1 00:00:03,520 --> 00:00:06,760 This is Geology Bites with Oliver Strimpel. 2 00:00:08,360 --> 00:00:12,000 In this episode, we'll be talking about geodesy, and 3 00:00:12,000 --> 00:00:15,080 particularly about accurately determining positions on the 4 00:00:15,080 --> 00:00:18,720 Earth's surface and monitoring how these positions change over 5 00:00:18,720 --> 00:00:22,360 time. Over the past few decades, the 6 00:00:22,360 --> 00:00:26,160 technology involved has improved dramatically and it has become 7 00:00:26,160 --> 00:00:30,480 much more affordable. How can we use this to address 8 00:00:30,480 --> 00:00:34,120 geological questions such as how the Earth deforms during and 9 00:00:34,120 --> 00:00:37,920 after large earthquakes or to see plate motions in high 10 00:00:37,920 --> 00:00:42,120 resolution? Tom Herring is a pioneer in high 11 00:00:42,120 --> 00:00:46,320 precision geodetic analytical methods and applications for 12 00:00:46,320 --> 00:00:49,720 satellite based navigation systems to study the Earth's 13 00:00:49,720 --> 00:00:52,960 surface. He is a professor in the Earth, 14 00:00:53,120 --> 00:00:56,240 Atmospheric and Planetary Sciences Department at the 15 00:00:56,240 --> 00:00:58,520 Massachusetts Institute of Technology. 16 00:00:59,280 --> 00:01:01,720 Tom Herring, welcome to Geology Bytes. 17 00:01:01,840 --> 00:01:03,680 Thank you, Oliver for inviting me to your show. 18 00:01:03,720 --> 00:01:07,480 It's a pleasure to be here. Let's start with the technology 19 00:01:07,640 --> 00:01:11,360 of high precision geodetic measurements and then we'll talk 20 00:01:11,360 --> 00:01:13,680 about some geological applications. 21 00:01:14,200 --> 00:01:17,520 First of all, what kind of accuracy can we achieve with 22 00:01:17,520 --> 00:01:21,120 these systems now? Well, in the best cases we can 23 00:01:21,120 --> 00:01:25,000 make position determinations at sub millimeter accuracies 24 00:01:25,000 --> 00:01:27,560 horizontally and about 3mm in the height. 25 00:01:27,880 --> 00:01:31,000 The height's worse than the horizontal components just 26 00:01:31,000 --> 00:01:34,240 because of the geometry of satellite orbits being above us 27 00:01:34,360 --> 00:01:36,360 and the atmospheric refraction effects. 28 00:01:37,080 --> 00:01:40,320 Getting centimeter accuracy with these systems is actually pretty 29 00:01:40,320 --> 00:01:43,720 easy these days. The accuracy does depend on the 30 00:01:43,720 --> 00:01:47,120 averaging time and the type of system that you use, and for 31 00:01:47,120 --> 00:01:51,280 geologic processes we often use 24 hour average positions to 32 00:01:51,280 --> 00:01:53,880 reduce the noise. This is where the best accuracy 33 00:01:53,880 --> 00:01:57,920 is, but with systems such as GPS we can also determine those 34 00:01:57,920 --> 00:02:00,720 positions every time a measurement's made, which could 35 00:02:00,720 --> 00:02:04,000 be rates up to as 10 times or 100 times per second. 36 00:02:04,440 --> 00:02:06,840 The accuracy of those measurements is generally around 37 00:02:06,840 --> 00:02:09,680 a few millimetres if it's in a very good environment. 38 00:02:10,400 --> 00:02:15,200 OK, what are the main types of geodesic measurement systems 39 00:02:15,200 --> 00:02:18,080 that can achieve such impressive accuracy? 40 00:02:18,480 --> 00:02:20,800 There's three main types. There's the satellite based 41 00:02:20,800 --> 00:02:23,440 types, which are the the GPS system. 42 00:02:23,440 --> 00:02:27,080 The Global Positioning System is 1 and it uses radio waves. 43 00:02:27,280 --> 00:02:30,760 There are also laser systems that can send laser pulses up to 44 00:02:30,760 --> 00:02:34,520 satellites and the moon with specially installed corner cube 45 00:02:34,520 --> 00:02:37,000 reflectors. Those are very expensive 46 00:02:37,000 --> 00:02:41,360 systems. The GPS system is the US1, and 47 00:02:41,360 --> 00:02:45,040 these days we add to that a European Galileo system, which 48 00:02:45,040 --> 00:02:48,960 is of similar design. The Chinese Baidu system, and 49 00:02:48,960 --> 00:02:53,520 the Russian Glona system, which is of the same era as the GPS 50 00:02:53,520 --> 00:02:56,520 system is combined together. People refer to this as the 51 00:02:56,520 --> 00:02:59,560 Global Navigation Satellite System, or GNSS. 52 00:03:00,080 --> 00:03:03,560 The more satellites you have, the easier it is to make 53 00:03:03,560 --> 00:03:06,720 position nations when you're in obstructed sort of views. 54 00:03:07,400 --> 00:03:10,200 The other big radio technique that we use is called very long 55 00:03:10,200 --> 00:03:13,960 baseline interferometry, and it uses radio signals from 56 00:03:13,960 --> 00:03:17,760 extragalactic sources and those need to be observed with very 57 00:03:17,760 --> 00:03:20,920 large telescopes because of the weak signals involved. 58 00:03:21,520 --> 00:03:25,880 And finally, we have synthetic aperture radar, which is a 59 00:03:25,920 --> 00:03:29,440 technique that can be used for imaging deformation on the 60 00:03:29,440 --> 00:03:33,440 surface based on the returned signals from radars installed 61 00:03:33,440 --> 00:03:37,520 generally on satellites. OK, can you give us a high level 62 00:03:37,520 --> 00:03:40,800 explanation of how these technologies work? 63 00:03:41,120 --> 00:03:43,680 Let's start with satellite based systems. 64 00:03:44,160 --> 00:03:47,840 Yes, these systems work by a technique called trilateration, 65 00:03:48,080 --> 00:03:51,280 and it's very similar to triangulation except that it 66 00:03:51,280 --> 00:03:55,560 uses distance measurements rather than angles, and that 67 00:03:55,760 --> 00:03:59,160 distance measurement is what allows us to achieve range 68 00:03:59,160 --> 00:04:03,440 measurements to satellites from the ground with millimeter level 69 00:04:03,440 --> 00:04:06,760 accuracy. For GNSS, there's actually two 70 00:04:06,760 --> 00:04:09,800 types of measurements that are made, two types of signals that 71 00:04:09,800 --> 00:04:14,120 are interpreted and one of them is readily available in all 72 00:04:14,120 --> 00:04:17,480 positioning things such as cell phones, boats, car navigation 73 00:04:17,480 --> 00:04:21,920 systems, and there's a second version which allows very high 74 00:04:21,920 --> 00:04:26,200 accuracy and that used to cost typically about $20,000 per 75 00:04:26,200 --> 00:04:28,520 receiver. The radio signals that are 76 00:04:28,520 --> 00:04:31,120 transmitted from the GNSS satellites have wavelengths 77 00:04:31,120 --> 00:04:35,320 around about 200 millimetres. They're encoded in a way that 78 00:04:35,320 --> 