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Methods: Electrical Resistivity

    The DC resistivity method is commonly used for site investigations where the following information is needed:
    WHAT IT MEASURES

    The electrical resistivity of a material is a measure of the ability of that material to transmit an electrical current. In the electrical resistivity method a DC circuit is established in the ground via cables and electrodes, and the ground acts as the resistor to complete the circuit. There are several different arrays that can be used to collect the data; however, the most common are Wenner, Schlumberger and dipole-dipole. Electrical resistivity data are typically displayed in 2D sections or profiles where they supply lateral and vertical electrical resistivity information about materials either directly below a given transect (much like a road cut); or between two boreholes in the case of ERT.


    The electrical resistivity method had its beginnings in the mining industry, but is now commonly used in the environmental and engineering businesses. Because the electrical resistivity of a material correlates well with grain size (and generally increases with increasing grain size) this method can be used not only to identify lateral and vertical boundaries between different materials but also to identify the lithology of the material. The electrical resistivity method is also sensitive to the chemistry of a material, and therefore can be used for many groundwater applications such as the delineation of saltwater and contaminant plumes (DNAPL and LNAPL), delineation of zones of weathered or fractured rock in competent rock, and delineation of degree of saturation of permeable materials. Because boundaries between electrical layers often correlate well with geologic contacts, the electrical resistivity method has many applications.


    The electrical resistivity of a material is a measure of the ease with which an electrical current can flow through that material. Since most minerals are insulators, electrical current flow through sedimentary soils and rocks is primarily electrolytic and takes place through pore spaces, along grain boundaries and through fractures. As a result, permeable materials (such as coarse sands or sandstones) are less resistive (or more conductive) when saturated than when dry. In addition, because ionic conduction is enhanced by the presence of dissolved salts in the pore fluid, soils and rocks saturated with saline or high-TDS groundwater will have significantly lower levels of resistivity than soils and rocks bearing fresh water. Degree of sorting is another important factor that influences electrical resistivity: for example, a well-sorted saturated sand or sandstone has many large void spaces between grains where water can flow, yielding a lower resistivity when saturated than a poorly sorted sand or sandstone. As mentioned above, an increase in grain size generally causes an increase in resistivity (e.g. coarse-grained materials such as gravel or cobbles have higher resistivity values than finer grained materials such as fine sands and silts). Clays are not permeable; however, the presence of clay minerals in a material will decrease the resistivity of that material for two reasons: 1) because clay minerals can combine with water and 2) because clay minerals tend to ionize and contribute to the supply of free ions in a material, thereby providing another path for current to travel along. Current flow through metamorphic or igneous rocks occurs mainly through fractures and along grain boundaries. As a consequence, unweathered and unfractured metamorphic and igneous rocks generally yield very high resistivity values.

    HOW IT WORKS

    Spectrum uses the Advanced Geosciences SuperSting R8/IP resistivity meter and associated cabling to acquire electrical resistivity data. The SuperSting is a system that allows automated acquisition of electrical resistivity data. Because it is automated it is quite efficient and relatively easy to use in the field. In practice, a linear array of electrodes is established in the ground and connected to the SuperSting meter. Once this is done, a known amount of current is introduced into the ground through a pair of electrodes (current electrodes). This current then travels through the ground and the electrical potential is commonly measured by two other electrodes (potential electrodes) some distance from the current electrodes, where the larger the separation between the current and potential electrodes the greater the depth of the measurement. Ohm’s Law (V=IR) is then used to calculate the apparent resistivity of the ground through which the current has traveled. During a SuperSting survey, many apparent resistivity measurements are made for a suite of electrode pair separations, and these apparent resistivity values are plotted on a two-dimensional section, where the surface location of the measurement is plotted versus the depth of the measurement. The automated resistivity data acquisition provided by the SuperSting allows for a tremendous amount of data to be acquired relatively quickly with very high lateral and vertical resolution, resulting in a 2D subsurface image representing the lateral and vertical variation of apparent resistivity along the line. Once the data have been acquired for a given transect, they can be downloaded to a field computer and subsequently viewed, color-contoured, and processed.


    Spectrum uses the EarthImager 2D software package (Advanced Geosciences, 2009) to interpret the data acquired with the SuperSting. Once the data are acquired, they are entered into EarthImager 2D, edited where necessary and merged with elevation information. Once this has been done, an inversion routine is applied to the data using a non-linear least squares algorithm. The final EarthImager product is a color-contoured model resistivity section that can be interpreted for subsurface geologic features of interest. This model section is then interpreted for features of interest.


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