This is Geology Bites with Oliver Strimpel.
In this episode, we'll be talking about geodesy, and
particularly about accurately determining positions on the
Earth's surface and monitoring how these positions change over
time. Over the past few decades, the
technology involved has improved dramatically and it has become
much more affordable. How can we use this to address
geological questions such as how the Earth deforms during and
after large earthquakes or to see plate motions in high
resolution? Tom Herring is a pioneer in high
precision geodetic analytical methods and applications for
satellite based navigation systems to study the Earth's
surface. He is a professor in the Earth,
Atmospheric and Planetary Sciences Department at the
Massachusetts Institute of Technology.
Tom Herring, welcome to Geology Bytes.
Thank you, Oliver for inviting me to your show.
It's a pleasure to be here. Let's start with the technology
of high precision geodetic measurements and then we'll talk
about some geological applications.
First of all, what kind of accuracy can we achieve with
these systems now? Well, in the best cases we can
make position determinations at sub millimeter accuracies
horizontally and about 3mm in the height.
The height's worse than the horizontal components just
because of the geometry of satellite orbits being above us
and the atmospheric refraction effects.
Getting centimeter accuracy with these systems is actually pretty
easy these days. The accuracy does depend on the
averaging time and the type of system that you use, and for
geologic processes we often use 24 hour average positions to
reduce the noise. This is where the best accuracy
is, but with systems such as GPS we can also determine those
positions every time a measurement's made, which could
be rates up to as 10 times or 100 times per second.
The accuracy of those measurements is generally around
a few millimetres if it's in a very good environment.
OK, what are the main types of geodesic measurement systems
that can achieve such impressive accuracy?
There's three main types. There's the satellite based
types, which are the the GPS system.
The Global Positioning System is 1 and it uses radio waves.
There are also laser systems that can send laser pulses up to
satellites and the moon with specially installed corner cube
reflectors. Those are very expensive
systems. The GPS system is the US1, and
these days we add to that a European Galileo system, which
is of similar design. The Chinese Baidu system, and
the Russian Glona system, which is of the same era as the GPS
system is combined together. People refer to this as the
Global Navigation Satellite System, or GNSS.
The more satellites you have, the easier it is to make
position nations when you're in obstructed sort of views.
The other big radio technique that we use is called very long
baseline interferometry, and it uses radio signals from
extragalactic sources and those need to be observed with very
large telescopes because of the weak signals involved.
And finally, we have synthetic aperture radar, which is a
technique that can be used for imaging deformation on the
surface based on the returned signals from radars installed
generally on satellites. OK, can you give us a high level
explanation of how these technologies work?
Let's start with satellite based systems.
Yes, these systems work by a technique called trilateration,
and it's very similar to triangulation except that it
uses distance measurements rather than angles, and that
distance measurement is what allows us to achieve range
measurements to satellites from the ground with millimeter level
accuracy. For GNSS, there's actually two
types of measurements that are made, two types of signals that
are interpreted and one of them is readily available in all
positioning things such as cell phones, boats, car navigation
systems, and there's a second version which allows very high
accuracy and that used to cost typically about $20,000 per
receiver. The radio signals that are
transmitted from the GNSS satellites have wavelengths
around about 200 millimetres. They're encoded in a way that
different satellites can be easily separated from each
other, and your ground receiver correlates the arrival time of
the codes from the satellites with a replica of that code and
measures the time difference between them.
That time difference is related to the range to the spacecraft,
but also to the errors in the clocks in both the spacecraft
and the ground receivers. The the satellites can have very
expensive clocks, but what we want to do is make the ground
receivers have very inexpensive ones.
And the great revolution that happened with GPS is that
multiple satellites can be seen at the same time.
So the position can be based on the measurements to three
satellites. And then to estimate the clock
you need a fourth satellite. And that allows you to have a
very cheap clock in your receiver, which makes these
receivers inexpensive. Typically these days with GNSS,
we can see 25 to 30 satellites at any one time anywhere in the
world. And that level of correlation
with the codes is called shooter range positioning, and that's
middle level positioning. The big thing that came from
GNSS innovation was that we can look at the phase of the radio
signal that's being transmitted. And since we can continuously
track the spacecraft, we can count the number of cycles of
phase as it goes through. And So what we end up getting is
this range change measurement between the spacecraft when we
first observe it and then as it moves through the sky.
