Geophysical Surveys at INEL/RWMC Cold Pit, Acid
Pit, and Pit 9
NOTE: The following is an executive summary of a report prepared for UNC
Geotech. This paper an be found in its entirety at the National Technical
Information Service (NTIS) under the identification number DOE/ID/12584-49
UNC Geotech, Prime Contractor at the U.S. Department of Energy Grand Junction Projects
Office (DOE/GJPO), conducted geophysical surveys of the Cold Pit, Acid Pit, and Pit 9 at
the Radioactive Waste Management Complex (RWMC) of the Idaho National Engineering
Laboratory (INEL) during a 3-week period beginning 30 January 1989. All geophysical field
data were acquired and processed in accordance with established quality-assurance
procedures, and interpreted by experienced geophysicists. This report describes the
surveys that were conducted and the results of the interpretation of those surveys.Locate and identify other debris within and directly adjacent
to the pits.
The approach was to define the areas geophysically through reconnaissance surveys and
then use those results to plan and conduct detailed surveys. The locations of the detailed
surveys were chosen to further define anomalous areas and / or to better define the
location of the pit boundaries or metallic debris.
Reconnaissance magnetic and electromagnetic surveys determined the geologic and
cultural background noise, identified anomalous area, and indicated the best geophysical
approach for subsequent work. Detailed magnetic and induction electromagnetic (EM) surveys
were conducted in selected areas over each pit. Detailed very low frequency
electromagnetic (VLF EM) surveys were not conducted over any of the pits due to the
limited results of the reconnaissance surveys. Refraction seismic surveys were conducted
within the Acid Pit along two profile lines selected from the results of the magnetic and
EM reconnaissance surveys. Additionally, an EM conductivity depth sounding was conducted
along one profile within the Acid Pit.
Generally, separate profiles of the total magnetic field, magnetic gradient, EM
conductivity, and EM inphase data were interpreted. The results were integrated and
presented on a contour or base map to show a better spatial representation of the
anomalous areas and / or the location of the pit boundaries or metallic debris. Through
the integrated use of the four techniques the locations of the pit boundary and / or
metallic debris were interpreted.
Pit boundary locations determined from geophysical interpretations can be no more
accurate than the sampling interval, in this case 1 meter. Also, a pit boundary often is
not physically distinct due, for example, to sloping of the pit walls. When metallic
objects are buried close together, the geophysical responses of the objects interfere,
making it difficult to identify and separate the individual responses. In some cases, an
integrated interpretation of the geophysical data indicated only the boundaries of the
collections of objects. In other cases, the data only suggested the presence or absence of
metallic debris. It was not possible to determine pit geometries since reliable
interpretations from the seismic data were prohibited by the presence of an approximately
1-meter-thick frozen top layer.
Geophysical techniques measure the results of a parameter contrast, which is then
interpreted to be related to a physical property (e.g., pit boundary, metallic debris
location, ect.). The uncertainties for the interpretations made in this report are on the
order of 1 to 2 meters. Numerical or statistical analyses of geophysical interpretations
are inappropriate given the large unknown distributions that are involved, the inherent
non-uniqueness of a solution, and the lack of relevant numerical and model studies.
Judgment based on experience is the only practical way to arrive at estimates of
confidence for a particular interpretation; therefore, no quantitative statistics based on
assumed distributions are presented in this report.
At the Cold Pit, the magnetic gradient and EM inphase data were more useful for
interpretation purposes than the other data sets. The geophysically interpreted pit
boundary varies from the historic boundary by as much as 3 meters in the southwest portion
of the survey area. In other areas, the interpreted boundary varies from the historic
boundary by no greater then 1 meter. Locations of seven distinct objects, or collections
of objects, were interpreted from the geophysical data; however, the interpreted outlines
of the objects represent a combination of geophysical art and science.
At the Acid Pit, the pit boundary was interpreted from the EM conductivity data s a
boundary of increased conductivity. The interpreted size of the pit is smaller than the
historic size. This discrepancy may be due to the sloping of the pit walls and / or a
concentration of the high conductivity material (acid) in the central portion of the pit.
There was no geophysical evidence to confirm the presence of a 3,000-gallon tank thought
to be within the Acid Pit. A magnetic anomaly, possible due to a tank, matched the modeled
response of an edge of a basalt flow. A depth section of apparent conductivity was
constructed from variably spaced EM conductivity data to a depth of approximately 50
meters, and suggested that the solvent (acid) may have migrated beneath the original base
of the pit. These data also indicated a geo-electric section with a marked change to lower
conductivity values at a depth of approximately 25 meters.
At Pit 9, the EM inphase was the most useful technique to determine the pit boundary.
Overall, the interpreted pit boundary is approximately 2 meters beyond the historic
A combination of EM inphase and magnetic data was used to outline the location of the
metallic debris at Pit 9. Metallic debris was interpreted to be present north of the
reported outline of the transuranic (TRU) waste drums. It was difficult to define the
specific types of objects buried within the pit other than to state that the large
magnetic anomalies present in portions of the pit are caused by large magnetic sources
other than steel drums.