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 which measures both the resistivity and the dielectric constant of the soil. This method is innovative because the water relaxation effects due to interfacial polarization are greatly reduced. This effect has plagued previous researchers and resulted in inaccurate measurements of the soil dielectric constant. 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 using a modified Clapp high-frequency transistor oscillator operating at 100 MHz. The CPT measured dielectric constant can be used to calculate the soil volumetric moisture content by directly calibrating to the soil of interest. A more general equation, such as Topp`s Universal Equation, can determine the volumetric moisture content. Details of the sensor design approach along with results from an extensive field evaluation at a DOE site follow. The field data presents soil moisture and resistivity measurements at three different sites on a DOE facility. These sites included measurements in the vadose and saturated zones. An extensive independent laboratory study was conducted to verify the accuracy and precision of the sensor.