00:04:37,720 different satellites can be easily separated from each 79 00:04:37,720 --> 00:04:41,960 other, and your ground receiver correlates the arrival time of 80 00:04:41,960 --> 00:04:46,320 the codes from the satellites with a replica of that code and 81 00:04:46,320 --> 00:04:48,440 measures the time difference between them. 82 00:04:48,840 --> 00:04:53,120 That time difference is related to the range to the spacecraft, 83 00:04:53,360 --> 00:04:57,480 but also to the errors in the clocks in both the spacecraft 84 00:04:57,480 --> 00:05:02,200 and the ground receivers. The the satellites can have very 85 00:05:02,200 --> 00:05:04,680 expensive clocks, but what we want to do is make the ground 86 00:05:04,680 --> 00:05:06,680 receivers have very inexpensive ones. 87 00:05:07,120 --> 00:05:10,360 And the great revolution that happened with GPS is that 88 00:05:10,360 --> 00:05:12,920 multiple satellites can be seen at the same time. 89 00:05:13,240 --> 00:05:17,960 So the position can be based on the measurements to three 90 00:05:17,960 --> 00:05:20,800 satellites. And then to estimate the clock 91 00:05:20,960 --> 00:05:23,640 you need a fourth satellite. And that allows you to have a 92 00:05:23,640 --> 00:05:26,040 very cheap clock in your receiver, which makes these 93 00:05:26,040 --> 00:05:29,920 receivers inexpensive. Typically these days with GNSS, 94 00:05:29,920 --> 00:05:34,160 we can see 25 to 30 satellites at any one time anywhere in the 95 00:05:34,160 --> 00:05:38,280 world. And that level of correlation 96 00:05:39,080 --> 00:05:42,680 with the codes is called shooter range positioning, and that's 97 00:05:42,680 --> 00:05:45,400 middle level positioning. The big thing that came from 98 00:05:45,400 --> 00:05:50,840 GNSS innovation was that we can look at the phase of the radio 99 00:05:50,840 --> 00:05:54,760 signal that's being transmitted. And since we can continuously 100 00:05:54,760 --> 00:05:58,720 track the spacecraft, we can count the number of cycles of 101 00:05:58,720 --> 00:06:01,840 phase as it goes through. And So what we end up getting is 102 00:06:01,840 --> 00:06:05,280 this range change measurement between the spacecraft when we 103 00:06:05,280 --> 00:06:08,200 first observe it and then as it moves through the sky. 104 00:06:08,720 --> 00:06:11,680 And that phase change is accurate to a couple of 105 00:06:11,680 --> 00:06:13,920 millimeters. And it's from that that we can 106 00:06:13,920 --> 00:06:17,080 determine these millimeter level positions down on the Earth's 107 00:06:17,080 --> 00:06:20,000 surface. We also need multiple 108 00:06:20,000 --> 00:06:23,360 frequencies because of the impacts of some of the 109 00:06:23,360 --> 00:06:28,040 atmospheric refraction effects. And that is the way, again, that 110 00:06:28,040 --> 00:06:31,200 we can eliminate some of the systematic errors of these types 111 00:06:31,200 --> 00:06:33,920 of measurements. That's impressive to get the 112 00:06:33,920 --> 00:06:37,600 cost down and I can see that if you can have a low cost clock, 113 00:06:37,600 --> 00:06:39,160 obviously that makes a big difference. 114 00:06:39,640 --> 00:06:45,480 But has the cost reduction also benefited from the same kind of 115 00:06:45,480 --> 00:06:49,120 thing as the integrated circuits have following Moore's Law over 116 00:06:49,120 --> 00:06:51,640 the decades? Yes, they have, although not for 117 00:06:51,640 --> 00:06:56,000 the same reason precisely the current techniques that are used 118 00:06:56,000 --> 00:06:58,320 to do millimetre level positioning, the receivers for 119 00:06:58,320 --> 00:07:03,280 that are typically cost around about $20,000 each and now 120 00:07:03,280 --> 00:07:08,160 they're less than about $700.00. And that reduction happens 121 00:07:08,320 --> 00:07:12,120 because of the way the GPS signal is transmitted. 122 00:07:12,400 --> 00:07:17,520 And so in the main part of the GPS system, there's additional 123 00:07:17,520 --> 00:07:21,840 codes that are transmitted with the signals that are required 124 00:07:21,840 --> 00:07:25,000 for anti spoofing protection for the US military. 125 00:07:25,520 --> 00:07:28,920 Those additional codes on the second frequencies that 126 00:07:28,920 --> 00:07:31,880 transmitted make it actually very difficult to track the 127 00:07:31,880 --> 00:07:36,960 signal and hence the cost of the receivers to have the technology 128 00:07:36,960 --> 00:07:40,320 to be able to do that. But starting about a decade ago, 129 00:07:40,520 --> 00:07:43,560 the military realized the importance of civilian 130 00:07:43,560 --> 00:07:45,720 applications of high precision positioning. 131 00:07:46,040 --> 00:07:50,160 And so they started to introduce 1/3 frequency on the spacecraft, 132 00:07:50,520 --> 00:07:53,920 which allows civilians to use that frequency. 133 00:07:54,240 --> 00:07:58,480 And they've also added some additional codes to the normal 134 00:07:58,840 --> 00:08:02,480 transmissions, which again make it easier for these receivers to 135 00:08:02,640 --> 00:08:04,640 receive these high accuracy signals. 136 00:08:05,600 --> 00:08:09,240 Now GPS satellites get launched at about 1:00 to 2:00 per year. 137 00:08:09,560 --> 00:08:14,040 So over the last decade, we now have about 17 or 18 of the 138 00:08:14,040 --> 00:08:17,840 satellites that are transmitting these signals readily available 139 00:08:17,840 --> 00:08:20,960 to civilians. These cheap receivers take 140 00:08:20,960 --> 00:08:23,320 advantage of that. They also take advantage of the 141 00:08:23,320 --> 00:08:26,360 fact that the Galileo system, which is now almost complete, 142 00:08:26,640 --> 00:08:29,360 transmit signals which are easily tracked, and the Chinese 143 00:08:29,360 --> 00:08:33,159 Baidu system, which is now on its third generation, also 144 00:08:33,159 --> 00:08:35,400 transmit signals that are easy to track. 145 00:08:35,640 --> 00:08:38,440 And so the combination of changing the way the signals are 146 00:08:38,440 --> 00:08:42,760 encoded has allowed these very inexpensive receivers to drop 147 00:08:42,760 --> 00:08:46,040 the price down. And even today with a bit of 148 00:08:46,040 --> 00:08:50,080 tweaking of the latest generations of cell phones are 149 00:08:50,080 --> 00:08:52,600 actually getting capable of doing this millimeter level 150 00:08:52,600 --> 00:08:54,680 positioning as well. OK. 151 00:08:54,680 --> 00:08:57,680 Let's move on to the second of the three technologies you 152 00:08:57,680 --> 00:09:02,080 mentioned the very long baseline interferometry, VLBI. 153 00:09:02,080 --> 00:09:04,720 How do these systems work? Yeah. 154 00:09:04,720 --> 00:09:09,040 So VLBI is the sort of older, bigger, literally sibling of 155 00:09:09,040 --> 00:09:11,880 GPS. So in GPS, we know what the 156 00:09:11,880 --> 00:09:13,720 codes are that are being transmitted. 