Impact structures in seismic

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I saw this lovely tweet from PGS yesterday:

Kudos to them for sharing this. It’s always great to see seismic data and interpretations on Twitter — especially of weird things. And impact structures are just cool. I’ve interpreted them in seismic myself. Then uninterpreted them.

I wish PGS were able to post a little more here, like a vertical profile, maybe a timeslice. I’m sure there would be tons of debate if we could see more. But not all things are possible when it comes to commercial seismic data.

It’s crazy to say more about it without more data (one-line interpretation, yada yada). So here’s what I think.

Impact craters are rare

There are at least two important things to think about when considering an interpretation:

  1. How well does this match the model? (In this case, how much does it look like an impact structure?)

  2. How likely are we to see an instance of this model in this dataset? (What’s the base rate of impact structures here?)

Interpreters often forget about the second part. (There’s another part too: How reliable are my interpretations? Let’s leave that for another day, but you can read Bond et al. 2007 as homework if you like.)

The problem is that impact structures, or astroblemes, are pretty rare on Earth. The atmosphere takes care of most would-be meteorites, and then there’s the oceans, weather, tectonics and so on. The result is that the earth’s record of surface events is quite irregular compared to, say, the moon’s. But they certainly exist, and occasionally pop up in seismic data.

In my 2011 post Reliable predictions of unlikely geology, I described how skeptical we have to be when predicting rare things (‘wotsits’). Bayes’ theorem tells us that we must modify our assigned probability (let’s say I’m 80% sure it’s a wotsit) with the prior probability (let’s pretend a 1% a priori chance of there being a wotsit in my dataset). Here’s the maths:

\( \ \ \ P = \frac{0.8 \times 0.01}{0.8 \times 0.01\ +\ 0.2 \times 0.99} = 0.0388 \)

In other words, the conditional probability of the feature being a rare wotsit, given my 80%-sure interpretation, is 0.0388 or just under 4%.

As cool as it would be to find a rare wotsit, I probably need a back-up hypothesis. Now, what’s that base rate for astroblemes? (Spoiler: it’s much less than 1%.)

Just how rare are astroblemes?

First things first. If you’re interpreting circular structures in seismic, you need to read Simon Stewart’s paper on the subject (Stewart 1999), and his follow-up impact crater paper (Stewart 2003), which expands on the topic. Notwithstanding Stewart’s disputed interpretation of the Silverpit not-a-crater structure in the North Sea, these two papers are two of my favourites.

According to Stewart, the probability P of encountering r craters of diameter d or more in an area A over a time period t years is given by:

\( \ \ \ P(r) = \mathrm{e}^{-\lambda A}\frac{(\lambda A)^r}{r!} \)

where

\( \ \ \ \lambda = t n \)

and

\( \ \ \ \log n = - (11.67 \pm 0.21) - (2.01 \pm 0.13) \log d \)

Astrobleme_prob.png

We can use these equations to compute the probability plot on the right. It shows the probability of encountering an astrobleme of a given diameter on a 2400 km² seismic survey spanning the Phanerozoic. (This doesn’t take into account anything to do with preservation or detection.) I’ve estimated that survey size from PGS’s tweet, and I’ve highlighted the 7.5 km diameter they mentioned. The probability is very small: about 0.00025. So Bayes tells us that an 80%-confident interpretation has a conditional probability of about 0.001. One in a thousand.

Here’s the Jupyter notebook I used to make that chart using Python.

So what?

My point here isn’t to claim that this structure is not an astrobleme. I haven’t seen the data, I’ve no idea. The PGS team mentioned that they considered the possiblity of influence by salt or shale, and fluid escape, and rejected these based on the evidence.

My point is to remind interpreters that when your conclusion is that something is rare, you need commensurately more and better evidence to support the claim. And it’s even more important than usual to have multiple working hypotheses.

Last thing: if I were PGS and this was my data (i.e. not a client’s), I’d release a little cube (anonymized, time-shifted, bit-reduced, whatever) to the community and enjoy the engagement and publicity. With a proper license, obviously.

References

Hughes, D, 1998, The mass distribution of the crater-producing bodies. In Meteorites: Flux with time and impact effects, Geological Society of London Special Publication 140, 31–42.

