Reading the Rainbow: How to Interpret an Interferogram
Since the early 1990s, scientists have used radar satellites to map movement or deformation of Earth’s surface. Interferometric Synthetic Aperture Radar (InSAR) calculates the difference between two radar images acquired by an orbiting satellite taken at different times but looking at the same place on Earth. This difference is called an interferogram and is essentially a map of surface deformation.
Deformation is one of the main ways that the Hawaiian Volcano Observatory monitors activity of Hawaiʻi’s volcanoes. Volcanoes can change their shape as magma moves in and out of their plumbing systems, as their slopes shift on faults or because of gravity, and when they have internal changes in pressure. Interferograms, together with GPS and tilt, help HVO scientists keep many eyes on a volcano when it’s getting bent out of shape.
Interferograms cover large areas of land—the entire Island of Hawaiʻi can fit into a single radar scene!—and provide centimeter-scale accuracy of ground motion. This means seeing changes of less than half an inch from space! However, interferograms can be tricky to read. This Volcano Watch will guide you through interpreting an interferogram, as well as explain how they are produced.
The difference in the distance to the ground between two satellite passes, known as the interferometric phase, is shown as fringes or bands of color in an interferogram. This difference includes deformation of the surface that occurred between passes, but it is also influenced by uncertainty in satellite orbits, topographic ambiguities, atmospheric conditions, and other sources of error. These all contribute to the interferometric phase. To get at the true movement of the ground, you have to compensate for these sources of error.
The first step in reading an interferogram is to determine “when” and “what.” The dates of each image answer the question of “when” the deformation occurred. Different satellites use different wavelengths, and that controls the amount of ground deformation represented per colored fringe—the “what”. In the interferogram shown here, the satellite used is the Italian Space Agency’s COSMO-SkyMed (CSK) system. The radar wavelength for CSK is X-band (approximately 3.3 centimeters in total length). Because an interferogram is made using radar waves that travel to Earth from the satellite and back, the deformation is calculated in terms of half the wavelength. This means that when reading a CSK interferogram, one fringe is equal to 1.65 centimeters (or 0.65 inches) of change between the two dates.
Step two is to count the colored fringes to determine the amount of deformation shown in an interferogram. Volcanic deformation is often concentric in shape, so start on the outside of the fringe pattern and count the number of color cycles from the edge of the deformed area to the center. Multiply the number of colored fringes by the half wavelength to determine the magnitude of the deformation.
Step three is to determine if the surface moved up or down. To do this, as you count fringes from the outside to the inside of a concentric pattern, note the sense of color changes. Does the pattern go blue-purple-yellow in towards the center, or blue-yellow-purple? The color scale at the bottom of the image reveals that blue-yellow-purple is an increasing trend, which in this interferogram, means that the ground is moving towards the satellite—it is inflating! The color sequence should always be defined on a scale bar in the interferogram, along with the half wavelength, because not all interferograms use the same color sequence, and not all radar satellites have the same wavelength.
The last step is to interpret the information you have collected. On the CSK interferogram shown here, there are 3 colored fringes inside Kīlauea Caldera, which means that the surface within this region moved towards the satellite, or inflated, by about 4.95 centimeters (1.95 inches) between April 6 and June 2, 2019. But remember, the colored fringes include not just displacement, but also other influences, like orbital errors and atmospheric anomalies. These sorts of anomalies are especially prevalent around tall mountains, like Mauna Loa and Mauna Kea, and areas of very steep topography, like the south flank of Kīlauea. In this case, atmospheric anomalies are probably not an issue, since the area of deformation is smaller than most weather patterns. We also compensate for orbital errors, since we know the satellite orbits very precisely. But topographic errors due to the massive changes that took place at the summit in 2018 might introduce a small amount of error to the measurement. Also, because radar satellites don’t look straight down on Earth, the deformation in interferograms is a combination of vertical and horizontal displacements, although vertical changes usually dominate.
And that’s how you read an interferometric rainbow.
For more information on how to read an interferogram, including a step-by-step infographic, check out UNAVCO’s outreach products online.
Volcano Activity Updates
Kīlauea Volcano is not erupting and its USGS Volcano Alert level remains at NORMAL. For definitions of USGS Volcano Alert Levels, go online.
Rates of deformation, gas release and seismicity on Kīlauea have not changed significantly over the past week.
Monitoring data have shown no significant changes in volcanic activity over the past week. Rates of seismicity across the volcano remain low. The two largest recorded earthquakes were a M2.4 at the summit on June 15, at a depth of 1.2 miles and a M3.4 in the Upper East Rift Zone on June 13, at a depth of 3.7 miles. One felt report was submitted for the M3.4 earthquake. Real-time sulfur dioxide emission rates from the summit remain low. Sulfur dioxide emission rates at Puʻu ʻŌʻō remain below detection limits when last measured on May 23.
Since early March, GPS stations and tiltmeters at the Kīlauea summit have recorded deformation consistent with slow magma accumulation within the shallow portion of the Kīlauea summit magma system (approximately 1 mile below ground level). However, gas measurements have yet to indicate significant shallowing of large volumes of melt. HVO continues to carefully monitor gas output at the Kīlauea summit and East Rift Zone for important changes.
Further east, GPS stations and tiltmeters continue to show motions consistent with refilling of the deep East Rift Zone magmatic reservoir in the broad region between Puʻu ʻŌʻō and Highway 130. This trend has been observed since the end of the 2018 eruption, however there is an indication on tiltmeters that this motion has been slowing down over recent weeks. While the significance of this pattern is unclear, monitoring data do not suggest any imminent change in volcanic hazard for this area.
Two earthquakes with three or more felt reports occurred in Hawaiʻi this past week: a magnitude-3.4 quake 13 km southeast of Volcano at 6 km depth on June 16 at 12:54 p.m. HST, and a magnitude-2.8 quake 14 km southeast of Volcano at 6.8 km depth on June 13 at 1:19 p.m. HST.
Hazards remain at the Lower East Rift Zone and summit of Kīlauea. Residents and visitors near the 2018 fissures, lava flows, and summit collapse area should heed Hawaiʻi County Civil Defense and Hawaiʻi Volcanoes National Park closures and warnings. Hawaiʻi County Civil Defense advises that lava flows from the 2018 eruption are primarily on private property; people are asked to be respectful and to not enter or park on private property.
The USGS Volcano Alert level for Mauna Loa remains at NORMAL. A slight increase in detected earthquakes was noted over the past month. GPS instruments show slow inflation of the summit magma reservoir. Gas and temperature data showed no significant changes the past month.
HVO continues to closely monitor both Kīlauea and Mauna Loa for any signs of increased activity.
Visit HVO’s website for Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Call (808) 967-8862 for weekly Kīlauea updates. Email questions to [email protected].
“Volcano Watch” is a weekly article and activity update written by the US Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week’s article was written by HVO Geophysicist Sarah Conway.