Impact of Wood Sample Shape and Size on Moisture Content Measurement Using a GPR-Based Sensor

Nondestructive, GPR-based real-time sensors have been shown to be an effective tool for measuring moisture content in wood samples within the forest products industry. Moisture content is derived from either the early time electromagnetic pulse amplitude or the pulse travel time through the wood sample. When used on samples that are comparable in size to the sensor antenna and wavelength within the sample, the GPR sensor will measure a moisture content that is dependent on the sample shape and size. In this study, we look at a cylindrical sample, representative of a typical wood log sample. We have used numerical modeling with both simplified, unshielded Hertzian dipole antennas and a realistic shielded GPR antenna system to estimate these effects. The numerical modeling shows that for measurements performed on the sides or ends of small diameter logs, the curvature of the log, and closeness of the log boundaries has a significant impact on the early time amplitude and signal travel time through the log that result in reduced accuracy in moisture content measurements. Wood logs have electrical properties that are anisotropic and vary radially within the log. Our numerical models are assumed to be isotropic and homogeneous. GPR data acquired on the ends of wood logs show similar dependency on log diameter to those observed in the modeling, and numerical model results compare reasonably well with data measured on measured logs. The numerical modeling results also demonstrate the importance of using realistic shielded antenna models in this application.