Publication Date:
2012-05-01
Description:
The transient electromagnetic method (TEM) is a recognized tool for determining the subsurface resistivity structure over a wide depth interval. A key requirement for an accurate characterization of the shallow part of this interval is a sufficiently wideband receiver system. The maximum depth of investigation, on the other hand, is determined by the late-time signal-to-noise ratio (S/N). It has been demonstrated that the use of compact wideband receiver coils tends to deteriorate the late-time S/N due to their inherent low sensitivity to time-varying magnetic fields. To overcome this problem, we used a bandwidth- and sensitivity-optimized parallel recording setup using two separated receiver coils optimized for measuring the early- and late-time part of the transient signal, respectively. Using this setup, we experienced substantial improvements in the achievable depth of investigation while retaining shallow resolution. Further noise reduction may be obtained by adapting the applied synchronous detection measuring scheme to the local noise conditions. This is especially important when TEM are carried out in culturally developed areas, which is often the case for environmental investigations. Such soundings frequently suffer from inferior signal quality due to EM noise from local cultural sources. Comparing the frequency-domain filtering effect of the applied synchronous detection measuring scheme with the actual noise frequency content allowed us to evaluate the relative importance of the many sources of noise encountered in the field. Further improvements could be obtained by tailoring the applied synchronous detection to better reject specific noise frequencies through modification of the properties of time-gate integration and gate stacking.
Print ISSN:
0016-8033
Electronic ISSN:
1942-2156
Topics:
Geosciences
,
Physics
Permalink
