News of the week

A geoscience and technology news round-up. If you spot anything we can highlight next week, drop us a line!

Using meteorite impacts as seismic sources on Mars

On Earth and Mars alike, when earthquakes (or Marsquakes) occur, they send energy into the planet's interior that can be used for tomographic imaging. Because the positions of these natural events is never known directly, several recording stations are required to locate these data by triangulation. The earth has an amazing array of stations but not Mars. 

Nick Teanby and James Wookey, geophysicists at the University of Bristol, UK (@UOBEarthScience on Twitter), invvestigated whether meteorite impacts on Mars provide a potentially valuable seismic signal for seeing into the interior of the planet. Because new craters can be resolved precisely from orbital photographs, accurate source positions can be determined without triangulation, and thus used in imaging. 

Investigation showed that seismicity induced by most meteorites is detectable, but only at short ranges, and good for investigating the near surface. Only the largest impacts, which only happen about once every ten years, are strong enough for deep imaging. Read more in their Physics of the Earth and Planetary Interiors paper here. Image credit: NASA/JPL.

Geomage acquires Petro Trace 

Seismic processing company, Geomage, has joined forces with Petro Trace Services in a move to become a full-workflow seismic processing service shop. The merging of these two companies will likely make them the largest geophysical service provider in Russia. Geomage has a proprietary processing technology called Multifocusing, and uses Paradigm's software for processing and interpretation. Click here to read more about the deal.

New bathymetric data for Google Earth

Google Earth now contains bathymetric data from more than two decades of seafloor scanning expeditions. The update was released on World Oceans Day, and represents 500 different surveys covering the size of North America. This new update will allow you to plan your next virtual underwater adventure or add more flair to your envrionmental impact assessment. Google Earth might have to seriously reconsider adapting their streetview name to what,... fishview? Wired.com has a nice demo to get you started. Image: Google Earth.

Workshop: open source software in geophysics

The AAPG's Petroleum Technology Transfer Council, PTTC, is having a workshop on open source software next week. The two-day workshop is on open software tools and reproducibility in geophysics, and will take place at the Houston Research Center in west Houston. Matt will be attending, and is talking about mobile tools on the Friday afternoon. There are still places, and you can register on the University of Texas at Austin website; the price is only $300, or $25 for students. The organizer is Karl Schleicher of UT and BEG.

This regular news feature is for information only. We aren't connected with any of these organizations, and don't necessarily endorse their products or services. Image of Mars credit: NASA/JPL-caltech/University of Arizona. Image of Earth: Google, TerraMetrics, DigitalGlobe, IBCAO.

F is for Frequency

Frequency is the number of times an event repeats per unit time. Periodic signals oscillate with a frequency expressed as cycles per second, or hertz: 1 Hz means that an event repeats once every second. The frequency of a light wave determines its color, while the frequency of a sound wave determines its pitch. One of the greatest discoveries of the 18th century is that all signals can be decomposed into a set of simple sines and cosines oscillating at various strengths and frequencies. 

I'll use four toy examples to illustrate some key points about frequency and where it rears its head in seismology. Each example has a time-series representation (on the left) and a frequency spectrum representation (right).

The same signal, served two ways

This sinusoid has a period of 20 ms, which means it oscillates with a frequency of 50 Hz (1/20 ms^-1). A sinusoid is composed of a single frequency, and that component displays as a spike in the frequency spectrum. A side note: we won't think about wavelength here, because it is a spatial concept, equal to the product of the period and the velocity of the wave.

In reflection seismology, we don't want things that are of infinitely long duration, like sine curves. We need events to be localized in time, in order for them to be localized in space. For this reason, we like to think of seismic impulses as a wavelet.

The Ricker wavelet is a simple model wavelet, common in geophysics because it has a symmetric shape and it's a relatively easy function to build (it's the second derivative of a Gaussian function). However, the answer to the question "what's the frequency of a Ricker wavelet?" is not straightforward. Wavelets are composed of a range (or band) of frequencies, not one. To put it another way: if you added monotonic sine waves together according to the relative amplitudes in the frequency spectrum on the right, you would produce the time-domain representation on the left. This particular one would be called a 50 Hz Ricker wavelet, because it has the highest spectral magnitude at the 50 Hz mark—the so-called peak frequency. 

Bandwidth

For a signal even shorter in duration, the frequency band must increase, not just the dominant frequency. What makes this wavelet shorter in duration is not only that it has a higher dominant frequency, but also that it has a higher number of sine waves at the high end of the frequency spectrum. You can imagine that this shorter duration signal traveling through the earth would be sensitive to more changes than the previous one, and would therefore capture more detail, more resolution.

