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Monday
Dec222014

2014 retrospective

At this time of year, we look back at the best of the blog — what were the most read, the most contentious, the most informative posts of the year? If you only stop by once every 12 months, this is the post for you!

Your favourites

Let's turn to the data first. Which posts got the most hits this year? Older posts have a time advantage of course, but here are the most popular new posts, starting with the SEG-Y double-bill:

It's hard to say exactly how much attention a given post gets, because they sit on the front page of the site for a couple of weeks. Overall we got about 150 000 pageviews this year, and I think Well tie workflow — the most-read old post on the site this year — might have been read (okay, looked at) 5000 or so times.

I do love how some of our posts keep on giving: none of the top posts this year topped this year's readership of some golden oldies: Well tie workflow (written in April 2013), Six books about seismic interpretation (March 2013), k is for wavenumber (May 2012), Interpreting spectral gamma-ray logs (February 2013), or Polarity cartoons (April 2012).

What got people talking

When it all goes quiet on the comments, I worry that we're not provocative enough. But some posts provoke a good deal of chat, and always bring more clarity and depth to the issue. Don't miss the comments in Lusi's 8th birthday...

Our favourites

We have our favourites too. Perhaps because a lot of work went into them (like Evan's posts about programming), or because they felt like important things to say (as in the 'two sides' post), or because they feel like part of a bigger idea.

Evan Matt
Well tie calculus Whither technical books?
How much rock ... Mt St Helens? The new open geophysics tools
Laying out a seismic survey Two sides to every story?

Where is everybody?

We don't collect detailed demographic data, but it's fun to see how people are reading, and roughly where they are. One surprise: the number of mobile readers did not rise this year — it's still about 15%. The top 10 cities were

  1. Houston, USA
  2. Calgary, Canada
  3. London, UK
  4. Perth, Australia
  5. Ludwigshafen, Germany
  6. Denver, USA
  7. Aberdeen, UK
  8. Moscow, Russia
  9. Stavanger, Norway
  10. Kuala Lumpur, Malaysia

Last thing

Thank You for reading! Seriously: we want to be a useful and interesting part of our community, so every glance at our posts, every comment, and every share, help us figure out how to be more useful and more interesting. We aim to get better still next year, with more tips and tricks, more code, more rants, and more conference reports — we look forward to sharing everything Agile with you in 2015.

If this week is Christmas for you, then enjoy the season. All the best for the new year!


Previous Retrospective posts... 2011 retrospective •  2012 retrospective2013 retrospective

Wednesday
Dec172014

Laying out a seismic survey

Cutlines for a dense 3D survey at Surmont field, Alberta, Canada. Image: Google Maps.There are a number of ways to lay out sources and receivers for a 3D seismic survey. In forested areas, a designer may choose a pattern that minimizes the number of trees that need to be felled. Where land access is easier, designers may opt for a pattern that is efficient for the recording crew to deploy and pick up receivers. However, no matter what survey pattern used, most geometries consist of receivers strung together along receiver lines and source points placed along source lines. The pairing of source points with live receiver stations comprises the collection of traces that go into making a seismic volume.

An orthogonal surface pattern, with receiver lines laid out perpendicular to the source lines, is the simplest surface geometry to think about. This pattern can be specified over an area of interest by merely choosing the spacing interval between lines well as the station intervals along the lines. For instance:

xmi = 575000        # Easting of bottom-left corner of grid (m)
ymi = 4710000       # Northing of bottom-left corner (m)
SL = 600            # Source line interval (m)
RL = 600            # Receiver line interval (m)
si = 100            # Source point interval (m)
ri = 100            # Receiver point interval (m)
x = 3000            # x extent of survey (m)
y = 1800            # y extent of survey (m)

We can calculate the number of receiver lines and source lines, as well as the number of receivers and sources for each.

