your zone of interest, be sure that the filter cutoff values are not too steep—usually an octave (a doubling of frequencies) is preferable, to prevent the Hanning effect (ringiness). For example, suppose your “raw” (unfiltered, ungained) migrated seismic contains a frequency range from 6-80 Hz within your zone of interest. You could try a filter setting, shaped like a trapezoid, of 20-40 / 50-80 Hz. That is, F1=20, F2=40, F3=50, and F4=80 Hz, where F1, and F4 are the zero amplitude points at the low, and high frequency cutoffs, respectively. The F2, and F3 points are the 100 percent amplitude points on the filter trapezoid.
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