Entries in reproducibility (3)


The Blangy equation

After reading Chris Liner's recent writings on attenuation and negative Q — both in The Leading Edge and on his blog — I've been reading up a bit on anisotropy. The idea was to stumble a little closer to writing the long-awaited Q is for Q post in our A to Z series. As usual, I got distracted...

In his 1994 paper AVO in tranversely isotropic media—An overview, Blangy (now the chief geophysicist at Hess) answered a simple question: How does anisotropy affect AVO? Stigler's law notwithstanding, I'm calling his solution the Blangy equation. The answer turns out to be: quite a bit, especially if impedance contrasts are low. In particular, Thomsen's parameter δ affects the AVO response at all offsets (except zero of course), while ε is relatively negligible up to about 30°.

The key figure is Figure 2. Part (a) shows isotropic vs anisotropic Type I, Type II, and Type III responses:

Unpeeling the equation

Converting the published equation to Python was straightforward (well, once Evan pointed out a typo — yay editors!). Here's a snippet, with the output (here's all of it):

For the plot below, I computed the terms of the equation separately for the Type II case. This way we can see the relative contributions of the terms. Note that the 3-term solution is equivalent to the Aki–Richards equation.

Interestingly, the 5-term result is almost the same as the 2-term approximation.

Reproducible results

One of the other nice features of this paper — and the thing that makes it reproducible — is the unambiguous display of the data used in the models. Often, this sort of thing is buried in the text, or not available at all. A table makes it clear:

Last thought: is it just me, or is it mind-blowing that this paper is now over 20 years old?


Blangy, JP (1994). AVO in tranversely isotropic media—An overview. Geophysics 59 (5), 775–781.

Don't miss the IPython Notebook that goes with this post.


Why we should embrace openness

Openness—open ideas, open data, open teams—can help us build more competitive, higher performing, more sutainable organizations in this industry.

Last week I took this message to the annual convention of the three big applied geoscience organizations in Canada: the Canadian Society of Petroleum Geologists (CSPG), the Canadian Society of Exploration Geophysicist (CSEG), and the Canadian Well Logging Society (CWLS). Evan and I attended the conference as scientists, but also experimented a bit with live tweeting and event blogging.

The talk was a generalization of the talk I did in March about open source software in geoscience. I wasn't sure at all how it would go over, and spent most of the morning sitting in technical talks fretting about how flaky and meta my talk would sound. But it went quite well, and at least served as some light relief from the erudition in the rest of the agenda. It was certainly fun to give an opinion-filled talk, and it started plenty of conversations afterwards.

You can access a PDF of the visuals, with commentary, from the thumbnail (left).

What do you think? Is a competitive, secretive industry like oil and gas capable of seeing value in openness? Might regulators eventually force us to share more as the resources society demands become scarcer? Or are we doomed to more mistrust and secrecy as oil and gas become more expensive to produce?

← Click the image for the PDF (6.8M)



Newton didn't need open source, so why do you?Free and open source software is catalyzing a revolution in subsurface science. As a key part of the growing movement to open access to data, information, and the very process of doing science, open software is not just for the geeks. It's a party we're all invited to. 

I have been in California this week, attending a conference in Long Beach called Mathematical and Computational Issues in the Geosciences, organized by the Society of Industrial and Applied Mathematicians. In 2009 I started being more active in my search for lectures and courses that lie outside my usual comfort zone. I have done courses in reservoir engineering and Java programming. I have heard talks on radiology and financial forecasting. It's like being back at university; I like it.

How did I end up at this conference? Last spring, I wrote a little review article about open source software (available here at dGB Earth Science's site). It was really just a copy-edited version of notes I had made whilst looking for free geoscience software and reading up on the subject for my own interest. After some brushes with open source, I was curious about the history behind the idea, how projects are built, and how they are licensed. At the same time, I also started a couple of Wikipedia articles about free software in geology and geophysics, as a place to list the projects I had come across. Kristin Flornes, of IRIS in Stavanger, Norway, saw the article and her colleagues got in touch about the conference.

The talk, which you can access via the thumbnail (left) or look at in Google Docs, is part FLOSS primer, part geo-FLOSS advert, part manifesto for a revolution of innovation. I hope the speaker notes are sufficient. 

What do you think? Is software availability or architecture or capable of driving change, or is it just a tool, passive and inert?

← Click the image for the PDF (6.9M)