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A9A Appendix A GEOPHYSICAL METHODS AND INSTRUMENTATION Total Field Magnetic Intens"LTD The magnetic field at any given point on the earth's surface is the vector sum of the earth's field combined with the induced and remnant fields of nearby magnetic objects. Most magnetometers measure the total intensity of the magnetic field. These are referred to as "Total Field" measurements (TF) and are recorded in units of nanoTesla (nT). In general, TF measurements are affected by the amount of magnetic material present in an object, the depth of burial of the object, the orientation of the object relative to the earth's field, the amount of remnant magnetism of the object, and the density of measurement points. A magnetometer is the device that is used to obtain the TF data. For this investigation, we used a Scintrex ENVI-Mag magnetometer. This instrument consists of a console and two total field magnetic sensors that are positioned one-half meter apart (vertically) at the top of an 6-ft aluminum staff. For this survey, we obtained TF data at measurement points distributed on 5-ft centers. The ENVI-Mag features a built-in memory that stores the magnetic data and survey grid information. The survey information can later be up-loaded to a computer for further processing. Metal Detection (V D) The MD method is used to detect near surface buried metal objects such as UST's, metal conduits, rebar in concrete, manhole covers, and various metallic debris. This is done by holding a combined radio transmitter-receiver unit above the ground and continuously scanning the surface, usually along linear traverses in a rectangular grid. The unit utilizes two orthogonal coils that are mounted on a common staff. One of the coils transmits an electromagnetic signal(primary magnetic field)which in turn produces a secondary magnetic field about subsurface metal objects. Since the receiver coil is orthogonal to the transmitter coil (that is, in a "null" position), it is unaffected by the primary field. However, a buried metal object is not in a similar "null" position. As a result, the secondary magnetic fields produced by buried metal objects are detected by the receiver coil. This causes the instrument to generate an audible tone. The pitch of the audible tone peaks when the instrument is directly over the metal object. We performed the MD survey using a Fisher TW-6 pipe and cable locator. Since the TW-6 does not record data that can be used for later computer processing, we marked the interpreted outlines of detected objects in the field. We also mapped their locations on Site Survey Logs. Ground Penetrating Radar (GPS Ground penetrating radar(GPR) is a method that provides a continuous, high resolution graphic cross-section depicting variations in the electrical properties of the shallow subsurface. The method is particularly sensitive to variations in electrical conductivity and electrical permittivity(the ability of a material to hold a charge when an electrical field is applied). A-1