USGS - science for a changing world

USGS Groundwater Information: Branch of Geophysics

*  Home *  Resources *  Research *  Publications *  About *  Contact Us *  Groundwater Information

Analysis of Borehole-Radar Reflection Data from Machiasport, Maine, December 2003

USGS Scientific Investigations Report 2005-5087

Prepared in cooperation with the United States Army Corps of Engineers New England District

By Carole D. Johnson and Peter K. Joesten



ABSTRACT

In December 2003, the U.S. Geological Survey, in cooperation with the U.S. Army Corps of Engineers, collected borehole-radar reflection logs in two boreholes in Machiasport, Maine. These bedrock boreholes were drilled as part of a hydrogeologic investigation of the area surrounding the former Air Force Radar Tracking Station site on Howard Mountain near Bucks Harbor. The boreholes, MW09 and MW10, are located approximately 50 meters (m) from, and at the site of, respectively, the locations of former buildings where trichloroethylene was used as part of defense-site operations. These areas are thought to be potential source areas for contamination that has been detected in downgradient bedrock wells.

This investigation focused on testing borehole-radar methods at this site. Single-hole radar-reflection surveys were used to identify the depth, orientation, and spatial continuity of reflectors that intersect and surround the boreholes. In addition, the methods were used to (1) identify the radial depth of penetration of the radar waves in the electrically resistive bimodal volcanic formation at the site, (2) provide information for locating additional boreholes at the site, and (3) test the potential applications of borehole-radar methods for further aquifer characterization and (or) evaluation of source-area remediation efforts.

Borehole-radar reflection logging uses a pair of downhole transmitting and receiving antennas to record the reflected wave amplitude and transit time of high-frequency electromagnetic waves. For this investigation, 60- and 100-megahertz antennas were used. The electromagnetic waves emitted by the transmitter penetrate into the formation surrounding the borehole and are reflected off of a material with different electromagnetic properties, such as a fracture or change in rock type. Single-hole directional radar surveys indicate the bedrock surrounding these boreholes is highly fractured, because several reflectors were identified in the radar-reflection data. There are several steeply dipping reflectors with orientations similar to the fracture patterns observed with borehole imaging techniques and in outcrops. The radar-reflection data showed that the vitrophyre in borehole MW09 was more highly fractured than the underlying gabbroic unit.

The velocities of radar waves in the bedrock surrounding the boreholes were determined using single-hole vertical radar profiling. Velocities of 114 and 125 meters per microsecond were used to determine the distance to reflectors, the radial depth of penetration, and the dip of reflectors. The bimodal volcanic units appear to be ideal for radar-wave propagation. For the radar surveys collected at this site, radar reflections were detected up to 40 m into the rock from the borehole. These results indicate that boreholes could conservatively be spaced about 15-20 m apart for hole-to-hole radar methods to be effective for imaging between the boreholes and monitoring remediation. Integrated analysis of drilling and borehole-geophysical logs indicates the vitrophyric formation is more fractured than the more mafic gabbroic units in these boreholes. There does not, however, appear to be a quantifiable difference in the radar-wave penetration in these two rock units.


To view the full report, you can select from two options:

Note:

This report contains a PDF file that you will need a PDF reader to view. If you do not already have a PDF reader installed, you can download the most recent free Adobe Acrobat Reader to view PDF files. Visit http://access.adobe.com for free tools that allow visually impaired users to read PDF files.

This report contains a movie (.avi) that you will need a media player to view. If you do not already have a media player installed, you can download free Windows Media Player or free RealPlayer.

Hypertext links and other references to non-USGS products, trade names, and (or) services are provided for information purposes only and do not constitute endorsement or warranty, express or implied, by the USGS, USDOI, or U.S. Government, as to their suitability, content, usefulness, functioning, completeness, or accuracy.


Citation: Johnson, C.D., and Joesten, P.K., 2005, Analysis of borehole-radar reflection data from Machiasport, Maine, December 2003: U.S. Geological Survey, Scientific Investigations Report 2005-5087, 44 p.

USGS Home Water
Climate and Land Use Change Core Science Systems Ecosystems Energy and Minerals Environmental Health Natural Hazards

Accessibility FOIA Privacy Policies and Notices

Take Pride in America logo USA.gov logo U.S. Department of the Interior | U.S. Geological Survey
URL: http://water.usgs.gov/ogw/bgas/publications/SIR2005-5087/index.html
Page Contact Information: Contact the OGW Branch of Geophysics
Page Last Modified: Thursday, 03-Jan-2013 20:04:36 EST