Unique Insight into the Seasonal Variability of Geophysical Properties of Field Soils: Practical Implications for Near Surface Investigations

Electromagnetic (EM) wave propagation methods are extensively used in geophysical prospection, such as archaeological and utility surveys. The signal penetration and attenuation depend strongly on the apparent permittivity and electrical conductivity of the soil which vary on a seasonal basis, affecting the detection of buried features, especially their detected depth. However, there is a significant lack of high quality, long term seasonal field monitoring data of EM properties in different soil conditions to aid the understanding of how these properties vary in field conditions. The research reported in this paper provides an important step forward in addressing the scarcity of data. Long term data are presented and analysed from bespoke time domain reflectometry (TDR) monitoring stations designed to enable collection of apparent relative dielectric permittivity (ARDP), bulk electrical conductivity (BEC) and temperature data at a high temporal resolution (hourly) from three remote sites with different soils over an extended period (16-23 months). As well as providing an extensive data set on this subject, the data highlighted the importance of using accurate EM soil data for geophysical prospection The greatest changes in geophysical properties for all sites were found in the near surface soils (< 0.5 m), where many buried utilities are found, with rapid wetting events and slower drying events greatly affecting both the ARDP and BEC. However, the most critical factor for determining these properties was the soil water holding capacity which in turn was a function of the clay mineralogy and content. Analysis of the ratio of energy loss to storage showed that the optimum time for GPR surveying was found to be during dry periods and when the soil temperature was low, displaying the significance of soil temperature on survey outcomes due to its significant effect on BEC. The results from this paper will aid survey planning thereby ensuring better detection rates.