Unique insight into the seasonal variability of geophysical properties of field soils: practical implications for near-surface investigations
Electromagnetic wave propagation methods are extensively used in geophysical prospecting, such as in archaeological and utility surveys. The signal penetration and attenuation of electromagnetic waves 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 electromagnetic properties in different soil conditions to aid the understanding of how these properties vary in field conditions. The results reported in this paper will contribute to addressing this scarcity of data. Longterm data are presented and analysed from bespoke time-domain reflectometry monitoring stations designed to enable collection of apparent relative dielectric permittivity, bulk electrical conductivity, and temperature data at a high temporal resolution (hourly) from three remote sites with different soils over an extended period of time (16–23 months). In addition to providing an extensive dataset, the data highlight the importance of using accurate electromagnetic soil data for geophysical prospecting. The greatest changes in geophysical properties for all sites are detected in the near-surface soils (< 0.5 m), where many buried utilities are generally found, with rapid wetting events and slower drying events greatly affecting both the apparent relative dielectric permittivity and the bulk electrical conductivity. However, the most critical factor for determining these properties is the soil water holding capacity, which, in turn, is a function of the clay mineralogy and content. An analysis of the ratio of energy loss to energy storage shows that the optimum time for ground penetrating radar surveying is during the dry periods and when the soil temperature is low, displaying the significance of soil temperature on survey outcomes due to its significant effect on the bulk electrical conductivity. The results from this paper will aid survey planning, thereby ensuring a better underground target detection rate.