Effect of Aluminum Neutron Probe Access Tubes on the Apparent Electrical Conductivity Recorded by an Electromagnetic Soil Survey Sensor

Correlating soil moisture content to apparent electrical conductivity (σa), derived from above-ground, electromagnetic induction (EMI) dipole sensors, requires capacitance or neutron probe moisture meters. To this end, plastic or metallic access tubes (ca. 0.5-2 m long, 40-50 mm internal diameter, and 1-2 mm thickness walls) are inserted vertically into the soil to allow the probe to be lowered for moisture readings at a series of soil depths. The impact of these tubes on measurements derived from above-ground EMI sensors, when the sensor is in proximity or adjacent to these buried tubes, in unknown. We report on the impact of widely used aluminum (Al), as well as popular plastic alternatives of polyethylene (PE), polyvinylchloride (PVC) access tubes on the lateral σa profiles of an EM38 EMI meter as it is moved along survey transects that pass beside the access tubes. There was no significant difference observed between the EMI meter readings of the bare soil and the vertical holes created to house the access tubes nor when the plastic access tubes were in place. However, the Al tubes showed a considerable variation in readings once the EM38 meter was within 50 cm of the tube location. A theoretical model, based on a single dipole transmitter and receiver coil, and a thin, cylindrical conducting shell located beneath the earth' s surface confirmed the horizontal eddy currents, traveling around the tube shell to be responsible for the observed deviation in the sensor response when in proximity to the metallic tube.