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NORCAL <br /> t <br /> Appendix A <br /> GEOPHYSICAL METHODS <br /> Vertical Magnetic Gradiometer (VMG) <br /> A magnetic gradiometer measures the vertical gradient of the earth's magnetic field. It <br /> consists of two total field magnetic sensors separated vertically by one-half meter. The <br /> magnetic field strength is measured simultaneously at both of these sensors. The difference <br /> in magnetic intensity between these measurements is proportional to the vertical gradient of <br /> the earth's magnetic field. Because the vertical gradient is constant with respect to time, the <br /> effect of diurnal variations is eliminated. Since a gradiometer is effected less by cultural <br /> features, it provides higher sensitivity and better resolution of near surface sources than total <br /> field magnetometers. Areas with significant amounts of buried metal typically produce <br /> 1 anomalously steep magnetic gradients. Because the gradiometer is sensitive to ferrous metal <br /> sources both above and below ground, site and vicinity surface conditions can affect survey <br /> results. <br /> We used an SCINTREX ENVI-MAP magnetometer to obtain the vertical magnetic gradient <br /> data. The instrument features a built-in memory that stores the vertical magnetic gradient and <br /> ' survey grid information. The information can be down loaded to a computer for further <br /> processing. <br /> ' Ground Penetrating Radar (GPR) <br /> Ground penetrating radar is a method that provides a continuous, high resolution cross-section <br /> ' depicting variations in the electrical properties of the shallow subsurface. The method is <br /> particularly sensitive to variations in electrical conductivity and electrical permittivity (the <br /> ability of a material to hold a charge when an electrical field is applied). <br /> The system operates by continuously radiating an electromagnetic pulse into the ground from <br /> a transducer (antenna) as it is moved along a traverse. Since most earth materials are <br /> ' transparent to electromagnetic energy, only a portion of the radar signal is reflected back to <br /> the surface from interfaces representing variations in electrical properties. When the signal <br /> encounters a metal object, however, all of the incident energy is reflected. The reflected <br /> signals are received by the same transducer and are printed in cross-section form on a <br /> ' graphical recorder. Depending upon depth and/or thickness the resulting records can provide <br /> information regarding the location of UST's, underground utilities, and variations in the shallow <br /> site materials. Generally, electrically conductive materials, such as clay, saturated silt, and <br /> rebar can reduce the penetration capability and limit radar performance. <br /> 1 <br /> 1 <br />