The future is uncertain

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Image: Repsol, SEG. Click for the abstract.

SEG Day 2. In the session entitled Exploration and Uncertainty Analysis, I was underwhelmed with the few talks that I attended, except for the last one of the session entitled, Measuring time-map uncertainty. 

Static uncertainty

It is commonly uttered that different data processing companies will produce different results; seismic processing is non-unique, and so on. But rarely do I get to see real examples of the kind of variances that can occur. Bruce Blake from Repsol showed seismic imaging results that came back from a number of contractors. The results were truly shocking. The example he showed was an extreme case of uncertainty caused by inadequate static solutions caused by the large sand dunes in Libya. The key point for me is exemplified by the figure shown on the right: the image from one vendor suggests a syncline, the image from the other suggest an anticline. Beware!

A hole in the theory

In the borehole sonic session, Xinding Fang, a student from MIT, reinforced a subtle but profound idea: it is tricky to measure the speed of sound in a rock when you drill a hole into it. The hole changes the stress field, and induces an anisotropic stiffness around the circumference of the borehole where sonic tools make their measurements. And since waves take the shortest travel path from source to receiver, speeds that are measured in the presence of an artificial stress are wrong.

Image: Xindang Fang, SEG. Click for the abstract.

The bigger issue here that Xinding has elucidated is that we routinely use sonic logs to make time-depth relationships and tie wells, especially in the absence of a check-shot survey. If it works, it works, but if ever discrepancies exists between seismic and well, the interpreter applies a stretch or a squeeze without much thought. Some may blame the discrepancy on dispersion alone, but that's evidently too narrow. Indeed, we rarely bother to investigate the reasons.

There's a profound point here. We have to drop the assumption that logs are the 'geological' truth upon which to hang an interpretation. We have to realize that the act of making the measurement changes the very thing we want to measure. 

Past, present, future SEG

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Today was the first day of the SEG Annual Meeting in Houston. 

Last night we wandered around the icebreaker, still buzzing from the hackathon. The contrast was crushing. The exhibition is gigantic — it's an almost overwhelming amount of marketing. My thoughts on what the exhibition hall is, and what it represents, are not fully formed and might be a bit... ranty, so I will save them for a more considered post. 

As usual, SEG kicked off with a general session — much better attended this year, but also much less ambitious. At least 300 members came to hear outgoing president David Monk's perspective on SEG's future. His address mostly looked backwards, however, at the trends over the last few years. I guess the idea is to extrapolate from there... But maybe we can do even better than recent years? We mustn't forget to do completely new and unexpected things too. 

At the end of his slot, Monk showed some animated renderings of SEG's new building in Tulsa. The movie was accompanied by an almost comically strident anthem — evidently it is a big deal. As well as having a smart new office, the real estate will turn in some smart new revenue from other tenants. Ground was broken on Friday, and the opening is expected to be in December 2014. As you see, the architects understood industrial geophysics quite well, opting for a large black box. 

At the end of the day, Canada strode home to yet another SEG Challenge Bowl victory as the University of Manitoba fought off the Autonomous University of Mexico and Colorado School of Mines to prove that, while Texas might be the home of the industry, Canada is the home of exploration geophysics. 

Where's all the geophysics? Evan is compiling some technical highlights from the day as I type. Stay tuned for that. 

If you're at the conference, tell us what you've enjoyed most about the first 24 hours.

Looking forward to SEG 2013

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The SEG Annual Meeting is coming! The program starts tomorrow with the DISC, and continues over the weekend with various other courses. It's not part of the conference, but we're looking forward to the Geophysics Hackathon, obviously. Curious? You're welcome to drop in.

The meeting boasts 124 technical sessions totalling over 1000 PowerPoint presentations. If you haven't looked at the list of expanded abstracts yet, I can't blame you, it's a massive amount of content and the website experience is, er, not optimal — and there's no helpful mobile app this year. [Update: The app came out today! Go get it, it's essential. Thank you Whitney at SEG for letting us know.] I've tried to pick out a few sessions that seem really exciting below.

Worst. App. Ever.Each day at 10:30 am, I will be doing a guest presentation at the Enthought booth, showing some novel geophysics tools that I've been making. They are powered by Python and Enthought's Canopy environment. Come by and I will show you that you can too! However, I need somebody to please go to this exhibition booth 'browser' and show me where the Enthought booth is. Worst. App. Ever.

