
A Brief Introduction to Ice-Penetrating Radar - DalekBaldwin
http://lindzey.github.io/blog/2015/07/27/a-brief-introduction-to-ice-penetrating-radar/
======
bluusteel
Really nice writeup! I wish more graduate students would blog about their
research in this way. Could be a great way to educate the public and convey
the importance of government research funding.

I wonder how big the reflection coefficient is for the air/ice interface. It
seems like it would be huge. So maybe ground based techniques offer better
coupling at the cost of not being able to survey as much area?

~~~
lindzey
We record on two channels, "high gain" and "low gain", separated by ~50dB. I
showed the high gain products in this post, and the surface absolutely does
saturate the detectors. The system was designed so that near-surface returns
don't saturate the low gain channel.

We transmit 8kW, and the air/ice surface reflection coefficient is ~0.08
(~-11dB). Flying at ~600m above the surface, spreading loss actually
contributes more to signal attenuation (1/(2*h)^2 ~= -62dB).

Our instrument is optimized for seeing through the entire ice sheet, mapping
deep layers and the bed. Other (also airborne) instruments operate at higher
frequencies, trading higher resolution for less penetration. I'm not super
familiar with groups using ground-based ice-penetrating radar, but one big
tradeoff is $$$. The airplane is hugely expensive to operate, whereas ground-
based just needs a snowmobile.

------
timeu
On a related note: My father is a geophysicist and he is using ground
penetrating radard for archeo prospection. It's really amazing. The software
can reconstruct entire ancient cities from the GPR signal.

------
ovis
I had the opportunity to collect and work with similar data during grad
school. I love the fact that these guys use a DC-3; very cool. We used a sled.

------
a2tech
Thats really pretty amazing. Filtering out the 'echos' would clearly make the
data more useful, but I can see how thats almost impossible without very
clever engineering

~~~
TTPrograms
There's some work in SAR stuff on that sort of thing. One sketch of an idea
would be to iteratively estimate the measurement including multi-path from
what you think the surfaces look like and then remove the "surfaces" that are
explained by echos off of single surfaces. The goal would be to get images
that explain the measurements you take using the fewest number of surfaces.

This is trickier than normal imaging, because if you ignore the multiple
reflections you're basically just inverting a matrix (a fourier transform in
the case of single line imaging). With multipath the measurement becomes
nonlinear in your scene and inversion isn't as trivial.

~~~
vonmoltke
Another alternative is to sweep the radar so you get multiple revisits from
different times and angles. Stacking the revisits at a location will suppress
the multipath since, in theory, the multipath components depend more strongly
on grazing and aspect angle than the "real" components.

~~~
lindzey
Absolutely! I only included our lowest-level data products in this post. We
don't sweep the radar side-to-side, but since it's on a moving platform, we
see the same thing from multiple angles. In the data I showed, there has been
some very limited stacking applied, mostly just to improve the signal-to-noise
ratio. The next level of processing would be focusing/migration (depending on
whether you're in the radar or seismic world) - it reasons about possible
incident angle and collapses the hyperbolas to a single point.

Unfortunately, this only improves the resolution along the flight track;
features parallel to the flight path and offset to the side are the hardest to
filter out.

------
fnordfnordfnord
An acquaintance of mine is prepping for a cruise to do some sea-ice
measurements. They'll be on the RV Sekuliak though, not an airplane.

