USGS Groundwater Information: Branch of Geophysics
For the last 30 years, the New York City Department of Environmental Protection (NYCDEP) has been constructing a water supply tunnel for New York City. Since 1998, the U.S. Geological Survey (USGS) New York Subdistrict Office in Coram has been using geophysical methods as part of a geotechnical collaboration with NYCDEP. USGS scientists Fred Stumm, Tony Chu, and Andy Lange have been applying a toolbox approach to collect geophysical data for characterizing the subsurface bedrock in advance of tunnel drilling.
According to Stumm, the use of multiple geophysical methods allows for more accurate and comprehensive site characterization. The USGS is using borehole geophysical methods including optical televiewer, acoustic televiewer, radar, and heat-pulse flowmeter to map bedrock contacts, fractures, and foliation. More recently Stumm and colleagues have been exploring the use of seismic tomography to increase the volume of rock being imaged. They are also working to integrate water-borne seismic reflection data collected on the East River and borehole seismic data to better map subsurface features. This multi-method 'toolbox' approach builds on research conducted by the Office of Ground Water, Branch of Geophysics (OGW BG) and others at sites such as the Toxic Substances Hydrology Program Mirror Lake Research Site.
"What we're really trying to do is provide information that is in support of safety issues for the tunnel," says Stumm. Detection of water-bearing fractures that could flood the tunnel or fracture systems that could cause tunnel failure contributes not only to project safety but reduces construction cost overruns by providing information on the condition of bedrock and the potential for ground water inflow. "I think we bring a unique package to the table; not only is it the borehole geophysics but it's the water resources that gives them a real picture of what is going on in the bedrock," Stumm explained.
OGW BG has assisted the project by providing training and interpretive assistance for new tools and methods at the start of the project, field support for borehole-radar surveys, and technical support for the seismic tomography. Says Stumm, "When working together, we really can get a lot of work done. It really helps us also with our program development. It's really been a good relationship, and I think it helps the cooperators appreciate the Survey's ability to be flexible."
What is it like working in the middle of Manhattan? "You feel like you are in a fishbowl!" explains Stumm. "It's amazing to do borehole geophysics on the sidewalk in a city like that."
In the News! Read the November 2008 news article about this project.
To learn more about this project, see http://ny.usgs.gov/pubs/wri/wri004276/. Do you have a project you would like to see highlighted in Profiles in Geophysics? Contact OGW BG at GS-W_OGW_BG_info@usgs.gov.