Calibration and Field Measurements of a Scalable Electromagnetic Induction System (SELMA-RB) for Agricultural Applications

In precision farming, more and more methods are being developed and used for efficient and environmentally friendly farming of agricultural land. Technical solutions for rapid mapping of soil parameters help to enable more efficient field cultivation. Non-invasive methods, such as electromagnetic induction (EMI), are advantageous for fast mapping. These systems measure the electrical conductivity of the soil and enable the determination of various soil parameters (e.g. soil stratification, water content, fertilizer concentration). 
For a depth-resolving measurement, which requires a large number of different coil separations and orientations, multiple surveys with different commercial EMI devices are usually necessary. To simplify the application in the field, a modular EMI system was developed that enables simultaneous measurements with flexible coil spacing.
A temperature drift correction and a model-based offset calibration were carried out as part of the measurement data pre-processing. Two approaches for calibrating the offset were tested.
In the first approach, the EMI device was positioned over a pool of water at different heights, with the water modeled as a homogeneous layer to calculate the offset. In the second approach, the system was calibrated by placing it at different heights above a natural soil of an agricultural area.
To evaluate the quality of the EMI measurements, the apparent soil conductivity was mapped with the SELMA-RB system (twelve separations) and a commercial CMD measuring system (six separations, CMD Mini Explorer) on a test field (230 m x 160 m) near Jülich, Germany. The field was measured within approximately one hour by pulling the device with an ATV at 6-8 km/h with 4 m line spacing. A comparison of the conductivity maps and the calibration data are presented.