Davis, J, 1986, Statistics and data analysis in geology, John Wiley & Sons, New York.

Stewart, SA (1999). Seismic interpretation of circular geological structures. Petroleum Geoscience 5, p 273–285.

Stewart, SA (2003). How will we recognize buried impact craters in terrestrial sedimentary basins? Geology 31 (11), p 929–932.

Are there benefits to pseudoscience?

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No, of course there aren't. 

Balance! The scourge of modern news. CC-BY by SkepticalScience.com

Balance! The scourge of modern news. CC-BY by SkepticalScience.com

Unless... unless you're a journalist, perhaps. Then a bit of pseudoscience can provide some much-needed balance — just to be fair! — to the monotonic barrage of boring old scientific consensus. Now you can write stories about flat-earthers, anti-vaxxers, homeopathy, or the benefits of climate change!*

So far, so good. It's fun to pillory the dimwits who think the moon landings were filmed in a studio in Utah, or that humans have had no impact on Earth's climate. The important thing is for the journalist to have a clear and unequivocal opinion about it. If an article doesn't make it clear that the deluded people at the flat-earth convention ("Hey, everyone thought Copernicus was mad!") have formed their opinions in spite of, not because of, the overwhelming evidence before them, then readers might think the journalist — and the publisher — agree with them.

In other words, if you report on hogwash, then you had better say that it's hogwash, or you end up looking like one of the washers of the hog.

Fake geoscience?

AAPG found this out recently, when the August issue of its Explorer magazine published an article by Ken Milam called Are there benefits to climate change? Ken was reporting on a talk by AAPG member Greg Wrightstone at URTeC in July. Greg wrote a book called Inconvenient Facts: The Science That Al Gore Doesn't Want You To Know. The gist: no need to be concerned about carbon dioxide because, "The U.S. Navy’s submarines often exceed 8,000 ppm (20 times current levels) and there is no danger to our sailors" — surely some of the least watertight reasoning I've ever encountered. Greg's basic idea is that, since the earth has been warmer before, with higher levels of CO2, there's nothing to worry about today (those Cretaceous conurbations and Silurian civilizations had no trouble adapting!) So he thinks, "the correct policy to address climate change is to have the courage to do nothing".

So far, so good. Except that Ken — in reporting 'just the facts' — didn't mention that Greg's talk was full of half-truths and inaccuracies and that few earth scientists agree with him. He forgot to remark upon the real news story: how worrying it is that URTeC 2018 put on a breakfast promoting Greg and his marginal views. He omitted to point out that this industry needs to grow up and face the future with reponsibility, supporting society with sound geoscience.

So it looked a bit like Explorer and AAPG were contributing to the washing of this particular hog.

Discussion

As you might expect, there was some discussion about the article — both on aapg.org and on Twitter (and probably elsewhere). For example, Mark Tingay (University of Adelaide) called AAPG and SPE out:

So did Brian Romans (Virginia Tech):

And there was further discussion (sort of) involving Greg Wrightstone himself. Trawl through Mark Tingay's timeline, especially his systematic dismantling of Greg's 'evidence', if your curiosity gets the better of you.

Response

Of course AAPG noticed the commotion. The September issue of Explorer contains two statements from AAPG staff. David Curtiss, AAPG Executive Director, said this in his column:

Milam was assigned to report on an invited presentation by Greg Wrightstone, a past president of AAPG’s Eastern Section, based on a recently self-published book on climate change, at the Unconventional Resources Technology Conference in July. Here was an AAPG Member and past section officer speaking about climate change – an issue of interest to many of our members, who had been invited by a group of his geoscience and engineering peers to present at a topical breakfast – not a technical session – at a major conference.

This sounds fine, on the face of it, but details matter. A glance at the book in question should have been enough to indicate that the content of the talk could only have been presented in a non-technical session, with a side of hash browns.

Anyway, David does go on to point out the tension between the petroleum industry's activities and society's environmental concerns. The tension is real, and AAPG and its members, are in the middle of it. We can contribute scientifically to the conversations that need to happen to resolve that tension. But pushing junk science and polemical bluster is definitely not going to help. I believe that most of the officers and members of AAPG agree. 

