Use of Advanced Borehole Geophysical Techniques to Delineate Fractured-Rock Ground-Water Flow, Faults, Foliation, and Fractures along the Western Part of Manhattan, New York

by Frederick Stumm, Anthony Chu, and Andrew D. Lange

U.S. GEOLOGICAL SURVEY
Open-File Report 01-196

ABSTRACT

Advanced borehole-geophysical methods were used by the U.S. Geological Survey in cooperation with the New York City Department of Environmental Protection (NYCDEP) to assess the geohydrology of crystalline bedrock at three test boreholes (W34ST-B, W37ST-A, and W55ST-A) on the western side of Manhattan Island, N.Y., in preparation for construction of a third water tunnel for New York City by the NYCDEP. The borehole-logging methods included natural gamma, single-point resistance, mechanical and acoustic caliper, focused electromagnetic induction, magnetic susceptibility, borehole- fluid temperature and resistivity, heat-pulse and EM (electromagnetic) flowmeter, borehole deviation, acoustic and optical televiewer, and borehole radar. The boreholes penetrated gneiss and other crystalline bedrock with an overall southwest-to-northwest dipping foliation. Most of the fractures encountered are either nearly horizontal or have moderate to high-angle northwest dip azimuths. Foliation within the tunnel-construction zone is southwest-to-northwest dipping, with fractures that average a northwest dip azimuth. EM and heat-pulse flowmeter logs obtained under pumping and ambient conditions, together with other geophysical logs, indicate transmissive fracture-flow zones in each borehole. The 60-megahertz directional borehole radar logs delineated the location and orientation of radar reflectors that would intersect the projection of the borehole above or below the drilled depth. Borehole data and interpretations may not reflect the exact location of the proposed tunnel-construction zone because of borehole deviation and constraints on borehole placement. 

Fracture indexes range from 0.25 to 0.44 fracture per foot of borehole. The three-dimensional orientation of the fractures indicates a highly transmissive fracture zone at 328 to 330 feet below land surface (BLS) (-287.73 to -289.73 feet elevation) in W34ST-B that may intersect the proposed tunnel-construction zone to about 220 feet northwest of the borehole. Analysis of pumping tests of each borehole indicates that their total transmissivity ranges from 11 to 360 feet squared per day; the highest transmissivity is at the middle borehole (W37ST-A). 

Citation: Stumm, F., Chu, A., and Lange, A.D., 2001, Use of Advanced Borehole Geophysical Techniques to Delineate Fractured-Rock Ground-Water Flow, Faults, Foliation, and Fractures along the Western Part of Manhattan, New York: U.S. Geological Survey Open-File Report 01-196, 46 p.

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