# Calculate the number of receiver and source lines.
rlines = int(y/RL) + 1
slines = int(x/SL) + 1

# Calculate the number of points per line (add 2 to straddle the edges). 
rperline = int(x/ri) + 2       
sperline = int(y/si) + 2

# Offset the receiver points.
shiftx = -si/2.
shifty = -ri/2.

Computing coordinates

We create a list of x and y coordinates with a nested list comprehension — essentially a compact way to write 'for' loops in Python — that iterates over all the stations along the line, and all the lines in the survey.

# Find x and y coordinates of receivers and sources.
rcvrx = [xmi+rcvr*ri+shifty for line in range(rlines) for rcvr in range(rperline)]
rcvry = [ymi+line*RL+shiftx for line in range(rlines) for rcvr in range(rperline)]

srcx = [xmi+line*SL for line in range(slines) for src in range(sperline)]
srcy = [ymi+src*si for line in range(slines) for src in range(sperline)]

To make a map of the ideal surface locations, we simply pass this list of x and y coordinates to a scatter plot:

 

Plotting these lists is useful, but it is rather limited by itself. We're probably going to want to do more calculations with these points — midpoints, azimuth distributions, and so on — and put these data on a real map. What we need is to insert these coordinates into a more flexible data structure that can hold additional information.

Shapely, Pandas, and GeoPandas

Shapely is a library for creating and manipulating geometric objects like points, lines, and polygons. For example, Shapely can easily calculate the (x, y) coordinates halfway along a straight line between two points.

Pandas provides high-performance, easy-to-use data structures and data analysis tools, designed to make working with tabular data easy. The two primary data structures of Pandas are:

  • Series — a one-dimensional labelled array capable of holding any data type (strings, integers, floating point numbers, lists, objects, etc.)
  • DataFrame — a 2-dimensional labelled data structure where the columns can contain many different types of data. This is similar to the NumPy structured array but much easier to use.

GeoPandas combines the capabilities of Shapely and Pandas and greatly simplifies geospatial operations in Python, without the need for a spatial database. GeoDataFrames are a special case of DataFrames that are specifically for representing geospatial data via a geometry column. One awesome thing about GeoDataFrame objects is they have methods for saving data to shapefiles.

So let's make a set of (x,y) pairs for receivers and sources, then make Point objects using Shapely, and in turn add those to GeoDataFrame objects, which we can write out as shapefiles:

# Zip into x,y pairs.
rcvrxy = zip(rcvrx, rcvry)
srcxy = zip(srcx, srcy)

# Create lists of shapely Point objects.
rcvrs = [Point(x,y) for x,y in rcvrxy]
srcs = [Point(x,y) for x,y in srcxy]

# Add lists to GeoPandas GeoDataFrame objects.
receivers = GeoDataFrame({'geometry': rcvrs})
sources = GeoDataFrame({'geometry': srcs})

# Save the GeoDataFrames as shapefiles.
receivers.to_file('receivers.shp')
sources.to_file('sources.shp')

It's a cinch to fire up QGIS and load these files as layers on top of a satellite image or physical topography map. As a survey designer, we can now add, delete, and move source and receiver points based on topography and land issues, sending the data back to Python for further analysis.

All the code used in this post is in an IPython notebook. You can read it, and even execute it yourself. Put your own data in there and see how it comes out!


NEWSFLASH — If you think the geoscientists in your company would like to learn how to play with geological and geophysical models and data — exploring seismic acquisition, or novel well log displays — we can come and get you started! Best of all, we'll help you get up and running on your own data and your own ideas.

If you or your company needs a dose of creative geocomputing, check out our new geocomputing course brochure, and give us a shout if you have any questions. We're now booking for 2015.

Thursday
Dec112014

The race for useful offsets

We've been working on a 3D acquisition lately. One of the factors influencing the layout pattern of sources and receivers in a seismic survey is the range of useful offsets over the depth interval of interest. If you've got more than target depth, you'll have more than one range of useful offsets. For shallow targets this range is limited to small offsets, due to direct waves and first breaks. For deeper targets, the range is limited at far offsets by energy losses due to geometric spreading, moveout stretch, and system noise.

