In starting to write the SOAR-E grant proposal to attain RADARSAT-2 images over Iran, Catherine e-mailed me her 2008 paper that uses an Iranian salt diapir as a case study. Originally the article was just so I could get the diapir coordinates, but I started to read it and found it quite interesting.
Radar topography of domes on planetary surfaces by Neish et al. (2008) introduced me to the technique of radarclinometry, which is to use radar images to produce topographic information. By measuring variations in radar image radiance, you can find relative elevations using "shape-from-shading" techniques. One of the largest controls on radar-backscatter values is the incidence angle at the surface. If you imagine that a structure is homogenous, you can attribute changes in backscatter to variations in incidence angle, i.e. slope. Once we know the slope, we can produce a topographic profile for the feature! Neat, eh?
There are a couple of methods for this that you can use.
A one-dimensional method determines the amount of slope on a surface by measuring the brightness of lines of pixels to make a topographic profile. This method doesn't give you any information perpendicular to your slope profile. There is a two-dimensional photoclinometry method that fits a digital topographic model to an image. This incorporates two-dimensions, but is both slow to process and is sensitive to artifacts in the radar backscatter images. Another method, which is employed here, uses a larger scale image of a specifically shaped feature, and adjusts the height accordingly. This is less vulnerable to backscatter variations than the other 2-D method.
The purpose of this paper is to determine if you can use radioclinometry to measure the heights and shapes of "viscously emplaced domes", with a specific interest in studying features on Saturn's moon Titan. The altimetry data for Titan is both sparse and poor resolution, so exploring alternative methods of gaining elevation information can have large impacts on how we study Titanean features.
What do shield volcanos and salt diapirs have in common? They are both viscously emplaced domes! That is, they both form by the slow movement of thick, soft, deformable material, and make roughly circular shapes. So, this study assumes a dome-shaped profile, and uses the aforementioned radarclinometry technique to measure the height profiles of viscously emplaced domes across the solar system. The authors use an Iranian salt diapir of known elevation, and pancake domes on Venus to test their methodology before applying the technique to Ganesa Macula - a 180 km across, circular, radar-dark feature on Titan. Ganesa Macula is suspected to have volcanic or cryovolcanic origins.
How did they do?
Well, after comparing different models, the authors found that they did a pretty good job of fitting the radarclinometry data they produced to the available topographic data available for the Iranian salt diapir. The terrestrial case study demonstrates a good proof-of-concept for this technique. In contrast, the radarclinometry profiles made for the Venusian pancake domes were consistent, but a little less than the altimetry. Nonetheless, the heights they measured for Ganesa Macula (2.0-4.9 km) fits previous constraints, so the results are promising. They also estimate the volume of Ganesa Macula ato 30,000-40,000 km^3. This is a significant volume, because if Ganesa Macula is made of a volcanic lava, based on Titan's heat production rates it would have to becomparable to the duration of Earth's Deccan Traps. Perhaps Ganesa Macula is the result of a rare event, which would explain why it is such a unique feature on Titan.
I expect that next time I write I'll have fixed the registration issue with my radar images. I look forward to showing you my PALSAR acquisitions!