IEEE Std 356-2020 pdf download

IEEE Std 356-2020 pdf download.IEEE Guide for Measurements of Electromagnetic Properties of Earth Media.
While measurements are clearly influenced by subsurface variations in conductivity, rugged topography has a similar effect since the current tends to follow the surface. Shallow conductive layers also can result in misleading potential variations (Telford et al. [B44]).
When the earth is horizontally layered, the results become a function of the electrode spacing a. Information about the conductivities of the two layers and the depth of the upper layer can be determined by multiple measurements using different spacings.
Automated inversion techniques for field survey results based on these models have been under investigation. For example, least squared error minimization techniques combined with a forward modeling numerical technique have been investigated by Merrick et al. [B38j and Shima [B43]. For in-field use, a simple calculator-based method is available (Merrick [B37]).
2.2 Field methodology
Both dc and ac (less than 60 Hz) current sources are used (Telford et al. [B44]). If the source is dc, the polarity should be reversed periodically to reduce the effect of electrolytic polarization (Keller and Frischknecht [B35]). This reversal is implemented a number of times each second. While a dc source clearly yields dc resistivity, the spontaneous potentials (SP) in the earth (the basis of another method of geophysical mapping) can significantly alter the results. This bias effect can be minimized by simply noting the potential before the current source is applied, and subtracting this value from the measured results. The ac technique eliminates the SP effect and allows the use of narrowband amplifiers tuned to the source frequency to improve the signal-to-noise ratio. The measured resistivity is usually lower than the true dc value and there can be interference through inductive coupling from the current supply leads.
The ac electrodes may be metallic conductors (e.g., steel, brass, aluminum, stainless steel). For dc currents, porous-pot electrodes filled with electrolyte are used (Keller and Frischknecht jB35], Lu and Macnae [B36]). The electrodes should penetrate the ground by at least 10 cm to enable adequate contact. The feed cables for both the current source and the potential measurements should be insulated from the ground.
Figure 4 represents a number of the common configurations used for resistivity mapping. An increase in the spacing between the electrodes results in a deeper effective area of measurement. The expressions given in Table 1 for the apparent conductivity u assume a uniform earth half-space, where I and V are the injected current and measured voltage, respectively. It is clear from these expressions that the spacing between the electrodes a is the most important parameter in determining the volume of earth to be associated with a resulting apparent resistivity datum.
The probe resistance can cause significant problems. While the four-electrode array is independent of the probe resistance, if this is too high, then insufficient current is injected into the earth. This results in poor quality voltage measurements and an associated loss in the maximum probing depth. Wait [B481 modeled the contact points as hemispherical electrodes coated with a layer of insulation to investigate the influence of contact resistance analytically.
In highly resistive earth, the technique can still be used eftèctively. In the Antarctic (Bentley [B31]), the current was provided by an 810 V voltage source and the voltmeter used had an input impedance of 1014 2.
Marine resistivity surveys have been successful in determining the resistivity of the sea floor and the layered structure beneath (Scott and Maxwell [B42]). In one example, a dipole-dipole array of nine electrodes is floated in a regularly spaced line behind a vessel. The first two inject current, and the following seven are non-polarizing potential difference electrodes. Six voltage measurements are made, and the apparent conductivity for the channel is calculated. These data are then inverted to give a horizontally layered interpretation that includes the resistivity of each layer and its depth.