Conducting Subsurface Environmental Investigations in Urban Watersheds - Urban Watersheds: Geology, Contamination, and Sustainable Development - page 131

Geologists conducting environmental investigations apply geophysical techniques for the
following reasons:
• To collect geologic and hydrogeologic data.
• To detect and map contaminant plumes.
• To detect and map subsurface features such as buried drums, utilities, tanks,
trenches, landfills, buried wastes, or other buried objects.
• To evaluate soil properties.
• To evaluate anthropogenic subsurface structures.
Geophysical investigations include the following techniques and benefits:
• Seismic refraction and reflection. Seismic refraction and reflection measurements
involve the measurement of seismic waves traveling through the subsurface
(ASTM 2000b). Stratigraphy, structure, and material properties can be evaluated
using seismic methods.
• Electrical resistivity. Electrical resistivity measures the apparent resistivity aver-
aged over a volume of material (ASTM 1999a). These measurements can be used
to evaluate the depth and thickness of geologic units, map saltwater intrusion
and contaminant plumes, locate buried wastes, and evaluate lateral changes in
geologic units.
• Electromagnetics. Electromagnetic methods measure the conductivity of sub-
surface materials and are frequently used to detect buried metal objects (ASTM
2001).
• Gravity survey. Gravity or microgravity surveys measure changes in subsurface
density (ASTM 1999b). Natural variations in the density of subsurface materials
such as soil and rock types, cavities, buried channels, faults, fractures, and joints
can be detected and evaluated using gravity surveys.
• Ground penetrating radar (GPR). GPR uses high-frequency electromagnetic waves
to evaluate subsurface strata (ASTM 2005b). GPR is one of the more popular geo-
physical techniques used in environmental investigations because it is easy to use
and economical, and employable in a variety of challenging accessibility situa-
tions. GPR can determine objects having a dielectric contrast with surrounding
areas, i.e., buried drums, disturbed soil, etc.
• Borehole geophysics. Borehole geophysics uses instruments to measure and record
different properties outside of a well or borehole as the instrument is lowered
down the borehole (Keys 1990; ASTM 2005c). Borehole geophysics is used to evalu-
ate well construction, lithology, permeability and porosity, and water quality.
Figure 4.6 contains seismic survey output from a site used to evaluate and map sedimen-
tary layers.
4.2.5 Ecological and Human Health Risk Assessment
After the investigation phase has been completed and the nature and extent of contamina-
tion have been defined, an environmental risk assessment may be conducted. A common