Geoscience Reference
In-Depth Information
The state of practice in shallow seismic investigations is good and improving. As noted
above, a number of field seismic methods are readily available for use. Improvements in
analysismethods,instrumentationandautomationassociatedwiththemethodsareoccur-
ring.Thenumberofknowledgeableengineeringpractitionersisalsoincreasing,andareas
ofapplicationaregrowing.Surface-wavetestingisthemostrapidlygrowingarea,duein
large part to the efficiency and cost-effectiveness of nonintrusive testing. Developments
are also occurring in two other aspects of field seismic testing. The first is profiling to
deeperdepthsinalltypesofgeologicsettings.Thesecondisperformingparametricstud-
ies in situ. The effects of parameters such as stress state, strain amplitude, and cyclic
loading leading to liquefaction are being evaluated in situ. Developments in both aspects
are briefly discussedbelow and arecovered in more detail inthe presentation.
2. Deeper seismic profiling
In the past decade, considerably deeper investigations have been required at critical sites
intheUnitedStates,withprofilingdepthsintherangeof125to450m.Intermediateand
deep wave-velocity profiles have been measured with two or three seismic methods at
severalofthesesites.Intermediate-depthprofilesaredefinedashavingmaximumdepths
in the range of 75 to 225m and deep profiles have depths exceeding 225m. (Shallow
profiles are defined as profiles less than 75m deep.) The seismic methods used in deeper
profiling have been downhole testing, surface-wave based tests and P-S suspension log-
ging as discussed below.
Intermediate and deep profiles have been measured at several locations including:
(1) Yucca Mountain, Nevada, (2) the northern Mississippi embayment, (3) the Salt
Lake Valley, Utah, and (4) the Hanford Site near Richland, Washington. The spectral-
analysis-of-surface-waves(SASW)methodwasusedateachlocation,andmeasurements
were performed over lateral extents ranging from 10 to 200km. The large vibrator used
as the SASW source is called “Liquidator”. Liquidator is shown in Figure7.1a. It is a
one-of-a-kind,low-frequencyvibratorthatisspeciallydesignedtogivehigh-forceoutput
Fig. 7.1. Large mobile vibrators used as controllable, high-energy seismicsources:
(a) thelow-frequency vibrator called Liquidator and (b) the tri-axial vibrator
called T-Rex
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