DELINEATION OF STRUCTURE AND GROUND-WATER-FLOW ZONES IN BEDROCK, ON THE SOUTHERN PART OF MANHATTAN, NEW YORK, THROUGH USE OF ADVANCED BOREHOLE-GEOPHYSICAL TECHNIQUES

Advanced borehole-geophysical techniques were used to assess the geohydrology of crystalline bedrock in 26 of 29 boreholes on the southern part of Manhattan Island, N.Y., in preparation for construction of a third water tunnel for New York City. The borehole-logging techniques included natural gamma, single-point resistance, short-normal resistivity, mechanical and acoustic caliper, magnetic susceptibility, borehole-fluid temperature and resistivity, borehole-fluid specific conductance, dissolved oxygen, pH, redox, heat-pulse flowmeter (at selected boreholes), borehole deviation, acoustic and optical televiewer, and borehole radar (at selected boreholes). Hydraulic head and specific-capacity test data were collected from 29 boreholes. The boreholes penetrated gneiss, schist, and other crystalline bedrock that has an overall southwest to northwest-dipping foliation. Most of the fractures penetrated are nearly horizontal or have moderate- to high-angle northwest or eastward dip azimuths. Fracture population dip azimuths are variable. Heat-pulse flowmeter logs obtained under pumping and nonpumping (ambient) conditions, together with other geophysical logs, indicate transmissive fracture zones in each borehole. The 60-megahertz directional borehole-radar logs delineated the location and orientation of several radar reflectors that did not intersect the projection at nine selected boreholes. Fracture indexes range from 0.12 to 0.93 fractures per foot of borehole. Analysis of specificcapacity tests from each borehole indicated that transmissivity ranges from 1 to 459 feet squared per day..