And each day at 11 am, there's a 2-hour mini-wikithon. Stop by the Press Room for a quick tour of SEG Wiki, and find out how you can help make it better.

Monday

With no technical presentations on Monday morning, it is safe to assume that most delegates will be wandering around the exhibition hall. A few may trickle over to the unenticing Opening Session which Matt and I found was horribly attended last year and the year before. Matt at least will be there, mostly out of morbid curiosity.

Continuing the Hackathon's theme on error and uncertainty, I will be diving into the session on Monday afternoon called

From 3-6 pm be sure to check out the always popular SEG Student Challenge Bowl. The global finals, are hosted by the crowd-pleasing past SEG president (and fellow Canadian) Peter Duncan. Top pairings from Universities across the world duking it out in a button-pushing quiz show. Come out, cheer on the students and test your own geophysics trivia from the audience.

Tuesday

The sessions that look appealing to me on Tuesday are

Wednesday

Agile's good friend Maitri Erwin is the instigator behind the Women's Networking Breakfast. All are welcome; consider yourself lucky to connect with Maitri. As for talks, I will try make an appearance at

The first one I know quite a bit about, but can always use a refresher, and the second one I know very little about, but it's been a hot topic for 3 or 4 years now. If we aren't worn out at the end of the day, we might find some tickets to the Bayou Bash.

Thursday & Friday

There are over a dozen workshops on both Thursday and Friday. As far as I can tell, they are basically more talks, each around a central theme. Don't ask me how this is in any way distinguishable from the technical program, and there is still a full suite of technical sessions conflicting on Thursday morning. It's a shame because I'm curious to attend the session Fractures, shale and well-log characterization but I don't want to miss Workshop 2, Grand challenges, which takes place all day on Thursday. Then on Friday there's Characterizing fractures (Workshop 15).

There are many other events going on, so if you see something good, make sure you tweet the rest of us about it: @EvanBianco, @kwinkunks, @maitri, @toastar, and lots of others — follow hashtag #SEG13. (Not #SEG2013, that's all marketing wonks).

If you'll be at the Annual Meeting, do look out for us, we'd love to meet you. If you won't be there, tell us what you'd like to hear about. News from the exhibition? Our favourite talks? Detailed minutes from the committee meetings? Let us know in the comments.

Wiki world of geoscience

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This weekend, I noticed that there was no Wikipedia article about Harry Wheeler, one of the founders of theoretical stratigraphy. So I started one. This brings the number of biographies I've started to 3:

  • Karl Zoeppritz — described waves almost perfectly, but died at the age of 26
  • Johannes Walther — started as a biologist, but later preferred rocks
  • Harry Wheeler — if anyone has a Wheeler diagram to share, please add it!

Many biographies of notable geoscientists are still missing (there are hundreds, but here are three): 

  • Larry Sloss — another pioneer of modern stratigraphy
  • Oz Yilmaz — prolific seismic theoretician and practioner
  • Brian Russell — entrepreneur and champion of seismic analysis

It's funny, Wikipedia always seems so good — it has deep and wide content on everything imaginable. I think I must visit it 20 or 30 times a day. But when you look closely, especially at a subject you know a bit about, there are lots of gaps (I wonder if this is one of the reasons people sometimes deride it?). There is a notability requirement for biographies, but for some reason this doesn't seem to apply to athletes or celebrities. 

I was surprised the Wheeler page didn't exist, but once you start reading, there are lots of surprises:

I run a geoscience wiki, but this is intended for highly esoteric topics that probably don't really belong in Wikipedia, e.g. setting parameters for seismic autopickers, or critical reviews of subsurface software (both on my wish list). I am currently working on a wiki for AAPG — is that the place for 'deep' petroleum geoscience? I also spend time on SEG Wiki... With all these wikis, I worry that we risk spreading ourselves too thinly? What do you think?

In the meantime, can you give 10 minutes to improve a geoscience article in Wikipedia? Or perhaps you have a classful of students to unleash on an assignment?

Tomorrow, I'll tell you about an easy way to help improve some geophysics content.

Seismic quality traffic light

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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.

Colouring maps

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Over the last fortnight, I've shared five things, and then five more things, about colour. Some of the main points:

  • Our non-linear, heuristic-soaked brains are easily fooled by colour.
  • Lots of the most common colour bars (linear ramps, bright spectrums) are not good choices.
  • You can learn a lot by reading Robert Simmon, Matteo Niccoli, and others.

