Improving Borehole Optical Stratigraphy (BOS)

T.J. Fudge

Ben Smith

Ed Waddington

What is BOS?

Hawley, 2008

BOS measures brightness of borehole wall with depth

Annual Layers are Visible

Allows rapid determination of past accumulation rate

Ability to observe seasonal firn compaction

Hawley, 2005

Why is getting in situ grain size and density important?

Densification of Firn

Trapping of Air in Firn

What causes the variations in brightness?

Can the physical characteristics of the firn, such as grain size and density, be inferred from the brightness variations?

Modeling the cause of brightness variations

-Photons enter the firn and move by scattering. -Keeps track of scattering events and position photon exits firn

Examine the effects of changes in grain size and density by using a Monte-Carlo model that tracks photons

the mean distance a photon travels between scattering events

Scattering Length:

snowgrain

photon track

Fir

n s

urf

ace

Scattering length

snow

icescat

rL

3

2

radius of snow grain

scatscatscat

QrrnL

2)(1

snow n(r)4

3r3ice

Density of scatters (grains)

Scattering efficiency, ~2

Scattering length

snow

icescat

rL

3

2

radius of snow grain

scatscatscat

QrrnL

2)(1

snow n(r)4

3r3ice

Density of scatters (grains)

Scattering efficiency, ~2

Brightness Variations can be causedby either grain size or density changes

Fine GrainCoarse Grain

Spreading of Light due to Scattering Length Change

Grainsize also affects albedo, but does not dominate scattering length brightness effects.

Multiple bounces are required to get large enough brightness changes

One limitation is that measurements cannot be made on cores

101 102 103 104

10-5

10-4

10-3

Sin

gle

-sca

tter

coa

lbe

do

geometric mean grain size, m

more absorptionless

absorption

New Device

Emphasis on measuring grain size

Currently designed for cores

New Device

Two light sources of different wavelengths

Measures spreading of light

Yellow (590nm)

Near Infrared (950nm)

0 10 20 30 40 50 60 70 8010-8

10-7

10-6

10-5

10-4

10-3

10-2

10-1

10-210-3

10-410-5

Inte

nsit

y (a

rbit

rary

uni

ts)

Scattering Lengths

101 102 103 104

10-5

10-4

10-3

10-2

10-1

Sin

gle

-sca

tter

coa

lbe

do

geometric mean grain size, m

6 8 10 12 14 16 18 20 22 24 260

0.2

0.4

0.6

0.8

1.0

Returned Intensity at 57.54 meters

Distance from source (mm)

Non

-dim

ensi

onal

ized

Inte

nsity

Modeling Results

590nm

950nm

Preliminary Results from Summit Greenland Firn Core

Light Sourcesare located closestto pixel 62

Preliminary Results from Summit Greenland Firn Core

Layer counting suggests annual layer thicknesses of 17-40cmat this depth, with 3-4 years per meter

Light Sourcesare located closestto pixel 62

Conclusions

BOS brightness variations can be caused by either changes in density or grains size.

The measurement are insufficient to determine physical properties of the firn.

A new device using two separate wavelengths of light and simplifying the geometry of the measurement yields promising results for inferring grain size.