# The right writing tools

Scientists write, it's part of the job. If writing feels laborious, it might be because you haven't found the right tools yet.

The wrong tools <cough>Word</cough> feel like a lot of work. You spend a lot of time fiddling with font sizes and not being sure whether to use italic or bold. You're constantly renumbering sections after edits. Everything moves around when you resize a figure. Tables are a headache. Table of contents? LOL.

If this sounds familiar, check out the following tools — arranged more or less in order of complexity.

### Markdown

If you've never experienced writing with a markup language, you're in for a treat. At first it might feel clunky, but it quickly gets out of the way, leaving you to focus on the writing. Markdown was invented by John Gruber in about 2004; it is now almost ubiquitous in tools for developers. It's very lightweight, but compatible with HTML and LaTeX math, so it has plenty of features. Styling is absent from the document itself, being applied enitrely in post-production, as it were. With help from pandoc, you can compile Markdown documents to almost any format (e.g. PDF or Word). As a result, Markdown is sufficient for at least 70% of my writing projects. Here's a sampling of Markdown markup, rendered on the right with no styling:

### Jupyter Notebook

If you've been following along with our X Lines of Python series, or any of our other code-centric content, you'll have come across Jupyter Notebooks. These documents combine Markdown with code (in more or less any language you can think of) and the outputs of code — data, charts, images, etc. More than containing code, a so-called kernel can also run the code: Notebooks are fully computable documents. Not only could you write a paper or book in a Notebook, many people use them to give presentations with fully interactive, live code blocks and widgets.

### LaTeX

I discovered LaTeX in about 1993 and it was love at first sight. I've always been a bit of a typography nerd, and LaTeX — like TeX, around which LaTeX is wrapped — really cares about typography. So you get ligatures, hyphenation, sentence spacing, and kerning for free. It also cares about mathematics, cross-references, bibliographies, page numbering, tables of contents, and everything else you need for publication-ready documents.

You can install LaTeX locally, but there are several ways to use LaTeX online, without installing anything — and you get the best of both worlds: markup with WYSIWYG editing. OverleafShareLaTex (which is merging with Overleaf), Authorea, and Papeeria are all worth a look, especially if you write scientific papers.

### When WYSISYG works

Sometimes you just want a couple of headings and some text, or you need to share a document with others. I often go for WYSISYG in those situations too — Google Docs is the best WYSIWYG editor I've used. When it supports Markdown too, which is surely only a matter of time, it will be perfect.

What about you, do you have a favourite writing tool? Share it in the comments.

### Matt Hall

Matt is a geoscientist in Nova Scotia, Canada. Founder of Agile Scientific, co-founder of The HUB South Shore. Matt is into geology, geophysics, and machine learning.

# Not getting hacked

The end of the year is a great time to look around at your life and sort stuff out. One of the things you almost certainly need to sort out is your online security. Because if you haven't been hacked already (you probably have), you're just about to be.

Just look at some recent stories from the world of data security:

There are plenty of others; Wired has been keeping track of them — read more here. Or check out Wikipedia's list.

Despite all this, I see hardly anyone using a password manager, and anecdotally I hear that hardly anyone uses two-factor authentication either. This tells me that at least 80% of smart people, inlcuding lots of my friends and relatives, are in daily peril. Oh no!

After reading this post, I hope you do two things:

• Start using a password manager. If you only do one thing, do this.
• Turn on two-factor authentication for your most vulnerable accounts.

### Start using a password manager

Please, right now, download and install LastPass on every device and in every browser you use. It's awesome:

• It stores all your passwords! This way, they can all be different, and each one can be highly secure.
• It generates secure, random passwords for new accounts you create.
• It scores you on the security level of your passwords, and lets you easily change insecure ones.
• The free version is awesome, and the premium version is only \$2/month.

There are other password managers, of course, but I've used this one for years and it's excellent. Once you're set up, you can start changing passwords that are insecure, or re-used on multiple sites... or which are at Uber, Yahoo, or Equifax.

One surprise from using LastPass is being able to count the number of accounts I have created around the web over the years. I have 473 accounts stored in LastPass! That's 473 places to get hacked... how many places are you exposed?

