wave propagation via laser ultrasound

IR laser focused on 19 mm lineLaser line source

Transmitted EM phase image of granite at 150 GHz

Measuring electrical and mechanical properties of rocks on the

submillimeter scale

JS, M. Batzle, M. Prasad, N. Greeney & A. Yuffa

Colorado School of Mines

Collaboration between Physics, Geophsysics and Petroleum Engineering

All data and software will be available

• http://mesoscopic.mines.edu

• http://physics.mines.edu/~jscales

• Common Ground free database of rock properties

High spatial resolution techniques now available

• Laser ultrasound

• millimeter/submillimeter wave EM

• Strain microscopy

• Acoustic microscopy (Prasad)

• Micro-CT scan (Batzle)

Motivation

• Complimentary measurements

• Submillimeter waves sample on same length scale as ultrasound.

• measurements fully noncontacting and can be done on same samples without other preparation.

Length scale of measurement easily controlled optically

'low' frequency normal mode

Laser spot size measured in microns

'high' frequency normal mode

But how to get local elastic properties from waveforms?

Electrical properties at sub-mm resolution

CSM submillimeter system covers from microwaves (8-10 GHz) to 1 THz (1000 GHz)

Allows us to do bulk dielectric spectroscopy And now, near-field scanning

ABMillimetre submm VNA

Funded by NSF MRI

Unique instrument

• measure amplitude and phase of the electric field over broad range of millimeter to submillimeter wave frequencies

• In free-space or in waveguide

• Produces linearly polarized Gaussian beams of high optical quality: quasi-optics

• Allows 'easy optics'

quasi-optics

Dielectric spectroscopy

Fit E field with 1D Fabry-Perot model to get complex permittivity

Measuring water content

Measuring anisotropy in shale

MMW rock physics applicationsScales and Batzle APL papers

• Measure organic content in rocks and oil/water emulsions

• Resolve sedimentation at the 100 micron level (implications for climate models)

• Check mixing models (such as Maxwell-Garnett)

Recent: cavity perturbation

• Have recently built ultra-high-Q millimeter wave cavity for measuring (e.g.,) conductivity of thin films.

• Use ultrasonic cavity perturbation to measure minute changes in samples

Getting high-resolution EM results

First work at 150 GHz

Greeney & JS, Appl. Phys. Letts. Bare teflon probes Later, went to higher frequency, 260 GHz Clad teflon in aluminum Small hole at tip to prevent leakage Weiss et al, J. Appl. Phys. Finite element modeling of tip surface coupling

Transmitted phase image of granite at 150 GHz

Transmitted phase image of shale at 150 GHz

Seeing inside dielectrics: rfid card @ 260 GHz

Seeing vascular structure

True near-field scanning

Tip-sample distance .2mm Wavelength about 1 mm

Can see standing waves in the shadow (backside of dime)

Circular drum modes

Tip-sample distance .6mm

small scale effects of Pyrolisis

McEvoy et al, 2009 oil shale conf.

Comparison with acoustic microscopy (M. Prasad's lab)

Laser ultrasound analog

Pulsed laser sources

• Pulses from 10 ns to 100 fs

• Looking at first arriving energy as we scan across the sample.

• Scanning resolution measured in nm

Measuring spatial strain in real time at video frame rates

• Illuminate a surface with laser speckle

• Take a picture of the speckle

• Apply a strain

• Take another picture

• Subtract the two

• The result is an interferogram

Electronic Speckle Pattern Interferometry

ESPI through a microscope

Speckle interferograms of concrete

Are grains floating?

Trick is in the image processing

Skeletonization by nonlinear pde filtering

conclusion

• Are acquiring independent high-spatial resolution data sets for relevant rocks

• Expect to have high-res mechanical properties soon.

• Batzle now has micro-CT scanner. Again, no rock prep required.

• Have a high-speed video camera for the ESPI