Development of a non-invasive modular electromagnetic induction (EMI) system with spatial high resolution for agricultural applications

The acquisition of high-resolution soil information is essential for more environmentally friendly and efficient management of agricultural areas in the context of precision farming. The electrical conductivity (EC) of the soil can be measured quickly and without direct contact using electromagnetic induction (EMI) systems. The EC can be related to soil properties such as soil water content, pore water electrical conductivity, nutrition, clay content and salinity. EMI devices provide an apparent conductivity value that averages electrical conductivity variations with depth. To reconstruct the depth-dependent conductivity from measured data, EMI devices with different coil separations between transmitter and receiver or coil orientations are required. For the measurement with different coil separations, measurements with several commercial devices are commonly combined. However, mutual interference between devices is problematic here, so that measurements with the individual devices must be carried out either one after the other or with sufficient spatial separation, which complicates data acquisition substantially. To simplify EMI data acquisition and to improve depth resolution, an EMI device is required that provides simultaneous measurements with a larger number of freely selectable coil distances. To achieve this, a modular scalable multi-coil system (SELMA) with one transmitter and 12 receiver coils was developed. In the first test configuration, the receiver coils are arranged in a coplanar configuration and equally distributed from 0.3 to 3.6 m in a straight line. The system currently operates at a transmission frequency of 20 kHz and is designed for a measurement range from 2 mS/m to 100 mS/m. The noise of the measured apparent electrical conductivity is below 1 mS/m at a measurement rate of 10 Hz. To achieve modularity, decentralised System-on-Chip modules are used for the data acquisition, which are connected to the control unit (PC) via Ethernet. In addition to the apparent conductivity values, temperatures, pressure, and acceleration are recorded. The reliability of the EMI measurements was checked by repeatedly measuring a transect using a custom-made sled.