157 00:09:13,880 --> 00:09:17,560 And so your receiver can create a copy of that code to correlate 158 00:09:17,720 --> 00:09:20,200 with the incoming signals to tell which satellite it's 159 00:09:20,200 --> 00:09:22,800 looking at and the range and phase to that satellite. 160 00:09:23,640 --> 00:09:26,960 In VOBI, we don't have that, but we have these extra galactic 161 00:09:26,960 --> 00:09:31,080 radio sources that can be seen from multiple telescopes at the 162 00:09:31,080 --> 00:09:34,400 same time. And so you can take each pair of 163 00:09:34,400 --> 00:09:39,240 telescopes and that very weak extra galactic signal is 164 00:09:39,240 --> 00:09:42,600 recorded at the sites and you can cross correlate them to find 165 00:09:42,600 --> 00:09:47,400 the time difference between the arrival of 1 signal at 1 166 00:09:47,400 --> 00:09:49,880 telescope and at the other. And again that time difference 167 00:09:49,880 --> 00:09:53,080 can be translated into a range and that allows you to do the 168 00:09:53,080 --> 00:09:57,160 relative positioning of these large radio telescopes. 169 00:09:57,760 --> 00:10:01,560 Now since these extragalactic radio sources transmit radio 170 00:10:01,560 --> 00:10:05,320 waves across a very wide bandwidth, these VOBI systems 171 00:10:05,560 --> 00:10:09,520 can make millimeter accuracy differences in range 172 00:10:09,520 --> 00:10:12,960 measurements because of that really wide band that they have. 173 00:10:12,960 --> 00:10:16,200 So it allows again the sub millimeter level positioning of 174 00:10:16,200 --> 00:10:18,640 these systems. However, the radio telescopes 175 00:10:18,640 --> 00:10:22,600 have to be very large, they have to have very low noise receivers 176 00:10:22,880 --> 00:10:26,320 and because the clocks are completely separated and they 177 00:10:26,360 --> 00:10:30,640 only can see one thing at a time, they need to have very 178 00:10:30,640 --> 00:10:33,040 accurate clocks. So these systems typically 179 00:10:33,040 --> 00:10:37,840 crossed 1 to $2,000,000 per unit and there is about 30 of them 180 00:10:37,880 --> 00:10:39,600 operating around the world at the moment. 181 00:10:39,960 --> 00:10:41,400 OK. Let's talk about the third of 182 00:10:41,400 --> 00:10:45,920 the technologies you mentioned the radar based one in SAR. 183 00:10:46,760 --> 00:10:50,480 In a previous episode of Geology Bites with Romar Jolive about 184 00:10:50,480 --> 00:10:53,800 the Turkey, Syria earthquake of 2023. 185 00:10:54,360 --> 00:10:59,160 We discussed how Insar was used to map the movements associated 186 00:10:59,160 --> 00:11:02,560 with that event. Can you remind us of how this 187 00:11:02,560 --> 00:11:04,240 technology works? Yeah. 188 00:11:04,240 --> 00:11:09,360 So ITZAR is a radar imaging technique and it looks at 189 00:11:09,360 --> 00:11:13,000 signals which are reflected from the Earth and transmitted 190 00:11:13,120 --> 00:11:15,600 normally from satellites. But you can also put this in 191 00:11:15,600 --> 00:11:17,840 aircraft and sometimes on balloons. 192 00:11:18,480 --> 00:11:21,560 The synthetic part of the name there comes from the fact that 193 00:11:21,560 --> 00:11:25,960 the motion of the spacecraft is used to synthesize effectively a 194 00:11:25,960 --> 00:11:29,640 very large band. And so with that you get very 195 00:11:29,640 --> 00:11:33,000 high resolution on the ground. And the pixel size in these 196 00:11:33,000 --> 00:11:36,680 images is typically of order one to two meters across. 197 00:11:37,640 --> 00:11:43,360 The interferometric part of the name comes from the fact that 198 00:11:43,360 --> 00:11:47,160 when you look at this pixel on the ground in one image, there's 199 00:11:47,160 --> 00:11:50,880 lots of little facets in there that reflect the signal back, 200 00:11:51,200 --> 00:11:54,960 and they sort of add together into the final amplitude and the 201 00:11:54,960 --> 00:11:57,960 phase of the signal that's received back at the spacecraft. 202 00:11:58,760 --> 00:12:03,520 If you come back six days later, 12 days later, and reimage that 203 00:12:03,520 --> 00:12:07,320 same pixel and those little pixel pieces have all stayed 204 00:12:07,320 --> 00:12:11,480 pretty much the same, then that phase is going to add to a very 205 00:12:11,480 --> 00:12:14,760 similar result. And if the pixel has changed 206 00:12:14,760 --> 00:12:18,040 position, you can actually see the change in position. 207 00:12:18,440 --> 00:12:24,360 So in inside, you take little groups of pixels, maybe 9 or 16 208 00:12:24,360 --> 00:12:27,600 of them, and you look at what the phase difference is between 209 00:12:27,600 --> 00:12:30,800 two images. And if the whole group of little 210 00:12:30,800 --> 00:12:34,360 pixels, which again is quite small in size, only, you know, 7 211 00:12:34,360 --> 00:12:38,040 or 8 meters across, if that has moved, you can tell that it's 212 00:12:38,040 --> 00:12:40,640 moved. And that can be typically done 213 00:12:40,640 --> 00:12:44,440 with accuracies, again, between a millimeter and a centimeter or 214 00:12:44,440 --> 00:12:46,400 so. The noise is about the 215 00:12:46,400 --> 00:12:49,160 millimeter. What you worry about with INSAA 216 00:12:49,360 --> 00:12:51,880 is that you're looking through the Earth's atmosphere when 217 00:12:51,880 --> 00:12:55,080 these signals come down, and the refractive index of the 218 00:12:55,920 --> 00:12:59,920 atmosphere changes the range as weather conditions change. 219 00:13:00,160 --> 00:13:04,560 And so that makes it difficult to know exactly whether you're 220 00:13:04,560 --> 00:13:07,600 looking at whether the Earth moved or whether there was a 221 00:13:07,880 --> 00:13:10,320 atmospheric effect. And they have many techniques 222 00:13:10,320 --> 00:13:14,440 now of using multiple images that try to reduce that 223 00:13:14,440 --> 00:13:16,960 atmospheric effect. So I suppose one difference is 224 00:13:16,960 --> 00:13:21,680 that the radar technique actually produces A spatial map, 225 00:13:21,920 --> 00:13:24,320 whereas the other two technologies are really more 226 00:13:24,520 --> 00:13:27,360 point to point measurements where you have your sensors. 