Introduction

There are a number of instruments available for kinematic soil mapping by geoelectrical methods. Thus, a potential user might ask him- or herself, which is the appropriate instrument, what does it measure, and how accurate can the measurement be? What is measured is largely influenced by the underlying measuring principle. From theory, we know that the depth response curves of geoelec-trical measurements could vary depending on the layering of conductive and resistive structures (Dabas and Tabbagh, 2003). It is assumed that the galvanic contact resistivity (GCR) and the electromagnetic induction (EMI) methods react different on soil layering. Most of the instruments used for soil mapping in agriculture are based on one of these two principles. A third method, based on capacitively coupled resistivity, has rarely been used in agricultural practice up to now. Thus, we will focus on GCR and EMI instruments.

The measurement error (accuracy and precision) could depend on both the instrument's design and the measuring principle. An example for the importance of the design of an instrument is temperature compensation. All geoelectrical measurements are known to be subjected to temperature drift. A part of this drift can be suppressed by the construction of the instrument. The measuring principle can influence precision (repeatability) as follows: EMI instruments react on metal objects (e.g., pulling vehicle) and ambient electrical interference to a considerable extent (Sudduth et al., 2001), and GCR may have problems under dry soil conditions due to high contact resistivity.

The objective of this study was to compare measurements of different instruments in different soilscapes under field conditions. We want to find out how depth of investigation is influenced by soil variability, which is inherent in different soilscapes. Additionally, precision of the instruments is tested.

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