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7kadwell&Rollo <br /> ' 3.1.1 Geophysical Survey <br /> The purpose of the geophysical survey was to obtain subsurface information to help determine <br /> the location of suspected underground structures, including USTs, sumps, and pipelines. The <br /> vertical magnetic gradient(VMG),terrain conductivity (TC), and ground penetrating radar <br /> (GPR)methods were used. The surveys were performed by NORCAL Geophysical Consultants <br /> ' Inc. (NORCAL) on 17, 18, and 19 August 1999. A copy of their report is included as <br /> Appendix B. The individual geophysical methods used in the survey are described below. <br /> 3.1.1.1 Vertical Magnetic Gradient ent Method (VMG), <br /> ' The VMG uses a magnetic gradiometer to measure the vertical gradient of the earth's magnetic <br /> field. It consists of two total field magnetic sensors separated vertically by one-half meter. <br /> ' Areas with significant amounts of buried metal typically produce anomalous steep magnetic <br /> gradients. A SCINTREX ENVI-MAP magnetometer was used to obtain the vertical magnetic <br /> ' gradient data. This instrument features a built-in memory that stores the vertical magnetic <br /> gradient and survey grid information. The information was then downloaded to a computer for <br /> processing. Specifically,the VMG method was used to determine the presence of buried ferrous <br /> metal that may indicate the presence of USTs, sumps, or pipelines. <br /> 1 <br /> 3.1.1.2 Electromagnetic Terrain Conductivity (TC) <br /> ' The electromagnetic TC method is used to measure variations in <br /> g subsurface electrical <br /> conductivity. The electromagnetic system utilizes two coils separated by a specified distance. <br /> One of these coils transmits a signal that induces current flow in the earth. This in turn creates a <br /> ' secondary magnetic field that is detected by the second coil. This secondary field is proportional <br /> to the induced current and can be measured. The measurement represents the conductivity of the <br /> ' volume of material sampled and is referred to as terrain conductivity. The electromagnetic <br /> survey was performed using a Geonics EM31-DL ground conductivity meter that is connected to <br /> ' 4 <br /> ' 25970119.DGD 13 December 1999 <br />