227 00:13:27,880 --> 00:13:30,600 Yes, these two are very complementary in a sense because 228 00:13:30,600 --> 00:13:35,120 with the GNSS type systems we can get relatively high density, 229 00:13:35,160 --> 00:13:39,440 you know, stations spaced by 5 to 10 kilometers in areas of 230 00:13:39,440 --> 00:13:43,320 tectonic activity and we can make measurements literally 231 00:13:43,320 --> 00:13:45,760 continuously. Most of the systems that run 232 00:13:45,920 --> 00:13:48,200 make measurements at least once every 30 seconds. 233 00:13:48,200 --> 00:13:50,200 So you get very high time resolution. 234 00:13:50,480 --> 00:13:53,160 But as you say, the spatial resolution is quite cross. 235 00:13:53,520 --> 00:13:56,480 But you can combine that with these inside measurements which 236 00:13:56,480 --> 00:13:58,520 have very high spatial resolution. 237 00:13:58,880 --> 00:14:03,920 But we only can acquire an image typically in a region once every 238 00:14:03,920 --> 00:14:08,680 12 days or six days. And so you have high spatial 239 00:14:08,680 --> 00:14:12,240 resolution, low time resolution, high time resolution, low 240 00:14:12,240 --> 00:14:14,440 spatial. And as we try to combine them 241 00:14:14,440 --> 00:14:17,240 together, one of the areas of research actually is how do we 242 00:14:17,240 --> 00:14:21,600 try to then generate a product which is essentially both high 243 00:14:21,600 --> 00:14:26,960 resolution in time and space from these combination of Insar 244 00:14:26,960 --> 00:14:29,960 and gene S s measurements? OK, let's move on to the 245 00:14:29,960 --> 00:14:32,800 application of these technologies to geological 246 00:14:32,800 --> 00:14:36,160 questions. How exactly does it help us 247 00:14:36,160 --> 00:14:40,440 understand what happens before, during and after a large 248 00:14:40,480 --> 00:14:42,080 earthquake? Yes. 249 00:14:42,080 --> 00:14:45,840 So what we were able to see very early on is plate motion. 250 00:14:46,120 --> 00:14:50,760 And as we monitor something, we can see the large Co seismic 251 00:14:50,760 --> 00:14:54,360 offset that happens during the time of an earthquake either 252 00:14:54,360 --> 00:14:56,680 through INSA or the GNSS measurements. 253 00:14:57,200 --> 00:15:01,320 And then when we look very carefully at these accumulated 254 00:15:01,320 --> 00:15:04,480 stream that's happening up until the time of the earthquake, the 255 00:15:04,480 --> 00:15:07,240 strain release that happens during the earthquake. 256 00:15:07,760 --> 00:15:10,440 And one thing which is very clear for many earthquakes now 257 00:15:10,480 --> 00:15:14,600 is that there is also a continued strain release after 258 00:15:14,640 --> 00:15:17,600 the earthquake that's called the post seismic deformation phase. 259 00:15:18,520 --> 00:15:23,880 That phase is very well measured with these modern techniques and 260 00:15:24,160 --> 00:15:28,680 can typically be of order 10 to 15% of the size of the Co 261 00:15:28,680 --> 00:15:32,360 seismic offsets. And so one thing we have noticed 262 00:15:32,680 --> 00:15:35,280 is that this post seismic part of the earthquake seems to be a 263 00:15:35,280 --> 00:15:39,640 much deeper process and so it spreads over a larger distance. 264 00:15:40,080 --> 00:15:44,720 We think it also potentially transfers stress from the region 265 00:15:44,720 --> 00:15:49,320 where the earthquake was happening to areas where stress 266 00:15:49,320 --> 00:15:52,000 is still building but no earthquake has happened yet. 267 00:15:53,040 --> 00:15:57,000 The other thing we can do with GNSS is we can actually see what 268 00:15:57,000 --> 00:15:59,720 happens during the earthquake itself when we have these high 269 00:15:59,720 --> 00:16:02,240 rate measurements. So we can see the arrival of 270 00:16:02,240 --> 00:16:06,200 seismic waves. And one of the first interesting 271 00:16:06,200 --> 00:16:10,600 cases of this was the Denali earthquake up in Alaska that 272 00:16:10,600 --> 00:16:12,600 happened a couple of decades ago. 273 00:16:13,080 --> 00:16:17,880 The direction of that earthquake rupture essentially sent seismic 274 00:16:17,880 --> 00:16:22,200 waves right across the whole of the US and all of the GNSS 275 00:16:22,200 --> 00:16:26,240 stations in the continental US region that we're tracking data 276 00:16:26,240 --> 00:16:31,280 at 1 Hertz rates could see these 1 to 2cm waves propagating 277 00:16:31,280 --> 00:16:34,080 across the US after that earthquake. 278 00:16:34,600 --> 00:16:38,160 Now that type of process is quite common to be able to 279 00:16:38,160 --> 00:16:42,560 observe in large earthquakes. The other phenomena that happens 280 00:16:43,080 --> 00:16:46,240 before some of these earthquakes, also depending on 281 00:16:46,240 --> 00:16:51,560 where you are, is that we have strain accumulation before the 282 00:16:51,560 --> 00:16:53,800 earthquake. But particularly in subduction 283 00:16:53,800 --> 00:16:59,200 zones, we often see this process called slow slip, where there 284 00:16:59,200 --> 00:17:02,120 seems to be a relaxing of that strain accumulation. 285 00:17:02,480 --> 00:17:06,480 And geodetically, it looks like you've had this earthquake in 286 00:17:06,480 --> 00:17:10,599 the region, but instead of it lasting only a few seconds, this 287 00:17:10,599 --> 00:17:14,599 earthquake takes maybe 10 to 20 days to happen. 288 00:17:15,040 --> 00:17:19,160 And that is something, again, very well observed in large 289 00:17:19,280 --> 00:17:22,839 subduction zone areas. What's not clear at the moment 290 00:17:22,839 --> 00:17:27,440 is what's the implication of these slow slip things for the 291 00:17:27,440 --> 00:17:28,880 future occurrence of earthquakes? 292 00:17:29,120 --> 00:17:32,680 One might imagine that they release stress, so they're like 293 00:17:32,680 --> 00:17:35,480 little earthquakes and stop the earthquake from happening. 294 00:17:36,000 --> 00:17:38,880 But what's more likely is they're relieving stress in one 295 00:17:38,880 --> 00:17:44,240 region, but transferring it to an area where that earthquake is 296 00:17:44,240 --> 00:17:46,760 more likely to happen. We just don't have enough 297 00:17:46,760 --> 00:17:50,280 examples of large earthquakes at the moment in subduction zones 298 00:17:50,640 --> 00:17:53,960 fully instrumented to be able to address that question as to what 299 00:17:53,960 --> 00:17:57,000 the impact of these types of events are before the events. 300 00:17:57,720 --> 00:18:03,400 Does this mapping of strain work as well in highly mountainous 301 00:18:03,400 --> 00:18:08,600 regions like for example the Himalayas in the 2015 Gorkha 302 00:18:08,600 --> 00:18:11,520 earthquake in Nepal? In high mountainous areas, we 303 00:18:11,520 --> 00:18:14,640 have a couple of issues. Logistically, if you're trying 304 00:18:14,640 --> 00:18:17,960 to install GNSS, it's tricky. That has been done. 305 00:18:17,960 --> 00:18:21,640 It's been done again with 50 to 100 kilometers sort of spacings 306 00:18:21,640 --> 00:18:23,520 across many of these types of areas. 