SalSA Telecon 10/18/06 1

Attenuation Length Measurement at Cote Blanche

Amy Connolly and Abby Goodhue

SalSA Telecon 10/18/06 2

Goals of Second Trip to Cote Blanche Transmit over long distances through solid

salt Make measurements at many different

distances Many more measurements can be made with

antennas against walls than in boreholes Drill a few boreholes at places where we made

wall measurements to compare results

SalSA Telecon 10/18/06 3

New System Diagram

Pockel’s Cell Driver: HV pulses at 125 kW

Frequency band filtered on receiving end: 50-300 MHz

300 MHz

SalSA Telecon 10/18/06 4

Antenna TransmissionTransmitting Antenna

Rated transmission band in air is 400-470 MHz Frequency band in salt is lowered and overlaps with

band we filtered for (50-300 MHz)

Antenna Transmission folded in with FFT of PCD pulse

SalSA Telecon 10/18/06 5

Transmission Map -1500’ Level

Transmitting station 27 ft. from the corner

Observed signals through up to 270 meters of salt!

SalSA Telecon 10/18/06 6

At Work Underground

SalSA Telecon 10/18/06 7

Preliminary Results from Cote Blanche Took the fourier transform of

the signal and binned it in three frequency bins

Difference between borehole and wall measurements is similar to the variation between measurements made against the wall at similar locations

Do not see a strong frequency dependence

SalSA Telecon 10/18/06 8

Comparison with First Trip On first trip, receiving

station was 35 ft. from corner & transmitting stations along a wall (similar setup as first two measurements on this trip)

Slope of first trip’s data is consistent with the second data set

Note: The points are not normalized to the same scale factor. Only the relative slopes should be considered

SalSA Telecon 10/18/06 9

First Fit

Fit only the measurements made after we went around the corner

-6

-5

-4

-3

-2

-1

0

1

0 50 100 150 200 250 300

Distance (m)

ln (

V *

d)

Arb

itar

ary

Un

its

50-150 MHz

150-250 MHz

250-350 MHz

SalSA Telecon 10/18/06 10

New Model- Fractured Salt Along Walls

Fracturing along walls would make the bulk of salt appear more lossy than it actually is

Explains additional attenuation seen for measurements made along the north wall

Consistent with the data from the first trip

Maximum path through “fractured salt” would be just before turning the corner

SalSA Telecon 10/18/06 11

Fit for Depth of Fractured Salt – Work in Progress

Use systematic errors of 75%: dominated by variation in borehole and against the wall measurements

Shape seems consistent with theory, but the errors are large and there are too few points in crucial region along the wall

Theta measured from straight across (radians)

ln(v

*d),

Arb

itrar

y U

nits

SalSA Telecon 10/18/06 12

Fit for Depth of Fractured Salt – Work in Progress Cont’d Fit is consistent with no fractured region We can possibly include points from the

previous Cote Blanche trip We are looking at Hockley data to see if it is

consistent with this theory Central value of the attenuation length in solid

salt only increases by 20% over our exponential fit (from 25.3 m to 30.1 m)

SalSA Telecon 10/18/06 13

Conclusions From This Measurement Fracture zone near the walls could be lossy

Would be bad for the planar array idea Need to focus on drilling holes in salt

Even deep salt measured seems to have an attenuation length that is too short

Contradiction with GPR Measurements? “ [GPR] transmission in Cote Blanche salt has been better than

that in any other mine visited” - Stewart & Unterberger (1976) They measured signals reflected from 3177 ft., giving a total

path length of 6354 ft. of salt Miners have told us that they can drill downward deep (over

50’) “easily” if we want