Minimizing Drift in Electrical Conductivity Measurements in High Temperature Environments using the EM-38

considered for use in agriculture by De Jong et al. (1979). Since then the technique has been used to map The EM-38 is a noninvasive instrument, commonly used for monia variety of physical quantities with which ECa correlates toring salinity, mapping bulk soil properties, and evaluating soil nutri(e.g., salinity, moisture, and clay content). Water conent status. Users in the Southwest USA have observed as much as 20% “drift” in the measurement of bulk soil electrical conductivity tent has been estimated from measurements of ECa by (ECa) with this instrument. This drift has usually been ignored or Kachanoski et al. (1988) and Sheets and Hendrickx (1995), compensated for by statistical procedures. We performed laboratory salinity by a number of authors (Corwin and Rhoades, and field experiments to determine if the drift is due to calibration 1982; Wollenhaupt et al., 1986; Hendrickx et al., 1992; instability of the instrument or to heating of the instrument by the Rhoades, 1993; Lesch et al., 1995a, 1995b; Rhoades et sun. In laboratory experiments, after a warm-up period, the instrument al., 1999), and inferring differences in mineralogy by provided constant readings in the range 25 to 40 C; above 40 C the Triantafilis et al. (2000). Increasingly, applications are response of the instrument was unpredictable. In field experiments, being identified in precision agriculture for determining where we placed the EM-38 in a fixed location we observed an unexnutrient status and potential yield (Corwin and Lesch, pected response at air temperatures below 40 C. Temperature sensors 2003; Corwin et al., 2003). in different locations on the instrument demonstrated that temperature differences between the instrument’s transmitting and receiving The EM-38 has been adapted for general mapping in coils and the control panel (CP) were as great as 20 C. As the instruagriculture, an example is the Lower Colorado Region ment is temperature compensated from this CP, erroneous compensaSalinity Assessment Program. This is a network of peotion occurred when the instrument was placed in direct sunlight. In ple and organizations that are committed to improving this study, we demonstrate that differential heating of the EM-38 is one the assessment of soil salinity in agricultural fields in cause of drift and erroneous bulk electrical conductivity measurement; the Southern Colorado region to guide management shading the instrument substantially reduced this problem, effectively decisions (http://www.ussl.ars.usda.gov/lcrsan/LCRhome. extending the reliable working temperature range by minimizing drift. htm; verified 7 Oct. 2003). Soil mapping survey units consisting of converted spray rigs, mounted with dual dipole EM-38 units and GPS, have been used to map T concept of using induced electromagnetic fields agricultural fields (Rhoades, 1993; Lesch et al., 1995a, to measure ground conductivity has been applied 1995b; Triantafillis et al., 2002). Data has been analyzed in the geosciences for more than 50 yr (Belluigi, 1948; using ESAP computer software to produce maps and Wait 1954, 1955, 1982). Induction methods were used exstatistical sampling plans (http://www.ussl.ars.usda.gov/ tensively for ore prospecting as metallic ore bodies can MODELS/esap-95.htm; verified 7 Oct. 2003). As this have substantial electrical conductivity (Keller and network of users has developed, large amounts of data Frischknecht, 1966). They were also used for well loghave been collected and some anomalous results have ging in the petroleum exploration industry (Keller and been observed. Frischknecht, 1966). Noninvasive instruments were first The term drift has been used to describe disparate values in EM-38 data, collected at different times from D.A. Robinson, I. Lebron, S.M. Lesch, and P. Shouse, USDA-ARS, the same location, that cannot be accounted for by George E. Brown, Jr. Salinity Lab., 450 W. Big Springs Road, changes in water content or soil temperature. The causes Riverside, CA 92507. Received 26 Feb. 2003. *Corresponding author ([email protected]). Abbreviations: CP, control panel; ECa, bulk soil electrical conductivPublished in Soil Sci. Soc. Am. J. 68:339–345 (2004).  Soil Science Society of America ity; Hi, induced magnetic field; Hp, primary magnetic field; Rx, receiving coil; Tx, transmitting coil. 677 S. Segoe Rd., Madison, WI 53711 USA