307 00:18:24,000 --> 00:18:27,560 With INSUR in high mountains, you have sort of 2 problems. 1 308 00:18:27,560 --> 00:18:31,040 is that the high mountains can actually shadow the radar 309 00:18:31,320 --> 00:18:35,240 returns because again the radars tend to look at the surface not 310 00:18:35,240 --> 00:18:39,240 directly down, but from an angle and that angle is somewhere 311 00:18:39,240 --> 00:18:45,760 between 45 to 65° off Nata. And so you can get shadowing. 312 00:18:46,080 --> 00:18:48,280 The other problem you have in the mountains is you can get a 313 00:18:48,280 --> 00:18:51,640 lot of snow. And so when snow falls on the 314 00:18:51,640 --> 00:18:55,240 ground, that reflection that comes back now is a completely 315 00:18:55,240 --> 00:18:57,760 different type of reflection than it was before. 316 00:18:58,080 --> 00:19:00,840 And so there is what we call no coherence in the image. 317 00:19:00,840 --> 00:19:04,600 Nearby pixels now just have random phase variations and so 318 00:19:04,600 --> 00:19:07,720 you lose the information on the deformation itself. 319 00:19:08,160 --> 00:19:11,880 But in general, insight has been very successful in mountainous 320 00:19:11,880 --> 00:19:14,040 regions. Can one turn that problem on its 321 00:19:14,040 --> 00:19:19,760 head and use the variation in snow depth to measure how 322 00:19:20,200 --> 00:19:23,640 actually ice and glaciers are thinning out? 323 00:19:24,040 --> 00:19:27,160 Yes, that's another very big application of the inside 324 00:19:27,160 --> 00:19:32,400 technique and also GNSS as well. The polar regions are really 325 00:19:32,400 --> 00:19:35,960 very well observed because all of the satellites that make 326 00:19:35,960 --> 00:19:39,600 these inside measurements are in high inclination orbits. 327 00:19:39,760 --> 00:19:43,080 And so the paths of the spacecraft, the ground tracks, 328 00:19:43,280 --> 00:19:47,160 cross very close together up in the polar regions and that gives 329 00:19:47,160 --> 00:19:50,640 you much more opportunity to be able to actually look at the 330 00:19:50,640 --> 00:19:53,120 deformation that's happening in those regions. 331 00:19:53,320 --> 00:19:57,080 Also some satellite systems, the European Sentinel systems for 332 00:19:57,080 --> 00:19:59,800 example, have pairs of satellites. 333 00:20:00,080 --> 00:20:02,360 And you have one measure satellite making a measurement 334 00:20:02,360 --> 00:20:05,840 and then very shortly after, another satellite can measure in 335 00:20:05,840 --> 00:20:08,680 the same region. And since glaciers can move very 336 00:20:08,680 --> 00:20:12,520 fast, they can move up to, you know, kilometers per year and 337 00:20:12,520 --> 00:20:16,040 several 10s of meters per day. You can actually see the 338 00:20:16,040 --> 00:20:19,680 deformation between those, you know, fairly rapid acquisitions. 339 00:20:20,080 --> 00:20:23,360 And this has been one of the great applications of Insar is 340 00:20:23,360 --> 00:20:27,320 looking at velocities of ice floes and how they change. 341 00:20:27,320 --> 00:20:32,040 Throughout the years we've seen tidal effects on glaciers which 342 00:20:32,040 --> 00:20:34,880 actually move out to the sea. The ocean type sort of 343 00:20:35,000 --> 00:20:38,240 accelerates and decelerates the motion of the glaciers since 344 00:20:38,240 --> 00:20:42,200 have lots of properties that allow us insights into the 345 00:20:42,200 --> 00:20:46,520 mechanics of the ice. And that in turn allows us to 346 00:20:46,520 --> 00:20:51,520 have a much better sense of as things like temperature change, 347 00:20:51,640 --> 00:20:54,280 water temperature, air temperature change, the light, 348 00:20:54,280 --> 00:20:58,040 the change in the nature of the ice dynamically, how fast can it 349 00:20:58,040 --> 00:21:01,880 move, What are the limits on how fast the glacia can change its 350 00:21:01,960 --> 00:21:05,040 motion, the amount it moves based on things like temperature 351 00:21:05,040 --> 00:21:07,840 changes. In the previous Geology by its 352 00:21:07,840 --> 00:21:13,200 episode with Chuck Demetz, we talked about high temporal and 353 00:21:13,200 --> 00:21:17,480 spatial resolution measurements of plate motions and 354 00:21:17,480 --> 00:21:20,480 particularly the northward movement of the Indian plate and 355 00:21:20,480 --> 00:21:23,840 the effect of the plates collision with Eurasia. 356 00:21:24,720 --> 00:21:28,640 Can we use the high spatial resolution of geology to get a 357 00:21:28,640 --> 00:21:32,600 better handle on plate motions? Oh, definitely we can measure 358 00:21:32,600 --> 00:21:36,560 plate motions at sub millimeter per year accuracies these days. 359 00:21:36,800 --> 00:21:39,680 But it is over the interval of time which we have collected 360 00:21:39,720 --> 00:21:42,920 data. And so places like India, for 361 00:21:42,920 --> 00:21:46,480 example, the Indian Eurasia collision, that's happening at 362 00:21:46,480 --> 00:21:49,280 around about 50 millimetres per year. 363 00:21:49,800 --> 00:21:53,520 And one of the interesting results that came out of early 364 00:21:53,520 --> 00:21:59,320 GNSS deployments in the late 1990s, early 2000s was that that 365 00:21:59,400 --> 00:22:03,240 motion of India into the Himalayan mountains, which you 366 00:22:03,240 --> 00:22:05,800 reflects in the building of that large mountain range. 367 00:22:06,600 --> 00:22:10,080 That motion is only half accommodated at that Himalayan 368 00:22:10,120 --> 00:22:11,800 front. The Himalayan mountains 369 00:22:11,800 --> 00:22:15,840 themselves are actually being pushed back towards Eurasia at a 370 00:22:15,840 --> 00:22:20,160 rate of about 20mm per year. And that 20mm per year is 371 00:22:20,160 --> 00:22:22,560 actually accommodated in defamation. 372 00:22:22,560 --> 00:22:25,640 That's happening on the other side of the Tibetan Plateau and 373 00:22:25,640 --> 00:22:31,920 that large scale pushing of the whole Tibetan Plateau by the 374 00:22:31,920 --> 00:22:34,880 Indian plate. That was not fully appreciated 375 00:22:34,880 --> 00:22:38,400 until we had GNSS measurements that allowed us to get fine 376 00:22:38,400 --> 00:22:43,320 spatial scale variations in velocity across these regions to 377 00:22:43,320 --> 00:22:46,440 be able to interpret what the mechanics again of how this 378 00:22:46,480 --> 00:22:48,360 happens. Wow, that's incredible. 