An efficient calibration procedure for correction of drift in EMI survey data

• Effects of drift on the quadrature and in-phase responses are researched.
• Need for a versatile drift compensation is demonstrated.
• A drift correction, using a calibration line crossing the survey area, is proposed.
• The procedure is shown to be suitable for various survey setups and drift effects.

Frequency domain electromagnetic induction (EMI) sensors with a small coil separation are used for near-surface surveys in a variety of domains. Regardless of the application, the instrument response(s) may suffer from a drift, meaning that a response at one given location may vary over time, despite no appreciable changes above or underneath the surface. Drift is unwanted as it may introduce global trends or abrupt changes in EMI data not related to the underground. In this paper, the effects of drift on the quadrature and in-phase responses of a ground-based system are researched by evaluating several multi-receiver EMI datasets. First, a stationary recording illustrates the need for a versatile drift compensation. For area surveys we propose an efficient drift correction procedure. To start, a calibration line that crosses the entire survey area within a short time frame is recorded. An approach to account for spatial offset between sensor midpoint and global navigation satellite system antenna is also detailed given the bearing it has on accurate localization. The residuals of coincident calibration and survey data can then be used to model and subtract the drift from the sensor data. This is performed by applying outlier detection and removal, followed by curve fitting of the comparison data. The developed procedure allows a near continuous evaluation of drift, without the need for ancillary data and is time efficient. The approach is shown to be suitable for various survey setups and drift effects.