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Appendix A <br /> GEOPHYSICAL METHODS and INSTRUMENTATION <br /> Vertical Magnetic Gradient (VMG) <br /> A magnetic gradiometer measures the vertical gradient of the earth's magnetic field. A <br /> gradiometer typically consists of two total field magnetic sensors separated vertically by one-half <br /> meter. The magnetic field strength is measured simultaneously at both of these sensors. The <br /> difference in magnetic intensity between these measurements is essentially the vertical gradient <br /> of the earth's magnetic field at a point midway between the sensors. Because the vertical <br /> gradient is constant with respect to time, the diurnal corrections commonly needed for total field <br /> magnetic data processing is eliminated. In addition, since a gradiometer is affected less by <br /> cultural objects such as building walls, light poles, etc. than a total field magnetometer, VMG <br /> provides higher sensitivity and better resolution of near surface sources. Areas with significant <br /> amounts of buried metal typically produce anomalously steep magnetic gradients. However, <br /> because the gradiometer is sensitive to ferrous objects both above and below ground, site and <br /> vicinity surface conditions can affect survey results. <br /> NORCAL's gradiometer is an SCINTREX ENVI-MAG magnetometer. The instrument features <br /> • a built-in memory that stores the vertical magnetic gradient and survey grid information. The <br /> information can be up loaded to a computer for further processing. <br /> Metal Detection (MD) <br /> The MD method is used to detect near surface buried metal objects such as metal conduits, <br /> rebar, manhole covers, and various metallic debris. This is done by holding a radio transmitter- <br /> receiver unit above the ground and continuously scanning the surface. The unit utilizes two <br /> orthogonal coils that are mounted on a common staff. One of the coils transmits an <br /> electromagnetic signal(primary magnetic field)which in turn produces a secondary magnetic field <br /> about the subsurface metal object. Since the receiver coil is orthogonal to the transmitter coil(that <br /> is, in a "null" position), it is unaffected by the primary field. However, a buried metal object is not <br /> in a similar"null" position and therefore the secondary electromagnetic field induces a signal in <br /> the receiver and generates an audible response from the unit. The peak of this response usually <br /> occurs when the unit is directly over the metal object. Our MD instrument for this investigation <br /> was a Fisher TW-6 pipe and cable locator. The TW-6 does not provide a recordable data output <br /> that can be used for later computer processing. Results are generally limited to marking the <br /> interpreted outlines of detected objects in the field and mapping their locations. <br /> Ground Penetrating Radar (GPR) <br /> Ground penetrating radar is a method that provides a continuous, high resolution cross-section <br /> of the shallow subsurface depicting variations in electrical properties. The method is particularly <br /> • sensitive to variations in electrical conductivity and electrical permittivity (the ability of a material <br /> to hold a charge when an electrical field is applied). <br /> A-1 <br />