379 00:22:48,360 --> 00:22:53,080 So really only about half the northward movement of India is 380 00:22:53,080 --> 00:22:56,360 actually accommodated by the building of the mountains, and 381 00:22:56,360 --> 00:22:59,360 another half is actually transmitted to the north. 382 00:22:59,800 --> 00:23:02,240 Yes, and you sort of see that in the topography. 383 00:23:02,240 --> 00:23:05,600 I suppose that's one of the things we've learnt from these 384 00:23:05,600 --> 00:23:10,040 GNSS measurements is wherever you have topography, you have 385 00:23:10,040 --> 00:23:12,360 defamation typically still ongoing. 386 00:23:12,360 --> 00:23:15,920 So if you look at North America, for example, once you get to 387 00:23:15,920 --> 00:23:20,680 Colorado and start moving across towards the Pacific, all of that 388 00:23:20,720 --> 00:23:23,520 material out there is actively deforming right now. 389 00:23:24,040 --> 00:23:27,040 And so it's same in the Tibetan Plateau, etcetera. 390 00:23:27,240 --> 00:23:30,640 Wherever you have topography, we tend to be seeing defamation 391 00:23:30,640 --> 00:23:33,680 happening within those regions, again at rates of a few 392 00:23:33,680 --> 00:23:36,600 millimeters per year, up to a centimeter per year depending on 393 00:23:36,640 --> 00:23:39,920 where we are. You've also used high precision 394 00:23:39,920 --> 00:23:43,280 geography to measure changes in the rotation of the Earth. 395 00:23:43,960 --> 00:23:46,800 Can you tell us how you did that and what you found out? 396 00:23:47,520 --> 00:23:50,440 Yes, the Earth's rotation is actually pretty wobbly as I'd 397 00:23:50,440 --> 00:23:53,720 like to say. So there's essentially 3 aspects 398 00:23:53,720 --> 00:23:56,080 to it. One is the rotation axis of the 399 00:23:56,120 --> 00:24:00,560 Earth is actually not fixed to the solid body of the Earth. 400 00:24:00,560 --> 00:24:04,720 It actually moves around, and that movement is about 10 meters 401 00:24:04,720 --> 00:24:06,480 in a year. It actually is a circular sort 402 00:24:06,480 --> 00:24:10,640 of motion because of some annual effects in the seasonal movement 403 00:24:10,640 --> 00:24:14,320 of fluids on the planet. It also has a drift to it, again 404 00:24:14,360 --> 00:24:17,440 of about 10 meters per century for the drift part. 405 00:24:18,560 --> 00:24:21,640 The spin rate of the Earth, how fast we spin, also changes 406 00:24:21,640 --> 00:24:24,880 dramatically, at least on a geodetic sense, throughout the 407 00:24:24,880 --> 00:24:29,280 year, and that for periods less than about a year is driven 408 00:24:29,480 --> 00:24:32,200 largely by changes in wind speeds in the atmosphere. 409 00:24:32,200 --> 00:24:35,560 And that's extremely well measured and documented now 410 00:24:35,920 --> 00:24:39,440 based essentially on our weather forecast models that allow us to 411 00:24:39,640 --> 00:24:44,240 calculate the angular momentum of the atmosphere to extremely 412 00:24:44,240 --> 00:24:48,120 high precision. And then in standard mechanics, 413 00:24:48,120 --> 00:24:50,240 the angular momentum of the whole system has to be 414 00:24:50,240 --> 00:24:52,440 conserved. So if the atmosphere's angular 415 00:24:52,440 --> 00:24:56,200 momentum increases, the solid Earth's angular momentum must 416 00:24:56,200 --> 00:24:59,600 decrease and we decrease it by changing the rotation rate. 417 00:25:00,560 --> 00:25:04,360 We can also see that there's a long term trend in our rotation 418 00:25:04,560 --> 00:25:06,400 rate as well. We're slowing down in our 419 00:25:06,400 --> 00:25:08,440 rotation. This has been happening ever 420 00:25:08,440 --> 00:25:11,960 since the Earth formed, essentially, and that is due to 421 00:25:11,960 --> 00:25:15,680 tidal forces, mostly from ocean tides coming in and out. 422 00:25:16,120 --> 00:25:19,280 And we can indirectly also measure that by these laser 423 00:25:19,280 --> 00:25:22,160 measurements that have been made to the Moon's retro reflectors, 424 00:25:22,640 --> 00:25:25,960 mostly put on by the Apollo missions and by the Russian 425 00:25:26,160 --> 00:25:29,440 missions that put Rovers on the Moon back in the 1970s. 426 00:25:29,840 --> 00:25:32,680 And that's about 38mm per year of motion. 427 00:25:32,840 --> 00:25:34,880 And that's been well measured and it's consistent with the 428 00:25:34,880 --> 00:25:38,560 slowing at the rate. The other big fluid for rotation 429 00:25:38,560 --> 00:25:40,080 rate is the fluid core of the Earth. 430 00:25:40,400 --> 00:25:43,560 And we actually need motion in the fluid core to generate the 431 00:25:43,560 --> 00:25:46,360 Earth's magnetic field, which is actually quite critical to 432 00:25:46,360 --> 00:25:50,560 protecting us from solar wind particles coming from the sun. 433 00:25:50,920 --> 00:25:53,880 And that motion of the fluid core, if it changes, then that 434 00:25:53,880 --> 00:25:58,360 also changes the rotation rate. And then finally, the Earth's 435 00:25:58,360 --> 00:26:03,080 rotation axis moves in a natural space, mainly due to the torque 436 00:26:03,080 --> 00:26:06,480 supplied by the Sun and the Moon on the Earth's secretarial 437 00:26:06,480 --> 00:26:10,400 bulge, and the presence of the fluid core in there has a 438 00:26:10,400 --> 00:26:13,840 dramatic effect on that. So one of the very early, very 439 00:26:13,840 --> 00:26:17,400 long baseline interferometry measurements was to actually be 440 00:26:17,400 --> 00:26:22,680 able to determine the flattening of the fluid core boundary based 441 00:26:22,760 --> 00:26:27,160 on how the Earth was rotating. And that showed that the fluid 442 00:26:27,160 --> 00:26:29,520 core was not in hydrostatic equilibrium. 443 00:26:29,800 --> 00:26:34,520 It was off by about 500 meters in the distance between the pole 444 00:26:34,520 --> 00:26:37,360 to the equator radius from what would be predicted. 445 00:26:38,040 --> 00:26:40,600 And that was a measurement that came directly out of Earth's 446 00:26:40,600 --> 00:26:44,160 rotation variations. So what are the time scales of 447 00:26:44,160 --> 00:26:47,880 these rotation period changes? So you said the atmospheric 1 is 448 00:26:47,880 --> 00:26:51,840 about annual. What about the ones that we can 449 00:26:51,920 --> 00:26:56,440 trace back to the movements of the core or the moon or other 450 00:26:56,440 --> 00:26:58,120 celestial impacts? Yeah. 451 00:26:58,120 --> 00:27:01,280 The atmosphere has a variety of frequencies in it and we do see 452 00:27:01,280 --> 00:27:05,200 a strong annual signal in there. We also see periods down at just 453 00:27:05,200 --> 00:27:08,120 10 days. There's natural resonance modes 454 00:27:08,120 --> 00:27:12,120 in the atmosphere that show up on the longest time scales 455 00:27:12,280 --> 00:27:15,480 longer than a year. Oh, those variations in terms of 456 00:27:15,480 --> 00:27:19,840 rotation rate or around about up to three milliseconds per day 457 00:27:19,840 --> 00:27:23,880 change in the length of the day. Now 3 milliseconds doesn't sound 458 00:27:23,880 --> 00:27:27,920 very much, but at times we are consistently 3 milliseconds 459 00:27:27,920 --> 00:27:30,920 slower. So every day it accumulates. 