Laserfiche WebLink
l <br /> Electromagnetic Terrain Conductivity(TC) <br /> Methodology I <br /> The terrain conductivity method uses the principle of electromagnetic induction to measure <br /> variations in subsurface electrical conductivity. These changes in conductivity can arise from natural " I <br /> changes in soil composition or from buried foreign objects. The method utilizes two coils separated <br /> by a fixed distance. One of these coils transmits a primary signal that induces a current flow <br /> (secondary signal) in the earth. The other coil senses this secondary signal. For measurement I <br /> purposes the secondary signal is broken down into both quadrature and in-phase components <br /> (relative to the primary current). The quadrature component is used to determine electrical <br /> conductivity and is measured in milliSiemens/meter(mS/m). The in-phase component also changes I <br /> with conductivity, but varies in a different way than the quadrature component. In-phase <br /> measurements are expressed in parts-per-thousand(PPT). <br /> When highly resistive material is encountered,as is the case for most earth material,there is a linear I <br /> relationship between the quadrature component and conductivity. When highlyconductive materials <br /> Eke metals are encountered,both quadrature and in-phase components can be quite large and their I <br /> behavior is often n on-linear. While this non-linear effect can make the measurement of both <br /> components useful in looking for buried metal, it is typically the quadrature component that is <br /> analyzed. This is because the quadrature component is affected by both metallic and non-metallic I <br /> materials, whereas the in-phase component is affected primarily only by metals. <br /> Instrumentation I <br /> The instrument typically used for shallow subsurface investigations is a Geonics, Ltd. EM31-DL <br /> terrain conductivity meter. This instrument consists of transmitting and receiving coils mounted at I <br /> opposites ends of a horizontal boom and a control console in between. The separation distance of <br /> the coils is approximately 12 feet. This translates into an effective sampling depth of approximately <br /> 18 feet. The device is carred by the operator at hip-level and TC readings are taken by pressing a I <br /> trigger button. The EM31 is connected to an Omnidata data recorder that automatically stores the <br /> TC values as well as station locations and any field notes. The data logger stores the data in a way <br /> that it can be up-loaded to a computer for processing. I <br /> Computer Processing <br /> TC data are typicallyprocessed in the field on a portable computer.The up-loaded data are converted `. I <br /> into a format suitable for contouring using the program SURFER from Golden Software. This <br /> program calculates an evenly spaced array of values (data grid)based on the measured field data. I <br /> These gridded values are then contoured to.produce TC contour maps for interpretation. <br /> Contour Map Intemretation I <br /> A-3 <br /> I <br />