Principle The Geonics EM-38 is a small, light weight electromagnetic induction instrument which has found applications in archaeology, agriculture, soil contamination, and studies to a depth of 1.5metres         The instrument itself houses a transmitter and a receiver which are placed about 1m apart in a non conductive casing. Inside the transmitter an alternating current (a/c) is applied to a copper coil which induces an electromagnetic wave, known as the primary magnetic field. These electromagnetic waves are sent out perpendicular to the coil direction and into the media being studied. Looking at a typical field situation, when this magnetic field comes into contact with the conductive material within the soil, a current in the soil matrix is created. With the creation of this current, new eddy currents are generated which give rise to a secondary magnetic field. The size of the secondary magnetic field is recorded by the receiver. The electrical conductivity (EC) of the site being measured is the ratio of the two magnetic fields, and the units are millisiemens per metre. EC = Secondary magnetic field (mS/m) Primary magnetic field The magnitude of the secondary magnetic field is contributed to by all the conductive materials in the soil, which include; 1.) amount of negative charges on the clay particles; 2.) clay content 3.) salt concentration in the soil solution 4.) soil moisture content The higher the size of the secondary magnetic field, higher the electrical conductivity of the medium. Once you have your reading it would take a more detailed analysis of the site to determine how the various conductive materials in the soil were contributing to the EC value.
Method The EM-38 has an effective range of 1.5m. We placed the instrument in the upright position on the soil surface and recorded the reading as the vertical reading for zero centimetres above the ground. The Em-38 was then placed lying on the soil surface to get the horizontal reading for zero centimetres above the soil surface. Vertical and horizontal readings were then taken at 20cm, 40cm, 60cm, 80cm, and 100cm above the soil surface. As the EM-38 is raised higher above the soil surface we are taking into account its effective range of 1.5m hoping to record the different values of electrical conductivity at different depths in the soil profile.

Analysis of Data

The EM data consists of measurements of electrical conductivity taken at six heights above the soil surface at each sampling point (viz. 0cm, 20cm, 40cm, 60cm, 80cm and 100cm). At each height, measurements were taken with the EM38 instrument in vertical and horizontal orientations. The data analysed here are the 0cm vertical orientation data.

These data were analysed by one-way analysis of variance against landuse (viz. pasture or cultivated ground). This analysis yielded a mean electrical conductivity for each landuse type. The residuals at each sampling point were calculated by subtracting the appropriate mean value from the measured value at each point. This set of residuals were then analysed by kriging using the Vesper (ACPA 2003) software package. A grid was created for the survey area and an exponential variogram was fitted. The kriging produced kriged residual predictions at one?metre intervals over the grid area.

The kriged residuals were then added to the ANOVA means, which were weighted for landuse type at each grid point in the following way. Sampling points were designated with an indicator variable of 0 for cultivated ground or 1 for pasture. The indicator variables were then kriged using the same settings as for the EM residuals, producing probability predictions for landuse at each grid point. The overall mean-kriging prediction of electrical conductivity was then obtained from the following formula:

EM prediction = Kriged residual + (Landuse probability x Pasture mean) + [(1 - Landuse probability) x Cultivated mean)]

These predictions were then displayed on a contour plot using the Minitab statistical package.

Results

The EM38 was used to measure the moisture content of the soil of both pasture and cultivated land, at the site near Narrabri. 110 measurements were randomly chosen over the area during the 5 day period. These figures were analysed using the programs JMP5.1, Minitab and Vesper1.6. From the analysis the following contour map of the study area was created.

Knowing that the soil at Narrabri has a very high clay content, we know the clay content alone, will be a major contributor of the readings taken from the EM38. However, the main goal of using the EM38 for these measurements is initially to find the electricial conductivity which, will be used to determine through analysis, the moisture content of the soil. From the contour map generated, it can be seen that there is a distinct boundary between the pasture and cultivated areas. These results were expected, as the cultivated field being in a fallow state has the characteristic of high water retention and low water loss. As compared to the pasture areas, which have a complete cover of vegetation, the consumption of water is high as a consequence of necessary water uptake. This is seen in the contour map where in the pasture, lower electrical conductivity results were obtained in areas where there is a complete cover of vegetation. The majority of the pasture area had a well established cover of grasses, and a smaller section between the road and cultivated area was inhabited by a less developed cover of vegetation. The remainding area was bare fallow which explains the high values of electrical conductivity obtained, and therefore a higher moisture content.

Problems with method

The EM38, although small, light, and convenient to use, is subject to electronic drift when extreme temperature changes occur during hot weather. It is also sensitive to irrelevant environmental and industrial electromagnetic noise like lightning and power lines. However the major disadvantage with the instrument is that EM technology responds to a wide range of metals, both ferrous and non-ferrous (Berle, 2001). The presence of any near-surface metal can affect the readings taken from the instrument. Metals that may cause interference with the EM38 measurements include:

• Any conducting material worn by the operator
• Presence of metal piping
• Presence of buried metal material
  

References

R. Berle. 2001. Working with the EM38 Earth Conductivity Meter: Geophysical Survey at the Hopeton Earthwork, Chillicothe, Ohio. Clay Cultural Resource Analysts, Inc. Lexington

Triantafilis J. ,Ahmed M.F. , Odeh I.O.A. 2002. Application of a mobile electromagnetic sensing system (MESS) to assess cause and management of soil salinization in an irrigated cotton-growing field

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