ESTIMATION OF NEAR-SURFACE SOIL DENSITY USING ELECTRICAL AND ELECTROMAGNETIC GEOPHYSICAL TECHNIQUES

Soil density is an important parameter for agricultural and geotechnical applications. In agriculture, soil compaction is a problem on both regional and global scales. In geotechnical applications, soil density estimates are essential for constructing foundations and earth-fill structures. Conventional methods of measuring soil density are point measurements and are not suitable for field-scale characterization. This experiment examines the potential of geophysical methods for rapid, field-scale soil density estimation.

The experiment was conducted over a 2 m x 1 m plot area in a loamy agricultural soil. Geophysical data were collected over the undisturbed plot area using Ground Penetrating Radar (GPR), electromagnetic induction (EM), and resistivity techniques. After the non-invasive geophysical data collection was completed, point measurements of electrical conductivity (EC) and dielectric permittivity (κ) were acquired every 12 cm using Time Domain Reflectometry (TDR). Lastly, soil samples were collected at the same locations to measure the bulk density and water content. The soil was then tilled, and the data collection was repeated with all techniques.

Data processing and interpretation is in progress, but preliminary results show that all geophysical techniques showed significant changes after tilling, as did the conventional measurements of soil density and water content. EC measured using resistivity and TDR techniques decreased after tilling, as did κ from TDR. EC from EM showed a slight increase after tilling, but the increase is within the error range of the instrument. Although GPR data processing is on-going, groundwave data were acquired using common- and variable-offset survey modes with 250 MHz antennas, and both travel time and amplitude data are being considered. To determine which geophysical techniques are most sensitive to changes in soil density, Archie’s law will be employed. Using EC measurements from TDR, EM, and resistivity techniques and water content estimates from GPR and TDR, the changes in EC due to changes in the soil tortuosity will be calculated. Comparing changes in tortuosity will indicate which geophysical techniques are most sensitive to changes in soil density, and thus might be most suitable for field-scale density estimation.