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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
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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
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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
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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%
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5COF & elasticity tensor - Kohnen - Colle Gnifetti
Connection
Measured Cij Measured eigenvalues
Integrationuniform distribution(Nanthikesan, 1994)
Cij for different fabrics
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6COF & elasticity tensor - Kohnen - Colle Gnifetti
COF → Elasticity tensor
Data: Kohnen
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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
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8COF & elasticity tensor - Kohnen - Colle Gnifetti
Detonation cord: • depth 2580 - 100 m• 40m intervals
VSP dataset
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9COF & elasticity tensor - Kohnen - Colle Gnifetti
VSP-velocities
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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
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12COF & elasticity tensor - Kohnen - Colle Gnifetti
Core – Seismic – Radar
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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
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14COF & elasticity tensor - Kohnen - Colle Gnifetti
ElViS reflectionsstacking = depth conversion velocity COF measurements
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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%
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16COF & elasticity tensor - Kohnen - Colle Gnifetti
Connecting seismics & radar
Seismic
VNMO
Radar
→ depthVRMS,P0
derive interval δ
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17COF & elasticity tensor - Kohnen - Colle Gnifetti
Deriving anisotropy
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COF & elasticity tensor - Kohnen - Colle Gnifetti 18
Conclusion
Connection Evaluation Comparison Derivation
Integration
Seismics & Radar
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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
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COF & elasticity tensor - Kohnen - Colle Gnifetti 20
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COF & elasticity tensor - Kohnen - Colle Gnifetti 21
VSP-velocities
Temperature correction
gradient 2.3 m/s/K
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COF & elasticity tensor - Kohnen - Colle Gnifetti 22
Radar
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COF & elasticity tensor - Kohnen - Colle Gnifetti 23
COF – Seismic – Radar
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COF & elasticity tensor - Kohnen - Colle Gnifetti 24
Vibro
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COF & elasticity tensor - Kohnen - Colle Gnifetti 25
Picked velocities VNMO
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COF & elasticity tensor - Kohnen - Colle Gnifetti 26
Calculated velocities VRMS,P0
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COF & elasticity tensor - Kohnen - Colle Gnifetti 27
Girdle orientation