Last time I finished on two questions:

  1. How many attributes can a seismic interpreter show with colour in a single display?
  2. On thickness maps should the thicks be blue or red?

One attribute, two attributes

The answer to the first question may be a matter of personal preference. Doubtless we could show lots and lots, but the meaning would be lost. Combined red-green-blue displays are a nice way to cram more into a map, but they work best on very closely related attributes, such as seismic amplitude of three particular frequencies. 

Here's some seismic reflection data — the open F3 dataset, offshore Netherlands, in OpendTect. 

A horizon — just below the prominent clinoforms — is displayed (below, left) and coloured according to elevation, using one of Matteo's perceptual colour bars (now included in OpendTect!). A colour scale like this varies monotonically in hue and luminance.

Some of the luminance channel (sometimes called brightness or value) is showing elevation, and a little is being used up by the 3D shading on the surface, but not much. I think the brain processes this automatically because the 3D illusion is quite good, especially when the scene is moving. Elevation and shape are sort of the same thing, so we've still only really got one attribute. Adding contours is quite nice (above, middle), and only uses a narrow slice of the luminance channel... but again, it's the same data. Much better to add new data. Similarity (a member of the family that includes coherence, semblance, and so on) is a natural fit: it emphasizes a particular aspect of the shape of the surface, but which was measured independently of the interpretaion, directly from the data itself. And it looks awesome (above, right).

Three attributes, four

OK, we have elevation and/or shape, and similarity. What else can we add? Another intuitive attribute of seismic is amplitude (below, left) — closely related to the strength of the reflected energy. Two things: we don't trust amplitudes in areas with low fold — so we can mask those (below, middle). And we're only really interested in bright spots, so we can edit the opacity profile of the attribute and make low values transparent (below, right). Two more attributes — amplitude (with a cut-off that reflects my opinion of what's interesting — is that an attribute?) and fold.

Since we have only used one hue for the amplitude, and it was not in Matteo's colour bar, we can layer it on the original map without clobbering anything. Unfortunately, there's no easy way for the low fold mask to modulate amplitude without interfering with elevation, because the elevation map needs to be almost completely opaque. What I need is a way to modulate a surface's opacity with an attribute it is not displaying with hue...

Thickness maps

The second question — what to colour thicks — is easy. Thicks should be towards the red end of the spectrum, sometimes not-necessarily-intuitively called 'warm' colours. (As I mentioned before in the comments, a quick Google image poll suggests that about 75% of people agree). If you colour your map otherwise, perhaps because you like the way it suggests palaeobathymetry in some depositional settings, be careful to make this very clear with labels and legends (which you always do anyway, right?). And think about just making a 'palaeobathymetry' map, not a thickness map.

I suspect there are lots of quite personal opinions out there. Like grammar, I do think much of this is a matter of taste. The only real test is clarity. Do you agree? Is there a right and wrong here? 

Five more things about colour

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Last time I shared some colourful games, tools, and curiosities, including the weird chromostereopsis effect (right). Today, I've got links to much, much more 'further reading' on the subject of colour...

The provocation for this miniseries was Robert 'Blue Marble' Simmon's terrific blog series on colour, which he's right in the middle of. Robert is a data visualization pro at NASA Earth Observatory, so we should all listen to him. Here's his collection (updated after the original writing of this post):

Perception is everything! One of Agile's best friends is Matteo Niccoli, a quantitative geophysicist in Norway (for now). And one of his favourite subjects is colour — there are loads of great posts on his blog. He also has a fine collection of perceptual colour bars (left) for most seismic interpretation software. If you're still using Spectrum for maps, you need his help.

Dave Green is a physicist at the University of Cambridge. Like Matteo, he has written about the importance of using colour bars which have a linear increase in perceived brightness. His CUBEHELIX scheme (above) adapts easily to your needs — try out his colour bar creator. And if this level of geekiness gets you going, try David Dalrymple or Gregor Aisch.

ColorBrewer is a legendary web app and add-in for ArcGIS. It's worth playing with the various colour schemes, especially if you need a colour bar that is photocopy friendly, or that can still be used by colour blind people. The equally excellent, perhaps even slightly more excellent, i want hue is also worth playing with (thanks to Robert Simmon for that one). 

In scientific publishing, the Nature family of journals has arguably the finest graphics. Nature Methods carries a column called Points of View, which looks at scientific visualization. This mega-post on their Methagora blog links to them all, and covers everything from colour and 3D graphics to broader issues of design and typography. Wonderful stuff.