### Two-factor authentication

Sure, it's belt and braces — but you don't want your security trousers to fall down, right?

Er, anyway, the point is that even with a secure password, your password can still be stolen and your account compromised. But it's much, much harder if you use two-factor authentication, aka 2FA. This requires you to enter a code — from a hardware key or an app, or received via SMS — as well as your password. If you use an app, it introduces still another layer of security, because your phone should be locked.

I use Google's Authenticator app, and I like it. There's a little bit of hassle the first time you set it up, but after that it's plain sailing. I have 2FA turned on for all my 'high risk' accounts: Google, Twitter, Facebook, Apple, AWS, my credit card processor, my accounting software, my bank, my domain name provider, GitHub, and of course LastPass. Indeed, LastPass even lets me specify that logins must originate in Canada.

### What else can you do?

There are some other easy things you can do to make yourself less hackable:

• Install updates on your phones, tablets, and other computers. Keep browsers and operating systems up to date.
• Be on high alert for phishing attempts. Don't follow links to sites like your bank or social media sites — type them into your browser if possible. Be very suspicious of anyone contacting you, especially banks.
• Don't use USB sticks. The cloud is much safer — I use Dropbox myself, it's awesome.

For more tips, check out this excellent article from Motherboard on not getting hacked.

### Matt Hall

Matt is a geoscientist in Nova Scotia, Canada. Founder of Agile Scientific, co-founder of The HUB South Shore. Matt is into geology, geophysics, and machine learning.

# Burning the surface onto the subsurface

Previously, I described a few of the reasons why we don't get a clean ground surface event on land seismic data like we do the water-bottom in marine seismic. In land data, the worst part of the image is right at the surface. But ground level is not just tricky to see, it's impossible to see. Since the vibe truck is on the ground, there's no reflection from that surface. Even if there was some kind of event there, processors apply a magic eraser to the top of the section — the mute — to erase the early arrivals. So it's not possible to see the ground in land data, and you can't pick what isn't there.

But I still want to know where the ground is. Why can't we slap a ground-level seismic 'reflection' event on the section?

### What you need

We need the ground level, which is in depth of course, in the time domain of the seismic section. To compute this, let's call it $$t_\mathrm{G}$$, we need three pieces of information at every trace location: the ground elevation $$G$$, the seismic reference datum (SRD) which I'll call $$D$$, and the replacement velocity $$V_\mathrm{r}$$.

$$t_\mathrm{G} = \frac{2 (G - D)}{V_\mathrm{r}}$$

Ground elevation.  If you're lucky, you'll be able to find the ground elevation corresponding to each trace stored in the trace headers. Ground elevation might be located in bytes 41-44 or 45-48 of the trace header, which correspond to the receiver group elevation and the surface elevation of the source, respectively. These should be the same for a stacked trace, but as with any meta-data to do with SEGY, this info could be hiding somewhere else, or missing altogether. And if you're that unlucky, you might have to comb through processing reports for the missing information. If you are even more unlucky (as I was in this example), you won't have any kind of processing report to fall back on and you'll have to concoct something else. In the accompanying Jupyter notebook, I resorted to interpolating a digitized elevation profile from a JPEG plot of the seismic line. So if you're all out of options, you might find refuge in those legacy plots!

This profile is particularly wonky, because the seismic reference datum (red) is not the same across the profile

Seismic reference datum. And to make life yet more complicated, the seismic reference datum is not flat across the profile. It goes downhill and then flattens out (red line below). Don't ask me what the advantages are of processing data to a variable datum, but whatever they are, I hope they offset the disadvantages of all-to-easily mistaking the datum to be flat.

The replacement velocity is given in the sidelabel of the raster image online (shown right). It's 10 000 ft/sec, or 3048 m/s.

Byte locations 53-56 and 57-60 are the standard trace header placeholders reserved for holding the datum elevation at the receiver group and the datum elevation at source. Again, for a stacked trace, these should be the same value. If these fields are zeros, then check the fields of the Trace Header Extension. If they turn up empty, and if the datum is horizontal, it might be listed in the file's text header.

