The Hygro-Mole's success has been proven both in the laboratory and in the Waste-management Education & Research Consortium (WERC) competition. This logging tool is designed to determine the value of low moisture content present in subsurface regions, at very low cost. The Hygro-Mole yields immediate results, and can be calibrated to specifically monitor multiple types of sub-surface material. This logging tool is a simple, non-invasive technique with virtually no waste stream, and very minimal health and safety hazards.
Designing the Hygro-Mole logging tool began in August of 1999, to determine the moisture content of salt rock formations at levels as low as 0.1%. Knowing the moisture content of salt formations is useful when predicting potential Disturbed Rock Zones (DRZ) that could cause structural instability. The US Department of Energy Waste Isolation Pilot Plant is an example of such a site where the detection of moisture and DRZ would be useful.
Numerous processes were researched and considered for moisture measurement before the development of the Hygro-Mole concept. To determine the best process, the evaluation criteria defined by the Environmental Protection Agency were reviewed1. Table I lists the advantages and disadvantages of potential processes.
The Hygro-Mole consists basically of a hygrometer and a dry-air line contained in a probe structure. The hygrometer's function is to measure the rate of humidity change in the headspace of the tool in a borehole relative to equilibrium saturations at the borehole temperature. The dry-air line removes initial moisture in the probe area. As moisture begins to "re-enter" the probe headspace, the partial pressure of the water in the air increases with time. The relative humidity is then plotted against time in order to obtain a humidification rate. The humidification rate is then correlated to the moisture content graphs for that specific matrix of concern.
The in-situ moisture content is thereby determined through mass diffusion modeling. Extensive bench scale testing validated the theory needed to support the hygrometer technique. Several trials were conducted on various media in order to generate reproducible results.
This study is based on the fact that given enough time, relative humidity in a confined chamber will rise to an equilibrium value when in contact with a higher moisture source2. In this case, sealing the probe chamber against the side (or bottom) of the borehole and allowing moisture content to reach equilibrium. The time required to reach equilibrium depends on the moisture present, as well as the matrix analyzed.
The equilibrium condition in the chamber humidity is the value measured by the hygrometer when the value stops changing. At this constant humidity level, dry air may be pumped into the chamber at a steady flow rate to remove moisture and invoke a driving force of moisture between the formation and the chamber. The rate of diffusion can then be derived by plotting the change in chamber humidity verses time, from 10% relative humidity (limit of the hygrometer used) to saturation. The diffusion rate is plotted against percent moisture to create a moisture graph. The moisture graph can then be used to find the percent moisture of samples tested of the same material.
The principle behind this procedure is that materials of different moisture contents dry at different rates. If the user knows what type of medium is being tested, with calibration, the moisture content can be determined.