A revolution in seismic acquisition?

We're in warm, sunny Calgary for the GeoConvention 2013. The conference feels like it's really embracing geophysics this year — in the past it's always felt more geological somehow. Even the exhibition floor felt dominated by geophysics. Someone we spoke to speculated that companies were holding their geological cards close to their chests, but the service companies are still happy to talk about (ahem, promote) their geophysical advances.

Are you at the conference? What do you think? Let us know in the comments.

We caught about 15 talks of the 100 or so on offer today. A few of them ignited the old whines about half-cocked proofs of efficacy. Why is it still acceptable to say that a particular seismic volume or inversion result is 'higher resolution' or 'more geological' with nothing more than a couple of sections or timeslices as evidence?

People are excited about designing seismic acquisition expressly for wavefield reconstruction. In a whole session devoted to the subject, for example, Mauricio Sacchi showed how randomization helps with regularization in processing, allowing us to either get better image quality, or to lower cost. It feels like the start of a new wave of innovation in acquisition, which has more than its fair share of recent innovation: multi-component, wide azimuth, dual-sensor, simultaneous source...

Is it a revolution? Or just the fallacy of new things looking revolutionary... until the next new thing? It's intriguing to the non-specialist. People are talking about 'beyond Nyquist' again, but this time without inducing howls of derision. We just spent an hour talking about it, and we think there's something deep going on... we're just not sure how to articulate it yet.

Unsolved problems

We were at the conference today, but really we are focused on the session we're hosting tomorrow morning. Along with a roomful of adventurous conference-goers (you're invited too!), looking for the most pressing questions in subsurface science. We start at 8 a.m. in Telus 101/102 on the main floor of the north building.

Well-tie workflow

We've had a couple of emails recently about well ties. Ever since my days as a Landmark workflow consultant, I've thought the process of calibrating seismic data to well data was one of the rockiest parts of the interpretation workflow—and not just because of SynTool. One might almost call the variety of approaches an unsolved problem.

Tying wells usually involves forward modeling a synthetic seismogram from sonic and density logs, then matching that synthetic to the seismic reflection data, thus producing a relationship between the logs (measured in depth) and the seismic (measured in travel time). Problems arise for all sorts of reasons: the quality of the logs, the quality of the seismic, confusion about handling the shallow section, confusion about integrating checkshots, confusion about wavelets, and the usability of the software. Like much of the rest of interpretation, there is science and judgment in equal measure. 

← Synthetic seismogram (right) from the reservoir section of the giant bitumen field Surmont, northern Alberta. The reservoir is only about 450 m deep, and about 70 m thick. From Hall (2009), Calgary GeoConvention. 

I'd go so far as to say that I think tying wells robustly is one of the unsolved problems of subsurface geoscience. How else can we explain the fact that any reasonably mature exploration project has at least 17 time-depth curves per well, with names like JLS_2002_fstk01_edit_cks_R24Hz_final?

My top tips

First, read up. White & Simm (2003) in First Break 21 (10) is excellent. Rachel Newrick's essays in 52 Things are essential. Next, think about the seismic volume you are trying to tie to. Keep it to the nears if possible (don't use a full-angle stack unless it's all you have). Use a volume with less filtering if you have it (and you should be asking for it). And get your datums straight, especially if you are on land: make certain your seismic datum is correct. Ask people, look at SEGY headers, but don't be satisfied with one data point.

Once that stuff is ironed out:

1. Chop any casing velocities or other non-data off the top of your log.
2. Edit as gently and objectively as possible. Some of those spikes might be geology.
3. Look at the bandwidth of your seismic and make an equivalent zero-phase wavelet.
4. Don't extract a wavelet till you have a few good ties with a zero-phase wavelet, then extract from several wells and average. Extracting wavelets is a whole other post...
5. Bulk shift the synthetic (e.g. by varying the replacement velocity) to make a good shallow event tie.
6. Stretch (or, less commonly, squeeze) the bottom of the log to match the deepest event you can. 
7. If possible, don't add any more tie points unless you really can't help yourself. Definitely no more than 5 tie points per well, and no closer than a couple of hundred milliseconds.
8. Capture all the relevant data for every well as you go (screenshot, replacement velocity, cross-correlation coefficient, residual phase, apparent frequency content).
9. Be careful with deviated wells; you might want to avoid tying the deviated section entirely and use verticals instead. If you go ahead, read your software's manual. Twice.
10. Do not trust any checkshot data you find in your project — always go back to the original survey (they are almost always loaded incorrectly, mainly because the datums are really confusing).
11. Get help before trying to load or interpret a VSP unless you really know what you are doing.

I could add some don'ts too...

