EM31大地电导率仪-技术说明5.pdfVIP

Technical Note TN-31 APPLICATION OF “DIPOLE-DIPOLE” ELECTROMAGNETIC SYSTEMS FOR GEOLOGICAL DEPTH SOUNDING Introduction In Geonics Limited Technical Note TN-30 “Why Doesn’t Geonics Limited Build a Multi- Frequency EM 31 or EM 38?” we addressed the issue of why Geonics had never produced multi-frequency versions of their well known EM 31 or EM 38. Indeed in that note two apparently excellent reasons were given for building such instruments. These were (i) to allow the geoelectrical sounding of a multi-layered earth rather than measuring the depth-averaged bulk conductivity, and (ii) to supply additional interpretive data for improving target diagnostics in identifying the nature of buried metal. However it was pointed out in the Technical Note that (i) unless the operating frequency range of a multi-frequency EM 31 or EM 38 extended up to the MHz frequency range it was impossible to resolve a multi-layered earth with such an instrument, and (ii) that the coil configuration of the EM 31 or EM 38 (or similar instruments) combined with the fact that they operated in the frequency domain meant that whilst they were indeed useful for indicating the presence of buried metal, they were extremely poor at accurately locating and identifying the nature of the buried metal. Indeed, it was for this reason that neither the EM 31 nor EM 38 were promoted by Geonics Limited for this latter application, although both instruments have seen a good deal of activity in the identification of buried metal. There is still controversy on both of these matters, and a competing commercial multi- frequency instrument claims to be superior to both the