### Convert elevation to time

By definition, the seismic reference datum is horizontal in the time-domain (red line below). Notice how the ground elevation – in the time domain – plots mostly as negative values (before) time zero. In other words, most of the ground is being cut-off by the top of the section. So, if we want to see it, we need to shift everything down into the field of view. Conceptually, this means adjusting the seismic reference datum so it floats entirely above the ground-level. Computationally, we can achieve this easily enough by padding the top of the data with zeros.

A time-domain representation of the ground-level along the seismic profile. The surface of the earth extends above the start of the seismic data for most of the locations along the profile.

### Make the ground a pickable event

As a final bit of post-processing, we could actually burn the ground-level into the data as a sort of synthetic seismic event. The reason I like this concept is that it alleviates the need to dig up old-processing reports, puzzle over missing header data, or worse, maintain and munge external text files containing elevation information. I say, let's make it self-contained. Let's put it directly into the data so that it can be treated like any other seismic reflection. Why would I do this?

• You can see where there might be fold, velocity or other issues related to topography.
• You can immediately see the polarity of the data.
• You could use the bandwidth of the data to make the pseudo-reflector, giving a visual hint to the interpreter.
• Keeping track of amplitude adjustments and phase rotations would be self-documenting and reversible.
• you could autotrack it to get a topographic map (or just get this from the processor).
• It looks cool!

Seismic profile with ground level SYNTHETICALLY SLAPPED ON TOP.  Bandlimited, of course, so you can Autotrack till your hearts content!

I've deliberately constructed a band-limited reflection, opposed to placing a sharp spike at ground-level. The problem with a spike is that it has infinite bandwidth. It contains higher frequencies than the image, so as Carl Reine commented on that last post, that might not play nice with seismic attributes. Also, there's the problem of selecting an amplitude value to assign to the spike: we don't want to introduce amplitudes that are ridiculously out of range of the existing data.

### The whole image

I hereby propose that this synthetic ground level trick adopted as the new standard for any land seismic processing and interpretation. The great thing is, it can be done just as easily by interpreters and seismic data technologists, as by the processing companies that create the rest of the image. I realize we're adding stuff to the data that isn't actually signal. We do non-real things to signals all the time. The question is, do the benefits outweigh the artificiality?

Here's the view of the entire section:

The whole section, ground level included.

The details of this exercise can be found in the this Jupyter Notebook.

References

The seismic is line 36_77_PR from the USGS data repository.

SEG Y rev 2 Data Exchange Format. SEG Technical Standards Committee. Draft 2.0, January, 2015.

# How to QC a seismic volume

I've had two emails recently about quality checking seismic volumes. And last month, this question popped up on LinkedIn:

We have written before about making a data quality volume for your seismic — a handy way to incorporate uncertainty into risk maps — but these recent questions seem more concerned with checking a new volume for problems.

### First things first

Ideally, you'd get to check the volume before delivery (at the processing shop, say), otherwise you might have to actually get it loaded before you can perform your QC. I am assuming you've already been through the processing, so you've seen shot gathers, common-offset gathers, etc. This is all about the stack. Nonetheless, the processor needs to prepare some things:

• The stack volume, of course, with and without any 'cosmetic' filters (eg fxy, fk).
• A semblance (coherency, similarity, whatever) volume.
• A fold volume.
• Make sure the processor has some software that can rapidly scan the data, plot amplitude histograms, compute a spectrum, pick a horizon, and compute phase. If not, install OpendTect (everyone should have it anyway), or you'll have to load the volume yourself.

There are also some things you can do ahead of time.

1. Be part of the processing from the start. You don't want big surprises at this stage. If a few lines got garbled during file creation, no problem. If there's a problem with ground-roll attentuation, you're not going to be very popular.
2. Make sure you know how the survey was designed — where the corners are, where you would expect live traces to be, and which way the shot and receiver lines went (if it was an orthogonal design). Get maps, take them with you.
3. Double-check the survey parameters. The initial design was probably changed. The PowerPoint presentation was never updated. The processor probably has the wrong information. General rule with subsurface data: all metadata is probably wrong. Ideally, talk to someone who was involved in the planning of the survey.
4. You didn't skip (2) did you? I'm serious, double check everything.

