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Entries in geophysics (88)

Tuesday
Sep172013

Ten ways to make a difference

SEG WikiAfter reading my remarks yesterday about geoscience wikis, perhaps you're itching to share some of what you know. Below are ten quick ways to get started. And if you're going to SEG next week, you're in luck: you'll find a quick way to get started. 

Ten things you can do

First, if you really just want to dive in, here are ten easy things you can do in almost any wiki. Let's use SEG Wiki as an example — but this applies equally well to SubSurfWiki, PetroWiki, or Wikipedia.

  1. Read it — find a page or category that interests you, and start exploring the content
  2. Edit it — nothing tricky, but if you find a typo or other small error, hit Edit and fix it (you can do this without logging in on Wikipedia, but most other wikis require you to make an account first. This isn't usually a deliberate effort to put you off — allowing anonymous editing results in an amazing amount robot spam. Yes, robot spam.)
  3. Share it — like most of the web, wikis need to be shared to survive. When you find something useful, share it.
  4. Add a profile — if you're an SEG member, you already have an account on SEG Wiki. Why not add some info about yourself? Go log in to SEG.org then click this link. Here's mine
  5. Add a sandbox — Edit your user page, add this: [[/Sandbox/]], then save your page. You'll see a red link. Click on it. Try some editing — you can do anything you like here. Again, here's mine — click Edit and copy my code. 
  6. Fix equations — most of the equations in the SEG Encyclopedic Dictionary are poorly formatted. If you know LaTeX, you can help fix them. Here's one that's been fixed. Here's a bad one (if it looks OK, someone beat you to it :)
  7. Add references — Just like technical papers, wikis need citations and references if they are to be useful and trusted. Most articles in SEG Wiki have citations, but the references are on another page. Here's one I've fixed. 
  8. Add a figure — Again, the figures are mostly divorced from their articles. The Q article shows one way to integrate them. Some articles have lots of figures. 
  9. Improve a definition — Many of the Dictionary definitions are out of date or unhelpfully terse. Long articles probably belong in the 'main' namespace (that is, not the Dictionary part) — so for example I split Spectral decomposition into a main article, apart from the short dictionary definition.
  10. Add an article — This may seem like a big step, but don't be shy. Be bold! We can worry later if the new article needs to be split or combined or renamed or reformatted. The point is to start.

Wiki markup takes a little getting used to, but you can get a very long way with a little know-how. This wiki markup cheatsheet will give you a head start.

One place you can start

SEG Annual MeetingAt the SEG Annual Meeting next week, I'll be hanging about the Press Room from 11 am till 1 pm every day, with John Stockwell, Karl Schleicher and some other wiki enthusiasts. We'd be happy to answer any questions or help you get started.

Bring your laptop! Spread the word! Bring a friend! See you there!

Wednesday
Sep112013

Seismic quality traffic light

We like to think that our data are perfect and limitless, because experiments are expensive and scarce. Only then can our interpretations hope to stand up to even our own scrutiny. It would be great if seismic data was a direct representation of geology, but it never is. Poor data doesn't necessarily mean poor acquisition or processing. Sometimes geology is complex!

In his book First Steps in Seismic Interpretation, Don Herron describes a QC technique of picking a pseudo horizon at three different elevations to correspond to poor, fair, and good data regions. I suppose that will do in a pinch, but I reckon it would take a long time, and it is rather subjective. Surely we can do better?

Computing seismic quality

Conceptually speaking, the ease of interpretation depends on things we can measure (and display), like coherency, bandwidth, amplitude strength, signal-to-noise, and so on. There is no magic combination of filters that will work for all data, but I am convinced that for every seismic dataset there is a weighted function of attributes that can be concocted to serve as a visual indicator of the data complexity:

So one of the first things we do with new data at Agile is a semi-quantitative assessment of the likely ease and reliability of interpretation.

This traffic light display of seismic data quality, corendered here with amplitude, is not only a precursor to interpretation. It should accompany the interpretation, just like an experiment reporting its data with errors. The idea is to show, honestly and objectively, where we can trust eventual interpretations, and where they not well constrained. A common practice is to cherry pick specific segments or orientations that support our arguments, and quietly suppress those that don't. The traffic light display helps us be more honest about what we know and what we don't — where the evidence for our model is clear, and where we are relying more heavily on skill and experience to navigate a model through an area where the data is unclear or unconvincing.

Capturing uncertainty and communicating it in our data displays is not only a scientific endeavour, it is an ethical one. Does it change the way we look at geology if we display our confidence level alongside? 

Reference

Herron, D (2012). First Steps in Seismic Interpretation. Geophysical Monograph Series 16. Society of Exploration Geophysicists, Tulsa, OK.

The seismic profile shown in the figure is from the Kennetcook Basin, Nova Scotia. This work was part of a Geological Survey of Canada study, available in this Open File report.

Monday
Jul222013

Invitation to a geophysics hackathon

Do you like to build things? Join us for two days of scientific software creation. We'll be in Houston on 21 & 22 September, right before the SEG Annual Meeting, building web and mobile apps to attack one of the unsolved problem themes we exposed in Calgary in May — error and uncertainty

Let's build something together

What displays, or calculators, or simulators, could you dream up to help understand, or compute, or visualize, or communicate, or reduce error and uncertainty in your work? How about stochastic synthetics? Well logs with error bars? Fuzzy inversion?

