Chem-Nuclear Geotech, Inc. (Geotech), personnel performed geophysical surveys<br>at Hickam Air Force Base (HAFB), Hawaii, to determine the presence or<br>absence of underground storage tanks. Geophysical survey techniques included<br>magnetic total field, magnetic gradient, and horizontal-loop electromagnetic<br>methods. This case study presents validated data from five sites.<br>Twenty-three sites at Hickam Air Force Base were designated for investigation<br>following interviews with HAFB personnel and examination of base records and<br>historical land use. Geophysical investigations were conducted at 10 sites to<br>confirm the removal of tanks and at 3 sites to establish the location of tanks<br>presumed abandoned in place. Tests were conducted at an additional 10 sites,<br>for which no historical tank disposition records were available, to determine<br>whether tanks were present.<br>Magnetic and electromagnetic (EM) instruments respond characteristically<br>to steel underground storage tanks because the tanks are ferromagnetic<br>and good electrical conductors. Determinations were made of an equivalent<br>magnetic susceptibility and a conductor EM-response parameter range<br>characteristic of steel storage tanks. The presence of other metal objects<br>within the surveyed areas affected the instrument readings, but these objects<br>usually produced significantly different response parameters.<br>In six areas, our interpretation of the geophysical data confirmed the<br>presence of 14 tanks and the probability of an additional 5 tanks.<br>Interpretation of the data indicated that no tanks were present at 16 of the<br>investigated areas. Survey results from one area were indeterminant because<br>of the limited scope of the measurements.<br>Exploratory excavations and augering were conducted at 9 of the 23 sites to<br>provide ground-truth for the geophysical interpretation of the collected<br>survey data. Conclusions derived from the geophysical data were validated at<br>eight of the nine sites.<br>This case study presents data from five of the investigated sites.<br>Underground storage tanks were uncovered at three of the sites, and the<br>absence of storage tanks was verified at one site. The fifth site illustrates<br>the need for an integrated, multiple-method approach for proper geophysical<br>investigations.


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