TDR-inspection of vast tracts of forests or plants. Tract of plants as an environmental hazard detector

摘要

A new method to monitor vast tracts of plants and forests was introduced in the former USSR in the 80's and was developed later in the USA. The method is based on a time domain reflectometer, TDR which deals with propagating a flat electromagnetic wave initiated by an electric pulse traveling along an electric line. While propagating, the pulse experiences reflections from the heterogeneities of the line, which is displayed on the screen of TDR-instrument. The novelty of the application of TDR for monitoring tracts of plants is that the probing line and the tract form a waveguide. The traveling wave, which is scanning the surrounding medium, experiences a reflection from any heterogeneity in plants medium: changing the density of plants, condition of green tissue, variation of moisture and so on. The reflected signal, which is seen on the screen of TDR-instrument, brings the information about the distribution of the physical condition of the tract along the probing line. The method can evaluate the flora for hundreds of miles. The influence of the density of plants and electro-physical characteristics of green media on propagating traveling waves was shown. Analysis of the contribution of the heterogeneity of plants to the complex dielectric permittivity of the medium, depending on the distance from the line, was accomplished. To evaluate the tract of plants as a whole, an effective dielectric permittivity of the tract, ef, was employed. A simple relation between filling coefficient F and, ef was derived. The filling coefficient shows the ratio of the volume of plants to the total volume. It was shown that in a wide band of probing frequencies up to hundreds of MHz the following relation applies; epsiv_ef = (1-F)^-1 For the tracts with developed surface of leaves rather than stems, the frequency dependence of the permittivity of the tissue can be neglected. The experiments, conducted on the 100m tract of the plants during the vegetation period i-nn a botanical garden, April- September, revealed the variation of epsiv_ef within 1.7-2.3 which correlated to the observed 30-80% variation of the water content in the plants. This process manifested on the screen of TDR-instrument as a variation of time between the reflected signals as well as their intensity.The value of ef for each portion of the tract can be found as a ratio of speeds of light and probing electromagnetic wave, based on the reading of the instrument: epsiv_ef ap (^C/_V)²Another novelty of the method is employing tracts of the plants as an environmental detector. Value eff depends on condition of the media around the plants, so the probing wave experiences reflection from the related heterogeneities, which are seen on the screen of the instrument. For instance, leaves of tobacco are pretty sensitive to sulfur dioxide, which results in their electro-physical characteristics.