APPLICATION OF GEOPHYSICS TO THE STUDY OF FROZEN GROUND IN NORTHERN NEW YORK

Frozen ground conditions in northern New York create environmental problems during the winter. Burials cannot take place because of the frozen ground and the installation of water pipelines must take into account the thickness of the frozen ground. Our approach to the geophysical study was to conduct d.c. resistivity and seismic refraction surveys under frozen ground conditions in the winter and unfrozen ground conditions in the summer. The seismic refraction survey conducted in the summer under unfrozen ground conditions indicated a 3 layer model consisting of 6 feet of low velocity silty sand underlain by a medium velocity clay layer extending to a depth of 40 feet. The third layer of high velocity is the Potsdam sandstone bedrock. Our interest was in the upper low velocity layer of 6 feet thickness where the frozen ground occurs.

An electrical resistivity survey conducted by the Wenner method at closely spaced electrode spacings was used to determine the vertical extent and resistivity of the frozen ground. This survey was effective in determining the thickness of 3 feet for the frozen ground and a resistivity of 1000 ohm-meters. Beneath the frozen ground the resistivity decreased to 40 ohm meters.

The seismic survey attempted to illustrate the problem of conducting seismic refraction surveys when frozen ground is present to create a low velocity layer beneath the higher velocity frozen ground. An offset of 2 feet and a 2 feet geophone interval enabled us to determine the velocity of frozen ground. By using the additional evidence of 3 feet thickness for frozen ground from the resistivity survey, and velocities V1, V2, V3 and intercept time (Ti) the thickness of the unfrozen low velocity layer was determined.

The research indicated a general increase in resistivity and seismic velocity in frozen ground. Also, the resistivity method is effective in determining its thickness. The frozen ground gives rise to the low velocity problem. However, if other geophysical data is available, it is possible to determine thickness of the unfrozen low velocity layer.