Application Of Borehole Geophysics At The Retsof Salt-Mine Collapse Site, Western New York

Borehole-geophysical methods were used in the hydrogeologic characterization of the valley fill and bedrock<br>at the site of the 1994 Retsof salt-mine collapse in western New York. Collapse of the mine roof and fracturing<br>of the overlying bedrock in the southern downdip section of the IO-square-mile salt mine resulted in flow of<br>ground water into the historically dry mine at a rate of more than 15,000 gallons per minute. A suite ofboreholegeophysical<br>logs, including caliper, gamma, electromagnetic induction, specific conductance, temperature,<br>acoustic televiewer, video camera, and impeller flowmeter, was collected from 17 test wells. Most of the logged<br>test wells were 700 to 850 feet deep and were completed with steel casing through the valley fill and finished as<br>open hole in the bedrock.<br>The valley-till sequence at the mine-collapse site was about 500 feet thick and the bedrock sequence, from<br>top of bedrock to the salt mine, was about 600 feet thick. Analysis of the borehole-geophysical logs and correlation<br>with reported drilling logs delineated (1) glaciolacustrine fine-grained sediments that restricted downward<br>flow of surface water and ground water from the overlying alluvium; and (2) ground-water flow zones in<br>glaciofluvial sand and gravel in the mixed deposits of the lower valley fill, near the top of bedrock, and in fracture<br>zones at the limestone-dolomite contact and in the lower part of the dolomite. Fractures that provided the vertical<br>hydraulic connection between the ground-water flow system and the mine were penetrated by several test wells<br>drilled within the collapse.<br>Borehole geophysics proved to be a highly efficient method for obtaining information on the hydrogeology<br>of the valley fill and bedrock at the Retsof salt-mine collapse site. This information was critical in evaluating<br>potential remedial actions at the site and developing a plan of study to assess the effects of the mine collapse on<br>the water resources of the area.