Unique insight into the seasonal variability of geophysical properties of field soils: practical implications for near‐surface investigations

Abstract

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. Nevertheless, 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.tected near-surface buried features will be reduced, thereby significantly reducing the risk of excavation.Several previous studies have shown links between geophysical and geotechnical properties for soil, including the soil particle size distribution (e.g.linear shrinkage (e.g., Thomas et al. 2010a;Thomas et al. 2010b), but by far the most important factor is the water content. These studies have primarily focused on testing the relationships in the laboratory over a range of volumetric water contents (VWCs) and temperatures, but with limited attention to the effects of the seasonal variation on the relationships between apparent permittivity and water content (e.g.

Long‐term monitoring to inform the geophysical detection of archaeological ditch anomalies in different climatic conditions

“…This section draws on the knowledge of the links between water content, temperature, soil type and geophysical properties, using them to identify the behaviour of archaeological and natural soils in the field and resulting changes to geophysical contrast. Generalized seasonal signal penetration effects in the natural soil and on GPR performance were discussed in another paper by the authors (Boddice et al, 2017) and by other authors (e.g., Curioni et al, 2017), but the current work focuses on the difference in EM properties between the archaeological and natural soils. Since only small differences existed in the ARDP‐VWC relationships between the archaeological and the natural soils on site and ARDP was not strongly affected by temperature, ARDP values provide a good proxy for water content, and differences in values can be mostly equated to differences in water movement and storage in the soil.…”

“…TDR is a technique for measuring the apparent relative dielectric permittivity (ARDP) in soils using EM signals in a coaxial transmission line, which has been widely used for experiments in geotechnical, hydrological and geophysical studies including tracking solutes (Amente et al, 2000), infiltration studies (Bachmair et al, 2009; West & Truss, 2006), contaminated land studies (Cataldo et al, 2002), monitoring the suitability of the soil for GPR (Boddice et al, 2017; Curioni et al, 2012), slope monitoring (Kim & Kim, 2007) and flood prediction (Menziani et al, 2003). The operating principles of the technique have been well described (Jones et al, 2002; Robinson et al, 2003; Topp et al, 1980) and will only be briefly described here.…”

“…For this study, monitoring stations were constructed based on the design of Curioni et al (2012). Construction, calibration and data processing are discussed in greater detail elsewhere (Boddice et al, 2012, 2017). A similar setup using the same equipment was used to take laboratory measurements that are described in another study (Thring et al, 2014).…”

“…This section draws on the knowledge of the links between water content, temperature, soil type and geophysical properties, using them to identify the behaviour of archaeological and natural soils in the field and resulting changes to geophysical contrast. Generalized seasonal signal penetration effects in the natural soil and on GPR performance were discussed in another paper by the authors (Boddice et al, 2017) and by other authors (e.g., Curioni et al, 2017), but the current work focuses on the difference in EM properties between the archaeological and natural soils.…”