In seismic surveying, one must choose a spacing interval between geophones along a receiver line. If phones are spaced close together, we can collect plenty of samples in a small area. If the spacing is far apart, the sample density goes down, but we can collect data over a bigger area. So there is a trade-off and we want to maximize both; high sample density covering the largest possible area.

What are useful offsets?

It isn't immediately intuitive why illuminating shallow targets can be troublesome, but with land seismic surveying in particular, first breaks and near surface refractions clobber shallow reflecting events. In the CMP domain, these are linear signals, used for determining statics, and are discarded by muting them out before migration. Reflections that arrive later than the direct wave and first refractions don't get muted out. But if these reflections arrive later than the air blast noise or ground roll noise — pervasive at near offsets — they get caught up in noise too. This region of the gather isn't muted like the top mute, otherwise you'd remove the data at near offsets. Instead, the gathers are attacked with algorithms to eliminate the noise. The extent of each hyperbola that passes through to migration is what we call the range of useful offsets.

The deepest reflections have plenty of useful offsets. However if we wanted to do adequate imaging somewhere between the first two reflections, for instance, then we need to make sure that we record redundant ray paths over this smaller range as well. We sometimes call this aperture; the shallow reflection is restricted in the number of offsets that it can be illuminated with, the deeper reflections can tolerate an aperture that is more open. In this image, I'm modelling the case of 60 geophones spanning 3000 metres, spaced evenly at 100 metres apart. This layout suggests merely 4 or 5 ray paths will hit the uppermost reflection, the shortest ray paths at small offsets. Also, there is usually no geophone directly on top of the source location to record a vertical ray path at zero offset. The deepest reflections however, should have plenty of fold, as long as NMO stretch, geometric spreading, and noise levels were good.

The problem with determining the range of useful offsets by way of a model is, not only does it require a velocity profile, which is easily attained from a sonic log, VSP, or velocity analysis, but it also requires an estimation of the the speed, intensity, and duration of the near surface events to be muted. Parameters that depend largely on the nature of the source and the local ground conditions, which vary from place to place, and from one season to the next.

In a future post, we'll apply this notion of useful offsets to build a pattern for shooting a 3D.

ipynb_icon.png
Click here for details on how I created this figure using the IPython Notebook. If you don't have it, IPython is easy to install. The easiest way is to install all of scientific Python, or use Canopy or Anaconda.

 

This post was inspired in part by Norm Cooper's 2004 article, A world of reality: Designing 3D seismic programs for signal, noise, and prestack time-migration. The Leading Edge 23 (10), 1007-1014.DOI: 10.1190/1.1813357

Tuesday
Dec092014

The new open geophysics tools

The hackathon in Denver was more than 6 weeks ago. I kept thinking, "Oh, I must post a review of what went down" (beyond the quick wrap-up I did at the time), but while I'm a firm believer in procrastination six weeks seems unreasonable... Maybe it's taken this long to scrub down to the lasting lessons. Before those, I want to tell you who the teams were, what they did, and where you can find their (100% open source!) stuff. Enjoy!


Geophys Wiz

Andrew Pethick, Josh Poirier, Colton Kohnke, Katerina Gonzales, and Elijah Thomas — GitHub repo

This team had no trouble coming up with ideas — perhaps a reflection of their composition, which was more heterogeneous than the other teams. Josh is at NEOS, the consulting and software firm, and Andrew is a postdoc at Curtin in Perth, Australia, while the other 3 are students at Mines. The team eventually settled on building MT Black Box, a magnetotellurics modeling web application. 

Last thing: Don't miss Andrew Pethick's write-up of the event. 

Seemingly Concerned Neighbours

Elias Arias, Brent Putman, Thomas Rapstine, and Gabriel Martinez — Github repo

These four young geophysicists from the Colorado School of Mines impressed everyone with their work ethic. Their tight-knit team came in with a plan, and proceeded to scribble up the coolest-looking whiteboard of the weekend. After learning some Android development skills 'earlier this week', they pulled together a great little app for forward modeling magnetotelluric responses. 