And that phase change is accurate to a couple of
millimeters. And it's from that that we can
determine these millimeter level positions down on the Earth's
surface. We also need multiple
frequencies because of the impacts of some of the
atmospheric refraction effects. And that is the way, again, that
we can eliminate some of the systematic errors of these types
of measurements. That's impressive to get the
cost down and I can see that if you can have a low cost clock,
obviously that makes a big difference.
But has the cost reduction also benefited from the same kind of
thing as the integrated circuits have following Moore's Law over
the decades? Yes, they have, although not for
the same reason precisely the current techniques that are used
to do millimetre level positioning, the receivers for
that are typically cost around about $20,000 each and now
they're less than about $700.00. And that reduction happens
because of the way the GPS signal is transmitted.
And so in the main part of the GPS system, there's additional
codes that are transmitted with the signals that are required
for anti spoofing protection for the US military.
Those additional codes on the second frequencies that
transmitted make it actually very difficult to track the
signal and hence the cost of the receivers to have the technology
to be able to do that. But starting about a decade ago,
the military realized the importance of civilian
applications of high precision positioning.
And so they started to introduce 1/3 frequency on the spacecraft,
which allows civilians to use that frequency.
And they've also added some additional codes to the normal
transmissions, which again make it easier for these receivers to
receive these high accuracy signals.
Now GPS satellites get launched at about 1:00 to 2:00 per year.
So over the last decade, we now have about 17 or 18 of the
satellites that are transmitting these signals readily available
to civilians. These cheap receivers take
advantage of that. They also take advantage of the
fact that the Galileo system, which is now almost complete,
transmit signals which are easily tracked, and the Chinese
Baidu system, which is now on its third generation, also
transmit signals that are easy to track.
And so the combination of changing the way the signals are
encoded has allowed these very inexpensive receivers to drop
the price down. And even today with a bit of
tweaking of the latest generations of cell phones are
actually getting capable of doing this millimeter level
positioning as well. OK.
Let's move on to the second of the three technologies you
mentioned the very long baseline interferometry, VLBI.
How do these systems work? Yeah.
So VLBI is the sort of older, bigger, literally sibling of
GPS. So in GPS, we know what the
codes are that are being transmitted.
And so your receiver can create a copy of that code to correlate
with the incoming signals to tell which satellite it's
looking at and the range and phase to that satellite.
In VOBI, we don't have that, but we have these extra galactic
radio sources that can be seen from multiple telescopes at the
same time. And so you can take each pair of
telescopes and that very weak extra galactic signal is
recorded at the sites and you can cross correlate them to find
the time difference between the arrival of 1 signal at 1
telescope and at the other. And again that time difference
can be translated into a range and that allows you to do the
relative positioning of these large radio telescopes.
Now since these extragalactic radio sources transmit radio
waves across a very wide bandwidth, these VOBI systems
can make millimeter accuracy differences in range
measurements because of that really wide band that they have.
So it allows again the sub millimeter level positioning of
these systems. However, the radio telescopes
have to be very large, they have to have very low noise receivers
and because the clocks are completely separated and they
only can see one thing at a time, they need to have very
accurate clocks. So these systems typically
crossed 1 to $2,000,000 per unit and there is about 30 of them
operating around the world at the moment.
OK. Let's talk about the third of
the technologies you mentioned the radar based one in SAR.
In a previous episode of Geology Bites with Romar Jolive about
the Turkey, Syria earthquake of 2023.
We discussed how Insar was used to map the movements associated
with that event. Can you remind us of how this
technology works? Yeah.
So ITZAR is a radar imaging technique and it looks at
signals which are reflected from the Earth and transmitted
normally from satellites. But you can also put this in
aircraft and sometimes on balloons.
The synthetic part of the name there comes from the fact that
the motion of the spacecraft is used to synthesize effectively a
very large band. And so with that you get very
high resolution on the ground. And the pixel size in these
images is typically of order one to two meters across.
The interferometric part of the name comes from the fact that
when you look at this pixel on the ground in one image, there's
lots of little facets in there that reflect the signal back,
and they sort of add together into the final amplitude and the
phase of the signal that's received back at the spacecraft.
