Resistivity surveys measure the ground's capacity to pass an electrical current. This property can be utilised for designing earthing systems for substations or specialist plant, and for measuring the corrosion susceptibility of a buried pipeline or other buried steel structures. Four equidistant electrodes are set-out in a fixed configuration. A low frequency current is applied across the two outer electrodes and the resultant voltage is measured across the inner electrodes. The voltage reading is converted by standard equations into a value representing the average resistivity of the ground between the electrodes. Resistivity depth readings are acquired by electrical soundings using an expanding electrode array centred on the same point. The depth penetration of the readings is directly proportional to the spacing of the electrodes.
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 knowledge of the ground resistivity. The information is obtained by a procedure known as soil resistivity testing using an expanding 4-electrode Wenner array (BS 7430). This measures soil 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 an 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.
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, pH and redox potential (see below), 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.
An important component of soil corrosivity assessment involves the measurement of soil redox potential. This measures the ability of a soil to ionise by the transferance of electrons either to or from a reference electrode. The measurement data indicates whether the soil is aerobic or anaerobic, and if iron oxides or nitrates in the soil have been reduced or are present in oxidized form
The insitu soil redox measuring system consists of a high impedance voltmeter, a platinum test electrode and a silver or copper reference electrode. The electrodes are inserted at a set interval into the ground below root level and the magnitude and polarity of the potential between the platinum and reference electrodes is measured. Results are corrected for soil temperature and PH and convereted to a hydrogen electrode equivalent, termed the redox potential (Eh).