At the mouth of Tampa Bay sits a giant concrete block and a steel pole extending to just below the surface. Another steel pole, connected to the first, appears above the water.
The seemingly simple pole setup, upon a closer look, is a buoy system featuring plenty of tech: an antenna, a sensor, solar panels, and a variety of other instruments. The "spar buoy" detects small changes in the sea floor — an important measurement when you need to guess the location and size of the next earthquake.
Geoscientists at the University of South Florida developed the shallow-water system.
Created with the help of an $822,000 grant from the National Science Foundation, the buoy was installed off Egmont Key in the Gulf of Mexico last year and has been producing data on the three-dimensional motion of the sea floor.
The underwater shifts being detected are often a precursor to deadly natural hazards, like earthquakes, volcanoes and tsunamis.
Geodesy is the science of measuring placement, position, and strain. Ultimately, the geodetic system in Tampa Bay will be able to detect small changes in the stress and strain the Earth’s crust, said USF School of Geosciences Distinguished Professor Tim Dixon.
"Measurement strain and displacement before an earthquake occurs can help us do a better job of forecasting the size and possibly location of these events when they occur," Prof. Dixon told Tech Briefs. "We can’t predict earthquakes, but we may be able to do a better job of forecasting where they’re going to be and how big they’re going to be."
The buoy rests on the sea bottom, thanks to a heavy concrete ballast. The connected steel pole, or spar buoy, has a GPS antenna and 3D orientation sensor, also known as a digital compass. Both instruments measure rotation and other movements, which can then be calculated to determine position of the ballast.
Measuring sea-floor motion in shallow coastal water is challenging due to strong and highly variable oceanographic effects. Tampa Bay, for example, has experienced major storms over the past year and is a very good test for the buoy's stability and orientation system.
The buoy has held up in the tough weather, according to the team's report.
Even in the presence of strong tidal currents, which can deflect the top of the buoy several meters, estimates for the anchor are 1–2 cm or better, said the researchers in the Journal of Geophysical Research-Solid Earth .
In an edited interview with Tech Briefs below, Prof. Dixon explains why this highly accurate data is so valuable to environmental researchers.
Tech Briefs: What does the technology look like?
Prof. Tim Dixon: It's basically a giant concrete block that sits on the sea floor. There’s a shackle on top of that block. There’s a rigid steel pole that goes up to the surface. To keep it vertical, we put a float on the end of it, near the surface, just below the surface so it’s always underneath the waves. Above that float, there’s a skinnier pole that weighs a little bit less, and on top of that we mount solar panels, batteries, the GPS antenna, the GPS receiver, and an iridium transmitter that sends the data back to us.
We try to make it work as much as possible like a land-based, high-precision GPS installation — almost like the GPS you have in your phone or your car. We’re measuring positions and changes in position, but it’s more accurate. It has a bigger antenna. It has a very stable base. We do lots of computer processing to get the accuracies down to a few millimeters. We wanted to recreate the characteristics of a land station as best as we could for the shallow water environment.
Tech Briefs: Why is it important to get these shallow measurements off-shore?
Prof. Tim Dixon: The problem with the subduction zone earthquakes — those are the big ones and the ones that create most tsunamis — is that all the action happens off-shore. Offshore measurement methods are expensive and don’t work in shallow water. That’s the basis of this buoy that we designed and tested.
Tech Briefs: Can you provide a simplified, theoretical example of how you go from the measurement data of your system to alerting a region about an earthquake?
Prof. Tim Dixon: The size of an earthquake is related to the size of the patch of plate boundary that slips in the earthquake.
If we could measure the size of these patches that are accumulating strain, you’d have a better idea of where and how big a subsequent earthquake is going to be. The problem is: It's hard to make these measurements offshore.
For the Japan earthquake that occurred in 2011, there was a very large component of offshore slip. Some estimates put the motion during the earthquake as high as 50 meters. So, if we’d had better strain accumulation data for that difficult-to-instrument offshore area, we might have seen a very large pre-earthquake strain accumulation on the patches that subsequently slip.
We want to measure the location of these "locked" patches that are accumulating strain, to find out where they are, how big they are, and how much strain they’re accumulating. Most people who study subduction-zone earthquakes think that this kind of data is going to be a key part to better understanding the earthquake process and possibly forecasting these things.
Tech Briefs: How has it been challenging to make detections at shallow levels?