### Crack open the data

OK, now you are ready for a visit with the processor. Don't fall into the trap of looking at the geology though — it will seduce you (it's always pretty, especially if it's the first time you've seen it). There is work to do first.

1. Check the cornerpoints of the survey. I like the (0, 0) trace at the SW corner. The inline and crossline numbering should be intuitive and simple. Make sure the survey is the correct way around with respect to north.
2. Scan through timeslices. All of them. Is the sample interval what you were expecting? Do you reach the maximum time you expected, based on the design? Make sure the traces you expect to be live are live, and the ones you expect to be dead are dead. Check for acquisition footprint. Start with greyscale, then try another colourmap.
3. Repeat (5) but in a similarity volume (or semblance, coherency, whatever). Look for edges, and geometric shapes. Check again for footprint.
4. Look through the inlines and crosslines. These usually look OK, because it's what processors tend to focus on.
5. Repeat (7) but in a similarity volume.

### Dive into the details

1. Check some spectrums. Select some subsets of the data — at least 100 traces and 1000 ms from shallow, deep, north, south, east, west — and check the average spectrums. There should be no conspicuous notches or spikes, which could be signs of all sorts of things from poorly applied filters to reverberation.
2. Check the amplitude histograms from those same subsets. It should be 32-bit data — accept no less. Check the scaling — the numbers don't mean anything, so you can make them range over whatever you like. Something like ±100 or ±1000 tends to make for convenient scaling of amplitude maps and so on; ±1.0 or less can be fiddly in some software. Check for any departures from an approximately Laplacian (double exponential) distribution: clipping, regular or irregular spikes, or a skewed or off-centre distribution:
1. Interpret a horizon and check its phase. See Purves (Leading Edge, October 2014) or SubSurfWiki for some advice.
2. By this time, the fold volume should yield no surprises. If any of the rest of this checklist throws up problems, the fold volume might help troubleshoot.
3. Check any other products you asked for. If you asked for gathers or angle stacks (you should), check them too.

Last of all, before actual delivery, talk to whoever will be loading the data about what kind of media they prefer, and what kind of file organization. They may also have some preferences for the contents of the SEG-Y file and trace headers. Pass all of this on to the processor. And don't forget to ask for All The Seismic

Have I forgotten anything? Are there things you always do to check a new seismic volume? Or if you're really brave, maybe you have some pitfalls or even horror stories to share...

### Matt Hall

Matt is a geoscientist in Nova Scotia, Canada. Founder of Agile Scientific, co-founder of The HUB South Shore. Matt is into geology, geophysics, and machine learning.

# Save the samples

A long while ago I wrote about how to choose an image format, and then followed that up with a look at vector vs raster graphics. Today I wanted to revisit rasters (you might think of them as bitmaps, images, or photographs). Because a question that seems to come up a lot is 'what resolution should my images be?'

### Forget DPI

When writing for print, it is common to be asked for a certain number of dots per inch, or dpi (or, equivalently, pixels per inch or ppi). For example, I've been asked by journal editors for images 'at least 200 dpi'. However, image files do not have an inherent resolution — they only have pixels. The resolution depends on the reproduction size you choose. So, if your image is 800 pixels wide, and will be reproduced in a 2-inch-wide column of print, then the final image is 400 dpi, and adequate for any purpose. The same image, however, will look horrible at 4 dpi on a 16-foot-wide projection screen.

Rule of thumb: for an ordinary computer screen or projector, aim for enough pixels to give about 100 pixels per display inch. For print purposes, or for hi-res mobile devices, aim for about 300 ppi. If it really matters, or your printer is especially good, you are safer with 600 ppi.

The effect of reducing the number of pixels in an image is more obvious in images with a lot of edges. It's clear in the example that downsampling a sharp image (a to c) is much more obvious than downsampling the same image after smoothing it with a 25-pixel Gaussian filter (b to d). In this example, the top images have 512 × 512 samples, and the downsampled ones underneath have only 1% of the information, at 51 × 51 samples (downsampling is a type of lossy compression).