You don't have to be a programmer — teams need ideas, they need science, they need design, and they need presentation skills. Please bring your creativity and your courage. Bring whatever you have, but mainly your brain

I'm in, what now? 

If want to take part, sign up at hackathon.io. If you have an idea already, start a project there. We are still filling in some blanks but can say that the event will be in downtown Houston, starting at 9 am on Saturday 21 Sept and running till 6 pm on Sunday 22 Sept. It will be free for students; there will probably be a small fee for professionals. We can only take 8 teams, so get in early and be sure not to miss out! 

If you'd like to help make this event happen, we'd love to have you as a sponsor for the event. The two main opportunities for sponsorship are the catering, and the prizes, but we're open to ideasDrop us a line.

Last thing: Please share this post with someone you know who loves to make things. Or help spread us on social media with the hashtag #geophysicshack. Cheers!

Thursday
Jul182013

Great geophysicists #8: d'Alembert


Jean-Baptiste le Rond d'Alembert was a French mathematician, born on 16 or 17 November 1717 in Paris, and died on 29 October 1783, also in Paris. His father was an artillery officer, but his mother was much more interesting. Having been a nun, she sought papal dispensation in 1714 for a new career as a fun-loving socialite, benefiting from the new government banknote printing scheme of John Law. She left her illegitimate child on the steps of Église St Jean Le Rond de Paris, whence he was taken to an orphanage. When his father returned from duty, he arranged for the boy's care.

Perhaps d'Alembert's greatest contribution to the world was helping Denis Diderot 'change the way people think' by editing the great Encyclopédie, ou Dictionnaire raisonné des sciences, des arts et des métiers of 1751. There were many contributors, but d'Alembert was listed as co-editor on the title page (left). This book was an essential ingredient in spreading the Enlightenment across Europe, and d'Alembert was closely involved in the project for at least a decade. 

But that's not why he's in our list of great geophysicists. As I mentioned when I wrote about Euler, d'Alembert substantially progressed the understanding of waves, making his biggest breakthrough in 1747 in his work on vibrating strings. His paper was the first time the wave equation or its solution had appeared in print:

Though Euler and d'Alembert corresponded on waves and other matters, and strongly influenced each other, they eventually fell out. For example, Euler wrote to Lagrange in 1759:

d'Alembert has tried to undermine [my solution to the vibrating strings problem] by various cavils, and that for the sole reason that he did not get it himself... He thinks he can deceive the semi-learned by his eloquence. I doubt whether he is serious, unless perhaps he is thoroughly blinded by self-love. [See Morris Kline, 1972]

D'Alembert did little mathematics after 1760, as he became more involved in other academic matters. Later, ill health gradually took over. He lamented to Lagrange (evidently an Enlightenment agony aunt) in 1777, six years before his death:

What annoys me the most is the fact that geometry, which is the only occupation that truly interests me, is the one thing that I cannot do. [See Thomas Hankins, 1970]

I imagine he died feeling a little hollow about his work on waves, unaware of the future impact it would have—not just in applied geophysics, but in communication, medicine, engineering, and so on. For solving the wave equation, d'Alembert, we salute you.

References

Read more on Wikipedia and The MacTutor History of Mathematics.

D'Alembert, J-B (1747). Recherches sur la courbe que forme une corde tenduë mise en vibration. (Researches on the curve that a tense cord forms [when] set into vibration.) Histoire de l'académie royale des sciences et belles lettres de Berlin, vol. 3, pages 214–219. Read on Google Books, with its sister paper, 'Further researches...'.

Portrait is a pastel by Maurice Quentin de La Tour, 1704–88.

Tuesday
May072013

A revolution in seismic acquisition?

We're in warm, sunny Calgary for the GeoConvention 2013. The conference feels like it's really embracing geophysics this year — in the past it's always felt more geological somehow. Even the exhibition floor felt dominated by geophysics. Someone we spoke to speculated that companies were holding their geological cards close to their chests, but the service companies are still happy to talk about (ahem, promote) their geophysical advances.

Are you at the conference? What do you think? Let us know in the comments.

We caught about 15 talks of the 100 or so on offer today. A few of them ignited the old whines about half-cocked proofs of efficacy. Why is it still acceptable to say that a particular seismic volume or inversion result is 'higher resolution' or 'more geological' with nothing more than a couple of sections or timeslices as evidence?

People are excited about designing seismic acquisition expressly for wavefield reconstruction. In a whole session devoted to the subject, for example, Mauricio Sacchi showed how randomization helps with regularization in processing, allowing us to either get better image quality, or to lower cost. It feels like the start of a new wave of innovation in acquisition, which has more than its fair share of recent innovation: multi-component, wide azimuth, dual-sensor, simultaneous source...

Is it a revolution? Or just the fallacy of new things looking revolutionary... until the next new thing? It's intriguing to the non-specialist. People are talking about 'beyond Nyquist' again, but this time without inducing howls of derision. We just spent an hour talking about it, and we think there's something deep going on... we're just not sure how to articulate it yet.

Unsolved problems

We were at the conference today, but really we are focused on the session we're hosting tomorrow morning. Along with a roomful of adventurous conference-goers (you're invited too!), looking for the most pressing questions in subsurface science. We start at 8 a.m. in Telus 101/102 on the main floor of the north building.

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