The Forgotten Tool That Shaped Leapfrog

Before the internet and mobile phones—back in the 1990s when I sported a Lady Di hairdo, this is how I measured bedding and other planar surfaces. Hairdos may have changed (thankfully), but my method hasn’t, and that’s what I’m going to share with you today.

Now, let’s talk gear.

I was as student, so I used the cheapest Brunton compass I could get my hands on—the just released plastic Brunton did the job just fine (although these days, at around USD $750, they are expensive!). However, any budget compass, such as a Suunto, will work, but the real secret weapon was an essential tool introduced to me as an undergraduate by Patrick Conaghan at Macquarie University in the 1980s. Without this instrument, rapid accurate measurements would not have been possible using a cheap compass.

Sure, what I am going to describe to you can be accomplished with a mobile phone or a high-end compass like the Freiberg.

But let’s be honest—those are pricey, and most students of geology don’t have money to burn. Plus, splurging on an expensive compass doesn’t magically improve your measurements or assist with your structural understanding. ?

In fact, I’d argue using this secret weapon will make you smarter as a structural geologist in the field

So, let’s cut through the nonsense and get back to the practical approach that works for any geologist for any budget, shall we?

The “Dip Frisbee”

This extra equipment that I’m referring to is the “Dip Frisbee”. That’s not its official name, but it was christened this name at Macquarie University because the device looked like a Frisbee (Figures 1 and 2).

The original device, designed by Pryor (1958), bore the rather sleep-inducing name “dip-direction indicator” and was featured in Potter and Pettijohn’s classic sedimentology textbook, Paleocurrents and Basin Analysis (1977, p.100). It was specifically designed to measure accurate palaeocurrent directions by placing the disk on cross-bed foresets, with only the dip-direction line marked on the disk (Figure 3).

Unlike the original's blunted edge (Figure 3), the Macquarie University version of the Dip Frisbee was a precision-machined, non-magnetic aluminium plate with two key modifications to the original design:

• It has a tapered edge; and
• narrow slots parallel to the strike and the dip line

These modifications were made by Patrick Conaghan who was my sedimentology lecturer and my Honours thesis supervisor and the design went through several iterations before it settled on a 2mm thick stainless steel version (Figure 4).

"This stainless steel version of the frisbee was very practicable because it was light to carry and structurally robust" — Pat Conaghan

Students often asked if they could keep a Dip Frisbee for themselves, as they quickly realised how effortless it made structural measurements. We laughed at how the rest of the world was still relying on folders to take readings in the field. It’s been over 40 years since my undergraduate days at Macquarie University, yet it seems little has changed. I’m still amazed that no one else has come up with anything as ingenious as the Dip Frisbee.

Measuring low angle dips

The Dip Frisbee is great for measuring low dip angles, as illustrated in Figures 5-7.

The slots also allow for easy marking of rocks, ensuring samples can be accurately oriented, as shown in the following images. This makes it simple to mark and orient samples for transport back to the office (Figures 9 and 10).

Measuring steep dip angles

For steeply dipping planes, such as foliated tectonites, the tapered edge can be wedged into cleaved rock, while the slots enable easy visualisation of strike and dip (Figure 11–15).

When measuring planar surfaces visible in two planes, the Dip Frisbee can be aligned by eye with the two exposed traces, as shown in Figures 17–19.

The Dip Frisbee provides a highly visual and intuitive way to measure planar surfaces, making it far more engaging and easier to grasp than using a mobile phone or a Freiberg compass.

Pat Conaghan recalls:

"I had to start putting numbers in the frisbees for record-keeping purposes because when loaned to undergraduates in field courses and Honours students and higher degree students they were so popular that it was hard to get them back"

The Dip Frisbee represents the tangent plane of a curved surface with strike and dip-line clearly marked, offering a practical visual aid for structural data acquisition.

Using this method, thousands of measurements in the field helped me to refine my structural thinking and develop techniques over the years—techniques that ultimately shaped the methods that are now programmed into Leapfrog software. If you compare the simple interface of Leapfrog structural data entry interface for the Moving Plane (Figure 20), it contrasts with the complex array of choices that you are faced using other software products.

Leapfrog's simple interface comes from my personal practical field experience using the Dip Frisbee, and knowing what is the most efficient and error-proof method of structural data entry that's available to us. For example, during my undergraduate years, a strong emphasis was made by Pat Conaghan and structural geology lecturer Chris Powell was for students to habitually record the dip-azimuth of the planar orientation instead of the strike-line azimuth.

This eliminated the confusion and various definitions used internationally of recording the strike orientation. If you don’t introduce this simple method, disaster can strike [pun intended!].

I analysed multiple regional strike and dip measurements of planar structures from mine sites, but in some cases, the company failed to specify whether they used the right-hand or left-hand rule for strike. As a result, thousands of measurements—representing years of work—had to be discarded.

The original Leapfrog software required users to enter planar data as dip-azimuth, preventing any data entry errors. Linear data pitch was always measured from the right-hand strike line, constrained between 0° and 180°—a simple, foolproof system.

It’s no accident that planar measurement disks (S) in Leapfrog are circular, not square—just like the Dip Frisbee I used in the field (Figure 21).

There is nothing more that is required to define an ellipsoid orientation in space as shown in my own software OREFIND (See cross-eyed stereo images of Figure 22).

It’s obvious when a software feature is designed by a programmer with no real geological field experience or geological knowledge. After I stopped contributing to Leapfrog’s original design, impractical, time-wasting features crept into Leapfrog Geo—features that actively hinder modelling and push geologists to the brink like lemmings to the slaughter. But that’s a topic for another time!

Obtaining rapid and accurate structural measurements need not be expensive

A key advantage of the Dip Frisbee approach is that you can easily create one using a plastic 360° protractor with a spirit level attached—both of which can be sourced from Alibaba for about USD $3 (Figure 21). Pair it with an inexpensive plastic compass like a Suunto, and you’re set to collect reliable structural measurements for under USD $40.

I’d say give it a go!

References

Potter, P. E., & Pettijohn, F. J. (1977). Paleocurrents and Basin Analysis (Second Edition). Springer-Verlag, New York.

Pryor, W. (1958). Dip Direction Indicator. Journal of Sedimentary Petrology, 28, 230.

Acknowledgements

I really appreciate the effort Pat Conaghan went into designing the Dip Frisbee back in the 1970s. This was a great demonstration of how great inventions can be made from improving an original idea made by others. A big thanks also to Paul Godin and Kathleen Kemp for being fantastic models back in the 1990s. If anyone knows Kathleen’s whereabouts, I’d appreciate any contact, as I haven’t been in touch her for decades. Last I heard, she was living in Melbourne (she is originally from Toronto).

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Jun Cowan is a structural geological consultant, specialising in interpreting mineral deposits at the deposit-scale. He is the conceptual founder of Leapfrog software, which is now used by many international mining and mineral exploration companies (Leapfrog software resulted from private R&D collaboration undertaken by a joint venture between SRK Consulting Australasia, where Jun worked, and New Zealand company, ARANZ). Out of his home in Fremantle, Western Australia, he consults to mineral industry clients around the world and enjoys sharing his crazy ideas with his clients, and with online colleagues. This and other articles, mainly focused on geological subjects, are available from LinkedIn.