Bats do it Dolphins do it Glaciologists do...

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Bats do it... Dolphins do it... Glaciologists do it Active Source Seismology: Make a sound and listen for an echo How long did it take for the echo? (velocity) That tells us how thick the ice is. Echos from subglacial layers That tells us what is below the ice 1

Transcript of Bats do it Dolphins do it Glaciologists do...

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Bats do it... Dolphins do it... Glaciologists do it

• Active Source Seismology: • Make a sound and listen for an echo • How long did it take for the echo? (velocity)

– That tells us how thick the ice is. • Echos from subglacial layers

– That tells us what is below the ice

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Discover ice/rock properties at depth

• Passive source seismology: wait for earthquakes to occur, record them, use their travel path, travel time, travel amplitudes for:

– Structure of the ice sheet and planet – Thermal and physical properties at great depth

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Important Results

• Glacier bed properties – Flow of glaciers depends on bed

• Physical properties <==> Elastic properties – Till properties: seismic velocities (P & S) – Soft vs. hard till - acoustic impedance – Areal distribution of sediments – (physical <==??==> electrical)

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Important Results

• Basic ice thickness, now superseded – Almost... some ice still troublesome for radar :)

• Water: Lakes and Ice Shelves • Water: sheets and channels • Water: moulins (to be done...)

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Important results

• Englacial layers indicating xtal orientation – Strongly oriented layers are 2-4% faster than

randomly oriented layers • Subglacial geologic structures: basins, faults

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Active Source Seismology

• When did the echo come? – How deep is the ice... and how thick are the sub-

glacial layers? • How strong is the echo?

– How rigid is the rock? Is it water-saturated? • What is the shear-wave velocity profile?

– Water.

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How to make a sound?

• Explosives • Sledgehammer • Vibrator truck • Compressed-air gun • …

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Refracted Energy

CriticallyRefracted

Reflection

Direct

V1

V2

z

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1.7

1.8

1.9

2.0

800 1600 2400Tw

o-W

ay

Tra

ve

lTim

e (

Se

co

nd

s)

Source - Receiver Offset (Meters)

0 4520

WAIS Divide

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Seismic velocity of body waves

• Velocity = (elastic modulii / density)1/2

• Reflection is related to change in seismic velocity and change in density.

• Reflection is related to phase change (P-SV)

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vp =

s43µ+ k

vs =

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Key Measurements

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Direct

V1

V2

h

Reflection Coefficient

R =⇢2v2 � ⇢1v1⇢2v2 + ⇢1v1

t(x) =1

v1

px

2 + (2h)2

x RS

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Till Properties - from time only

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© Nature Publishing Group1986

Blankenship et al., 1986

Vp = 1600 m/sVs = 150 m/s

© Nature Publishing Group1986

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Reflection Coefficient

22Holland and Anandakrishnan, 2009

A1 = A0 ⇥R(✓)⇥ �1 ⇥ exp(�↵d1)�1 = Spreading Losses; Free Surface

↵ = Attenuation

d1 = PathLength

R(✓) = Reflectivity

✓ = Angle

A0 = SourceAmplitude

A1 = ReceiverAmplitude

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Multiple Bounce @ Normal

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Direct

V1

V2

h

Reflection Coefficient

x RS

Multiple Reflection

A0 =A2

2

A1

d12

R(✓1) =A1

A0

1

�1exp(↵d1)

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Seismic params• Firn: Vp = 1-3 km/s, 𝛒=.4-.8 g/cc • Ice: Vp = 3.9 km/s, Vs = 1.9 km/s, 𝛒=0.92 g/cc

– Highly anisotropic, small temperature dependence

– Water: Vp=1.5 km/s, Vs = 0 • Tills: Vp = 1.6-2.4 km/s, Vs = is .05-1 km/s, 𝛒

depends on porosity - important for fast flow • Sediments vs. crystalline rocks: roughness,

erosion potential, fracture potential, water transport... 24

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Seismic AVO Theory

P-P Reflection

θiICE

SUBGLACIAL BED

VS, ice

VP, iceρice

VS, bed

VP, bedρbed

REFLECTION!!!

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Reflection Amplitude vs Offset

• AVO or AVA - partitioning of energy at an interface that includes P and S conversions

• so-called Zoeppritz equations – see Shuey (1985), Aki & Richards (1980) – https://bruges.readthedocs.io/en/latest/

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R(✓) ⇠= R(0) +G sin2(✓)

G =1

2

�vpvp

� 2v2sv2p

✓�⇢

⇢+ 2

�vsvs

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27GMT 2006 Mar 2 11:20:48 Volumes/UserP/Users/sak/GMT/2poles.gmt

South Pole

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Radar from the S. Pole

South Pole Lake region

UTIG Radar Data

5 km

100 m

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0 10 20 30 40 50 60 70 80 90-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

Angle of Incidence (degrees)

Be

d R

efl

ec

tiv

ity

Bedrock

Water

Frozen/Consolidated Seds.

Soft/Dilatant Seds.

Vp (m/sec) Vs (m/sec) Density (kg/m³)

3780 1860 920

5800 3200 2800

3000 1200 2200

1650 200 1700

1500 0 1000

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Ice over Water

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Seismic body wave velocity

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Vp=

s�43µ + k

Vs=

⇢⇢=density

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Influences on modulii and density---velocity

• Rock type, porosity – Till vs. basement; water content; deformation

rate • Fluid presence and type

– Water • Temperature, microcracks, fabric, ...

– Flow law of ice

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Summary

• Seismic energy is sensitive to changes in elastic properties.

• Reflection seismic profiling can image at & below the base of the ice sheet.

• For fast-flowing glaciers (and rapidly-changing glaciers), conditions at the base & below the base are extremely important.

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What are the elastic modulii?

• Bulk modulus or incompressibility (k) • Shear modulus or rigidity (μ) • Young’s modulus (E) • Poisson’s ratio (ν) • Any two, along with density will define the

seismic body wave velocity

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Bulk modulus (k)

35k =Applied hydrostatic pressure P

Fractional volume change �v/v

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Shear modulus or Rigidity

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µ =shear stress ⌧

shear strain ✏

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Young’s Modulus

37E =uniaxial stress �11 = �F/A

uniaxial strain ✏11 = �L/L

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Poisson’s Ratio

38E =radial strain ✏22 = �W/W

axial strain ✏11 = �L/L