Well tie guys

Michaël Montouchet, Graham Dawes, Mark Roberts

It was terrific to have pro coders Graham and Michaël with us — they flew from the UK to be with us, thanks to their employer and generous sponsor ffA GeoTeric. They hooked up with Mark, a Denver geophysicist and developer, and hacked on a well-tie web application, rightly identifying a gap in the open source market, so to speak (there is precious little out there for well-based workflows). They may have bitten off more than they could chew in just 2 days, so I hope we can get together with them again to finish it off. Who's up for a European hackathon? 

Five hour wonders

Rowan Cockett and Seogi KangGitHub repo for SimPEG

These two characters from UBC didn't get going till Sunday morning, but in just five hours they built a sweet web app for forward modeling the DC resistivity response of a buried disk. They weren't starting from scratch, because Rowan and others have spent months honing SimPEG, a rich open-source geophysical library, but minds were nonetheless blown.

Key takeaway: interactivity beyond sliders for the win.

Pick This!

Ben Bougher, Jacob Foshee, Evan Bianco, and an immiscible mixture of Chris Chalcraft and me — GitHub repo

Wouldn't you sometimes like to know how other people would interpret the section you're working on? This team, a reprise of the dream team from Houston in 2013, built a simple way to share images and invite others to interpret them. When someone has completed their interpretation, only then do they get to see the ensemble — everyone else's interpretations — in a heatmap. Not only did this team demo live software at pickthis.io, but the audience provided the first crowdsourced picks in real time. 

We'll be blogging more about Pick This soon. We're actively seeking ideas, images, interpreters, and financial support. Keep an eye out.

What I learned at this hackathon

  • Potential fields are an actual thing! OK, kidding, but three out of five teams built potential field modeling tools. I wasn't expecting that, and I think the judges were impressed at the breadth. 
  • 30 hours is easily enough time to build something pretty cool. Heck, 5 hours is enough if you're made of the right stuff. 
  • Students can happily build prototypes alongside professional developers, and even teach them a thing or two. And vice versa. Are hackathons a leveller of playing fields?
  • We need to remove the road blocks to more people enjoying this event. To help with this, next time there will be a 1-day bootcamp before the hackathon.
  • After virtually doubling in size from 2013 to 2014, it's clear that the 2015 Hackathon in New Orleans is going to be awesome! Mark your calendar: 17 and 18 October 2015.

Thank you!

Thank you to the creative, energetic geophysicists that came. It was a privilege to meet and hack with you!

Thank you to the judges who gave up their Sunday teatime to watch the demos and give precious feedback to the teams: Steve Adcock, Jamie Allison, Maitri Erwin, Dennis Cooke, Chris Krohn, Shannon Bjarnason, David Holmes, and Tracy Stark. Amazing people, one and all.

A final Thank You to our sponsors — dGB Earth Sciences, ffA GeoTeric, and OpenGeoSolutions. You guys are totally awesome! Seriously.

Tuesday
Nov252014

It's the GGGG (giant geoscience gift guide)

I expect you've been wondering what to get me and Evan for Christmas. Wonder no more! Or, if you aren't that into Agile, I suppose other geoscientists might even like some of this stuff. If you're feeling more needy than generous, just leave this post up on a computer where people who love you will definitely see it, or print it out and mail it to everyone you know with prominent red arrows pointing to the things you like best. That's what I do.

Geology in the home

Art!

Museums and trips and stuff

Image is CC-BY by Greg Westfall on Flickr.

Geo-apparel

Blimey... books!

Who over the age of 21 or maybe 30 doesn't love getting books for Christmas? I don't!... not love it. Er, anyway, here are some great reads!

  • How about 156 things for the price of three? Yeah, that is a deal.
  • They're not geological but my two favourite books of the year were highly geeky — What If? by Randall Munroe and Cool Tools by Kevin Kelly.
  • Let's face it, you're going to get books for the kids in your life too (I hope). You can't do better than Jon Tennant's Excavate! Dinosaurs.
  • You're gonna need some bookends for all these books.

Still stuck? Come on!

All of the smaller images in this post are copyright of their respective owners, and I'm hoping they don't mind me using them to help sell their stuff.