1. Don't just match the well picks to the seismic horizons. I have seen this go wrong lots of times, including places where 'everyone knows what the picks are'.
2. Don't copy time-depth tables, well logs, or synthetics from one interval to another or one well to another — that way madness lies. If you need time-depth where you don't have it, the best idea is to build a velocity model.
3. Don't use a different approach, wavelet, window, etc., for different intervals or different wells unless you have a really good reason (e.g. you're in different 2D vintages, or there are dramatic, mappable changes in lithology). 
4. Don't tie wells to 2D seismic lines you have not balanced yet, unless you're doing it as part of the process of deciding how to balance the seismic. 
5. Don't create multiple, undocumented, obscurely named copies or almost-copies of well logs and synthetics, unless you want your seismic interpretation project to look like every seismic interpretation project I've ever seen (you don't).

Well ties are one of those things that get in the way of 'real' (i.e. fun) interpretation so they sometimes get brushed aside, left till later, rushed, or otherwise glossed over. Resist at all costs. If you mess them up and don't find out till later, you will be very sad, but not as sad as your exploration manager.

Update on Saturday, April 27, 2013 at 10:19 AM by Matt Hall

Can't resist posting this most excellent well tie. Possibly the best you'll ever see.

Picture by Maitri, licensed CC-BY-NC-SA

The elements of seismic interpretation

I dislike the term seismic interpretation. There. I said it. Not the activity itself, (which I love), just the term. Why? Well, I find it's too broad to describe all of the skills and techniques of those who make prospects. Like most jargon, it paradoxically confuses more than it conveys. Instead, use one of these three terms to describe what you are actually doing. Note: these tasks may be performed in series, but not in parallel.

Visualizing

To visualize is to 'make something visible to the eye'. That definition fits pretty well in what we want to do. We want to see our data. It sounds easy, but it is routinely done poorly. We need context for our data. Being able to change the way our data looks, exploring and exaggerating different perspectives and scales, symbolizing it with perceptually pleasant colors, displaying it alongside other relevant information, and so on.

Visualizing also means using seismic attributes. Being clever enough to judge which ones might be helpful, and analytical enough to evaluate from the range of choices. Even more broadly, visualizing is something that starts with acquisition and survey planning. In fact, the sum of processes that comprise the seismic experiment is to make the unseen visible to the eye. I think there is a lot of room left for bettering our techniques of visualization. Steve Lynch is leading the way on that.

Digitizing

One definition of digitizing is along the lines of 'converting pictures or sound into numbers for processing in a computer'. In seismic interpretation, this usually means capturing and annotating lines, points, and polygons, for making maps. The seismic interpreter may spend the majority of their time picking horizons; a kind of computer-assisted drawing. Seismic digitization, however, is both guided and biased by human labor in order to delineate geologic features requiring further visualization. 

Whether you call it picking, tracking, correlating or digitizing, seismic interpretation always involves some kind of drawing. Drawing is a skill that should be celebrated and practised often. Draw, sketch, illustrate what you see, and do it often. Even if your software doesn't let you draw it the way an artist should.

Modeling

The ultimate goal of the seismic interpreter, if not all geoscientists, is to unambiguously parameterize the present-day state of the earth. There is after all, only one true geologic reality manifested along only one timeline of events.

Even though we are teased by the sparse relics that comprise the rock record, the earth's dynamic history is unknowable. So what we do as interpreters is construct models that reflect the dynamic earth arriving at its current state.

Modeling is another potentially dangerous jargon word that has been tainted by ambiguity. But in the strictest sense, modeling defines the creative act of bringing geologic context to bear on visual and digital elements. Modeling is literally the process of constructing physical parameters of the earth that agree with all available observations, both visualized and digitized. It is the cognitive equivalent of solving a mathematical inverse problem. Yes, interpreters do inversions all the time, in their heads.

Good seismic interpretation requires practising each of these three elements. But indispensable seismic interpretation is achieved only when they are masterfully woven together.

Recommended reading
Steve Lynch's series of posts on wavefield visualization at 3rd Science is a good place to begin.

Six books about seismic interpretation

Last autumn Brian Romans asked about books on seismic interpretation. It made me realize two things: (1) there are loads of them out there, and (2) I hadn't read any of them. (I don't know what sort of light this confession casts on me as a seismic interpreter, but let's put that to one side for now.)

Here are the books I know about, in no particular order. Have I missed any? Let us know in the comments!

Introduction to Seismic Interpretation

AAPG
Amazon.com
Google Books

Bruce Hart, 2011, AAPG Discovery Series 16. Tulsa, USA: AAPG. List price USD 42.

This 'book' is a CD-based e-book, aimed at the new interpreter. Bruce is an interpreter geologist, so there's plenty of seismic stratigraphy.

A Petroleum Geologist's Guide to Seismic Reflection

Wiley
Amazon.com
Google Books

William Ashcroft, 2011. Chichester, UK: Wiley-Blackwell. List price USD 90.

I really, really like this book. It covers all the important topics and is not afraid to get quantitative — and it comes with a CD containing data and software to play with. 

