CORRESPONDENCE_2005-2006 - Laserfiche WebLink

Home Browse Search

• ' ELECTRICAL RESISTIVITY SURVEYS ' Rational Electrical resistivity is the physical property of a material that resists the flow of electrical current. ' The electrical resistivity of earth materials is directly affected by moisture content and permeability. Typically, electrical resistivity decreases as permeability and moisture content increases. The ' resistivity of earth materials is also greatly effected by the concentration of dissolved salts or free ions in the saturating fluid. Generally, fine-grained materials such as clays have a lower electrical resistivity than coarse grained materials such as sands and gravels. The presence of fluids that have ' a high concentration of dissolved salts or free ions can significantly decrease the electrical resistivity of both fine and coarse-grained materials. Electrical properties of rock can vary greatly depending upon degree of weathering and fracturing, as well as composition. Rock formations that are deeply buried and not exposed to chemical weathering are generally impermeable, contain little water, and have a relatively high electrical ' resistivity. Conversely, highly weathered and fractured rock that contains moisture typically has lower resistivity. Variations in the electrical properties may also be caused by changes in subsurface materials due to landfill debris. Typically, residential trash and construction debris, such as wood ' and concrete, will increase electrical resistivities. Based on the above relationships, geophysical methods that measure the electrical resistivity of the ' subsurface can be used to determine the depth and/or lateral extent of possible landfill material. Methodology The electrical resistivity of the subsurface is measured using a galvanic resistivity method. This ' consists of transmitting electrical current into the earth through a pair of grounded metal electrodes, and measuring the resulting potential drop across the second pair of grounded metal electrodes. There are a variety of electrode arrangements(arrays)that can be used. The dipole-dipole electrode ' configuration is typically used because it provides information on both the depth and lateral extent of subsurface electrical properties. The dipole-dipole array consists of four electrodes that are placed in the ground in a collinear arrangement. One pair of adjacent electrodes is used to transmit current into the earth and is referred to as the current dipole.The second pair of electrodes is used to measure the resulting potential drop, ' and is referred to as the potential dipole. Both dipoles have the same length. To begin a profile,a reading is taken with the dipoles separated by their common length.Subsequent readings are taken as the potential dipole is moved along the profile while the current dipole remains stationary. The separation between dipoles is always a multiple of the dipole length. As the ' A-4