The extreme end member case of infinite resolution is known mathematically as a delta function. Composing a signal of essentially zero time duration (notwithstanding the sample rate of a digital signal) takes not only high frequencies, but all frequencies. This is the ultimate broadband signal, and although it is impossible to reproduce in real-world experiments, it is a useful mathematical construct.

What about seismic data?

Real seismic data, which is acquired by sending wavelets into the earth, also has a representation in the frequency domain. Just as we can look at seismic data in time, we can look at seismic data in frequency. As is typical with all seismic data, the example below set lacks low and high frequencies: it has a bandwidth of 8–80 Hz. Many geophysical processes and algorithms have been developed to boost or widen this frequency band (at both the high and low ends), to increase the time domain resolution of the seismic data. Other methods, such as spectral decomposition, analyse local variations in frequency curves that may be otherwise unrecognizable in the time domain. 

High resolution signals are short in the time domain and wide or broadband in the frequency domain. Geoscientists often equate high resolution with high frequency, but that it not entirely true. The greater the frequency range, the larger the information carrying capacity of the signal.

In future posts we'll elaborate on Fourier transforms, sampling, and frequency domain treatments of data that are useful for seismic interpreters.

For more posts in our Geophysics from A to Z posts, click here.

To free, or not to free?

Yesterday, Evan and I published our fourth mobile app for geoscientists. It's called AVO*, it does reflectivity modeling, and it costs $2. 

Two bucks?? What's the point? Why isn't it free? Well, it went something like this...

- So, this new app: is it free?
- Well, all our apps are free. I guess it's free.
- Yeah, we don't want to stop it from spreading. If it wants to spread, that is...
- But does free look... worthless? I mean, 'you get what you pay for', right? Look at all the awesome stuff we pay for: Amazon web services, Squarespace web hosting, Hover domain hosting, awesome computers,...
- So what would we charge?
- What do other people charge? 
- There are no 'other people'... But there are technical apps for oil and gas out there. Most of them cost $1.99, some are $4.99, one or two are $9.99. Who knows how many downloads they get? 
- I bet the total revenue is constant: if you charge $1 and get 1000 downloads, then you'll get 100 at $10. But that's an experiment you can never do—once you've charged some amount, you can't really go up. Or down.
- How do other people decide what to charge?
- I guess traditionally you might use a cost-plus model: the cost of production, plus a profit margin.
- What's our cost of production?
- Well, a few days of time... let's call it $5000. If we wanted to make $10 000, and only expect 500 people to even be in the market... It doesn't work. No-one will pay $20 for a cell phone widget.
- Won't they just expense it?
- Maybe... I don't think the industry is quite there yet.
- Hmm... I downloaded an app for $20 once [a seismograph]. And another for $10 [a banjo tuner]. I don't even think about paying $1 or $2. That amount is basically free. $1 is free.
- But a buck... isn't it just a pain to even get your credit card out?
- Well, once you're set up in Google Checkout, or iTunes or whatever, it's essentially one click. And then we get a sense of the real user base. The hard core!
- Yeah... right now about 50% of people who install an app nuke it a few days later.
- At least if it's under $5 we probably won't have to deal with refunds and other nonsense.
- Arrgghhhh... why is this so hard? 
- Let's make it $2. 
- Let's make it free.
- But this app is awesome. Awesome shouldn't be free. Awesome is never free. Awesome costs. 
- But isn't this really just a thing that says "Agile is awesome, check us out, hire us"? It's marketing.
- Maybe... but it's useful too. It works. It does something. It has Science Inside™. People will get $1-worth out of it every time they use it. If this was a <insert energy software empire> app it would cost $10 000.
- Can we ask people to pay what they want? Like what Radiohead did with In Rainbows?
- No because they're already huge. They invoke mass hysteria in grown men. We don't invoke mass hysteria. Among anyboy.
- Damn. OK. Let's make it nearly free. As-good-as-free. Free-ish. Pseudo-free. Free*.
- $2?
- $2.

So the app costs a toonie, and we promise you won't regret it for a second. If you can't afford it, email us and we'll send you a free one. If you really hate it, email us and we'll send you $3.

What is AVO?

I used to be a geologist (but I'm OK now). When I first met seismic data, I took the reflections and geometries quite literally. The reflections come from geology, so it seems reasonable to interpret them as geology. But the reflections are waves, and waves are slippery things: they have to travel through kilometres of imperfectly known geology; they can interfere and diffract; they emanate spherically from the source and get much weaker quickly. This section from the Rockall Basin in the east Atlantic shows this attenuation nicely, as well as spectacular echo reflections from the ocean floor called multiples:

Data from the Virtual Seismic Atlas, contributed by the British Geological Survey.

