There’s an old joke in the exploration business. A mining company hires a geophysics company to survey their property. The president of the mining company looks at all the colorful diagrams that result and asks, “Okay, so where’s the deposit?” The geophysicist shrugs and replies, “Where do you want it to be?”
I might chalk this up to rivalry between geologists and geophysicists, except that I’ve heard geophysicists tell the joke.
I can explain the real reason with another geo-joke. One of the tools geophysicists use is called Induced Polarization (IP). But folks in exploration like to say that IP really stands for “it’s pyrite.” That’s because IP can detect many minerals, including worthless pyrite (iron sulfide—fool’s gold) which is far more common than real gold.
Further causing early hair loss among geologists is that sometimes gold occurs with or in pyrite. So you can’t just ignore it when you find pyrite; you have to spend time and money testing it to see if the fool’s gold comes with real gold. IP also detects copper sulfides and other metallic minerals which may or may not contain gold, or may or may not be valuable themselves.
It’s complicated.
The business is like working out enormous puzzles bigger than mountains—but after running the pieces through a planet-sized blender, so they no longer fit well together.
Don’t get me wrong: I’m not saying that geophysics isn’t useful.
But it’s important to understand what it is and what it can do—and what it can’t.
Conceptually, geophysics is the study of the physical properties of the earth—including its rocks. Obviously, that’s a good thing for explorers; the more we understand the rocks we’re exploring, the more likely we are to find what we’re looking for, if it’s there.
In practice, however, geophysics is really a science still in its infancy. One of its greatest limitations is that the most frequently used tools in the geophysicist’s toolbox are noninvasive tests that take measurements of distant materials that remain unseen. These tools include several forms of IP, magnetic surveys, and radiometric (radiation) surveys.
The thing is that identical readings can be caused by different things. Multiple readings from multiple tools help reduce the number of possible explanations for the readings. Uranium, for example, is far from the only radioactive element. So is potassium—and let’s not forget carbon-14. At the end of the day, what these tools give us is a theory about what might be causing the readings. It’s not a fact. It’s not an image. It’s not a map. It’s a hypothesis.
A vital geophysical tool that’s different from the others is the seismic survey. The big difference is that when a seismic survey shoots sound waves through the Earth, they are reflected back by the various structures encountered. Thus, the result of a seismic survey is an actual image.
But don’t get too excited. The image is like an X-ray, which might show where my stomach is, but not what I had for lunch. Seismic can show where one rock type gives way to another, if they’re different enough. But that doesn’t tell us what’s in the rocks. In the same way IP is great for finding sulfides, which so often turn out to be worthless pyrite, a seismic survey could easily detect quartz veins—but these too are common and usually barren.
Why not combine the two? We could shoot 3D seismic and run the other tools over the same area, but two things hold us back. The first is that while seismic can pinpoint structures in the Earth’s crust, the other tools are better at finding larger targets. The second is that 3D seismic is expensive.
This is why we often see seismic surveys used in exploration for oil, but not metals. Oil is often found in a structural “trap” in the earth. If we can see a good trap in the right setting, the odds are not bad that we’ll find oil. And the same well we use to explore for it becomes our “mine” to exploit it, quickly paying us back if the well flows at good volumes.
The astute reader might ask: “If we have a gold-bearing quartz vein right on surface, couldn’t we use seismic to tell us which way it dips into the Earth and how big it is?”
Yes, we could. But that wouldn’t tell us if it continues to bear gold at depth. We know that even very rich veins vary in how much metal they contain, often having higher- and lower-grade zones. That means we’d still have to do just as much drilling to know what we have—and a couple initial exploration holes could give us the orientation of the vein in the Earth for much less money than a seismic survey.
Like I said: it’s complicated.
But again, I’m not knocking geophysics. The giant puzzles geologists try to work out are so complicated, they are happy to have all the help they can get. Most geophysics gives us theories rather than images, but those theories are often pretty good. When you’re not Superman and can’t see through rock, a good theory can be a great help.
Key takeaways:
- The colorful maps and cutaway sections we get as results of geophysical surveys are not maps or pictures. Unless we’re looking at seismic data, they are not images at all. They are depictions of theories that could explain the readings taken. They can be useful, but don’t be too impressed. Large grains of salt are advised.
- When we do look at those colorful cartoons, the same colors do not always mean the same thing. A magnetic low could be a bad thing, for example, if it means our metals have been dissolved and removed by fluids moving through the Earth’s crust. But it could be a good thing if it means fluids have come through, removing worthless magnetic minerals and depositing valuable non-magnetic minerals.
- Let’s say that we are looking for something that might result in magnetic and chargeability highs in a survey. We look at the map-like survey results and see highs in both measurements in the same place (“coincident highs”). For reasons having to do with how minerals are deposited, the best mineralization is often on the margins of such zones, rather than in the bull’s eye.
- In some settings, especially when exploring for veins, it’s common to see companies do no geophysics at all. Even with an unlimited budget, it might be better to just drill.
- In other settings, especially when exploring for big, disseminated targets, it’s common to see a lot of geophysics before any drilling is done at all. The areas of visible alteration and the chemical signature on the surface can be so vast that even a large deposit is like a needle in a haystack. And needles are magnetic…
Bottom line: which tools to use and how much to use them often comes down to questions of budget and timing. Making these calls is no simple task.
That’s one of many reasons why, before I invest, I want to see management with experience with the specific type of deposit in question. If I’m putting my money into a company exploring a copper porphyry, for instance, I want a chief geologist with decades of copper porphyry exploration under his or her belt. A career exploring for gold in quartz veins would not prepare him or her to make the right call on the geophysics, and many other things. (What’s a porphyry? See this article for the basics on different types of deposits.)
Caveat emptor,
