HR: 14:05h
AN: NS33A-03    [Abstracts]
TI: Electrical Resistivity Imaging of Tidal Fluctuations in the Water Table at Inwood Hill Park, Manhattan
AU: * Kenyon, P M
EM: pkenyon@sci.ccny.cuny.edu
AF: City College of New York, Earth and Atmospheric Sciences Dept., J-106, Convent Ave. at 138th St., New York, NY 10031 United States
AU: Kassem, D
AF: City College of New York, Earth and Atmospheric Sciences Dept., J-106, Convent Ave. at 138th St., New York, NY 10031 United States
AU: Olin, A
AF: City College of New York, Earth and Atmospheric Sciences Dept., J-106, Convent Ave. at 138th St., New York, NY 10031 United States
AU: Nunez, J
AF: City College of New York, Earth and Atmospheric Sciences Dept., J-106, Convent Ave. at 138th St., New York, NY 10031 United States
AU: Smalling, A
AF: City College of New York, Earth and Atmospheric Sciences Dept., J-106, Convent Ave. at 138th St., New York, NY 10031 United States
AB: Inwood Hill Park is located on the northern tip of Manhattan and has been extensively modified over the years by human activities. In its current form, it has a backbone of exposed or lightly covered bedrock along the Hudson River, adjacent to a flat area with two tidal inlets along the northern shore of Manhattan. The tidal motions in the inlets are expected to drive corresponding fluctuations in the water table along the borders of the inlets. In the Fall of 2002, a group of students from the Department of Earth and Atmospheric Sciences at the City College of New York studied these fluctuations. Electrical resistivity cross sections were obtained with a Syscal Kid Switch 24 resistivity meter during the course of a tidal cycle at three locations surrounding the westernmost inlet in the park. No change was seen over a tidal cycle at Site 1, possibly due to the effect of concrete erosion barriers which were located between the land and the water surrounding this site. Measurements at Site 2 revealed a small, regular change in the water table elevation of approximately 5 cm over the course of a tidal cycle. This site is inferred to rest on alluvial sediments deposited by a small creek. The cross sections taken at different times during a tidal cycle at Site 3 were the most interesting. They show a very heterogeneous subsurface, with water spurting between blocks of high resistivity materials during the rising portion of the cycle. A small sinkhole was observed on the surface of the ground directly above an obvious plume of water in the cross section. Park personnel confirmed that this sinkhole, like others scattered around this site, is natural and not due to recent construction activity. They also indicated that debris from the construction of the New York City subways may have been dumped in the area in the past. Our conclusion is that the tidal fluctuations at Site 3 are being channeled by solid blocks in the construction debris, and that the sinkholes currently present result from removal of sediments from below, as a result of the tidal fluctuations.
DE: 0925 Magnetic and electrical methods
DE: 1899 General or miscellaneous
DE: 4560 Surface waves and tides (1255)
SC: Near-Surface Geophysics [NS]
MN: 2005 Joint Assembly