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• NORCAL <br /> Pacific Environmental Group, Inc <br /> July 10, 1996 <br /> Page 2 <br /> Vertical Magnetic Gradiometer <br /> A magnetic gradiometer measures the vertical gradient of the earth's magnetic field <br /> It consists of two total field magnetic sensors separated vertically by one-half meter <br /> The magnetic field strength is measured simultaneously at both of these sensors The <br /> difference in magnetic intensity between these measurements is proportional to the <br /> vertical gradient of the earth's magnetic field Because the vertical gradient is <br /> constant with respect to time, the effect of diurnal variations is eliminated Since a <br /> gradiometer is effected less by cultural features, it provides higher sensitivity and <br /> better resolution of near surface sources than total field magnetometers Areas with <br /> significant amounts of buried metal typically produce anomalously steep magnetic <br /> gradients Because the gradiometer is sensitive to ferrous metal sources both above <br /> and below ground, site and vicinity surface conditions can affect survey results <br /> We used an EDA OMNI IV tie-line magnetometer to obtain the vertical magnetic <br /> . gradient data The instrument features a built-in memory that stores the vertical <br /> magnetic gradient and survey grid information The information can be down loaded <br /> to a computer for further processing <br /> Ground Penetratinq Radar <br /> Ground penetrating radar is a method that provides a continuous, high resolution <br /> cross-section depicting variations in the electrical properties of the shallow <br /> subsurface The method is particularly sensitive to variations in electrical conductivity <br /> and electrical permittivity (the ability of a material to hold a charge when an electrical <br /> field is applied) <br /> The system operates by continuously radiating an electromagnetic pulse into the <br /> ground from a transducer (antenna) as it is moved along a traverse Since most earth <br /> materials are transparent to electromagnetic energy, only a portion of the radar signal <br /> is reflected back to the surface from interfaces representing variations in electrical <br /> properties When the signal encounters a metal object, however, all of the incident <br /> energy is reflected The reflected signals are received by the same transducer and <br /> are printed in cross-section form on a graphical recorder Depending upon depth <br /> and/or thickness the resulting records can provide information regarding the location <br /> of UST's, underground utilities, and variations in the shallow site materials <br /> Generally, electrically conductive materials, such as saturated clay and rebar can <br /> reduce the penetration capability and limit radar performance <br />