460 00:27:31,120 --> 00:27:35,760 And so after about 300 odd days, if you're doing 3 milliseconds 461 00:27:35,760 --> 00:27:38,760 per day, you're up to almost a second change in time. 462 00:27:39,120 --> 00:27:41,880 And we do try to account for that in the way that we do our 463 00:27:41,880 --> 00:27:45,160 civilian time keeping relations to atomic time. 464 00:27:46,040 --> 00:27:50,880 So the other time scales that are there are over decades. 465 00:27:51,320 --> 00:27:55,680 And there we see again multi millisecond changes in the 466 00:27:55,680 --> 00:28:00,080 length of day of decades. But now if we were to try to 467 00:28:00,080 --> 00:28:02,000 explain that with the atmosphere, we just don't have 468 00:28:02,000 --> 00:28:05,160 enough angular momentum in the atmosphere to have those large 469 00:28:05,160 --> 00:28:08,720 variations. And so the place where we 470 00:28:08,720 --> 00:28:11,920 believe all of that is coming from is the fluid core of the 471 00:28:11,920 --> 00:28:15,640 Earth. And over the last 20 years or 472 00:28:15,640 --> 00:28:19,720 so, we've seen the fluid core losing angular momentum and the 473 00:28:19,720 --> 00:28:23,440 Earth gaining it and the Earth spinning up over those periods. 474 00:28:24,200 --> 00:28:26,520 And then as I said of a really long time periods, we have this 475 00:28:26,520 --> 00:28:30,600 tidal dissipation that happens due to the energy loss that 476 00:28:30,600 --> 00:28:33,960 happens as you have water flowing in and out during the 477 00:28:33,960 --> 00:28:37,880 tidal sequence of the Earth. I think you implied from what 478 00:28:37,880 --> 00:28:41,400 you said earlier that you've been able to measure the change 479 00:28:41,400 --> 00:28:46,600 in geographical pole, namely that the axis of rotation of the 480 00:28:46,640 --> 00:28:49,760 Earth wanders with respect to the crust. 481 00:28:50,640 --> 00:28:54,480 We're used to magnetic poles wandering quite a bit. 482 00:28:54,760 --> 00:28:58,240 How much does the geographic pole move? 483 00:28:58,800 --> 00:29:02,440 Yeah, the geographic pole moves typically around about 10 meters 484 00:29:02,640 --> 00:29:06,880 over a year, and over a century it'll also secularly drift by 485 00:29:06,880 --> 00:29:10,680 about 10 meters. And that drift over that period 486 00:29:10,680 --> 00:29:14,360 of time, that's due to the Earth's response to the removal 487 00:29:14,360 --> 00:29:17,400 of ice at the end of the last Ice Age, which ended about 488 00:29:17,400 --> 00:29:20,320 10,000 years ago. When you put all that ice on the 489 00:29:20,320 --> 00:29:24,360 Earth's crust, it pushes it down and the material in the mantle 490 00:29:24,360 --> 00:29:27,040 as well. And when you take the ice away, 491 00:29:27,400 --> 00:29:30,840 there's an elastic response that happens instantly, but then 492 00:29:30,840 --> 00:29:34,960 there's a viscoelastic response when materials slowly come back 493 00:29:34,960 --> 00:29:37,120 in. That process is still happening 494 00:29:37,520 --> 00:29:40,160 around Hudson's Bay. For example, all of the land 495 00:29:40,160 --> 00:29:45,840 around Hudson's Bay is moving upwards at about 15mm per year 496 00:29:45,840 --> 00:29:51,400 at the moment due to that rebound and that change in the 497 00:29:51,400 --> 00:29:54,320 surface of the height and the material in the Earth's mantle, 498 00:29:54,560 --> 00:29:57,240 that's changing the moments of inertia of the Earth. 499 00:29:57,640 --> 00:30:02,000 And so the Earth's rotation axis essentially wants to try to 500 00:30:02,000 --> 00:30:05,640 rotate centered on the maximum moment of inertia of the Earth, 501 00:30:06,080 --> 00:30:09,600 and that internal change is moving that maximum moment of 502 00:30:09,600 --> 00:30:11,800 inertia. And that's what we're able to 503 00:30:12,080 --> 00:30:16,920 see in these measurements. The dramatic lowering of cost 504 00:30:16,920 --> 00:30:20,360 that you talked about of the satellite based systems means we 505 00:30:20,360 --> 00:30:24,080 can use them for a much larger range of applications. 506 00:30:24,600 --> 00:30:28,080 Can you give us some examples? Yes, we have lots of different 507 00:30:28,080 --> 00:30:30,400 applications. One thing we've been doing is we 508 00:30:30,400 --> 00:30:34,400 have GPS receivers that we put on tops of very tall buildings 509 00:30:34,400 --> 00:30:36,720 to see how they deform throughout the year. 510 00:30:36,960 --> 00:30:39,360 We've seen thermal responses to the buildings. 511 00:30:39,680 --> 00:30:43,080 We've seen buildings moving, shaking during earthquakes, you 512 00:30:43,080 --> 00:30:45,760 know, up to 20 centimeter type motions of the building. 513 00:30:46,280 --> 00:30:48,400 It turns out for the buildings we've looked at that all of 514 00:30:48,400 --> 00:30:51,520 these large motions are fully within the specifications of the 515 00:30:51,520 --> 00:30:52,880 safety stances with the building. 516 00:30:52,880 --> 00:30:56,800 So it's not an immediate danger to any of these buildings. 517 00:30:57,280 --> 00:31:02,400 When we build large dams, we can also have two processes that can 518 00:31:02,760 --> 00:31:06,520 be studied there. One is that as the dam fills, 519 00:31:06,840 --> 00:31:11,160 the land around the dam actually gets pushed down by the weight 520 00:31:11,160 --> 00:31:13,200 of the water. That's very well observed. 521 00:31:13,440 --> 00:31:17,280 And so we can also put GPS receivers on the faces of the 522 00:31:17,280 --> 00:31:22,320 dams to make sure that the dam is not being pushed out by the 523 00:31:22,320 --> 00:31:25,600 water excessively as it is filled with water. 524 00:31:26,120 --> 00:31:29,560 They've had dams in Southern California, which had this GPS 525 00:31:29,560 --> 00:31:35,000 monitoring put on the dam just to verify that the damned face 526 00:31:35,000 --> 00:31:38,440 is actually remaining stable and not suffering any damage from 527 00:31:38,440 --> 00:31:41,800 these earthquakes. There's also some rather 528 00:31:41,800 --> 00:31:44,640 unanticipated things that we've used these systems for. 529 00:31:45,240 --> 00:31:49,760 One is that the GPS systems need to be able to see the whole sky 530 00:31:49,760 --> 00:31:53,760 above them, but the antennas also can actually see 531 00:31:53,760 --> 00:31:56,680 reflections from below the antenna. 