### Careful with those screenshots

The other conundrum is how to get an image of, say, a seismic section or a map.

What could be easier than a quick grab of your window? Well, often it just doesn't cut it, especially for data. Remember that you're only grabbing the pixels on the screen — if your monitor is small (or perhaps you're using a non-HD projector), or the window is small, then there aren't many pixels to grab. If you can, try to avoid a screengrab by exporting an image from one of the application's menus.

For seismic data, you'd like to capture sample as a pixel. This is not possible for very long or deep lines, because they don't fit on your screen. Since CGM files are the devil's work, I've used SEGY2ASCII (USGS Open File 2005–1311) with good results, converting the result to a PGM file and loading into Gimp

Large seismic lines are hard to capture without decimating the data. Rockall Basin. Image: BGS + Virtual Seismic Atlas.If you have no choice, make the image as large as possible. For example, if you're grabbing a view from your browser, maximize the window, turn off the bookmarks and other junk, and get as many pixels as you can. If you're really stuck, grab two or more views and stitch them together in Gimp or Inkscape

When you've got the view you want, crop the window junk that no-one wants to see (frames, icons, menus, etc.) and save as a PNG. Then bring the image into a vector graphics editor, and add scales, colourbars, labels, annotation, and other details. My advice is to do this right away, before you forget. The number of times I've had to go and grab a screenshot again because I forgot the colourbar...

The Lenna image is from Hall, M (2006). Resolution and uncertainty in spectral decomposition. First Break 24, December 2006, p 43-47.

1 Comment

### Matt Hall

Matt is a geoscientist in Nova Scotia, Canada. Founder of Agile Scientific, co-founder of The HUB South Shore. Matt is into geology, geophysics, and machine learning.

# 5 ways to kickstart an interpretation project

Last Friday, teams around the world started receiving external hard drives containing this year's datasets for the AAPG's Imperial Barrel Award (IBA for short). I competed in the IBA in 2008 when I was a graduate student at the University of Alberta. We were coached by the awesome Dr Murray Gingras (@MurrayGingras), we won the Canadian division, and we placed 4th in the global finals. I was the only geophysical specialist on the team alongside four geology graduate students.

### Five things to do

Whether you are a staff geoscientist, a contractor, or competitor, it can help to do these things first:

1. Make a data availability map (preferably in QGIS or ArcGIS). A graphic and geospatial representation of what you have been given.
2. Make well scorecards: as a means to demonstrate not only that you have wells, but what information you have within the wells.
3. Make tables, diagrams, maps of data quality and confidence. Indicate if you have doubts about data origins, data quality, interpretability, etc.
4. Background search: The key word is search, not research. Use Mendeley to organize, tag, and search through the array of literature
5. Use Time-Scale Creator to make your own stratigraphic column. You can manipulate the vector graphic, and make it your own. Much better than copying an old published figure. But use it for reference.

All of these things can be done before assigning roles, before saying who needs to do what. All of this needs to be done before the geoscience and the prospecting can happen. To skirt around it is missing the real work, and being complacent. Instead of being a hammer looking for a nail, lay out your materials, get a sense of what you can build. This will enable educated conversations about how you can spend your geoscientific manpower, division of labour, resources, time, etc.

### Read more, then go apply it

In addition to these tips for launching out of the blocks, I have also selected and categorized blog posts that I think might be most relevant and useful. We hope they are helpful to all geoscientists, but especially for students. Visit the Agile blog highlights list on SubSurfWiki.

I wish a happy and exciting IBA competition to all participants, and their supporting university departments. If you are competing, say hi in the comments and tell us where you hail from.

# When to use vectors not rasters

In yesterday's post, I looked at advantages and disadvantages of various image formats. Some chat ensued in the comments and on Twitter about making drawings and figures and such. I realized I hadn't been very clear: when I say 'image', I really mean 'raster' or 'bitmap'. That is, a discretized (pixel-based) grid of data.

