CORRESPONDENCE_2005-2006 - Laserfiche WebLink

Home Browse Search

t • ' Appendix A ' ELECTROMAGNETIC /VERTICAL MAGNETIC GRADIENT SURVEYS ' Methodology ' Electromagnetic (EM) The electromagnetic method is used to measure variations in subsurface electrical conductivity that ' maybe due to buried foreign objects or changes in subsurface materials. The electromagnetic system utilizes two coils separated by a specified distance. One of these coils transmits a time-varying electromagnetic signal(primary magnetic field)which induces current flow in the earth. This in turn ' creates a secondary magnetic field which is detected by the receiver coil. The secondary signal is complex and has both quadrature and in-phase components. The amplitude of the quadrature component is proportional to the electrical conductivity of the subsurface materials. The in-phase component is proportional to conductivity, but is also affected by electrical properties associated with metal objects. The instrument displays the quadrature component in units of milliSiemens/meter(mS/m). Since this measurement represents the conductivity of the volume of material sampled, rather than individual layers, it is an apparent value and is referred to as terrain conductivity(TC).The instrument displays the in-phase component(IPC)value in units of parts per thousand (ppt). ' Electromagnetic surveys are typically performed using a Geonics EM31-DL ground conductivity meter connected to an Omnidata data recorder. The EM31 has a fixed coil separation of 12 feet, ' which results in a total depth of investigation of approximately 10 to 15 feet depending upon local site conditions. The data recorder automatically stores the EM values as well as station locations and annotations regarding cultural features. Vertical Magnetic Gradient(VMG) ' Vertical magnetic gradient surveys are used to determine the presence of buried ferrous objects. A magnetic gradiometer measures the vertical gradient of the earth's magnetic field. It consists of two total field magnetic sensors separated vertically by one-half meter. The magnetic field strength is measured simultaneously at both of these sensors. The difference in magnetic intensity between these measurements is proportional to the vertical gradient of the earth's magnetic field. Because ' the vertical gradient is constant with respect to time, the effect of diurnal variations is eliminated. Therefore, a gradiometer provides higher sensitivity and better resolution of near surface sources than total field magnetometers. Areas with significant amounts of buried metal typically produce ' anomalously steep magnetic gradients. Since it is sensitive to ferrous metal sources both above and below ground, site and vicinity surface conditions can affect survey results. ' A-1