2013 Honorary LecturerSponsored by Shell

Pacific South

Dave Isles

Perth, Australia

Aeromagnetics–A Driver for Discovery & Development of Earth Resources

 
Pre-Tour Article

Aeromagnetics: Take the time... get the value

Dave Isles

Few (if any) geological settings can conceal their secrets from aeromagnetic surveys.
One of the most telling (and successful) aeromagnetic applications I have experienced, was in a Carboniferous limestone package where the rocks were (almost) nonmagnetic and the aeromagnetic signals were around 2nT. This package is explored for both hydrocarbons and lead-zinc deposits and aeromag has had a role to play in both scenarios. I will show you this example in my lecture.

Geophysical training channels us in to thinking aeromagnetic "anomalies", advanced modeling/3D inversion and drilling the big highs (or lows) to make discoveries... but aeromag is much more than this... The subtle geological clues in aeromag are much more likely to be the keys to discovery than the obvious ones... but it takes time to observe, record and interpret these subtle clues.

If we take the time, we get the value.

What information is actually in magnetic data?

  • Lithology/stratigraphy?
    Yes, different rock assemblages have different magnetic  mineral assemblages
  • Structure?
    Definitely! We see folds, faults, fractures and deep-seated lineaments
  • Metamorphism?
    Yes, metamorphism can create and destroy magnetic minerals
  • Metasomatism?
    Yes, again, mag destruction and mag creation can be seen magnetometer surveys
  • Mineralization?
    Both hard rock and hydrocarbon mineral systems may include magnetic minerals...

But can we see these variations in AEROmagnetics?

My word you can. Aeromag will see magnetic mineral concentrations of less than 0.005% and, given appropriate volumes, will see magnetic rock bodies to depths of many kilometers.

Surely aeromag does its best work looking for iron ore?

We try, but the best iron-ore bodies are dominated by nonmagnetic or weakly magnetic minerals like haematite and goethite. The magnetic magnetite ore-bodies are the "leftovers".

But minerals like gold aren't magnetic. Surely you can't use aeromag for gold exploration?

Aeromag has played a leading role in gold exploration for nearly 30 years. It has been critical to discovery in Australia, and is now playing much the same role in the burgeoning African gold exploration scene… and in porphyry copper exploration in the Andes and Southeast Asia!! Aeromag is now part of the planning in most mineral exploration programs.

But not hydrocarbons, right?

Wrong. Aeromag not only gives a rapid and inexpensive view of basin architecture (and has done so since the 1950s) but we now see structural and stratigraphic signatures in the sedimentary section and, even more exciting, we sometimes see direct responses from salt and from magnetic mineral alteration assemblages in oil and gas fields.

OK, aeromag can do everything! How does it work and what are its limitations?

Modern magnetometers and modern acquisition practice allow us to see tiny magnetic mineral variations and magnetic rocks at depths of many kms. Most rocks have some magnetic mineral content and this will be recorded in aeromagnetic survey data largely irrespective of depth and cover type (vegetation, water, sand, etc). In an ideal (local) situation, we can assemble a 3D map of magnetic mineral content that will reflect some and sometimes most of the geological attributes mentioned above.

In practice we focus on particular geological attributes and tune our acquisition, processing and interpretation strategies to these attributes. A common thread is to cherry-pick the information we need (or think we need) from the survey data and largely ignore the wealth of other geological data recorded. THIS IS A BIG MISTAKE…..HUGE!

We are very rarely in a position to predict what an aeromag survey will deliver for us, and (in my experience) surprises are the norm. So let us not try to be too clever and prescriptive, and let us read the geological story presented to us by the aeromagnetics (hand in hand with pre-existing geological data, of course) and be on the look-out for the clues to discovery.

OK, how do we do this? What hardware and software do we need?

The hardware is easy. The neck-top computer, duly trained to be receptive of geological concepts. A keen eye and an open (geological) mind are good hardware "accessory options." A passion for new discovery and sound, basic geoscientific training is the software to be installed on the neck-top computer. That is, we use our eyes and our geoscientific brain to observe, record and interpret the patterns in aeromagnetic images. Fancy imaging and modeling has an important place, but neither has value unless it is applied in the appropriate geological context.

There's a theme developing here. Are you saying that aeromagnetics is a geological and not geophysical tool?

It is a geoscientific tool. Some understanding of the physics of the method is needed but the primary input to aeromagnetic interpretation is geology. Without a geological theme, the aeromag has very little value. Successful aeromagnetic interpretation requires input from both disciplines, and ideally a team including specialists in both. As a minimum, the interpreter should have a working knowledge of both, and an appreciation of the strengths and weaknesses of each of the data types. The "best" person to interpret a data set is the one most closely engaged with the project objectives. Remotely based "specialist advisors" are rarely the best choice.

So what is your objective in delivering the course/lecture?

To inspire people to look deeply and carefully into aeromag images and find the clues that lead to discovery.

How will you do this?

Case studies have always been a great learning experience for me. The four studies that I present in the lecture show how aeromag can peel back the cover and flesh out an otherwise sketchy geological picture. The Kalgoorlie study shown is the ‘model' application and I will show more recent examples where aeromag has been a major ingredient in commercial discovery.

Why are you doing this?

I have had 40 years of fun in geophysical exploration, mostly revolving around aeromag applications, and I feel the need to share and perpetuate this fun!

The Kalgoorlie goldfield

Figure:

The Kalgoorlie goldfield
(scale bar is 2 kilometres)

  1. Bedrock exposure (green)
     
  2. 200m spaced aeromagnetics
     
  3. Interpreter's observations
     
  4. Compiled interpretation

The magnetic rock unit dgm is the Golden Mile Dolerite. It is the dominant host to the c. 80Moz of gold in the district.

 

Data courtesy of Fugro Airborne Surveys and GSWA.