If you come back six days later, 12 days later, and reimage that
same pixel and those little pixel pieces have all stayed
pretty much the same, then that phase is going to add to a very
similar result. And if the pixel has changed
position, you can actually see the change in position.
So in inside, you take little groups of pixels, maybe 9 or 16
of them, and you look at what the phase difference is between
two images. And if the whole group of little
pixels, which again is quite small in size, only, you know, 7
or 8 meters across, if that has moved, you can tell that it's
moved. And that can be typically done
with accuracies, again, between a millimeter and a centimeter or
so. The noise is about the
millimeter. What you worry about with INSAA
is that you're looking through the Earth's atmosphere when
these signals come down, and the refractive index of the
atmosphere changes the range as weather conditions change.
And so that makes it difficult to know exactly whether you're
looking at whether the Earth moved or whether there was a
atmospheric effect. And they have many techniques
now of using multiple images that try to reduce that
atmospheric effect. So I suppose one difference is
that the radar technique actually produces A spatial map,
whereas the other two technologies are really more
point to point measurements where you have your sensors.
Yes, these two are very complementary in a sense because
with the GNSS type systems we can get relatively high density,
you know, stations spaced by 5 to 10 kilometers in areas of
tectonic activity and we can make measurements literally
continuously. Most of the systems that run
make measurements at least once every 30 seconds.
So you get very high time resolution.
But as you say, the spatial resolution is quite cross.
But you can combine that with these inside measurements which
have very high spatial resolution.
But we only can acquire an image typically in a region once every
12 days or six days. And so you have high spatial
resolution, low time resolution, high time resolution, low
spatial. And as we try to combine them
together, one of the areas of research actually is how do we
try to then generate a product which is essentially both high
resolution in time and space from these combination of Insar
and gene S s measurements? OK, let's move on to the
application of these technologies to geological
questions. How exactly does it help us
understand what happens before, during and after a large
earthquake? Yes.
So what we were able to see very early on is plate motion.
And as we monitor something, we can see the large Co seismic
offset that happens during the time of an earthquake either
through INSA or the GNSS measurements.
And then when we look very carefully at these accumulated
stream that's happening up until the time of the earthquake, the
strain release that happens during the earthquake.
And one thing which is very clear for many earthquakes now
is that there is also a continued strain release after
the earthquake that's called the post seismic deformation phase.
That phase is very well measured with these modern techniques and
can typically be of order 10 to 15% of the size of the Co
seismic offsets. And so one thing we have noticed
is that this post seismic part of the earthquake seems to be a
much deeper process and so it spreads over a larger distance.
We think it also potentially transfers stress from the region
where the earthquake was happening to areas where stress
is still building but no earthquake has happened yet.
The other thing we can do with GNSS is we can actually see what
happens during the earthquake itself when we have these high
rate measurements. So we can see the arrival of
seismic waves. And one of the first interesting
cases of this was the Denali earthquake up in Alaska that
happened a couple of decades ago.
The direction of that earthquake rupture essentially sent seismic
waves right across the whole of the US and all of the GNSS
stations in the continental US region that we're tracking data
at 1 Hertz rates could see these 1 to 2cm waves propagating
across the US after that earthquake.
Now that type of process is quite common to be able to
observe in large earthquakes. The other phenomena that happens
before some of these earthquakes, also depending on
where you are, is that we have strain accumulation before the
earthquake. But particularly in subduction
zones, we often see this process called slow slip, where there
seems to be a relaxing of that strain accumulation.
And geodetically, it looks like you've had this earthquake in
the region, but instead of it lasting only a few seconds, this
earthquake takes maybe 10 to 20 days to happen.
And that is something, again, very well observed in large
subduction zone areas. What's not clear at the moment
is what's the implication of these slow slip things for the
future occurrence of earthquakes?
One might imagine that they release stress, so they're like
little earthquakes and stop the earthquake from happening.
But what's more likely is they're relieving stress in one
region, but transferring it to an area where that earthquake is
more likely to happen. We just don't have enough
examples of large earthquakes at the moment in subduction zones
fully instrumented to be able to address that question as to what
the impact of these types of events are before the events.
Does this mapping of strain work as well in highly mountainous
regions like for example the Himalayas in the 2015 Gorkha
earthquake in Nepal? In high mountainous areas, we
have a couple of issues. Logistically, if you're trying
to install GNSS, it's tricky. That has been done.
