Author: Mike Cline, T/X Resources
This was initially going to be the fourth posting in my seismic inversion-related series (see the 03/08/08, 03/12/08, and 03/16/08 postings), with the title “Seismic Inversion—Frequency Sensitivity Analysis”. However, after thinking about the subject for a while, I decided to expand the scope, and shorten the title a bit, to be more general in nature. After all, a study of frequency-related seismic responses can not only be applied to inversion, but can also be used to illustrate the complications of seismic correlations between different datasets, as well as why spectral decomposition can better highlight a variety of seismic features at different frequencies.
The image below is a series of synthetic seismograms which resulted from the convolution with four different zero-phase wavelets—the wavelets and frequencies are posted at the top of the display. Since they were going to be the input for inversion examples, the synthetics are all relative amplitude (ie. no AGC, or Automatic Gain Control amplitude equalization).
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Author: Mike Cline, T/X Resources
I recently wondered why I hardly ever see anyone using the Seismic Inversion tool, found in SMT’s TracePak module? Maybe you’ve thought about using it, but didn’t understand it well enough, or maybe tried it once, and the results didn’t match anything in the well(s). Like anything new, if you don’t under- stand it, it’s going to be difficult to use it properly. So, I thought that it would be helpful to explain some of the benefits and pitfalls of using inversion, for those interested.
First, for those unfamiliar with inversion, what is it, and how do we use it in our interpretation? You could think of seismic inversion as the reverse of a synthetic seismogram processing flow, and we use inversion to get some idea about rock properties. For an example, the portion of the seismic inversion in the image below, was generated from a synthetic seismogram in the well at the center of the line. Normally, you would generate the inversion from an actual seismic line, but I wanted an optimum response for this example. I’ve also posted three well logs from this well: the spontaneous potential (aka. SP log) in blue, the deep resistivity (RES log) log in magenta, and the acoustic impedance (AI log) in red.
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Author: Mike Cline, T/X Resources
Here are the results from another one of my Do-It-Yourself spreadsheet applications. This one creates a pseudo-shear log from a sonic log, using John Castagna’s “Mudrock Line” technique.
Often, shear sonic logs are difficult to acquire, so we may end up using a “default” shear log suggested by our AVO programs, which commonly use either a Poisson’s Ratio of 0.25, or a Vp/Vs ratio of about 2.0 to calculate the shear log.
However, there is a much better way, which Castagna illustrates in his classic 1985 paper (long title) “Relationships Between Compressional-Wave and Shear-Wave Velocities in Clastic Silicate Rocks”. In his article, he demonstrates that there is an excellent linear relationship between the compressional, and shear velocities of clastic silicate rocks, a technique which I have used many times, with successful end-results for many of my clients.
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Author: Mike Cline, T/X Resources
The Amplitude versus Angle (AVA) graph below was derived from another spreadsheet that I imported into EditGrid (see the previous Online Spread- sheet Calculations posting).
For the original spreadsheet, I used the Shuey equation from his classic AVO paper, “A Simplification of the Zoeppritz Equations”, published in Geophysics in 1985. The Shuey equation has been found to produce a good approxi- mation of results (up to about 30 degrees) that can be achieved using the more complex Zoeppritz Equation, and is much easier to program in a spreadsheet, than the latter.
The red and blue curves in the image below, represent the calculated AVA response of a simple three-layer model—in this case, a gas sand, with shales above and below the sand. However, in this example, I elected to “push the limits” and generated values up to an angle of 45 degrees. With Zoeppritz values posted in the pink, and cyan circles, you can see that the Shuey values compare quite well with the Zoeppritz values, even past 30 degrees in this case.
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Author: Mike Cline, T/X Resources
For those of you who may be interested in computer hardware technology, read on and I’ll explain:
Most of us use computers in our current jobs, right (duhh…, we are SMT users after all)? It’s also well established that the continual advances in computer hardware technology allows us to crunch more geoscience data, and do it easier, faster and cheaper by the day. However, what may be less well known by some, is that video gamers are currently helping to drive much of the hardware advances in their quest for faster computers, and video cards. Being able to display more realistic scenes, and action, in their games may be helping keep many of us employed (for now)!
