Ground resistivity surveys measure the capacity of the ground to pass an electrical current. This property can be used for measuring the corrosion susceptibility of a buried pipeline, or designing the installation of an earthing system. Standard resistivity surveys use four equidistant electrodes spaced in a fixed configuration. A low frequency current is applied across the outer two electrodes and the resultant voltage is measured across the inner electrodes. The resultant voltage reading is converted by standard equations into a resistivity value, representing the average resistivity of the ground between the electrodes. To take resistivity depth readings, electrical soundings are taken using an expanding electrode array centred on the same point. The depth penetration of the readings is directly proportional to the electrode spacing.
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Earthing systems provide a safe connection between an electrical circuit and the ground. They are used for the dissipation of electrical faults, grounding lightning strikes and maintaining the correct operation of electrical equipment. Proper design of an earthing system requires a knowledge of the ground resistance. The information is obtained by resistivity testing using an expanding 4-electrode array (BS 7430), which measures resistivity as a function of depth at a series of locations. Earth testing is particularly important in areas of high resistivity ground where currents do not readily dissipate. In these conditions obtaining an earth is problematic, and information on ground resistivity is required to much greater depths.
The resistance of an installed earth rod can also be tested to verify the earthing system. Using a specially adapted testing procedure, the earth rod is connected with a two-electrode resistivity system (IEEE 81 1983). The electrodes are so configured that the system measures the resistance of the earth rod directly.
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A knowledge of soil corrosivity is critical for the effective design of cathodic protection measures, or predicting the lifetime of a buried steel structure. Factors such as soil composition, moisture content, pore water chemistry and pH control the ground resistivity, which is the main diagnostic factor.
Using a four-electrode resistivity system (BS 1377), specific sections of a proposed pipeline route are targeted to measure the resistivity of each lithology encountered and investigate special situations such as fault zones or infilled channels. Surveys are designed to take readings down to pipeline depth. Deeper readings are required near water courses and other cross-overs, where the pipeline has to be embedded at greater depths. The resultant resistivity data is converted into corrosivity factors and integrated into the design of effective cathodic protection measures.