The editor of Explorer, Brian Ervin, had this to say:

For the record, none of our coverage of any issue or any given perspective on an issue should be taken as an endorsement — explicit or implicit — of that perspective. Also, the EXPLORER is — quite emphatically — not a scientific journal. Our content is not peer-reviewed. [...] No, the EXPLORER exists for an entirely different purpose. We provide news about Earth science, the industry and the Association, so our mission is different and unrelated to that of a scientific publication.

He goes on to say that he knew that Wrightstone's views are not popular and that it would provoke some reaction, but wanted to present it impartially and "give [readers] the opportunity to evaluate his position for themselves".

I just hope Explorer doesn't start doing this with too many other marginal opinions.

I'd have preferred to see AAPG back-pedal a bit more energetically. Publishing this article was a mistake. AAPG needs to think about the purpose, and influence, of its reporting, as well as its stance on climate change (which, according to David Curtiss, hasn't been discussed substantially in more than 10 years). This isn't about pushing agendas, any more than talking about the moon landings is about pushing agendas. It's about being a modern scientific association with high aspirations for itself, its members, and society.

Once is never

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Image by ZEEVVEEZ on Flickr, licensed CC-BY. Ten points if you can tell what it is...


Image by ZEEVVEEZ on Flickr, licensed CC-BY. Ten points if you can tell what it is...

My eldest daughter is in grade 5, so she's getting into some fun things at school. This week the class paired off to meet a challenge: build a container to keep hot water hot. Cool!

The teams built their contraptions over the weekend, doubtless with varying degrees of rule interpretation (my daughter's involved HotHands hand warmers, which I would not have thought of), and the results were established with a side-by-side comparison. Someone (not my daughter) won. Kudos was achieved.

But this should not be the end of the exercise. So far, no-one has really learned anything. Stopping here is like grinding wheat but not making bread. Or making dough, but not baking it. Or baking it, but not making it into toast, buttering it, and covering it in Marmite...

Great, now I'm hungry.

The rest of the exercise

How could this experiment be improved?

For starters, there was a critical component missing: control. Adding a vacuum flask at one end, and an uninsulated beaker at the other would have set some useful benchmarks.

There was a piece missing from the end too: analysis. A teardown of the winning and losing efforts would have been quite instructive. Followed by a conversation about the relative merits of different insulators, say. I can even imagine building on the experience. How about a light introduction to thermodynamic theory, or a stab at simple numerical modeling? Or a design contest? Or a marketing plan?

But most important missing piece of all, the secret weapon of learning, is iteration. The crucial next step is to send the class off to do it again, better this time. The goal: to beat the best previous attempt, perhaps even to beat the vacuum flask. The reward: $20k in seed funding and a retail distribution deal. Or a house point for Griffindor.

Einmal ist keinmal, as they say in Germany: Once is never. What can you iterate today?

Test driven development geoscience

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Sometimes I wonder how much of what we do in applied geoscience is really science. Is it really about objective enquiry? Do we form hypotheses, then test them? The scientific method is largely a caricature — science is more accidental and more fun than a step-by-step recipe — but I think our field sometimes falls short of even basic rigour. Go and sit through a conference session on seismic attribute analysis some time and you'll see what I mean. Let's just say there's a lot of arm-waving and shape-ology. 

Learning from geeks

We've written before about the virtues of the software engineering community. Innovation has been so rapid recently, that I think it's a great place to find interpretation hacks like pair picking. Learning about and experiencing the amazing productivity of programmers is one of the reasons I think all scientists should learn to program (but not learn to be a programmer). You'll find out about concepts like version control, user-centered design, and test-driven development. Programmers embrace these ideas to a greater or lesser degree, depending on their goals and those of the project they're working on. But all programmers know them.

I'm especially into test-driven development at the moment. The idea is that before implementing a new module or feature, you write a test — a short program that gives the new thing some input, inspects the output, and compares it to a known answer. The first version of the code will likely fail the test. The idea is to refactor the code until it passes the test. Then you add that test to a suite that runs every time you build anything in the same project, so you know your new thing doesn't get broken by something else later. And you aren't tempted to implement features that weren't part of the test.

