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How is water in the ground measured? (GeoShort) · PlanetGeo: The Geology Podcast

• 0:08
  SPEAKER_00: Okay, welcome to Planet Geo Shorts, where we answer listener questions, deal with misconceptions, and just kind of elaborate in about three or four minutes.
• 0:17
  SPEAKER_01: Today we're gonna answer a question.
• 0:19
  SPEAKER_01: Yeah.
• 0:20
  SPEAKER_01: And the question deals with the issue of how do we measure groundwater?
• 0:24
  SPEAKER_01: We're dealing with water that's hundreds of feet below our surface, hundreds to sometimes thousands of feet down below the surface.
• 0:31
  SPEAKER_01: So, how do we measure this?
• 0:32
  SPEAKER_01: How do we know what's going on?
• 0:34
  SPEAKER_01: So what do you think, Jess?
• 0:37
  SPEAKER_00: Yeah, so I mean it's a great question.
• 0:39
  SPEAKER_00: Absolutely excellent question.
• 0:40
  SPEAKER_00: And we have some pretty rudimentary techniques that we use to measure groundwater, and then we have some really advanced stuff as well.
• 0:47
  SPEAKER_00: And the rudimentary stuff is just what you'd imagine.
• 0:49
  SPEAKER_00: Like drill a hole under the ground and stick a tape measure down there and see when you hit water.
• 0:54
  SPEAKER_00: I mean, that's basically it.
• 0:55
  SPEAKER_00: Like, where is the water table, which means where is the water saturating the sediment or the rock beneath you?
• 1:03
  SPEAKER_00: So that's the really rudimentary stuff, and and you can do that in your backyard.
• 1:07
  SPEAKER_01: Yeah, the geological surveys have these wells all over the place, and they they're monitoring wells, and they drop literally a measuring tape down there, and when it detects water, it beeps at them.
• 1:16
  SPEAKER_00: Yeah, exactly.
• 1:17
  SPEAKER_00: And so they have these constant measuring stations which are always measuring the groundwater, measuring it in real time, and sending data back home and all that stuff.
• 1:25
  SPEAKER_00: So, you know, but that's that's still pretty rudimentary stuff.
• 1:28
  SPEAKER_01: It is, and it's very set to the location where you have the wells and so on.
• 1:33
  SPEAKER_01: Now, so there's another thing that is really fascinating to me, and it's called Grace, and it stands for Gravity Recovery and Climate Experiment.
• 1:41
  SPEAKER_00: Yeah, this is so cool.
• 1:43
  SPEAKER_01: And this is a NASA project.
• 1:45
  SPEAKER_01: What they essentially rely on, they use satellites that so you're talking about basically measuring groundwater levels from space.
• 1:54
  SPEAKER_00: So this is unintuitive, right?
• 1:55
  SPEAKER_00: Because it's not using a video camera to like look for groundwater or anything like that, right?
• 2:00
  SPEAKER_01: Right.
• 2:00
  SPEAKER_01: No, it relies on mass.
• 2:02
  SPEAKER_01: The fact that you know, with so groundwater in the pore space of the rock below the surface and so on, it takes up space and it has a a lot of mass because we're talking about a lot of groundwater.
• 2:12
  SPEAKER_01: Right.
• 2:12
  SPEAKER_01: And mass has has gravity.
• 2:15
  SPEAKER_01: And so if you have an area with a lot of groundwater in it, it's gonna exert a stronger gravitational pull than something that doesn't have a lot of water in it.
• 2:24
  SPEAKER_01: Okay, it's kind of like looking at mountains.
• 2:26
  SPEAKER_01: Mountains have a lot of mass.
• 2:28
  SPEAKER_01: Right.
• 2:28
  SPEAKER_01: We humans don't feel that mass, but these satellites do.
• 2:31
  SPEAKER_01: So what we have are these satellites that are out orbiting the earth constantly, they travel 137 miles apart from each other.
• 2:39
  SPEAKER_00: So it's a pair of satellites that are traveling with each other, basically.
• 2:43
  SPEAKER_01: It's a pair, yeah.
• 2:45
  SPEAKER_01: So as the lead satellite comes onto an area that has a higher gravitational pull, it that satellite picks up speed.
• 2:53
  SPEAKER_01: And then the satellite trailing it detects that distance between it and then will pick up speed to make up the difference.
• 3:01
  SPEAKER_01: And then when the satellite leaves the area where there's not a lot of groundwater, that lead satellite will slow down, and the trailing satellite will feel that and slow down likewise.
• 3:12
  SPEAKER_01: So they're measuring the distance between them, and that translates into the mass of the groundwater that is below them.
