Firn variability derived from a statistical analysis of airborne ice penetrating radar

Thwaites Glacier catchment, West Antarctica

--

Cyril Grima, D.M. Schroeder, D.D. Blankenship, D.A. Young

2013 IGS International Symposium on radioglaciology

Lawrence, Kansas, USA

September 9th, 2013

2

=

+

Scattering (incoherent)

+++ Roughness

+++ Non-deterministic structure (volume)+ Permittivity

Reflectance (coherent)

+++ Permittivity

+++ Deterministic structure (layering)+ Roughness

TOTAL POWER

= Reflected power (Pc)

= Scattered power (Pn)

Because of the scattered part, the surface echo is stochastic

0

22

00 .2

1...,,

ds

aJAJAsaApHK

Amplitude distributions fitted with Homodyned K-envelope (HK)

Increasing roughness

HK allows within a footprint:

few and clustered scatterers+

one specular reflector

[Dutt & Greenleaf, 1994; Chitroub, 2002; Ward et al., 2006; Destrempes and Cloutier, 2010]

where J0 is the 0th order Bessel function of the first kind

22

2s

a

Demonstrated with SHARAD (Mars) [Grima et al., 2012]

4

HiCARS radar [Peters et al, 2005,2007]

f = 60 MHz ( = 5 m) f = 15 MHz

Footprint (along/cross-track) 30-50 m / 250-350 m

Ice thickness sensitivity 5-10 m

Amplitude distributions obtained along-track

1000 consecutive observations each

5

dBdB

REFLECTANCE (Pc) SCATTERING (Pn)

correlation between amplitudes & HK distribution < 95%i.e. less confident results due to roughness and/or permittivity heterogeneities

6

U

e U

h 4

21

where U = Pc/Pn

LASER (@ 100 m baseline) RADAR (@ 5-50 m baseline)

Small Perturbation Method (SPM)+ Nadir approximation+ Large correlation length

[Grima et al, 2012][Grima et al., in prep.]

---- SPM 1dB limit

7

2)2(2 hkcePr

Small Perturbation Method+ Nadir approximation+ Large correlation length

11rwhere

[Grima et al, 2012]

2 2.2 2.4 2.6 2.8 3

= 0.4-0.5

Real dielectic constantfor the first 5-10 m of firn

8

= 0.4-0.5

0 0.1 0.2 0.3 0.4 > 0.5Slopes [°]

The anomaly ( > 2.5) is a vein (30-60 km wide) whose northern boundary matches a slope break (0.5°) across the whole dataset coverage (~500 km)

9

What could explain = 0.4 - 0.5in a 5-10-m thick slice of ice ?

Ice composition/structure Maximum expected range Corresponding

Crystals shape ad size[Achammer and Denoth, 1994; Mätzler, 1996]

Random orientation ~ 0

Temperature[e.g. Mätzler and Wegmüller, 1987]

-40 to 0 °C < 0.04

Density (dry ice)[e.g. Kovacs, 1995]

350 to 917 kg.m-3 ~ 1.5

Wetness[e.g. Frolov and Machoeret, 1999]

0 to 10 % ~ 2.1

Neutral impurities[Looyenga, 1965]

0 to 1 vol.ppm < 10-5

Ionic impurities[e.g. Hallikainen, 1992; Fujita, 2000]

0 to 100 vol.ppm ~ 0.05

10

Consistent with a 550-kg.m-3 critical density at < 9 min this region, as modeled by [Ligtenberg et al., 2011]

[Kovacs et al., 1995]

6

3

10845

1'.

mkg

Anomly could be + 200 kg.m-3

11

3-layers model [from Mouginot et al, 2009]

1

(1)

Solutions for = 0.4-0.5

0 = 1

1 = ?? (> 2 )

2 = 2.3-2.4

??

(Atmosphere)

An upper high-permittivity layer with a subwavelenght thickness generates

constructive interferences

5-10

m

12

Dual frequency analysis (HF/VHF) to solve permittivity/layering ambiguities

Reflectance/scattering components extracted from the signal

500 x 30 km permittivity anomaly detected over Thwaites catchment, coincident with a prominent slope break

Higher density firn or wet snow layer Both indicative of a higher densification rate Implications for surface mass balance at regional scale

Application to bed interface

Thank you CreSIS for travel grant !