Determination of anisotropic ice fabric using seismic data

Anja Diez, Olaf Eisen, Coen Hofstede, Ilka Weikusat, Ulrich Polom, Thomas Bohlen, Astrid Lambrecht, Christoph Mayer, Heinrich Miller

2COF & elasticity tensor - Kohnen - Colle Gnifetti

Crystal orientation fabric (COF) & elasticity tensor• example: vertical single maximum fabric• connection: eigenvalues – elasticity tensor

Kohnen station• vertical seismic profiling• comparison COF – seismic – radar

Colle Gnifetti• velocity analysis• derivation of anisotropy

Overview

3COF & elasticity tensor - Kohnen - Colle Gnifetti

Seismic anisotropy

Ice core:

COF eigenvalues: ʎ1< ʎ2< ʎ3

Radar:

linear relation to dielectric constant ɛ‘ Seismic:

dependence on elasticity tensor Cij

c-axis

4

Example: vertical single max

NMO-velocities (interval)Thomsen parameter δ, γ

COF & elasticity tensor - Kohnen - Colle Gnifetti

Depth conversion

stacking velocity ≠

depth conversion velocity

20%

7%

5COF & elasticity tensor - Kohnen - Colle Gnifetti

Connection

Measured Cij Measured eigenvalues

Integrationuniform distribution(Nanthikesan, 1994)

Cij for different fabrics

6COF & elasticity tensor - Kohnen - Colle Gnifetti

COF → Elasticity tensor

Data: Kohnen

7COF & elasticity tensor - Kohnen - Colle Gnifetti

Kohnen Station• Dronning Maud Land• ~ 750 km southeast of Neumayer III• EDML (length 2774 m)

Seismic• profile, wideangle, vertical seismic profiling • explosives, vibroseis

Radar • 60, 600 ns pulse

Kohnen station

8COF & elasticity tensor - Kohnen - Colle Gnifetti

Detonation cord: • depth 2580 - 100 m• 40m intervals

VSP dataset

9COF & elasticity tensor - Kohnen - Colle Gnifetti

VSP-velocities

Best agreement• Gammon (1983)• Jona (1952)• Bennett (1968)

Gusmeroli (2012, JGR)• Borehole sonic logging 23 kHz• Best agreement: Dantl (1968)

Dependency of seismic waves on frequency

Comparison elasticity tensors

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11COF & elasticity tensor - Kohnen - Colle Gnifetti

Englacial reflections? Stacked trace

Stacked traces

12COF & elasticity tensor - Kohnen - Colle Gnifetti

Core – Seismic – Radar

13COF & elasticity tensor - Kohnen - Colle Gnifetti

Colle Gnifetti• Switzerland• 4500 m a.s.l.• KCI ice core (length 62 m)

Seismic data• profile• ElViS: P- and SH-wave

Radar data

Colle Gnifetti

14COF & elasticity tensor - Kohnen - Colle Gnifetti

ElViS reflections

stacking = depth conversion velocity COF measurements

15COF & elasticity tensor - Kohnen - Colle Gnifetti

Depth shift

Seismics: VElViS

KCI: VRMS,P0

VNMO

• density• temperature• COF

Bed reflection P-wave depth difference → 4 m 7% 1%

16COF & elasticity tensor - Kohnen - Colle Gnifetti

Connecting seismics & radar

Seismic

VNMO

Radar

→ depthVRMS,P0

derive interval δ

17COF & elasticity tensor - Kohnen - Colle Gnifetti

Deriving anisotropy

COF & elasticity tensor - Kohnen - Colle Gnifetti 18

Conclusion

Connection Evaluation Comparison Derivation

Integration

Seismics & Radar

COF & elasticity tensor - Kohnen - Colle Gnifetti 19

Thank You!

Thanks to: Pascal Bohleber, Reinhard Drews, Günther Druivenga, Jens Eichler, Sepp Kipfstuhl, Daniel Steinhage and the Kohnen logistic team

COF & elasticity tensor - Kohnen - Colle Gnifetti 20

COF & elasticity tensor - Kohnen - Colle Gnifetti 21

VSP-velocities

Temperature correction

gradient 2.3 m/s/K

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Radar

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COF – Seismic – Radar

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Vibro

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Picked velocities VNMO

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Calculated velocities VRMS,P0

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Girdle orientation