Management of Commercially Generated Radioactive Waste - U.S. ...

5.16
Gravity analysis can detect small variations in the earth's gravity field (Dobrin
1960). The variations of principal interest to repository siting result from lateral varia-
tions in subsurface rock density. Density variations may result from deformed strata,
faults, igneous intrusives, diapirs, breccia pipes, or lithologic changes.
Magnetic methods detect variations in the earth's magnetic field (Dobrin 1960).The mag-
netic variations (anomalies) of interest to site studies are due to lateral changes in mine-
ral content (especially magnetite) or to variations in the remnant magnetism of igneous
rocks. Subsurface structures like anticlines or faults can be detected if they result in
lateral changes of the above properties (Fabiano 1976).
A variety of electrical methods (Dobrin 1960 and Keller 1966) are used in geophysical
exploration; all depend upon detecting variations in the electrical resistivity of the media
through which a current flows. Subsurface resistivity is highly variable and strongly
influenced by the amount and the nature of fluids in the rocks. For this reason, such
hydrologic features as dissolution of salt, ground-water tables, and porosity variations are
particularly amenable to electrical prospecting methods.
Seismic exploration methods are perhaps the most useful geophysical tools for obtaining
accurate representations of the subsurface geology at individual sites (Dobrin 1960). They
rely on the reflection or refraction of seismic (acoustic) signals due to contrasts in velo-
city or acoustic impedance (the product of seismic velocity and rock density). Acoustic
signals are usually introduced into the earth by explosive sources or vibrating or impacting
masses. Seismic reflection surveys are particularly useful in mapping the attitude and con-
tinuity (or lack thereof) of subsurface rock beds. Other methods and equipment utilized for
seismic reflection can be selected for the specific site and parameters (i.e., depths,
dimensions) of interest. These parameters are often defined to provide information from
depths of more than 1,000 meters (Vail et al. 1978). Special field parameters and techni-
ques (high-resolution seismic) are available to explore accurately the shallower depths of
interest for repositories.
Geophysical logs in well bores are a powerful tool for correlating and interpreting
subsurface geologic conditions, including the condition and fluid content of subsurface
rocks (Dobrin 1960). They supplement cores and rock samples and furnish a vertically con-
tinuous record of certain physical properties for each borehole. Many types of logs are
used. Focused resistivity logs provide a reliable measure of in-situ rock and fluid charac-
teristics. Microresistivity logs measure the properties of small volumes of rock just
behind the borehole wall and thus permit the boundaries of permeable and/or electrically
resistive formations to be sharply defined. Gamma-ray logs indicate the clay content of
various formations and are valuable in making lithologic interpretations in clastic rock
sequences. Neutron logs are useful for identifying porous rock strata and rock densities.
These logs respond mainly to the hydrogen content of the formation and indicate the presence
of water, oil, or hydrogen-bearing minerals. Acoustic logs measure the velocity of sound
in rock units and can also help determine the porosity of a formation.