GEORES  GEOPHYSICAL RESISTIVITY INSTRUMENT

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Measuring Soil Resistivity

Uses of Resistivity Measurements

With intelligent and experienced assessment of the results of resistivity surveys, it can be used to detect unnatural irregularities in the soil; concentration of mineral deposits, buried masses, caves, etc. Various types of soil have different resistivities and areas with such differences or displaying a sudden change in resistivity can be shown up in a survey. For example, where there is a stratum of sand or rock below a top soil of loam. Also, such techniques have been used successfully in locating archaeological remains by showing sudden changes in resistivity in the vicinity of buried objects and thereby indicating their position. It is possible with experience and knowledge to deduce from results of a resistivity survey, the nature of the mineral deposit or buried object. The use of resistivity measurements can lead to a reduction in the number of boreholes that would otherwise have to be made in order to adequately determine underground conditions, etc.

Factors Affecting Resistivity

Various conditions of the soil causes differing resistivities and the differences found between them are quite considerable. Dry soil is generally speaking, a good insulator, and only becomes a conductor when water is added to it. Water can dramatically reduce (with a few exceptions) the resistivity of the soil. But water too, in its pure, demineralized state, is also a good insulator and not until salts are dissolved in it does it become a conductor (conduction in soil is largely electrolytic in nature). So to a considerable extent the resistivity of the soil is a measure of the resistivity of the water held in the soil which in turn is dependent upon the amount of salts dissolved in the water. By salts, is meant more than just common sodium chloride, it encompasses all the naturally occuring chemicals in the soil.

The amount of water held in the soil is governed by the weather conditions, time of the year, nature of the sub-soil, etc. To a certain degree the temperature of the soil has an effect upon resistivity, lower temperatures causing higher resistivities, therefore, climatic changes have a bearing on this subject. Over the course of a year, resistivity changes can be quite marked especially for layers of the soil near to the surface. When water is frozen, its resistance increases so it might be expected that when soil freezes in severe winter weather its resistivity increases. Below the frost penetration level the effects of temperature on resistivity are almost negligible.

Fluctuations occur also in very wet seasons, natural salts may be washed out by heavy rainfall. Likewise, the rising and falling of the natural water table level will considerably effect the resistivity at the particular depth where the table normally occurs. Below the water table level, soil resistivities are generally far more constant that they are above this level, where the seasonal factors take effect. Because of the changes in the moisture content of the soil near the surface, it may well be considered adviseable to sink earth electrodes deeper to gain the advantage of more stable resistivity conditions.

The resistivity is not a constant parameter, but varies from area to area; with the weather conditions and with the type and nature of the soil. It changes for different depths below the surface and for different strata formations in the ground. It cannot be assumed that once measured the resistivity will remain constant. The actual resistivity must be obtained by measurement at any one time because of all these effects.

The Wenner Principle of Resistivity Measurement

Several methods of earth resistivity measurement have been devised, but many of them are variations of the principle developed by Dr. F. Wenner of the United States Bureau of Standards. The basis of this principle is given below (and in how to use link of this website) with the application when Geores Geophysical Resistivity Instrument is used as measuring instrument. The method is sometimes known as the "four electrodes method" because four electrodes (spikes) are inserted into the ground and connected to the terminals of the instrument.

The four spikes are equally spaced in a straight line and driven into the ground such that the depth of insertion is 1/20 of the distance between the spikes. Current is passed through the earth via the two outer spikes, which are connected to the current terminals C1 and C2 of the instrument. The voltage appearing between inner spikes connected to the potential terminals P1 and P2 of the instrument as a result of the current flowing, is measured.

The Wenner method gives the resistivity to a depth equal to the spike seperation distance "a". In this way different soil structures, strata or buried objects and caves could be identified. Performing depth surveys in this manner over any particular area enables the contour of an underground stratum to be found.

Since the resistivity is obtained to a depth equal to the seperation distance between the spikes, by the Wenner principle, then if the seperation were too small, the buried object might not show up because there would be little change in resistivity. Similarly, if the seperation distance is too large, small objects might be missed for the same reason. So, it is necessary to estimate the depth required at the start of the resistivity survey when looking for buried objects. Usualy a survey is conducted for a depth 50% greater than the suspected depth of the object in question.

Depth Surveys

The point at which the depth survey is to be carried out is chosen. The Wenner principle is again used but this time the test spike spacing is not maintained constant at "a". Test spikes are hammered into the ground with this point as the mid-position of the system. Different spacings are used yielding resistivities at depths equivalent to those spacings. The changes in soil consistency, the next mineral strata or the buried object is at a depth equal to the test spike seperation distance when a sudden change in resistivity occurs.

Practical Considerations

Surveying does take time. Take great care when testing so that each individual test is reliable and accurate. Take care to connect the leads securely. Take care to connect the leads correctly, errors of 50% can result by wrong connections. For work at shallow depths, a frame to hold test spikes firm, so that they can be pushed into the ground at the correct spacing and penetration, will be found useful. When it is not possible to drive test spikes to the required depth water the area with a saline solution.

 

 

 

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