Description
The Advanced In Situ Moisture Logging System measures moisture content and soil density around access tubing within the soil/rock medium. These measurements determine the changes in fluid potential that, in turn, are indicative of leakage/contaminant movement from a waste site. The device is self-contained and programmable, making the application faster and easier, with longer access tubing capabilities than are available in conventional technology. The device provides an alternative for vadose zone monitoring scheme, instead of costly monitor well networks.
The conventional means of deploying such a probe is to lower a logging tool down an access tube to a depth of several meters or possibly tens of meters. The electronics to operate and process data from the probe are in a housing that sits at the top of the access casing. The cable used to lower the probe down the access tube serves to support cabling and to provide electronic communications. The signal/noise ratio may be impaired with the increasing cable length. Therefore, a practical limitation on hole depth exists.
The new Troxler Probe is self-contained. In other words, the electronics required to take a reading and to program subsequent readings of moisture content and soil density are contained within the housing of the device itself. The probe is drawn through an access tube via a support cable that requires no electronic communications to the top of the casing. Therefore, the length of the access tubing is not prohibitive, and long horizontal tubes may be used if a delivery system is properly engineered.
The proposed choice for a delivery system is to employ a constant velocity winch or one that can be hooked up to a datalogger or laptop computer to record cable take-up rate and time and/or length. Such a system has recently become available from Mount Sopris Instrument Co., Inc., Golden, CO. Other devices exist to monitor in situ moisture content and soil density; however, they are general point source devices. The advantage of a downhole logging device is its continuous data collection along the length of the access tubing. Therefore, a greater spatial coverage is obtained.
With the advent of the horizontal/directional drilling technologies, the applications of the downhole self-contained monitoring devices are significantly enhanced. One could envision a network of horizontally drilled access tubes beneath a facility that coupled with the right monitoring technology, could provide an early-warning system for leak detection.
In addition, information from downhole logging devices is essential in evaluating the performance of such remedial alternatives as capping or soil venting strategies and serving as input to risk assessment modeling methodologies. In deep vadose zone regimes, this type of device/monitoring system may be used instead of expensive monitor well installations.
Technical Performance
The neutron source/detector consists of an americium/beryllium combination (10 mCi). The source and detector are in close proximity to each other, and the probe counts thermalized ``slow'' neutrons that have contacted hydrogen atoms. This technique is significantly different from those used in the mineral and oil industries, where the source and detector are widely spaced and the probe counts ``fast'' neutrons. This device's count ratio is generally linear with respect to percent moisture. Its configuration allows for quantification of the moisture content to within ±2% volumetric water content when calibrated to a given soil and has an approximate radius of influence of about 30 cm. The mineral/oil industry probes integrate soil moisture over much larger volumes, with much less accuracy and precision. The gamma source for measuring soil density is an 8-mCi Cs-137 configuration.
The neutron source/detector can be used in vertical as well as horizontal access tubes. Existing vertical and/or horizontal access tubes are necessary, but beyond that, the device is self-contained.
If the tool becomes stuck inside a hole, there are a number of options to dislodge it. If a surface-to-surface hole is being used, the tool may be dislodged by pulling it from either end. If that fails, the Ditchwitch technology may be employed to retrieve the tool. In the event that the Ditchwitch fails, the last resort would be to excavate the tool, assuming that there are not too many contaminants in the soil. The tool is typically about 30-40 ft below the surface.
The first trials found that the device needed to be more rugged for use in the field; however, subsequent testing showed it to work as intended. The tool is quite reliable and easy to use, but there is always a need for caution when using a device containing nuclear material. There are no user-serviceable parts, but the user can recalibrate the tool if needed.
Cost. The start-up costs are $18,000 for the probe and $16,000 for the winch. Because of these high costs, Troxler Electronics Laboratories, Inc., will lease this tool. No routine maintenance costs are expected.
Projected Performance
One major technical challenge is to increase the reproducibility of the results. A second major challenge is to define adequately the accuracy and precision of the method, as well as to further develop the delivery system.
Waste Applicability
Since this technology is specifically used to detect the change in fluid concentrations in arid soils attributed to leaks/contaminant migration from a waste site, the technology is waste independent.
Status
The technology is currently in the field demonstration and testing stage and will be available at the end of FY93. To date, there are no logging tool manufacturers that have developed a similar tool.
Regulatory Considerations
Compliance with the Occupational Safety and Health Administration regulations is required for hazardous waste operations and for the protection of occupational workers from electrical power, winch operation, and radiation exposure. In addition, permits may be required for drilling at hazardous waste sites. Since the tool utilizes low-level radioactive materials, it must be stored in a shielded case with the proper security. Transportation of the device must meet Department of Transportation requirements for nuclear materials.
Potential Commercial Applications
This technology can be used at a wide variety of sites with several different remediation processes. It can be used at any site where monitoring of the vadose zone is needed to detect the potential for contaminant movement from waste sites. Such information is critical to the characterization of waste sites and the monitoring of waste remediation activities and post-closure performance of remediated waste sites. It offers an alternative vadose zone monitoring scheme, instead of costly monitor well networks.
Baseline Technology
Since most cabled nuclear logging devices are not self-contained, they do not lend themselves to long horizontal access tubing applications.
Intellectual Property Rights
The patent owner is Troxler Electronics Laboratories, Inc., #5155356, ``Apparatus and Method for Detecting Subterranean Leakage from a Large Storage Vessel.''
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References
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