Interpretation of Three-Dimensional Seismic Data

AAPG
Amazon.com
Google Books

Alistair Brown, AAPG Memoir. Tulsa, USA: AAPG. List price USD 115.

This book is big! Many people think of it as 'the' book on interpretation. The images are rather dated—the first edition was in 1986—but the advice is solid.

First Steps in Seismic Interpretation

SEG
Amazon.com
Google Books

Donald Herron, SEG. Tulsa, USA: SEG. List price USD 62.

This new book is tremendous, if a little pricey for its size. Don is a thoroughly geophysical interpreter with deep practical experience. A must-read for sub-salt pickers!

3D Seismic Interpretation

Cambridge
Amazon.com
Google Books

Bacon, Simm and Redshaw, 2003. Cambridge, UK: Cambridge. List price USD 80.

A nicely produced and comprehensive treatment with plenty of quantitative meat. Multi-author volumes seem a good idea for such a broad topic.

Elements of 3D Seismology

PennWell
Amazon.com
Google Books

Chris Liner, 2004. Tulsa, USA: PennWell Publishing. List price USD 129.

Chris Liner's book and CD are not about seismic interpretation, but would make a good companion to any of the more geologically inclined books here. Fairly hardcore.

The rest and the next

Out-of-print and old books, or ones that are less particularly about seismic interpretation:

• Practical Seismic Interpretation, Michael Badley, 1985
• Seismic Interpretation: The Physical Aspects, Nigel Anstey, 1977
• Pitfalls in Seismic Interpretation (Tucker & Yorston, 1973) and Pitfalls Revisited (Tucker, 1982)
• An Introduction to Geophysical Exploration, Keary, Brooks & Hill, 2002

An exciting new addition will be the forthcoming book from Wiley by Duncan Irving, Richard Davies, Mads Huuse, Chris Jackson, Simon Stewart and Ralph Daber — Seismic Interpretation: A Practical Approach. Look out for that one in 2014.

Watch out for our book reviews on all these books in the coming weeks and months.

Interpreting spectral gamma-ray logs

Before you can start interpreting spectral gamma-ray logs (or, indeed, any kind of data), you need to ask about quality.

Calibrate your tool...

The main issues affecting the quality of the logs are tool calibration and drilling mud composition. I think there's a tendency to assume that delivered logs have been rigorously quality checked, but... they haven't. The only safe assumption is that nobody cares about your logs as much as you. (There is a huge opportunity for service companies here — but in my experience they tend to be focused on speed and quantity, not quality.)

Calibration is critical. The measurement device in the tool consists of a thallium-laced NaI crystal and a photomultiplier. Both of these components are sensitive to temperature, so calibration is especially important when the temperature of the tool is changing often. If the surface temperature is very different from the downhole—winter in Canada—calibrate often.

Drilling mud containing KCl (to improve borehole stability) increases the apparent potassium content of the formation, while barite acts as a gamma-ray absorber and reduces the count rates, especially in the low energies (potassium).

One of the key quality control indicators is negative readings on the uranium log. A few negative values are normal, but many zero-crossings may indicate that the tool was improperly calibrated. It is imperative to quality control all of the logs, for bad readings and pick-up effects, before doing any quantitative work.

...and your interpretation

Most interpretations of spectral-gamma ray logs focus on the relationships between the three elemental concentrations. In particular, Th/K and Th/U are often used for petrophysical interpretation and log correlation. In calculating these ratios, Schlumberger uses the following cut-offs: if uranium < 0.5 then uranium = 0.5; if potassium < 0.004 then potassium = 0.001 (according to my reference manual for the natural gamma tool).

In general, high K values may be caused by the presence of potassium feldspars or micas. Glauconite usually produces a spike in the K log. High Th values may be associated with the presence of heavy minerals, particularly in channel deposits. Increased Th values may also be associated with an increased input of terrigenous clays. Increases in U are frequently associated with the presence of organic matter. For example, according to the ODP, particularly high U concentrations (> 5 ppm) and low Th/U ratios (< 2) often occur in black shale deposits.

The logs here, from Kansas Geological Survey open file 90-27 by Macfarlane et al. shows a quite overt interpretive approach, with the Th/K log labelled with minerals (feldspar, mica, illite–smectite) and the Th/U log in uranium 'fixedness', a proxy for organic matter.

Sounds useful. But really, you can probably find just a paper to support just about any interpretation you want to make. Which isn't to say that spectral gamma-ray is no use — it's just not diagnostic on its own. You need to calibrate it to your own basin and your own stratigraphy. This means careful, preferably quantitative, comparison of core and logs. 

Further reading 

• Ross Crain's Petrophysical Handbook is a mine of well logging goodness
• Paul Glover's MSc course notes are very comprehensive on most well logging topics
• Fabricius et al. (2003). Geological Survey of Denmark and Greenland Bulletin 1, 349–366. A case stuudy from the Danish Central Graben.