April 1997
TECHNICAL DESCRIPTION OF THE MINIATURIZED PULSER
The field rugged, miniaturized pulser/sampler card was developed through a consortium of private and public concerns to provide a platform to take TDR measurement to the field. This consortium recognized that to truly transform TDR measurement to a field robust technology, there must be a replacement for the present large sized and high power consumption equipment. The HYPERLABS card represents the product of that transformation effort.
The consortium was organized about the U.S. Army Corps of Engineers (USACE) Construction Productivity Advancement Research (CPAR) program. This program is designed to commercialize advancements in construction technology, and therefore focuses upon both technical and institutional development. A combination of HYPERLABS (the private sector), Northwestern University's Infrastructure Technology Institute (ITI) (the academic sector), and the U.S. Bureau of Mines (USBM) (another technology developer/user) combined efforts to develop the instrument. The U.S. Army Corps of Engineers then field tested the equipment at its Waterways Experiment Station in Vicksburg Mississippi.
The HYPERLABS TDR pulser/sampler, about the size of an external modem when encapsulated, is shown in Figure 1 in the instrument enclosure at the USACE test. The pulser card itself is shown in Figure 2. This enclosure houses a computer for control, 12 vt solar charged battery and a cellular phone. This entire system is designed to operate on power supplied by the solar cells. The HYPERLABS TDR box is about 1/7th the size of the Tektonix 1502 TDR tester that it replaces.
The over-all design objectives included:
COMMERCIALIZATION AND FIELD TESTING
Today a typical commercialization time line normally involves a government agency sponsored demonstration and verification of the system. This field verification and demonstration is being shared by the USAEC and ITI. The USAEC phase involved TDR measurement of water level, and has been completed. The ITI phase will involve measurement of ground deformation in conjunction with the Ohio Department of Transportation. These two projects will be described below.
USAEC Demonstration of Miniaturized System to Measure Water Levels
The USAEC was interested in deploying TDR to retrofit previously installed piezometers in dams. These piezometers measure pore water pressures within and below dam foundations, and are employed to determine dams stability and safety. The TDR system offers advantages over pressure transducer technology as described by Dowding et al. in their article, "Remote Piezometric Water Pressure Measurement with Time Domain Reflectometry" or Dowding and Huang in "Ground Water Pressure Measurement with Time Domain Reflectometry."
The measurement demonstrated by the USAEC combines the miniaturized TDR pulser with a hollow coaxial cable. The hollow cable is perforated to allow water to freely rise between the inner and outer conductors to produce a very large reflection at the air- water interface.
The beta version HYPERLABS card field testing was successfully completed in Nov of 1996 at a demonstration of TDR technology at the USACE Waterways Experiment Station in Vicksburg. This demonstration drew USAEC representatives from around the country as well as representatives form Campbell Scientific, who assist in the manufacture of the device. Dr. Kevin O'Connor, Director of USBM TDR research until the closure of the USBM and now President of GeoTDR, Dr. Glenn Nicholson, of the USAEC, and Prof Charles Dowding of NU's ITI lead the discussion during the demonstration.
As shown in Figure 1, the total instrumentation system consists of a cellular phone for communication, a portable computer for control, the HYPERLABS pulser, and a solar cell charged battery power source. This systems sits atop the well being measured. A single pulser can measure more than one well if a multiplexer is added to the instrument suite.
ITI Demonstration of Miniaturized System to Measure Rock Deformation
Ohio DOT's interest in monitoring TDR cable deformation presents an opportunity to demonstrate the advantages of remote monitoring with the miniaturized pulser. The TDR cable has been installed adjacent to a section of I-70 that overlies a abandoned coal mine that collapsed in 1995(???) in hopes of providing an early warning system. Daily monitoring of the TDR cable response would improve the early warning capabilities.
ITI is currently designing a trial unit to remotely monitor the TDR cable. The system is similar to that employed in the USAEC demonstration, only it involves two TDR systems. 1) the solar powered miniaturized pulser and cellular phone system as well as 2) a 110 Vt and hard line connect telephone system. The solar system is similar to that employed by the USAEC except that a specially designed field computer will be substituted for the lap top computer. A photograph of the prototype version of this solar powered system is shown on the roof of the ITI laboratory in Figures 3 and 4. The 110 Vt powered system is being employed as a backup in case there are too many sunless days at the field site. The side by side comparison will allow determination of the cost effectiveness of the solar powered alternative. The two systems are to be installed in June of 1997.
Remote measurement of rock deformation at depth provides early warning of surface disruption. As shown in "Telemetric and Multiplexing Enhancements of Time Domain Reflectometry," the movement at depth is detected before it reaches the surface. And the movement at depth is measured by the distortion of the cable, which produces a TDR voltage reflection.