Geoscience Reference
In-Depth Information
Shear Wave Velocity (m/sec)
COV
No. of Profiles
0
500
1000
1500
0.0
0.5
1.0
0
5
10
0
0
COV
= Coefficient
of Variation
= σ / mean
Median
16th and 84th
Percentile
40
40
80
80
120
120
(a) (b) (c)
Fig. 7.2. Statistical analysis of intermediate-depth V s profiles inan alluvial deposit from
SASW testing: (a) median and
one standard deviation, (b) coefficient of variation,
and (c) number of profiles (fromLin, 2007)
±
(a peak force of about 90kN) in the low-frequency range (about 1 to 4Hz). Liquidator
ispartofashared-useequipmentsitethatisoperatedbytheUniversityofTexasatAustin
(nees@utexas) and funded by the U.S. National Science Foundation (NSF) as part of
the George E. Brown, Jr. Network for Earthquake Engineering Simulation (NEES).
Statisticalanalysisofnine,intermediate-depthV s profilesofthealluviumattheHanford
Site is presented in Figure7.2. The median V s profile and the associated coefficient of
variation(COV)areshowninFigure7.2aandb,respectively.Theseresultscomparewell
with measurements from independent downhole tests, with the average median value of
V s from each test method within 4% over the same test depths. However, due to various
constraints, downhole testingwas only performed toan average depth of about 70m.
Deep V p and V s profiles have also been measured by the downhole and P-S suspension
logging methods at the Hanford Site. In this case, testing was performed in three bore-
holes spaced around the footprint of one set of buildings. Testing was conducted to a
maximum depth of about 440m. In the downhole test, generating measurable compres-
sion(P)andshear(S)wavesatdepthsof300to400mwaschallengingduetothealternat-
inglayersofsoilinterbedsandbasalt.Therefore,thetriaxialvibrator,called“T-Rex”,was
usedastheseismicsource.(MeasurementswiththeP-Ssuspensionloggerwereeasierto
perform at the deeper depths since the wireline tool containing the source and receivers
is lower to the test depths.) T-Rex is another large vibrator operated by nees@utexas.
T-Rex is capable of generating large dynamic forces in any of three directions (X, Y, or
Z directions). The peak forces are about 270kN in the vertical (Z) direction and about
135kNineitherhorizontaldirection.Thevibrationdirectioncanbechangedatthetouch
of a button. These capabilities, combined with the ability to prescribe the source signal,
made T-Rex an excellent source for deep downhole profiling. The use of 20- to 50-Hz,
fixed-sine input signals with 4 to 10 cycles worked well. Example travel-time records
are presented in Figure7.3. Sets of P- and S-wave records at depths of 137 and 290m
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