26 January 2012

Deformation and GPS


We get a LOT of questions about volcanoes, including how to Know if they'll Blow. There are a number of ways we can track magma movement at depth, including deformation and "LP's" - long-period seismic tremor that is indicative of fluid movement. At late stages of unrest, we will start seeing "VT's" - short-period volcanic tremor that is indicative of shallow rock-breaking - and increases in CO2 and H2S gases. There is at least the possibility that we can detect early movement of magma at 30 - 40 km depths using magnetotelluric systems, but so far there hasn't been funding to try this. As I write this, deformation reaches out the longest time ahead of all these detection systems to give us warning of an impending eruption.

The term "deformation" is used by specialists in ground movement in the geosciences; these guys themselves are called "geodesists". Geodesists measure movement as a component of strain along an active fault, to try to get a sense of the energy accumulating that could lead to an earthquake. Deformation is used in volcanology to look for - and then track - inflation in a volcanic edifice. Deformation is done in several ways:

  1. By surveying the ground with high precision. This has been done at Yellowstone since the mid-1920's, and those early data have helped us get a much better sense of how the huge caldera moves and breathes. 
  2. By deploying tiltmeters. Originally these were long tubes of water laid out over the ground. If the ground under the flank of a volcano started tilting, it would show up in amplified movement of water in vertical tubes at the end of the long tube. Modern tiltmeters are ultra-sensitive cylinders placed in a vertical hole in the volcanic rock, then packed in with sand. The signal from these devices and all the following systems is generally telemetered back to a recording and monitoring system. 
  3. By using radar satellites - this is called InSAR for Interferometric Synthetic Aperture Radar. If two images can be captured over the same volcano, they can be used to make interferograms. These are colored, Moire patterns - generated with enormous mathematical calculations to geometrically correct and ratio each pixel to another, called "rubber sheeting" - that will show inflation over the surface of a volcano and its environs. Each rainbow-colored ring-set represents one radar wavelength (typically 5 - 15 centimeters) of uplift. These often form a bulls-eye centered over an inflating volcano or deflating caldera, and I've seen several gorgeous examples at Ngiragongo volcano, in Central Africa; at Pavlof, Akutan, Okmok, Shishaldin, and many other volcanoes in the Aleutians, and at Mauna Loa and Kilauea volcanoes in Hawai'i. 
  4. Gravity level-lines. This is like survey leveling, but is done by making repeat measurements with a gravity meter over a line of stations every six months or so. All other things (including the water table) being equal, an inflating volcano will show up as a decrease in the gravity field - the gravimeter is being moved farther away from the Earth's center, and the pull of gravity falls off as (1/radius distance squared). I did this to monitor magma moving into the Harrat Rahat volcanic field east of Madinah al-Munawarrah ("Medina") in Saudi Arabia. Seismic telemetry also showed small earthquakes associated with this magma movement at the same time. The events died out by 1995, causing a lot of people to breathe a collective sigh of relief, but this kind of one-again-off-again restive behavior is not at all unusual for a volcano.
  5. Telemetered GPS. These use the same GPS satellites you and I utilize in our cars or when hiking, but the precision measurements made by geodesists (the formal name for the deformation guys) are made using different signals from the same satellites.
  6. We also instrument volcanoes with sensitive analog, and ultra-sensitive broadband seismic sensors. Some of these data are telemetered, some are recorded and just stored in the instrument box on a small hard-drive until retrieval the following summer. That is, unless bears decide to play ball with one. One over-winter seismic network campaign at Katmai in Alaska found 5 of 11 very expensive stations had been trashed by bears before they could get back and retrieve them. 
GPS is a fascinating field, and applies far beyond the earth sciences. A brief run-down might be useful here.

The Global Positioning System was first envisioned by DARPA - the Defense Advanced Research Projects Agency of the Department of Defense - during the 1980's. Navigation at that time was complex and difficult, and getting any sort of location precision over vast distances including oceans was very important to some people. Like, the people targeting ballistic missiles, for instance.

