Monday, May 29, 2017

Back to the Arctic: An anomalously radar-rough area

Hi, hi!

For this week's update, let's return up north to Axel Heiberg Island, NU.  Recently, I've been revisiting my radar and spectral images in preparation for our upcoming field season. In July, four of us will be visiting the island to ground-truth what we've been seeing in the satellite imagery, with a specific focus on the salt diapirs I've been studying. This will help us better understand the surface texture of the diapirs, how rough they are, and why they are producing the signatures we see in radar.

Part of the Canadian High Arctic, including Axel Heiberg Island, and most of Ellesmere Island. Boxed represents study area for Elise's research. Image credit at bottom right.

One area, not made of salt, is quite curious. There is nothing abnormal about it in the true-colour satellite imagery.

Overview of study area on Axel Heiberg Island. This "wall-and-basin structure" contains a high abundance of salt diapirs. Image credit same as above. Box outlines "weird" area.

But yet it appears very rough in C-Band radar.  More rough than the diapirs, even! Check it out:

RADARSAT-2 C-Band ciruclar polarization ratio (CPR) image over study area. The purple box surrounds anomalous radar rough area. The red circle outlines a large salt diapir for comparison.
That is super rough! How odd! But it definitely isn't salt, because the area doesn't show an anhydrite/gypsum signature in the spectral images:

Spectral (ASTER TIR) band composition of study site. Boxes are in same features as above image. Salt appears as dark maroon.  The mysterious radar-rough feature appears as dark blue.
Curiouser and curiouser! The rough area is appearing blue in the spectral image.  I'm not sure what that corresponds with in this band ratio - something to look up perhaps. However, we can definitely say it isn't salt.  Now, I know there are volcanic intrusions in the area, particularly in the Isachsen formation, so one of my ideas is that it could be made of lava. Lava is radar-rough.

I pulled up Harrison and Jackson's (2014) geological map of the site.  The area is mapped as Isachsen formation! At first I thought that confirmed my hypothesis that the area is lava, but then I looked at the detailed geological description of the units.  Whereas other Isachsen areas are mapped as basalt flows, or sedimentary units with localized volcanics, this area is just mapped as being limestone and siltstone.  No lava.  That is weird, because limestone shouldn't be producing a really rough signature. 

If we have time on one of our helicopter days in the field this July, I'd like to check it out!

RADARSAT-2 Data and Products (c) MacDonald, Dettwiler and Associates, Ltd. (2016) - All Rights Reserved. RADARSAT is an official trademark of the Canadian Space Agency.

1 comment:

  1. Hmm, so it's limestone, eh? We did see a lot of rough limestone around Haughton - the Allen Bay Formation. I wonder if it's similar stuff? Are there other 'smooth' limestone formations in this area?