Prof. Tim Dixon: There are various ways of measuring motions of the sea floor. Many of them rely on acoustic techniques where you put a sound pulse into the water and measure how long it takes to go from position A to position B, or position A and B and back again. We know the speed of sound, so you can convert time into distance.
The problem is that, in the ocean, the speed of sound can be quite variable. The speed depends on the salinity and the temperature of the water, both of which change a lot, and they especially change a lot in the coastal areas where the tidal currents are strong.
So the acoustic techniques have been used in deeper water offshore, in waters deeper than 1 km, but for the shallow coastal areas, the acoustic techniques are much less useful because of noise interference problems.
Tech Briefs: What was your moment of inspiration?
Prof. Tim Dixon: I read a paper by an Italian group who developed a very similar buoy, off of Naples. They were worried about volcano deformation from a volcano located very close to Naples. And they wanted a technique to measure the vertical motions of the sea floor.
So they made a buoy, very similar to the one we did. The only difference is that theirs only measured the vertical component. It turns out, with this geometry and this buoy technology, it’s relatively straightforward to measure the vertical component but not the horizontal component of motion.
I read this paper, and I was astonished at the vertical accuracy that they were getting. I thought “Boy, wouldn’t it be great, if we could do the same for the horizontal components of motion.” I put my thinking cap on and came up with a way that I thought might work to recover the horizontal component.
Tech Briefs: How were you able to measure horizontal motion?
Prof. Tim Dixon: It involves measuring the tilt and orientation of the buoy. We do that with an array of tilt meters and a magnetic compass that gives us very-high-precision orientation measurements. By adding the tilt information to the GPS information, we can solve the equations for both vertical and horizontal components of motion.
That was the innovation. The Italian team came up with the first way to make a buoy, and we added this tilt measurement to add the horizontal component of motion.
Tech Briefs: What other kinds of measurements are being made on this buoy?
Prof. Tim Dixon: We make a whole bunch of measurements. There are meteorological packages on there. We measure solar radiance. All of those, however, are kind of extra measurements. The oceanographic team is going to be mounting some stuff on there to measurement turbulence in the atmosphere at the ocean atmosphere interface; that’s of interest for transfers of energy and momentum from the ocean into the atmosphere. Hurricane growth depends on that transfer.
My interest is just the geodetic measurements. So what that means is, at the end of the day, all we get out of the system is the 3D position (x, y, z) of that anchor, and the millimeters of change from the day before. There’s a whole host of instruments on the thing now, and you can certainly add more because have the space and power, but my interest is focused on getting the anchor position with extremely high accuracy.
Tech Briefs: Where has this been used and tested so far?
Prof. Tim Dixon: So far, only in one location. It’s sitting at the mouth of Tampa Bay, and has been in the water for a little over a year. The location is close to home base, so it doesn’t cost us a lot of money to take a boat out to change a battery or instrument.
Another reason that we put it there: This location experiences extremely high tidal current. The buoy is leaning over one way to the east, and leaning over to the other way to the west by a large amount, much larger than it would for what I would call a preferred installation offshore in these subduction zones.
The reason we put it in Tampa Bay is that we wanted a huge deflection signal to ensure that our corrections for tilt were working. If the tilts are small, then any number of tilt correction algorithms might work, but you wouldn’t know if they were the optimum ones. In this location, the tilts are so large that if there’s any problem at all in the tilt correction, we’re going to see it.
We’re pretty happy with the way things are operating now. The buoy gets deflected several meters during some of the higher tides off the vertical. When we do the correction, it brings that anchor position back down to around a centimeter, so that’s pretty good.
Tech Briefs: What's next?
Prof. Tim Dixon: This current version's design is only good up to a depth of about 40 meters. The next evolution in the design is to make it available for deeper waters. We think it, in principle, can go up to possibly 200 meters, certainly 150. We’re in the process of writing a proposal to NSF to do that.
Tech Briefs: What’s most exciting to you about this technology and its possibilities?
Prof. Tim Dixon: There are applications in volcano forecasting, in monitoring of offshore oil and gas reservoirs, but the thing that excites me is that I think we can get this buoy tested and deployed in some subduction zones around the world. I think we might be able to greatly improve our ability to understand and possibly forecast these big subduction zone earthquakes and tsunamis, the kind that was so devastating in Japan a few years ago.
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