In the late 1980's I worked in the Venezuelan jungle, where our main form of navigation was using 1:250,000-scale airborne radar (SLAR) maps. These were assembled by flight strips - and it was not unusual to find splice errors as large as 3 kilometers. Basically that means I could be standing on a rock - and half of the rock was 2 miles along the strip edge from the other half of the rock. I have been on a helicopter traveling for an hour over trackless forest using a half-meter-sized roadmap of the country (except there are no roads in the jungle) and crudely-penciled lines with the azimuth and distance for the site we wanted to visit. If that helicopter's fuel line had a single bug in it, we would have dropped down into the trees. Even assuming we had survived such a crash (the incident statistics gave me a 50% chance of this), how would you call in a rescue helicopter? How in the world would you tell them where you were?!?

I first began using a GPS device in the early 1990's in Saudi Arabia. In the northern reaches of the country there is a vast plain that is dead flat for hundreds of kilometers in all directions. Some of our guys had accidentally strayed across the Iraqi border because there is no way to know where the line arbitrarily drawn by the British a century earlier actually was. The first GPS units were incredibly slow, the size of a Betty Crocker cookbook, and didn't always work - but the idea fascinated me. With a radio, I could then precisely tell people where I was. 


Since then, hand-held GPS devices have shriveled to matchbox sizes, strap to your wrist, and have maps built in. You can program them, collect precise tracks... the list of bells and whistles goes on and on.

But how do they work? What actually is out (or up) there?

The American GPS constellation has at any given time about 24 active satellites and a few loitering spares, and each one transmits a very faint signal on two freqs - digital signal for hand held use and another digital carrier that is used for precision location acquisition - I'm talking centimeter-size precision here. BOTH frequencies are encrypted... they belong to the military, and for a long time the signals were deliberately "fuzzed" - this was called Selective Availability, or SA for short. If you had the key and a certain type of book-sized device, you could get very precise locations - within 10's of meters. But DOD didn't want someone else using those signals to pop an artillery round on top of one of their military outposts. Even to this day, if you try to use a receiver and go faster than a commercial airliner (as in: a ballistic missile) it won't work. It has a built-in fail-safe.

In the meantime, the rest of the world has become incredibly dependent on the GPS constellation. I could never summarize adequately all the ways and places where it is used right now.

If you are surveying - or trying to see if two points on the opposite sides of a volcano are moving apart from other (uh-oh), then you need great precision. It can now be as good as a bit over a centimeter horizontally and 2-3 centimeters vertically. In part this difference in precision is because for horizontal solutions you can subtract the atmosphere effect from two different near-horizon satellites - and triangulate better. For vertical elevations, you have only satellites in one direction (not beneath your receiver).

GPS signals all use the same frequency, but the signals are encoded to separate the satellites. Both transmitted signals from each are encoded, so you can't use one for a ballistic missile guidance system unless you own the codes. As I said, above a certain aircraft speed, GPS won't work.

Well, the Russians certainly didn't want to be dependent on something that the Americans could fuzz - or even turn off. So despite their crushing economic difficulties, they turned the best Russian minds onto building their own constellation. This is called GLONASS, and the signal is not encoded, the energy transmitted is greater, so the signal-to-noise ratio is 5 times or 15 db better. Because of this, the signal penetrates tree canopy, so I could use it in the jungle! Woo-HOO! The GLONASS system also uses 3 different frequencies, so you can reduce ambiguities and calculate better differential atmospheric corrections.

These GNSS (Global Navigation Satellite Systems) are so precise that they routinely calculate and correct for relativistic effects! There are also huge atmosphere effects that must be compensated for - dense air masses here and and ionized layers there. GLONASS even works on new American and European hand-held devices when the GPS signals are poor due to a poor view of the constellation - if you've ever been in steep canyons in Utah or New York City, you will know what I mean here.

Not to be outdone, the European Union is now experimenting with their own GNSS (Global Navigation System) called GALILEO. This is a purely civilian system with three frequencies, and is scheduled to come online in 2015 - they are testing 2 satellites in orbit right now.

For the same reasons, the Chinese have started their own COMPASS satellite GNSS system, and it likewise is coming on line rapidly - there are 6 satellites in orbit already, and thee would have been more if a recent Russian rocket system hadn't crashed. Not to be left behind, the American version of GNSS - the only one that should technically be called "GPS", is being upgraded.