532 00:31:56,680 --> 00:32:00,480 That's what we call multipath for positioning the antenna. 533 00:32:00,480 --> 00:32:03,240 That's considered noise, and we'd like to try to eliminate 534 00:32:03,240 --> 00:32:05,320 it. But people have taken advantage 535 00:32:05,360 --> 00:32:09,720 of that reflected signal to tell them something about what is 536 00:32:09,760 --> 00:32:12,480 actually causing the reflection on the ground surface. 537 00:32:12,800 --> 00:32:16,760 And so here people have used it to do soil moisture measurements 538 00:32:16,760 --> 00:32:20,240 because typically as soils get moist, they reflect signals 539 00:32:20,240 --> 00:32:22,520 better, so that reflected signal becomes stronger. 540 00:32:22,520 --> 00:32:26,800 We can detect that if there's snow in the region, as the snow 541 00:32:26,800 --> 00:32:30,080 builds up, there's a reflection of the snow and you can see snow 542 00:32:30,080 --> 00:32:32,360 height. If vegetation is growing, if you 543 00:32:32,360 --> 00:32:35,600 put this in your field, you'll be able to see it, the signals 544 00:32:35,600 --> 00:32:38,680 growing, etcetera. One thing we worry about and we 545 00:32:38,680 --> 00:32:40,720 have to take into account with all of these geodetic 546 00:32:40,720 --> 00:32:43,880 measurements is that we have a refractive medium from the 547 00:32:43,880 --> 00:32:49,360 Earth's atmosphere and we also have a layer of electrons high 548 00:32:49,360 --> 00:32:53,240 above the atmosphere called the ionosphere and we can monitor 549 00:32:53,240 --> 00:32:54,800 those. We have to monitor those to do 550 00:32:54,800 --> 00:32:57,440 accurate positioning. And so we can use these 551 00:32:57,440 --> 00:32:59,760 measurements to help actually do weather forecasting. 552 00:33:00,200 --> 00:33:03,120 And for the ionosphere, we can monitor the strengths of 553 00:33:03,120 --> 00:33:08,200 magnetic storms and we've even been able to monitor the impacts 554 00:33:08,240 --> 00:33:11,760 on the atmosphere of tsunami waves coming from large 555 00:33:11,880 --> 00:33:16,360 earthquakes, etcetera. The areas which commercially 556 00:33:16,360 --> 00:33:20,280 have been very viable are things like precision farming, where 557 00:33:20,800 --> 00:33:25,080 the large combined harvesters for example can be controlled by 558 00:33:25,080 --> 00:33:30,160 GPS, but more importantly as the field is harvested it knows 559 00:33:30,160 --> 00:33:33,360 precisely where it is. It can also use the multi 560 00:33:33,400 --> 00:33:37,040 spectral scanner technologies that we often put in spacecraft 561 00:33:37,320 --> 00:33:40,400 to get a measure of the yield coming from that particular part 562 00:33:40,400 --> 00:33:43,440 of the field. The next year when that field is 563 00:33:44,240 --> 00:33:48,920 used, the system can determine where it needs to put more 564 00:33:48,920 --> 00:33:51,280 fertilizer to maximize the yield. 565 00:33:51,760 --> 00:33:54,600 We've also seen recently lots of drone displays. 566 00:33:54,600 --> 00:33:58,720 There was a massive 1 of 1500 drones set a Guinness World 567 00:33:58,720 --> 00:34:03,560 Record for Christmas 2024. And these drones, which map out 568 00:34:03,560 --> 00:34:06,200 these wonderful patterns in the sky with lights are all 569 00:34:06,200 --> 00:34:09,239 controlled by these GPS receivers where the drones have 570 00:34:09,239 --> 00:34:11,679 to be able to position themselves while they're flying 571 00:34:11,880 --> 00:34:14,840 to within a few centimeters of their designated needed position 572 00:34:14,840 --> 00:34:19,440 to make the image look correct. So in general, we're seeing this 573 00:34:19,440 --> 00:34:24,400 ability, I think, of personal positioning to get us to a few 574 00:34:24,400 --> 00:34:27,400 centimeters while we're walking around. 575 00:34:27,600 --> 00:34:31,679 And so in the future, when you go someplace, it's almost 576 00:34:31,679 --> 00:34:34,000 certain that there will be actually no debate as to which 577 00:34:34,000 --> 00:34:36,400 door you actually have to enter through in the building. 578 00:34:36,600 --> 00:34:38,560 It's not just knowing the building, but actually knowing 579 00:34:38,560 --> 00:34:41,120 which door to go to. I think there's a lot of 580 00:34:41,120 --> 00:34:43,520 potential growth for those types of things. 581 00:34:44,560 --> 00:34:46,239 What are you working on at the moment? 582 00:34:46,719 --> 00:34:49,199 So we're very interested in these slow slip events that are 583 00:34:49,199 --> 00:34:52,120 occurring in these large subduction zones and getting 584 00:34:52,120 --> 00:34:54,760 better temporal and spatial correlation from them, 585 00:34:55,400 --> 00:35:00,080 extracting as much noise out of there as we can to be able to 586 00:35:00,200 --> 00:35:03,200 get a better sense of what their relationship is to earthquakes. 587 00:35:04,360 --> 00:35:07,320 Fluids moving around on the planet, and primarily water on 588 00:35:07,320 --> 00:35:11,280 the surface is something which effects our position 589 00:35:11,280 --> 00:35:14,240 determinations because it can load the crust, it can expand 590 00:35:14,240 --> 00:35:17,400 soils. And so we want to get a better 591 00:35:17,400 --> 00:35:20,480 sense of how we can exploit these measurements to understand 592 00:35:20,480 --> 00:35:25,960 what subsurface water is doing. As a rule of thumb #1 millibar 593 00:35:25,960 --> 00:35:29,000 change in atmospheric pressure actually moves the Earth's 594 00:35:29,000 --> 00:35:33,000 surface by about 1/2 a millimeter, and things like 10 595 00:35:33,000 --> 00:35:34,920 millibar pressure changes are very common. 596 00:35:35,200 --> 00:35:36,800 And so we see this in our stations. 597 00:35:36,800 --> 00:35:40,040 We as weather systems move across the stations, we can see 598 00:35:40,040 --> 00:35:44,200 those types of motions. Tom Herring, thank you very 599 00:35:44,200 --> 00:35:46,240 much. And thank you, this was fun. 600 00:35:47,600 --> 00:35:50,280 To see pictures and illustrations that support this 601 00:35:50,280 --> 00:35:55,520 podcast, go to geologybytes.com, where you'll also find a subject 602 00:35:55,520 --> 00:35:57,480 matter index of all the episodes. 603 00:35:57,920 --> 00:36:01,280 There you can also give me feedback which I welcome, as 604 00:36:01,280 --> 00:36:04,280 well as sign up to get my emails about new episodes.