Survey Methods Used at Site Investigations at Radford Army Ammunition
surveying is utilized at waste sites to locate various subsurface
features that may be present at the site while being minimally invasive
of environmental media. At Radford Army Ammunition Plant (RFAAP)
the features most commonly of interest are areas of possible buried
waste/debris, the top of bedrock, and rock type. Geophysics can
also provide information regarding other subsurface geological features
e.g., bedrock fractures and groundwater properties. Collecting geophysical
information prior to formulating investigation work plans is efficient
and cost effective, because sampling locations and depths can be
optimized to provide data in areas of interest with a minimum amount
of sampling. Geophysical surveying at RFAAP uses an integrated borehole
and surface approach. The following paragraphs describe and illustrate
the equipment used.
use various probes to measure different properties of the soil and
rock penetrated by a well. The fluid probe measures thermal and
electrical properties of groundwater in a well. The natural gamma
probe estimates soil and rock clay content by measuring naturally
occurring radiation in penetrated soils and rocks. The induction
probe emits a magnetic field to measure soil and rock electrical
properties. When run with the natural gamma probe, the combined
data helps define rock type, depth of potential weathered or water-producing
zones, and calibrates surface geophysical methods. These probes
were deployed using a motorized winch with the data recorded and
monitored by a laptop computer (Figure 1).
Left hand photograph is a logging winch, cable, and laptop. The
right hand drawing is a fluid probe 21-in long by 1.5-in diameter
(courtesy Layne-Christiansen Colog, Golden CO).
and Horizontal Seismic Profiling
profiling uses a device known as a geophone that is lowered into
a monitoring well in order to measure the rate at which a mechanically-induced
vibration is transmitted through the soil and rock penetrated by
profiling is similar to vertical profiling except that a linear
array of geophones are placed on the ground surface (see Figure
2), and an low-energy impact source is used to generate the requisite
vibration. Low-energy impact sources such as sledgehammers and weight
drops are typically used for environmental investigations. Figure
3 shows examples of a sledgehammer (left picture) and the accelerated
weight drop (AWD) energy sources employed at RFAAP. Both sources
use a steel plate place on the ground surface as the impact point
in order to impart vibration into the ground. The plate also serves
to minimize damage to the ground surface. Ground vibrations generated
by the AWD are barely noticeable by human senses at distances of
10-20 ft from the source.
Geophone (left) and Seismic Cable Layout (Anniston Army Depot, Alabama).
Seismic Energy Sources: Sledge Hammer (left) and drop weight (right)
profiling is used to measure variations in the electrical properties
of the subsurface, which are correlated to the underlying geology
and hydrogeology. Like the seismic survey, a linear array of receivers
(in this case electrodes) is placed on the ground surface. During
the survey, two of the electrodes are used to impart a low-amperage
electric current (less than 2% of a household current) into the ground,
and the other electrodes are used to measure voltage changes along
Figure 4 shows
an array of electrodes with connecting cable (yellow) in the left
picture, and the controller and recording gear in the right picture.
The electrodes are approximately 12 inches long and 3/8 inch in
diameter, and are inserted from 3 to 8 inches into the ground. Like
the seismic survey, little to no damage is done to the ground surface
during the collection of the resistivity data.
Resistivity cable and electrode layout plus controller and recording
gear (Hunter Army Air Field, Georgia).
instruments (EM-31 and EM-34) are used to measure the electrical
conductivity of the soil near the ground surface. EM methods are
often used to locate possible buried metallic objects.
The EM instruments
are carried over the shoulder with data collected continuously as
the technician moves over a pre-established grid (Figure 5). The
instrument's information is stored in a portable data logger for
later processing. When the survey is complete, the data are downloaded
into a computer and processed to create color profile maps. The
EM data can be used to help map the location of metallic objects,
and the aerial extent of buried waste and/or geologic features.
EM31 survey at SWMU 51, Radford Army Ammunition Plant.
integrated geophysical survey methods employed at RFAAP provide
complimentary information from different probes and equipment. The
synthesis of this data provides the most extensive and thorough
subsurface picture that can be developed prior to initiating environmental
media sampling. Performing geophysical surveys prior to the creation
of investigation work plans allows the RFAAP Restoration Team to
target sampling locations to verify the geophysics while locating
areas that would be of primary interest if waste had been or is
being released to the environment.