Combining Proximal and Penetrating Soil Electrical Conductivity Sensors for High-Resolution Digital Soil Mapping

Abstract

Proximal ground conductivity sensors produce high spatial resolution maps that integrate the bulk electrical conductivity (EC_a) of the soil profile. For meaningful interpretation, variability in conductivity maps must either be inverted to profile conductivity or be directly calibrated to profile properties. Penetrating apparent electrical conductivity (EC_a–P) sensors produce high depth resolution data at relatively fewer spatial locations. The objectives of this research were to (i) investigate the profile source of EC_a in claypan soils via a detailed examination of EC_a–P profiles; (ii) examine the potential for feature detection with EC_a–P in claypan soils; and (iii) determine if EC_a sensors can be calibrated to EC_a–P features. Two study areas were chosen representing the claypan soils of north-east Missouri, USA. Profile conductivity was measured at high depth resolution on soil cores using a miniaturised Wenner conductivity probe and in the field using a conductivity penetrometer. Proximal ground conductivity was mapped with one direct contact sensor and two non-contact sensors, providing five distinct coil/electrode geometries. Increasing EC_a–P was observed below the claypan, correlated with decreasing clay and water content and increasing bulk density. Depth to the claypan was successfully calibrated to derivative peaks on EC_a–P profiles (R ² = 0.72, p < 0.001). Relationships between EC_a and EC_a–P features were poor (R ²∼ 0.21) to good (R ²∼ 0.87) on a field-specific basis. Results show that EC_a–P can be used for calibration of EC_a to the depth to claypan.