Fundamentals and Practice of Electrical Measurements
3.5 Resistivity Measurement
3.5.2 Measurement of Specific Soil Resistivity
There is a direct and an indirect method of measuring specific soil resistivity.
The direct method is carried out in the laboratory on a soil sample using a soil box as shown in Fig. 3-16. The resistivity of a soil specimen of cross-section, S1, and length, /, is measured and the specific resistivity determined:
Equation (3-42) is not valid for conducting systems consisting of several conduct- ing phases (e.g., steel pipeline in soil). Figure 3-15 shows an example for the mea- sured results (3).
The effects of frequency are within the accuracy of the measurement for elec- trode spacings up to 100 m and at the usual measuring frequency of 110 Hz. Two electrode resistance bridges work mostly with audio frequencies (800 to 2000 Hz) and give strongly deviating values. For measuring grounding resistances of small sections of extended installations, the most suitable is a ground meter with 25 kHz [36]. With plastic-coated casing pipes, the capacitive resistance can be smaller than the ohmic grounding resistance of holidays, which can then be better mea- sured by switching dc on and off.
for soil with fj.r = 1:
definite beats. If the amplitudes are not too large, it is even possible to make a null comparison by adjusting until the swings either side of the zero are equal. Some data on resistance measuring instruments are given in Table 3-2. All four-electrode resistivity measuring instruments can basically be used to measure grounding resistivities by connecting the two terminals, El and E2.
The grounding or penetration depth of the electrical resistance in conductors is, according to Eq. (3-42), dependent on the specific resistance and the frequency.
The penetration depth, ?, is the distance at which the field strength has fallen by 1/e; (Jir is the relative permeability [35]:
Fig. 3-15 Penetration depth, t, by ac as a function of the frequency,/. (1) Copper, (2) steel; t in millimeters; (3) pipeline of DN 200 steel; (4) and (5) in soil: t in kilometers.
tivity of less cohesive soils can be determined with approximate accuracy if the interfacial resistivity of the end surfaces is eliminated by using the four-electrode process. Current and voltage leads are separated according to the Wenner method [37]. With uniform current distribution, Eq. (3-43) also applies if / stands for the spacing of the inner electrodes [38].
The most commonly applied indirect method of measuring soil resistivity using the four-electrode arrangement of Fig. 3-14 is described further in Section 24.3.1. The measured quantities are the injected current, /, between the electrodes A and B, and the voltage, U, between the electrodes C and D. The specific soil resistivity follows from Eq. (24-41). For the usual measuring arrangement with equally spaced electrodes a - b, it follows from Eq. (24-41):
If the specific soil resistivity varies vertically with depth, t, an apparent specific resistivity can be obtained from a combination of the resistivity of the upper and When making measurements in the soil box, it has to be remembered that soil samples can change from their original condition and this will have an effect on the resistivity. Soil resistivity measurements in the soil box only give accurate results with cohesive soils. However, the order of magnitude of the specific resis-
Fig. 3-16 Soil box for determining the specific soil resistivity (dimensions in millimeters).
lower layers. Figure 3-17 shows the ratio of the apparent value, p, to the value of the upper layer, p}, in its dependence on the ratio of the soil resistivity p^/p2 for different values of the ratio alt [39]. It is recognizable that the influence of the lower soil layer is only significant when a > t. It is hardly possible to determine the true value of the lower soil layer since even with a/t = 5, only about half of the value of the soil resistivity, p2, °f me lower layer is obtained. In estimating soil resistivity, different values of a should be used at least up to the lower edge of the buried object.
The Wenner method is chiefly used to determine the grounding resistance along the pipeline track and the installation positions for cathodic structures. Local lim- ited soil resistivity is most clearly determined from the grounding resistance of an inserted Shephard rod (see Fig. 3-18). Soil stratification can be recognized from the apparent specific soil resistivity, p, by the Wenner method, if a is varied.
Since the Wenner formula [Eq. (24-41)] was deduced for hemispherical elec- trodes, measuring errors appear for spike electrodes. To avoid errors in excess of 5%, the depth of penetration must be less than a/5. Soil resistivity increases greatly under frost conditions. While electrodes can be driven through thin layers of frost, soil resistivity measurements deeper than 20 cm in frozen ground are virtually impossible.
With four-electrode measurements effected from the surface, an average soil resistivity over a larger area is obtained. The resistivity of a relatively localized layer of earth or pocket of clay can only be accurately measured by using a spike elec- trode. Figure 3-18 gives dimensions and shape factors, F0, for various electrodes.
Fig. 3-17 Apparent specific electrical soil resistivity p in the case of two
different layers of soil with the resistivities p1 down to a depth of t and p2 to a depth below t (explanation in the text).
Since the Wenner rod is mechanically somewhat delicate, it is only used in loose soils or in bore holes. For all measuring rods, the specific soil resistivity is equal to the product of the measured ac resistance and the shape factor F0, which is deter- mined empirically.
Soil resistivity measurements can be affected by uncoated metal objects in the soil. Values that are too low are occasionally obtained in built-up urban areas and in streets. Measurements parallel to a well-coated pipeline or to plastic-coated cables give no noticeable differences. With measurements in towns it is recommended, if The simplest design of a spike electrode is represented by the Shepard rod in Fig. 3-18a, which uses the right-hand electrode simply as an auxiliary ground, and measures with the left electrode the grounding resistance of the insulated stainless steel tip, which is proportional to the soil resistivity. The Columbia rod (Fig. 3-18b) uses the shaft of the rod as a counter electrode. Since the rod usually moves sideways as it is driven down, this method can easily give values that are too high. Both methods presuppose that the steel point is always in good contact with the soil; if this is not the case, too high values are obtained. To eliminate measuring errors at the electrode point, current and voltage electrodes are separated from each other on the Wenner rod as in Fig. 3-18c. Instead of Eq. (3-44), the following equation is valid for the Wenner rod because the current spreads out in all directions:
Fig. 3-18 Arrangement and dimensions of rod electrodes (dimensions in millimeters): (a) Shepard rod (FQ = 5.2 cm); (b) Columbia rod (FQ = 3.4 cm); (c) Wenner rod (FQ = 38 cm).
possible, that two measuring arrangements be selected that are at right angles to each other and when using impressed current anodes, to make the measurements with increased electrode spacings of 1.6, 2.4 and 3.2 m [40].