INVESTIGATING THE SUBSURFACE STRUCTURE OF THE CHESTNUT HILL EARTH EMBANKMENT DAM USING MAGNETIC AND RESISTIVITY MEASUREMENTS

Information on the subsurface structure of the Chestnut Hill Reservoir earth embankment dam is needed to evaluate the long-term integrity of the dam for water storage and flood prevention. Geophysical surveying was conducted to gather information on the subsurface structure of the dam. In this study, a magnetometer and resistivity system were used to search for subsurface materials with metallic or electrically conductive properties. High and low magnetic total-field measurements were taken at fifty different stations, distanced 7.5m apart for one minute each. The magnetic field measurements gradually increased in value from south to north. These magnetometer data indicate two large peaks in the magnetic field strength closest to the two block houses on the dam, which were partially constructed from metal materials. Resistivity measurements were taken at three equidistant locations along the dam. The apparent resistivity of subsurface materials up to 6m below the top of the dam was measured using a Schlumberger Array. At all three locations, apparent resistivity values were relatively low at approximately 3m below the top of the dam, suggesting the presence of water. It is possible that water infiltrated the material at the bottom of the dam. The magnetometer data do not exhibit any materials with magnetic properties that stood out during the geophysical survey. The apparent resistivity data are consistent with the water level of the reservoir.