Ground resistivity surveys measure the ground's capacity for passing electrical currents. Factors such as soil composition, moisture content, pore water chemistry and pH control ground resistivity, which is the main diagnostic for the corrosion susceptibility of buried steel structures. Generally low values of ground resistivity correspond to high values of soil corrosivity and vice-versa. A knowledge of soil corrosivity is critical for the effective design of Cathodic Protection measures or predicting the lifetime of a buried steel structure.
Standard resistivity surveys use four equi-spaced electrodes 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 resistivity value. This represents the average ground resistivity between the electrodes, with depth range proportional to electrode spacing.
The resistivity method can be customised for measuring soil corrosivity around buried steel structures such as pipelines. Specific sections of a proposed pipeline are targeted, in order to assess the corrosivity 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 taken near water courses or other crossings, where the pipeline is embedded at greater depth. The resultant data is used for the design of effective Cathodic Protection measures.