It's been done again with 50 to 100 kilometers sort of spacings
across many of these types of areas.
With INSUR in high mountains, you have sort of 2 problems. 1
is that the high mountains can actually shadow the radar
returns because again the radars tend to look at the surface not
directly down, but from an angle and that angle is somewhere
between 45 to 65° off Nata. And so you can get shadowing.
The other problem you have in the mountains is you can get a
lot of snow. And so when snow falls on the
ground, that reflection that comes back now is a completely
different type of reflection than it was before.
And so there is what we call no coherence in the image.
Nearby pixels now just have random phase variations and so
you lose the information on the deformation itself.
But in general, insight has been very successful in mountainous
regions. Can one turn that problem on its
head and use the variation in snow depth to measure how
actually ice and glaciers are thinning out?
Yes, that's another very big application of the inside
technique and also GNSS as well. The polar regions are really
very well observed because all of the satellites that make
these inside measurements are in high inclination orbits.
And so the paths of the spacecraft, the ground tracks,
cross very close together up in the polar regions and that gives
you much more opportunity to be able to actually look at the
deformation that's happening in those regions.
Also some satellite systems, the European Sentinel systems for
example, have pairs of satellites.
And you have one measure satellite making a measurement
and then very shortly after, another satellite can measure in
the same region. And since glaciers can move very
fast, they can move up to, you know, kilometers per year and
several 10s of meters per day. You can actually see the
deformation between those, you know, fairly rapid acquisitions.
And this has been one of the great applications of Insar is
looking at velocities of ice floes and how they change.
Throughout the years we've seen tidal effects on glaciers which
actually move out to the sea. The ocean type sort of
accelerates and decelerates the motion of the glaciers since
have lots of properties that allow us insights into the
mechanics of the ice. And that in turn allows us to
have a much better sense of as things like temperature change,
water temperature, air temperature change, the light,
the change in the nature of the ice dynamically, how fast can it
move, What are the limits on how fast the glacia can change its
motion, the amount it moves based on things like temperature
changes. In the previous Geology by its
episode with Chuck Demetz, we talked about high temporal and
spatial resolution measurements of plate motions and
particularly the northward movement of the Indian plate and
the effect of the plates collision with Eurasia.
Can we use the high spatial resolution of geology to get a
better handle on plate motions? Oh, definitely we can measure
plate motions at sub millimeter per year accuracies these days.
But it is over the interval of time which we have collected
data. And so places like India, for
example, the Indian Eurasia collision, that's happening at
around about 50 millimetres per year.
And one of the interesting results that came out of early
GNSS deployments in the late 1990s, early 2000s was that that
motion of India into the Himalayan mountains, which you
reflects in the building of that large mountain range.
That motion is only half accommodated at that Himalayan
front. The Himalayan mountains
themselves are actually being pushed back towards Eurasia at a
rate of about 20mm per year. And that 20mm per year is
actually accommodated in defamation.
That's happening on the other side of the Tibetan Plateau and
that large scale pushing of the whole Tibetan Plateau by the
Indian plate. That was not fully appreciated
until we had GNSS measurements that allowed us to get fine
spatial scale variations in velocity across these regions to
be able to interpret what the mechanics again of how this
happens. Wow, that's incredible.
So really only about half the northward movement of India is
actually accommodated by the building of the mountains, and
another half is actually transmitted to the north.
Yes, and you sort of see that in the topography.
I suppose that's one of the things we've learnt from these
GNSS measurements is wherever you have topography, you have
defamation typically still ongoing.
So if you look at North America, for example, once you get to
Colorado and start moving across towards the Pacific, all of that
material out there is actively deforming right now.
And so it's same in the Tibetan Plateau, etcetera.
Wherever you have topography, we tend to be seeing defamation
happening within those regions, again at rates of a few
millimeters per year, up to a centimeter per year depending on
where we are. You've also used high precision
geography to measure changes in the rotation of the Earth.
Can you tell us how you did that and what you found out?
Yes, the Earth's rotation is actually pretty wobbly as I'd
like to say. So there's essentially 3 aspects
to it. One is the rotation axis of the
Earth is actually not fixed to the solid body of the Earth.
It actually moves around, and that movement is about 10 meters
in a year. It actually is a circular sort
of motion because of some annual effects in the seasonal movement
of fluids on the planet. It also has a drift to it, again
of about 10 meters per century for the drift part.
