Line current filtering of fixed-loop transient electromagnetic data
R. S. Smith and S. A. Whitaker
Journal name: Geophysical Prospecting
Issue: Vol 41, No 8, November 1993 pp. 969 - 982
Info: Article, PDF ( 1.85Mb )
The VLF filtering technique of Karous and Hjelt has been applied to fixed-loop step-response transient electromagnetic data. This allows the data measured in each channel to be converted to an equivalent current-density pseudosection.
For a conductive half-space, the maximum value of the equivalent current density starts near the transmitter loop and migrates outwards as a function of delay time. The rate of migration tends to increase as a function of delay time, with the increase being faster for a surficial conductive layer than it is for a half-space.
Theoretical and field examples show that the currents tend to be more persistent in the relatively conductive areas, so that a pseudosection which is the average of the current densities at all delay times will highlight the more conductive zones.
In resistive ground, it is not so critical to average the pseudosections as a particular delay time may give a better idea of the conductivity structure. For example, the latest possible delay time will reveal the most conductive features.