Impedance is the product of density and velocity. Despite the complexity of seismic reflections, all is not lost. Even geologists interpreting seismic know that the strength of seismic reflections can have real, quantitative, geological meaning. For example, amplitude is related to changes in acoustic impedance Z, which is equal to the product of bulk density ρ and P-wave velocity V, itself related to lithology, fluid, and porosity.

Flawed cartoon of a marine seismic survey. OU, CC-BY-SA-NC.

But when the amplitude versus offset (AVO) behaviour of seismic reflections gets mentioned, most non-geophysicists switch off. If that's your reaction too, don't be put off by the jargon, it's really not that complicated.

The idea that we collect data from different angles is not complicated or scary. Remember the classic cartoon of a seismic survey (right). It's clear that some of the ray paths bounce off the geological strata at relatively small incidence angles, closer to straight down-and-up. Others, arriving at receivers further away from the source, have greater angles of incidence. The distance between the source and an individual receiver is called offset, and is deducible from the seismic field data because the exact location of the source and receivers is always known.

The basic physics behind AVO analysis is that the strength of a reflection does not only depend on the acoustic impedance—it also depends on the angle of incidence. Only when this angle is 0 (a vertical, or zero-offset, ray) does the simple relationship above hold.

Total internal reflection underwater. Source: Mbz1 via Wikimedia Commons.Though it may be unintuitive at first, angle-dependent reflectivity is an idea we all know well. Imagine an ordinary glass window: you can see through it perfectly well when you look straight through it, but when you move to a wide angle it suddenly becomes very reflective (at the so-called critical angle). The interface between water and air is similarly reflective at wide angles, as in this underwater view.

Karl Bernhard Zoeppritz (German, 1881–1908) was the first seismologist to describe the relationship between reflectivity and angle of incidence. In this context, describe means write down the equations for. Not two or three equations, lots of equations.

The Zoeppritz equations are very good model for how seismic waves propagate in the earth. There are some unnatural assumptions about isotropy, total isolation of the interface, and other things, but they work well in many real situations. The problem is that the equations are unwieldy, especially if you are starting from seismic data and trying to extract rock properties—trying to solve the so-called inverse problem. Since we want to be able to do useful things quickly, and since seismic data are inherently approximate anyway, several geophysicists have devised much friendlier models of reflectivity with offset.

I'll take a look at these more friendly models next time, because I want to tell a bit about how we've implemented them in our soon-to-be-released mobile app, AVO*. No equations, I promise! Well, one or two...

News of the week

This week has been fairly quiet for geoscience and technology news, so we're hijacking our own post to highlight a couple of Agile* changes you might have missed. The first one is this very feature—our News of the week post. More or less every Friday we round up some geoscience news with an oil & gas or technology angle. If you spot something you think we should include, please scribble a quick note to hello at agilegeoscience dot com!

Another new feature on our site is subscription by email. Every blog post comes right to your email inbox, so you won't miss any Agile* goodness. Go to the SUBSCRIBE box on the right (under the tag cloud), enter your email address and hit Subscribe. It's that easy! No password to remember, 100% spam free, and you can unsubscribe any time. Powered by Google.

If you're a regular reader then you know all about our new mobile apps. At the moment, for completely practical reasons, they are only available for Android™ devices. We just upgraded our first app, Volume*, a prospect volumetrics tool—now you can save and recall prospects! In the next couple of days we will launch our first über-app, AVO*. Visit the wiki for a sneak peak.

The wiki? Yes, last month we launched AgileWiki, an experiment in sharing knowledge about the subsurface. We know that much of what we know and do as industrial, applied scientists is proprietary—that's what in-house corporate wikis and knowledge bases are for. But some of it at least is basic, foundational, and generic in nature. And that's what AgileWiki is for. Join in, share what you know!

You might have noticed we've started dabbling a bit with video, and have a nascent YouTube channel. Today, the focus is on our mobile apps, but we are planning features on seismic interpretation workflows and other fun things. And we're open to feedback and suggestions on this effort, please let us know what you think!

We work hard to be interesting and relevant, not self-promoting and commercial. But occasionally people ask us what we actually do. So we made a one-pager setting out our stall. If you need some help doing something weird and wonderful, or just tricky and time-consuming, keep us in mind! We love helping people.

This ends the public service announcement. Back to our regular news feature next week!

Update on Tuesday, June 7, 2011 at 11:26 AM by Matt Hall

A quick follow up on one of the items in this post. We released AVO* into the Android Market yesterday. After much debate, we decided to experiment with charging for this app... It's $2. Reason for being so low: we want it to spread. Reason for not being free: it's awesome. Point your barcode reader at the QR code to install it...