The spin rate of the Earth, how fast we spin, also changes
dramatically, at least on a geodetic sense, throughout the
year, and that for periods less than about a year is driven
largely by changes in wind speeds in the atmosphere.
And that's extremely well measured and documented now
based essentially on our weather forecast models that allow us to
calculate the angular momentum of the atmosphere to extremely
high precision. And then in standard mechanics,
the angular momentum of the whole system has to be
conserved. So if the atmosphere's angular
momentum increases, the solid Earth's angular momentum must
decrease and we decrease it by changing the rotation rate.
We can also see that there's a long term trend in our rotation
rate as well. We're slowing down in our
rotation. This has been happening ever
since the Earth formed, essentially, and that is due to
tidal forces, mostly from ocean tides coming in and out.
And we can indirectly also measure that by these laser
measurements that have been made to the Moon's retro reflectors,
mostly put on by the Apollo missions and by the Russian
missions that put Rovers on the Moon back in the 1970s.
And that's about 38mm per year of motion.
And that's been well measured and it's consistent with the
slowing at the rate. The other big fluid for rotation
rate is the fluid core of the Earth.
And we actually need motion in the fluid core to generate the
Earth's magnetic field, which is actually quite critical to
protecting us from solar wind particles coming from the sun.
And that motion of the fluid core, if it changes, then that
also changes the rotation rate. And then finally, the Earth's
rotation axis moves in a natural space, mainly due to the torque
supplied by the Sun and the Moon on the Earth's secretarial
bulge, and the presence of the fluid core in there has a
dramatic effect on that. So one of the very early, very
long baseline interferometry measurements was to actually be
able to determine the flattening of the fluid core boundary based
on how the Earth was rotating. And that showed that the fluid
core was not in hydrostatic equilibrium.
It was off by about 500 meters in the distance between the pole
to the equator radius from what would be predicted.
And that was a measurement that came directly out of Earth's
rotation variations. So what are the time scales of
these rotation period changes? So you said the atmospheric 1 is
about annual. What about the ones that we can
trace back to the movements of the core or the moon or other
celestial impacts? Yeah.
The atmosphere has a variety of frequencies in it and we do see
a strong annual signal in there. We also see periods down at just
10 days. There's natural resonance modes
in the atmosphere that show up on the longest time scales
longer than a year. Oh, those variations in terms of
rotation rate or around about up to three milliseconds per day
change in the length of the day. Now 3 milliseconds doesn't sound
very much, but at times we are consistently 3 milliseconds
slower. So every day it accumulates.
And so after about 300 odd days, if you're doing 3 milliseconds
per day, you're up to almost a second change in time.
And we do try to account for that in the way that we do our
civilian time keeping relations to atomic time.
So the other time scales that are there are over decades.
And there we see again multi millisecond changes in the
length of day of decades. But now if we were to try to
explain that with the atmosphere, we just don't have
enough angular momentum in the atmosphere to have those large
variations. And so the place where we
believe all of that is coming from is the fluid core of the
Earth. And over the last 20 years or
so, we've seen the fluid core losing angular momentum and the
Earth gaining it and the Earth spinning up over those periods.
And then as I said of a really long time periods, we have this
tidal dissipation that happens due to the energy loss that
happens as you have water flowing in and out during the
tidal sequence of the Earth. I think you implied from what
you said earlier that you've been able to measure the change
in geographical pole, namely that the axis of rotation of the
Earth wanders with respect to the crust.
We're used to magnetic poles wandering quite a bit.
How much does the geographic pole move?
Yeah, the geographic pole moves typically around about 10 meters
over a year, and over a century it'll also secularly drift by
about 10 meters. And that drift over that period
of time, that's due to the Earth's response to the removal
of ice at the end of the last Ice Age, which ended about
10,000 years ago. When you put all that ice on the
Earth's crust, it pushes it down and the material in the mantle
as well. And when you take the ice away,
there's an elastic response that happens instantly, but then
there's a viscoelastic response when materials slowly come back
in. That process is still happening
around Hudson's Bay. For example, all of the land
around Hudson's Bay is moving upwards at about 15mm per year
at the moment due to that rebound and that change in the
surface of the height and the material in the Earth's mantle,
that's changing the moments of inertia of the Earth.
