Volcano Watch: It’s all about perspective; how to interpret an interferogram

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week’s article is by Tyler Paladino, a postdoctoral fellow with the U.S. Geological Survey.

Interferograms are images created by radar satellites that show how the ground has changed shape, or deformed, through time.

Today, we’ll dive into one of the trickiest parts of interpreting interferograms: the perspective of satellites.

Volcanologists use interferograms to detect unrest by observing how the surface of a volcano is deforming in response to migrating magma beneath the ground. We can see changing subsurface magma reservoirs, their connections and new paths magma creates as it travels closer to the surface.

While interferograms can be beautiful and informative, they are also difficult to interpret.

A June 2019 “Volcano Watch” article discussed how to read fringes, the repeating cycles of color that often make bullseye or butterfly wing patterns, to figure out how much ground movement is occurring.

To summarize quickly:

• Count the fringes, starting from the outside of the feature toward the inside. Keep track of the sequence the color is cycling. Is it red-yellow-blue or blue-yellow-red?
• Use the figure key to work out which direction the ground is moving, either toward or away from the satellite. The change in distance between the ground and satellite is called “range change.”
• Multiply the number of fringes by the range change value of a single fringe (also given in the figure key).

There you have it. You calculated displacement and its direction. Or have you?

In reality, it’s a bit more complicated.

Two Synthetic Aperture Radar images — separated in time by a few weeks — are combined from satellites orbiting around Earth to create interferograms.

If the ground moved enough within the timespan of the two images, fringes will be visible.

Where this gets tricky is how the satellite looks at Earth’s surface.

Synthetic Aperture Radar satellites don’t look straight down. They look to the side, often about 30 degrees from vertical.

What effect does this have on interferogram interpretation?

If an area on the ground moved exactly upward by 5 inches, from a Synthetic Aperture Radar satellite’s perspective, less than 5 inches of range change will be recorded.

But why is this?

Let’s pretend someone is shining a flashlight at you. If they shine it directly at you, it’s quite blinding!

But if they tilt the light just a bit off-center, the light will still be visible, but not nearly as blinding. If the light is tilted fully away from you, you might see nothing at all.

The intensity of the light hasn’t changed. What changed is your perspective.

Going back to the example above, a similar phenomenon is happening, but instead of light intensity, we’re measuring range change.

If the motion of the ground is tilted away from the Synthetic Aperture Radar satellite’s look direction, less of the actual motion will be measured than is physically occurring.

If the ground were to move 5 inches directly toward the satellite, it would indeed record 5 inches of range change. In this manner, the direction of the ground motion is important, as is the orientation of the satellite.

To illustrate how satellite orientation affects an interferogram, we created a simple model that shows a spherical inflating magma body — like an inflating balloon underground — and its effect on the surface viewed from multiple satellite perspectives.

In these models, the magma body is in the exact center (white star), but that’s not what we see.

We instead see a bullseye that looks somewhat skewed and shifted either to the west or east, depending on the viewing geometry. This is because of the phenomena we just discussed.

Since Synthetic Aperture Radar satellites look to the side, the range change we see as fringes is only a part of the actual motion of the ground if that motion is not in line with the satellite’s line-of-sight.

Since inflation results in the ground moving upward and outward, there will be some motion captured and other motion missed or minimized.

For example, if motion is perpendicular, or close to perpendicular, to the line-of-sight, the satellite will see barely any motion at all, just like the decrease in intensity as a light is being shined away from you.

This results in the entire measured deformation pattern to be shifted, as outward perpendicular motion away from the satellite will be small while motion toward the satellite will be larger.

Interferograms posted by the Hawaiian Volcano Observatory will provide an arrow denoting the satellite flight direction and a perpendicular bar that shows its look direction.

Using this information and your newfound knowledge of Synthetic Aperture Radar perspectives, you can more accurately interpret how a volcano is deforming.

In a future “Volcano Watch,” we’ll discuss how perspective changes more complex signals such as propagating dikes.

Spoiler alert: it gets even more complicated.

Volcano Activity Updates

Kīlauea is not erupting. Its U.S. Geological Survey Volcano Alert level is at Advisory.

Earthquake rates beneath Kīlauea’s summit and upper-to-middle East Rift Zone area decreased during the past week and were less than half that of the previous week.

About 70 earthquakes were located beneath the summit, with fewer than 100 located in the upper-to-middle East Rift Zone region.

Ground deformation rates in the summit region showed steady inflation during the past week, while ground deformation rates near the Sept. 15-20 middle East Rift Zone eruption site have slowed.

Future intrusive episodes and eruptions could occur with continued magma supply.

Mauna Loa is not erupting. Its Volcano Alert Level is at Normal.

Two earthquakes were reported felt in the Hawaiian Islands during the past week:

• A magnitude-3.6 event at 6:45 a.m. Dec. 10 located 27 miles southwest of Captain Cook at a depth of 19 miles.
• A magnitude-3.3 event at 4:26 p.m. Dec. 9 located 11 miles southeast of Pāhala at a depth of 20 miles.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the volcano observatory’s website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.