oa Prospecting for a Municipal Water Supply using Multiple Geophysical Methods, Long Lake, NY

As part of a continuing effort to find a potable water supply that meets both hydrogeologic, legal and political criteria, a geophysical Investigation was conducted on the Stanton Hill property east of Long Lake, New York. Seismic refraction, GPR, gravity, VLF and magnetic surveys were conducted to characterize the properties of both unconsolidated materials and bedrock Two seismic lines, each 440 feet in length were established along gravel roads. Explosive energy sources were placed at five locations along each line and at 200 feet off-end locations. the 24 channel data was interpreted using “critical distance” and non-linear refraction travel time Tomography methods. GPR, gravity, VLF and magnetic surveys were completed along the seismic lines and over much of the area of Investigation. A major discontinuity was detected near the midpoint of Line 1. Northwest of the discontinuity, bedrock, with a velocity of 9000 ft/s, is detected; its surface slopes downward to the southeast. South east of the discontinuity bedrock with a velocity of 15,000 ft/s is found with surface sloping upwards to the southeast A third segment with velocity of 13,500 ft/s and surface parallel to the ground surface makes up the southeast end of the profile. the overburden has a velocity of 6000 ft/s which is likely a dense till, and appears thickest over the bedrock contact. Coincident with this interpreted contact. GPR depth-slice imaging indicates a zone of high-amplitude reflectors, caused by an increase in water content or change in mineralogy, 35 to 55 feet below grade. Bouguer gravity values show an increase of over 35 microgals coincident with the bedrock profile. An increase in conductivity was also observed in the VLF data coincident with the deep pocket of unconsolidated material and bedrock contact. Magnetic data were inconclusive, biased by iron-rich rock placed as a cover on the gravel road.<br>A bedrock well, drilled in proximity to, but unfortunately up-dip from this contact and tens of feet from the thickest low velocity area determined by the seismic, confirmed the presence of a highly weathered rusty gneiss overlying a highly competent black, iron rich rock. However, it did not yield more than 10 GPM. Information from the Geophysics did, however, enable our client to situate two other wells in thicker sections of the highly weathered gneiss to produce yields sufficient to meet the town's water supply requirements.