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Geophysical Surveys at INEL/RWMC Cold Pit, Acid Pit, and Pit 9

by
J.C. Hasbrouck

 

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.

The objective of the geophysical surveys were to:

Define the boundaries of each pit,

Locate the 3,000 gallon tank thought to be buried in the Acid Pit,

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 boundary.

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.

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