Development of a CPT deployed probe for in situ measurement of volumetric soil moisture content and electrical resistivity

The authors have developed a cone penetration testing (CPT) probe that measures both electrical resistivity and volumetric soil moisture content. This method is innovative, as the effect of electrical conductivity on the measurement of soil moisture has been minimized. The design of our sensors consists of four concentric rings spaced along the penetration rod with insulators in between. The outer two rings determine the soil resistivity; the inner two rings measure the capacitance with the use of a modified Clapp high-frequency transistor oscillator operating at 100 MHz. The CPT-measured volumetric soil moisture can be used to back-calculate other properties such as the soil dielectric constant and, for saturated soil, the dry and wet density. Details of the sensor design approach along with results from an extensive field evaluation at a Department of Energy (DOE) site follow. The field data presents soil moisture and resistivity measurements at three different sites at a DOE facility. Included from these sites are measurements in the vadose and saturated zones. An extensive independent laboratory study was conducted to verify the accuracy and precision of the sensor. (C) 1998 John Wiley & Sons, Inc.