### What are vector graphics?

Click to enlarge — see a simulation of the difference between vector and raster art.What I was not writing about was drawings and graphics combining text, lines, and images. Such files usually contain vector graphics. Vector graphics do not contain descriptions of pixels, but instead they contain descriptions and positions of text, paths, and polygons. Example file formats are:

• SVGScalable Vector Graphics, an open format and web standard
• AI — a proprietary format used by Adobe Illustrator
• CDRCorelDRAW's proprietary format
• PPT — pictures in Microsoft PowerPoint are vector format
• SHP — shapefiles are a (mostly) generic vector format for GIS

One of the most important properties of vector graphics is that you can rescale it without worrying about changing the resolution — as in the example (right).

### What are composite formats?

Vector and raster graphics can be combined in all sorts of ways, and vector files can contain raster images. They can therefore be used for very large displays like posters. But vector files are subject to interpretation by different software, may be proprietary, and have complex features like guides and layers that you may not want to expose to someone else. So when you publish or share your work it's often a good idea to export to either a high-res PNG, or a composite page description format:

• PDFPortable Document Format, the closest thing to an open, ubiquitous format; stable and predictable.
• EPSEncapsulated PostScript; the precursor to PDF, it's rarely called for today, unless PDF is giving you problems.
• PSPostScript is a programming and page description language underlying EPS and PDF; avoid it.
• CGMComputer Graphics Metafiles are best left alone. If you are stuck with them, complain loudly.

### What software do I need?

Any time you want to add text, or annotation, or anything else to a raster, or you wish to create a drawing from scratch, vector formats are the way to go. There are several tools for creating such graphics:

Judging by figures I see submitted to journals, some people use Microsoft PowerPoint for creating vector graphics. For a simple figure, this may be fine, but for anything complex — curved or wavy lines, complicated filled objects, image effects, pattern fills — it is hard work. And the drawing tools listed above have some great advantages over PowerPoint — layers, tracing, guides, proper typography, and a hundred other things.

Plus, and perhaps I'm just being a snob here, figures created in PowerPoint make it look like you just don't care. Do yourself a favour: take half a day to teach yourself to use Inkscape, and make beautiful figures for the rest of your career.

### Matt Hall

Matt is a geoscientist in Nova Scotia, Canada. Founder of Agile Scientific, co-founder of The HUB South Shore. Matt is into geology, geophysics, and machine learning.

# How to choose an image format

Choosing a file format for scientific images can be tricky. It seems simple enough on the outside, but the details turn out to be full of nuance and gotchas. Plenty of papers and presentations are spoiled by low quality images. Don't let yours be one! Get to know your image editor (I recommend GIMP), and your formats.

### What determines quality?

The factors determining the quality of an image are:

• The number of pixels in the image (aim for 1 million)
• The size of the image (large images need more pixels)
• If the image is compressed, e.g. a JPG, the fidelity of the compression (use 90% or more)
• If the image is indexed, e.g. a GIF, the number of colours available (the bit-depth)

Beware: what really matters is the lowest-quality version of the image file over its entire history. In other words, it doesn't matter if you have a 1200 × 800 TIF today, if this same file was previously saved as a 600 × 400 GIF with 16 colours. You will never get the lost pixels or bit-depth back, though you can try to mitigate the quality loss with filters and careful editing. This seems obvious, but I have seen it catch people out.

### JPG is only for photographs

Click on the image to see some artifacts.The problem with JPG is that the lossy compression can bite you, even if you're careful. What is lossy compression? The JPEG algorithm makes files much smaller by throwing some of the data away. It 'decides' which data to discard based on the smoothness of the image in the wavenumber domain, in which the algorithm looks for a property called sparseness. Once discarded, the data cannot be recovered. In discontinuous data — images with lots of variance or hard edges — you might see artifacts (e.g. see How to cheat at spot the difference). Bottom line: only use JPG for photographs with lots of pixels.