Since I don't seem to have exhausted the subject yet, we'll save a couple of practical topics for next time:

  1. A thought experiment: How many attributes can a seismic interpreter show with colour in a single display?
  2. Provoked by a reader via email, we'll think about that age old problem for thickness maps — should the thicks be blue or red?

Five things about colour

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The fact that colour is a slippery subject is powerfully illustrated by my favourite optical illusion. Look at this:

Squares A and B are the same shade of grey. It's so hard to believe that you might need to see the proof to be convinced. 

Chromostereopsis is a similarly disarming effect that you may have noticed on maps with bright spectrum colour bars. Most people perceive blue and red on different depth planes, so the pseudo-3D effect can work in your favour and make the map 'pop' (This is not a good reason to use a spectrum colour bar, however... more on this next time). I notice that at least one set designer knows about the effect, making William Shatner pop on the TV show Have I Got News For You:

Color is a fun way to test your colour intuition. The game starts easy, but is very hard by the end as you simulatneously match colour tetrads. The first time I played I managed 9.8, which I am not-very-secretly quite pleased about. But I haven't been able to repeat the performance.

X-Rite's Online Color Challenge is also tough. You have to sort the very subtle colours into order. It takes a while to play but is definitely worth it. If your job depends on spotting subtle effects in images (like seismic data, for example) then stand by to learn something about your detection system. 

Color blindness will change how these games work, of course, and should change how we make maps, figures, and slides. Since up to about 5% of a large audience might be colour blind, you might want to think about how your presentations look to them. You can easily check with Vischeck and correct images for colourblind people with the Daltonizer. They can still be beautiful, but you can avoid certain colour combinations and reach a wider audience.

I have lots more links about colour to share in the next post, including some required reading from Rob Simmon and Matteo Niccoli, among others. In the meantime, have you come across any handy colour tools, or has colour ever caught you out? Let us know in the comments.

The image of William Shatner is copyright and courtesy of Hat Trick Productions Ltd, London, UK, and used with permission.

First appearance datum at Green Point

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Armed with the Geologic Field Guide of Newfoundland, last week I ventured to one of the most intensely scrutinized outcrops in the world. Green Point in Gros Morne National Park provides continuous exposure to more than 30 million years of sediment accumulation in the Iapetus ocean. The rocks formed in deep water near the base of the ancient continental slope. It was awesome and humbling.

In January 2000, the International Union of Geological Sciences designated Green Point as a Global Stratotype Section and Point (GSSP). That's an official international reference point for the geologic time scale. I learned after the fact that there are only a handful of these in North America.

Researchers and students at Memorial University and elsewhere studied more than 10,000 fossils from Green Point, using tiny conodonts and delicate graptolites to locate the boundary between the Cambrian and Ordovician periods, 488 Ma in the past. They have narrowed it down to a single layer, Bed 23, that contains the first appearance of the conodant species, Iapetognathus fluctivagus.

To the best of my estimatation, I have indicated the location of Bed 23 with the white dashed line in the figure to the right, and with the pointing figure of my *ahem* geologic scale marker in the photograph below.

Snapshots from the Outcrop

Being the massive natural exhibition that it is, there are likely volumes of things to observe and measure at Green Point. I had no agenda whatsoever, but here are four observations that caught my interest:

  1. Cavities from core plugs at regularly spaced intervals. Each piece taken and studied as part of an international scientific experiment, aimed at accurately identifying major turning points in earth's history. 
  2. Small scale fault with some antithetic joints reminiscent of some artifacts I have seen on seismic.
  3. and 4. A faulted limestone conglomerate bed. Shown from two different points of view. I am increasingly curious about the nature of the aperture of deformation zones. Such formidable forces, such a narrow region of strain.

I left with a feeling that I am sure is felt by most geologists leaving a site of extreme interest. Did I make enough observations? Did I collect enough data? I wish I had a GigaPan, or maybe portable LiDAR station. I feel reconnected to the vastness of scales over which earth processes occur, and the heterogeneity caused by well-understood systems playing out over inconceivable expanses of time. 

I'd like to flip the outcrop 120° counterclockwise, and build another stupid seismic model. What could mathematicians, programmers, and geoscientists do at this outcrop? A digital playground for integration awaits.

Great geophysicists #8: d'Alembert

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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.