• 3:20
  SPEAKER_01: So it's like this gigantic scale in the sky.
• 3:24
  SPEAKER_01: It is absolutely amazing.
• 3:26
  SPEAKER_01: We're talking about Jesse, we're talking about measuring microns of meters.
• 3:31
  SPEAKER_01: Like we're I mean, we're talking about very small distances between these satellites, yeah, and it translates into mass below them.
• 3:39
  SPEAKER_00: Right.
• 3:39
  SPEAKER_00: So yeah, just look, I'm just looking at the website right now, and the mission has they have a little elapsed time dial here.
• 3:45
  SPEAKER_00: It's been ongoing for 6,834 days in 15 hours so far.
• 3:50
  SPEAKER_00: That's pretty cool.
• 3:52
  SPEAKER_01: So the first mission lasted 15 years before it ran out of fuel.
• 3:56
  SPEAKER_01: Fifteen years.
• 3:57
  SPEAKER_01: Wow.
• 3:57
  SPEAKER_01: So in 2018, they launched what's called Grace Foe for follow-on.
• 4:04
  SPEAKER_01: So now we have this same system, but more technologically advanced, um, that is still measuring groundwater levels.
• 4:13
  SPEAKER_01: And what it does, Jesse, is this gives us an ability to track water movement below the surface from satellites in the sky.
• 4:21
  SPEAKER_01: Absolutely amazing.
• 4:22
  SPEAKER_00: And it gives us prediction ability.
• 4:23
  SPEAKER_00: So, you know, basically what it's doing, you can go out, you can log on to the NASA Grace website and you can look at are you in a drought stage in your area?
• 4:31
  SPEAKER_00: And and it also has a prediction for 60 and 90 days out in advance of the you know, the weather patterns in the groundwater capacity, basically.
• 4:39
  SPEAKER_00: So it's extremely powerful technology and extremely technologically advanced stuff.
• 4:44
  SPEAKER_01: And it also gives us the ability to do this over the entire globe, right?
• 4:47
  SPEAKER_01: Which is unbelievable.
• 4:49
  SPEAKER_01: We can measure groundwater levels anywhere in the world, not just the United States, not just the United Kingdom, not just you know the Middle East or anything like that.
• 4:57
  SPEAKER_01: It's all over the place.
• 4:58
  SPEAKER_00: Yeah, so cool.
• 4:59
  SPEAKER_00: And then the last technology that we're gonna highlight here in which we measure groundwater is another sort of really technically advanced thing called magnetotellurics.
• 5:08
  SPEAKER_00: And this is basically using the resistivity of groundwater or the conductivity of water and detecting this beneath Earth's surface.
• 5:16
  SPEAKER_00: And I only really know about this because I have a future colleague that's just been hired at Penn State who does this kind of work, and she gave a talk uh about her science.
• 5:25
  SPEAKER_00: And basically, they're detecting aquifer systems on the east coast of North America.
• 5:30
  SPEAKER_00: So, this is freshwater aquifer systems that extend out underneath the ocean, out like 350 kilometers out underneath the ocean.
• 5:38
  SPEAKER_00: They're detecting it with these techniques, which is basically using using electromagnetism in Earth's atmosphere and detecting how that is transferred in the subsurface and determining if it's freshwater or groundwater.
• 5:49
  SPEAKER_00: And so, I mean, the physics is beyond me, but it's super interesting stuff.
• 5:52
  SPEAKER_00: And so it's another way to detect groundwater.
• 5:55
  SPEAKER_00: Did you go to that talk?
• 5:56
  SPEAKER_00: Oh, yeah, I went.
• 5:57
  SPEAKER_00: Um, yeah, her name is Dr.
• 5:58
  SPEAKER_00: Chloe Gustavson, and uh she does this amazing research, and and she's we're we're very lucky to be able to hire her.
• 6:04
  SPEAKER_00: She hasn't started at Penn State yet, but once she does, uh we should we should have her on brilliant Geo, actually.
• 6:09
  SPEAKER_00: Yeah, um Yeah, so with that, an excellent listener question that uh maybe got a little bit long-winded.
• 6:15
  SPEAKER_01: No, this was a Geo Short kind of yeah, Geo Short-ish.
• 6:20
  SPEAKER_01: Anyway, awesome stuff.
• 6:22
  SPEAKER_00: Yeah, totally awesome stuff.
• 6:23
  SPEAKER_00: Absolutely.
• 6:23
  SPEAKER_00: Keep sending us your listener questions.
• 6:25
  SPEAKER_00: We love them and we're happy to answer them.
• 6:27
  SPEAKER_00: And uh stay tuned for more Planet Geo.