All four of these GNSS systems use L-band frequencies to resolve ambiguities and increase precision - and penetrate the ionosphere. What does L-band mean? Look at your personal GPS system and the smallest dimension on it will give you an idea of the wavelength for L-band.

The navigation problem is more than just triangulation - three satellites near the horizon would serve for this; two would give you two possible location solutions, three would mean only one possible solution. But there are four unknowns, since you are measuring how long a stretch of space and air that your signal must travel. The precision of your timing thus becomes utterly critical, the speed of light being so huge (300,000 km/second), and hand-held GNSS devices cannot carry $100,000 maser clocks. Thus, you must use a 4th satellite to help solve for the 4th unknown: 3 for position, 1 for a clock reference for your receiver

There are a few more complications. You really need to use a reference ground station to get really good differential distance calculations - to do good back-corrections for the changing satellite orbits, the complex and varying atmosphere, snow cover, etc... However, during the Tohoku earthquake in early 2011, all of Japan jerked eastward, so geodesists couldn't see the whole shift with really great precision because their reference station also moved.

So how does this help volcanologists? As I said earlier, if two telemetered GNSS receivers are moving away from each other, and there is a volcano in between them (this is happening right now with Mauna Loa, the largest volcano on Earth), then you are being given a warning that something is coming.

In 1989 we didn't have such a warning before Redoubt volcano in Cook Inlet of Alaska erupted. A KLM Boeing 747 flew right into the ash cloud - and lost all four engines in rapid succession. I've got a recording of the captain's voice as she tries to guide her flight crew in Dutch and talk with flight control in Anchorage in English. Her voice rises steadily a full octave before she finally yelled "Anchorage we have lost all four engines, we are in a fall. We can use all the help you can offer." They managed to restart two of the engines, and made a rough landing at Anchorage International airport. No lives were lost - but the repairs to that Boeing 747 cost $80 million.

To put that in perspective, when I served as chief scientist for volcano hazards for the US Geological Survey, my entire science team budget was less than $20 million.

There was another interesting GNSS application that you will find fascinating - I sure did. When Mount St Helens erupted on 1 October, 2004, we had just a week of accelerating seismic racket on our network beforehand for a warning. The extrusion was first seen on October 12 - and by pure luck I got the first photo of the new "spine" from a helicopter orbiting the steaming and fractured Crater Glacier. The dacite extrusion - 700 degrees C at where it was coming up from the talus slope at its base - came out like a tube of squeezed gray toothpaste. It resembled the back of a whale, so that became its name: The Whale. It moved south through crumbling talus and ice until it hit the remaining south rim of the 1980 eruption. The geodesists wondered when it actually reached the wall - When Did The Whale Hit the Wall? A check of a GPS station on the other side, on the outside south slope of the volcano, answered the question. On November 17, 2004, that station suddenly started moving south. Was it an effect of snow on the antenna? No, because the only direction it moved was south - by about 10 cm. The entire crater wall was shoved southward by 4 inches.

I'll never forget the elation of scientists using GPS technology to answer a real question about an erupting volcano. But GNSS systems provide us more than just answers to our scientific curiosity.

In 2006 a sharp-eyed geodesist in Anchorage, Alaska, was routinely checking data from several GPS units installed on Augustine volcano in the middle of Cook Inlet, south of Anchorage. This had erupted in 1979 and nearly killed David Johnston, one of our brightest young geologists who was later killed during the 1980 lateral blast, the opening eruption salvo of Mount St Helens.

In August 2006 this geodesist noticed some differential movement apart - the first subtle inflation was starting - and notified the Scientist-in-Charge. A close checking and monitoring effort was triggered - and sure enough, the signal was real, showing above all the background noise - and it was continuing. Federal and State Emergency entities, along with the FAA, were put on notice. In Late December the first VT's started appearing on the seismometers. As they accelerated in frequency and amplitude, the USGS issued a warning: an eruption is imminent in hours or days. One day later, on January 16, 2007, Augustine erupted, and dusted Anchorage with ash. International flights were cancelled or re-routed for three days - but not a single aircraft was damaged, not a single life was lost.

Yeah! This stuff works!

~~~~~

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