A simple field calibration procedure for EM38 units when undertaking multi-temporal surveys

Abstract

Multi-temporal electromagnetic induction (EMI e.g. EM38) surveys of fields may provide a greater insight into soil water dynamics, and when deployed as such may better inform irrigation or fertiliser management with a view to improving water use efficiency. Measuring absolute soil moisture content and subsequent changes over time requires detection of apparent electrical conductivity (ECa) changes as small as a few mS/m, and this requires an accuracy in instrument calibration and performance that is not achievable by relying solely on 'standard' instrument nulling and calibration procedures. In-field calibration has been reported by other workers as a major factor resulting in high levels of divergence from theoretically modelled responses especially at low conductivity ranges and significant instrument drifts have also been observed in this present work. To this end, a simple and inexpensive calibration rig and procedure has been developed to monitor the gain and offset of EM38 sensors in response to a constant q-coil at the beginning and end of each survey. The 'response curve' can be used to derive instrument-specific temperature correction algorithms (assuming the case temperature is monitored and logged during subsequent surveys) and also be used as the basis of a conversion algorithm to correct multi-temporal survey data to a common instrument performance characteristic.