### Formats in a nutshell

Rather than list advantages and disadvantages exhaustively, I've tried to summarize everything you need to know in the table below. There are lots of other formats, but you can do almost anything with the ones I've listed... except BMP, which you should just avoid completely. A couple of footnotes: PGM is strictly for geeks only; GIF is alone in supporting animation (animations are easy to make in GIMP).

All this advice could have been much shorter: use PNG for everything. Unless file size is your main concern, or you need special features like animation or georeferencing, you really can't go wrong.

There's a version of this post on SubSurfWiki. Feel free to edit it!

### Matt Hall

Matt is a geoscientist in Nova Scotia, Canada. Founder of Agile Scientific, co-founder of The HUB South Shore. Matt is into geology, geophysics, and machine learning.

# Polarity cartoons

...it is good practice to state polarity explicitly in discussions of seismic data, with some phrase such as: 'increase in impedance downwards gives rise to a red (or blue, or black) loop.'
Bacon, Simm & Redshaw (2003), 3D Seismic Interpretation, Cambridge

Good practice, but what a mouthful. And perhaps because it is such a cumbersome convention, it is often ignored, assumed, or skipped. We'd like to change that. Polarity is important, everyone needs to know what the wiggles (or colours) of seismic data mean.

### Two important things

Click the image to find your favorite colorbarSeismic data is about contrasts. The data are an abstraction of geological contrasts in the earth. To connect the data to the geology, there are two important things you need to know about your data:

1. What do the colours mean in terms of digits?
2. What do the digits mean in terms of impedance?

So whenever you show seismic to someone, you need to answers these questions for them. Show the colourbar, and the sign of the digits (the magnitude of the digits is not very important; amplitude are relative). Show the relationship between the sign of the digits and impedance.

### Really useful

To help you show these things, we have created a table of polarity cartoons for some common colour scales.

1. Decide if you want to use the American–Canadian convention of a downwards increase in impedance resulting in a positive amplitude, or the opposite European–Australian convention. Sometimes people talk about SEG Normal polarity — the de facto SEG standard is the American convention.
2. Choose whether you want to show a high impedance layer sandwiched between low impedance ones, or vice versa. To make this decision, inspect your well ties or plot impedance against lithology. For example, if your sands are relatively fast and dense, you may want to choose the hard layer option.
3. Select a colourmap that matches your displays. If you need another, you can download and edit the SVG file, or email us and we'll add it for you.
4. Right-click on a thumbnail, copy it to your clipboard, and paste it into the corner of your section or timeslice in PowerPoint, Word, or wherever. If the thumbnail is too small or pixelated, click the thumbnail for higher resolution.

With so many options to choose from, we hope this little tool can help make your seismic discussions a little more transparent. What's more, if you see a seismic section without a legend like this, then are you sure the presenter knows about the polarity of their data? Perhaps they do, but it is an oversight to assume that you should know as well.

What do you make your audience assume?

# How to cheat at spot the difference

Yesterday I left you, dear reader, with a spot the difference puzzle. Here it is again, with my answer:

Notice how my answer (made with GIMP) is not just a list of differences or a squiggly circle around each one. It's an exact map of the location and nature of every difference. I like the effect of seeing which 'direction' the difference goes in: blue things are in the left image but not the right. One flaw in this method is that I have reduced the image to a monochrome image; changes in colour only would not show up.

Another way to do it, a way that would catch even a subtle colour change, is to simply difference the images. Let's look at a detail from the image—the yellow box; the difference is the centre image:

The right-hand image here is a further processing of the difference, using a process in ImageJ that inverts the pixels' values, making dark things bright and viceversa. This reveals a difference we would probably never have otherwise noticed: the footprint of the lossy JPEG compression kernel. Even though the two input images were compressed with 98% fidelity, we have introduced a subtle, but pervasive, artifact.

So what? Is this just an image processing gimmick? It depends how much you care about finding these differences. Not only was it easier to find all the differences this way, but now I know for certain that I have not missed any. We even see one or two very tiny differences that were surely unintentional (there's one just next to the cat's right paw). If differences (or similarities) mean a lot to you, because a medical prognosis or well location depends on their identification, the small ones might be very important!

Here's a small tutorial showing how I made the line difference, in case you are interested →