Innovative Technology Reveals Past, Present and Future of Water Resources (Part 1)
USGS Scientists Jim Cannia and Jared Abraham discuss the use of geophysics, a powerful new tool, in water resources studies in Nebraska
Rachael McLeod: Welcome to the fifth episode of Nebraskast where we talk with real USGS scientists about the important water resources work they’re doing all over Nebraska. My name is Rachael McLeod and I’m here with Jim Cannia of the Nebraska Water Science Center, he’s a hydrologist. I’m also here with Jared Abraham. He’s with the US Geological Survey also, out of Denver. What office are you with?
Jared Abraham: I’m with the Crustal Imaging and Characterization Team.
Rachael McLeod: OK and we’re going to talk a little bit today just sort of generally about the kind of work you two gentlemen do. We have a lot of different ways of trying to gather information about our water resources. Probably the oldest and best-known is drilling but you guys do something a little bit different. Just give us a general overview of the kind of technology that you use to gather data about water resources in the state.
Jim Cannia: Well, Rachael I think that’s a great question. As a matter of fact, we spent part of the day today explaining this technology and this team-wide effort to do this. Basically, what we do is we take traditional geologic and hydrogeologic methods and we combine them with geophysical methods. We’ll let Jared talk a little bit about geophysics end of it. I’m a traditional hydrologist but I can see the future and the future is doing this type of work.
The end result of all of this is good groundwater management based on good groundwater models, based on solid hydrogeologic frameworks.
Rachael McLeod: What about the geophysics, Jared? Some people referred to it as kind of a CAT scan of the Earth. Explain a little bit where that notion comes from.
Jared Abraham: Well, I think the analogy comes from the sense that geophysics allows you to look into the Earth without disturbing it, seeing the unseen. So just like a CAT scan -- when you go in for a CAT scan or an MRI, they’re not cutting into your body to see what’s in there. They’re actually imaging it from the outside. Geophysics is the study of the physics of the Earth and so we’re using physical properties. Those properties are electrical conductivity, magnetic susceptibility, velocity and using those technologies and techniques to sense these things and then correlating those to the geology. The physics of the Earth meaning the physical parameters that relate to geology, the geo, geophysicist.
Rachael McLeod: And what kind of questions are we able to answer with this data, Jim?
Jim Cannia: One of the things that’s facing the state of Nebraska right now is integrated water management of surface and groundwater. What we find is this computer-generated modeling gives us very unique and robust answers provided they have solid frameworks to rest upon. What does that mean? It means we need to know which direction the water is moving. We need to know how long it takes to get there. The only way you can come to those types of conclusions is to understand the subsurface. Traditionally, we’ve used things like boreholes, outcrop mapping, aquifer tests, etcetera. Now we’re on the very beginning of applying actual tools that can do this without invasive means.
Rachael McLeod: And what does the data collection process look like when you’re using this kind of technology?
Jared Abraham: Well, there’s different types of geophysical systems or sensors. The system that we’re deploying here in Nebraska is an airborne method, so putting it in at an airplane or helicopter, we are flying over the surface and measuring the electrical properties.
Rachael McLeod: Can you talk a little bit more about exactly what you’re measuring and what that tells you about the Earth’s properties as it relates to groundwater?
Jared Abraham: Sure. In Nebraska, we’re looking at a system that’s mostly governed by sands and gravels which are the aquifers, and silts and clays which are the aquitards. So the nice thing for the geophysicist is we come in there and say “Okay, sand and gravel, what’s their physical properties?” Well, the physical properties are they’re typically resistive meaning they don’t conduct electrical charge and energy as opposed to clays and silts which do conduct electrical charges better.
Now it’s about the contrasts. What I do is I make pictures of the Earth of where things are more resistive or less resistive. It takes ground truthing and a geological knowledge to put that into the images that the geologist and hydrologist use.
Rachael McLeod: And the ground truthing is what helps you make sure that the data you’re collecting from the air matches up with what reality is.
Jared Abraham: Absolutely. Traditionally, geophysics has been used in exploration industries: mineral, oil and gas, etc. And what they do is they use geophysics to fill in between known points, outcrops and boreholes or to place boreholes better. Drill here to find an aquifer. Drill here to find a mineral deposit. We try to connect the dots, really.
It’s like being in a room that’s completely dark and you turn on a pencil beam, OK. You know what’s under that pencil beam exactly. That’s a borehole. You believe it, you can see it. When you have the geophysics, it’s like turning on a light in the room. Now, not only do you see where that pencil beam was but you see maybe the chair that was next to the table, maybe a cabinet over in the corner.
Rachael McLeod: The geology in Nebraska is quite varied from east to west. Does this technology work all over the state?
Jim Cannia: So far, we’ve been successful in the majority of the state. We have tried several different terrains. Presently, we’ve tried heliborne electromagnetics which is somewhere in the middle of the tools that we can apply. We can use surface geophysics which has a much more specific and narrower band of resolution. It’s high-resolution equipment.
Heliborne electromagnetics is somewhere in the middle with kind of a medium resolution and we just recently tried heliborne time domain and Jared can talk a little more about that but it sees very deeply but it’s much more coarse. So where we’ve been trying this, we’ve been successful. There are areas where if you take a tool that shouldn’t be used there, you’re not going to be successful. But overall, I would say that we’ve met with success in most of the areas that we’ve tried this.
Rachael McLeod: And then just give us an overview of the kinds of different geology that you are working with here in the state.
Jim Cannia: In 2007, we started in the eastern part of Nebraska with heliborne electromagnetic surveys in the glacial till zone. We tried three different areas: heavy, medium and light glacial tills. We were very successful in the light and medium. The heavy glacial tills was just too conductive for the heliborne to work.
We’ve also tried the panhandle in Nebraska which is the opposite end of the state but also the opposite extreme geologically. Very recent geologic valleys filled with sands and gravels sitting on top of conductive materials, perfect for HEM. The results that we had out there are bar none some of the best we’ve seen. We’ve also tried it in the center of the state in the Sand Hills using a different technology, heliborne time domain. We just recently got those results back. It looks like we are seeing quite deep but the resolution seems to be a little less but quite a bit more work needs to be done on that.
Rachael McLeod: So we talked about the application of the data with modeling but you’re also looking at climate change. I’ve seen some of the pictures where you found some paleochannels. Can you talk a little bit more about those applications?
Jim Cannia: Yes. One of the things that we’re working with on climate change is how do we use the past to predict the future? What we’re able to do is use these new tools and conditions in sequence with our traditional methods to map the past climate and the depositions that took place because of it.
An example would be to look at peat fens in the Sand Hills. Peat fens are a product of biological activity. They are precursors to coal seams. They are very well illuminated by our geophysical methods. We can map those at depth throughout the Sand Hills and it gives a hint to a wetter, more vibrant type of situation. The other thing that we see are areas where it’s sand on top of those peats. The only way that could happen is a drier climate mobilizing the sand dunes, making the sand go out into what used to be a wetland, so we have all of these indicators of climate change that has happened in the last thousand years.