And so the Earth's rotation axis essentially wants to try to
rotate centered on the maximum moment of inertia of the Earth,
and that internal change is moving that maximum moment of
inertia. And that's what we're able to
see in these measurements. The dramatic lowering of cost
that you talked about of the satellite based systems means we
can use them for a much larger range of applications.
Can you give us some examples? Yes, we have lots of different
applications. One thing we've been doing is we
have GPS receivers that we put on tops of very tall buildings
to see how they deform throughout the year.
We've seen thermal responses to the buildings.
We've seen buildings moving, shaking during earthquakes, you
know, up to 20 centimeter type motions of the building.
It turns out for the buildings we've looked at that all of
these large motions are fully within the specifications of the
safety stances with the building.
So it's not an immediate danger to any of these buildings.
When we build large dams, we can also have two processes that can
be studied there. One is that as the dam fills,
the land around the dam actually gets pushed down by the weight
of the water. That's very well observed.
And so we can also put GPS receivers on the faces of the
dams to make sure that the dam is not being pushed out by the
water excessively as it is filled with water.
They've had dams in Southern California, which had this GPS
monitoring put on the dam just to verify that the damned face
is actually remaining stable and not suffering any damage from
these earthquakes. There's also some rather
unanticipated things that we've used these systems for.
One is that the GPS systems need to be able to see the whole sky
above them, but the antennas also can actually see
reflections from below the antenna.
That's what we call multipath for positioning the antenna.
That's considered noise, and we'd like to try to eliminate
it. But people have taken advantage
of that reflected signal to tell them something about what is
actually causing the reflection on the ground surface.
And so here people have used it to do soil moisture measurements
because typically as soils get moist, they reflect signals
better, so that reflected signal becomes stronger.
We can detect that if there's snow in the region, as the snow
builds up, there's a reflection of the snow and you can see snow
height. If vegetation is growing, if you
put this in your field, you'll be able to see it, the signals
growing, etcetera. One thing we worry about and we
have to take into account with all of these geodetic
measurements is that we have a refractive medium from the
Earth's atmosphere and we also have a layer of electrons high
above the atmosphere called the ionosphere and we can monitor
those. We have to monitor those to do
accurate positioning. And so we can use these
measurements to help actually do weather forecasting.
And for the ionosphere, we can monitor the strengths of
magnetic storms and we've even been able to monitor the impacts
on the atmosphere of tsunami waves coming from large
earthquakes, etcetera. The areas which commercially
have been very viable are things like precision farming, where
the large combined harvesters for example can be controlled by
GPS, but more importantly as the field is harvested it knows
precisely where it is. It can also use the multi
spectral scanner technologies that we often put in spacecraft
to get a measure of the yield coming from that particular part
of the field. The next year when that field is
used, the system can determine where it needs to put more
fertilizer to maximize the yield.
We've also seen recently lots of drone displays.
There was a massive 1 of 1500 drones set a Guinness World
Record for Christmas 2024. And these drones, which map out
these wonderful patterns in the sky with lights are all
controlled by these GPS receivers where the drones have
to be able to position themselves while they're flying
to within a few centimeters of their designated needed position
to make the image look correct. So in general, we're seeing this
ability, I think, of personal positioning to get us to a few
centimeters while we're walking around.
And so in the future, when you go someplace, it's almost
certain that there will be actually no debate as to which
door you actually have to enter through in the building.
It's not just knowing the building, but actually knowing
which door to go to. I think there's a lot of
potential growth for those types of things.
What are you working on at the moment?
So we're very interested in these slow slip events that are
occurring in these large subduction zones and getting
better temporal and spatial correlation from them,
extracting as much noise out of there as we can to be able to
get a better sense of what their relationship is to earthquakes.
Fluids moving around on the planet, and primarily water on
the surface is something which effects our position
determinations because it can load the crust, it can expand
soils. And so we want to get a better
sense of how we can exploit these measurements to understand
what subsurface water is doing. As a rule of thumb #1 millibar
change in atmospheric pressure actually moves the Earth's
surface by about 1/2 a millimeter, and things like 10
millibar pressure changes are very common.
And so we see this in our stations.
We as weather systems move across the stations, we can see
those types of motions. Tom Herring, thank you very
much. And thank you, this was fun.
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