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GEOPHYSICAL SURVEY FACT FILE

SOIL RESISTIVITY TESTING


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INTRODUCTION

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 buried pipelines and other steel structures. Four equidistant electrodes are set-out in a fixed configuration; a low frequency current is applied across the two outer electrodes with the resultant voltage measured across the inner electrodes. The voltage reading is then 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. Our firm provides a variety of resistivity testing services for engineering infrastructure, new plant installation and electricity substations:



Earth rod testing using a 2-electrode resistivity system
earth rod testing

APPLICATION 1: SOIL RESISTIVITY & EARTH TESTING

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. The correct design of an earthing system is dependant upon knowledge of the local ground resistivity. This is measured as a function of depth at a series of locations around the site, using an expanding four electrode Wenner array (BS 7430). The procedure is known as soil resistivity or earth resistance testing. Correct measurement is particularly important in areas of high resistivity ground, where electrical currents do not readily dissipate. In these conditions obtaining an earth can be problematic, and information on ground resistivity is required to much greater depths for successful installation of an earthing system .

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 configured so that the system measures the resistance of the earth rod directly.






Soil resistivity testing using a 4-electrode Wenner array
soil resistivity testing

APPLICATION 2: SOIL CORROSIVITY INVESTIGATIONS

A knowledge of soil corrosivity is critical for the effective design of cathodic protection measures, or predicting the effective lifetime of a steel structure such as a tank that is to be buried within the ground. 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.







Soil redox testing on the side of a road cutting
soil redox testing


APPLICATION 3: INSITU SOIL REDOX TESTING

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 if the soil is aerobic or anaerobic, and whether 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).


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