Fail — Refactor — Pass

Imagine test-driven development geology (or any other kind of geoscience). What would that look like?

  • When planning wells, we often do write tests — they're called prognoses. But the comparison with the result is rarely formalized or quantified, especially outside the target zone. Once the well is drilled, it becomes data and we move on. No-one likes to dwell on the poorly understood or error-prone, but naturally that's where the greatest room for improvement is.  
  • When designing a new seismic attribute, or embarking on a seismic processing project, we often have a vague idea of success in our heads, and that's about it. What if we explicitly defined an input test dataset, some wells or bits of wells, and set 'passing' performance criteria on those? "I won't interpret the reprocessed seismic until it improves those synthetic correlation coefficients by 40%."
  • When designing a seismic survey, we could establish acceptable criteria for trace density, minimum offset, azimuth distribution, and recording time, then use these as a cost function to find the best possible survey for our dollars. Wait, perhaps we actually do this one. Is seismic acquisition unusually scientific? Or is it an inherently more linear problem?

What do you think? Can you see ways to define 'success' before you begin, then somewhat quantitatively compare your results with that? Ideas wanted!

A long weekend of Atlantic geology

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The Atlantic Geoscience Society Colloquium was hosted by Acadia University in Wolfville, Nova Scotia, this past weekend. It was the 50th Anniversay meeting, and attracted a crowd of about 175 geoscientists. A few members were able to reflect and tell stories first-hand of the first meeting in 1964.

It depends which way you slice it

Nova Scotia is one of the best places for John Waldron to study deformed sedimentary rocks of continental margins and orogenic belts. Being the anniversary, John traced the timeline of tectonic hypotheses over the last 50 years. From his kinematic measurements of Nova Scotia rocks, John showed the complexity of transtensional tectonics. It is easy to be fooled: you will see contraction features in one direction, and extension structures in another direction. It all depends which way you slice it. John is a leader in visualizing geometric complexity; just look at this animation of piecing together a coal mine in Stellarton. Oh, and he has a cut and fold exercise so that you can make your own Grand Canyon! 

The application of the Law of the Sea

In September 2012 the Bedford Institute of Oceanography acquired some multibeam bathymetric data and applied geomorphology equations to extend Canada's boundaries in the Atlantic Ocean. Calvin Campbell described the cruise as like puttering from Halifax to Victoria and back at 20 km per hour, sending a chirp out once a minute, each time waiting for it to go out 20 kilometres and come back.

The United Nation's Convention on the Law of the Sea (UNCLOS) was established to define the rights and responsibilities of nations in their use of the world's oceans, establishing guidelines for businesses, the environment, and the management of marine natural resources. A country is automatically entitled to any natural resources found within a 200 nautical mile limit of its coastlines, but can claim a little bit more if they can prove they have sedimentary basins beyond that. 

Practicing the tools of the trade

Taylor Campbell, applied a post-stack seismic inversion workflow to the Penobscot 3D survey and wells. Compared to other software talks I have seen in industry, Taylor's was a quality piece of integrated technical work. This is even more commendable considering she is an undergraduate student at Dalhousie. My only criticism, which I shared with her after the talk was over, was that the work lacked a probing question. It would have served as an anchor for the work, and I think is one of the critical distinctions between scientific pursuits and engineering.

Image courtesy of Justin Drummond, 2014, personal communication, from his expanded abstract presented at GSA 2013.

Practicing rational inquiry

Justin Drummond's work, on the other hand, started with a nugget of curiosity: How did the biogeochemical cycling of phosphorite change during the Neoproterozoic? Justin's anchoring question came first, only then could he think about the methods, technologies and tools he needed to employ, applying sedimentology, sequence stratigraphy, and petrology to investigate phosphorite accumulation in the Sete Lagoas Formation. He won the award for Best Graduate Student presentation at the conference.

It is hard to know if he won because his work was so good, or if it was because of his impressive vocabulary. He put me in mind of what Rex Murphy would sound like if he were a geologist.

The UNCLOS illustration is licensed CC-BY-SA, by Wikipedia users historicair and MJSmit.