em results - late channels, page-7

EM surveys involve switching current on and off in a transmitter loop and then taking measurements in the off time. A channel is a single measurement taken at some discrete time period (micro seconds to milli seconds typically) after the transmitter current is switched off. An early channel is measured shortly after switch off, and a late channel is after a longer period. Each channel represents a measurement over a tiny window of time. A typical survey might measure 20-30 channels per pulse of current in the transmitter loop/array. The exact time windows or channels are adjustable so they differ from survey to survey, but there will always be early and late channels, as these terms are relative to switch off. The pulse in the transmitter generates eddy currents in the rock, and these decay with time. In resistive rocks, the decay occurs quickly but in very conductive rocks the eddy currents can persist for longer. Also, the EM pulse moves away from the transmitter with time. So late channels can tell you stuff about either very conductive things (eddy currents persisting for long times) or about deeper things (because the pulse from the transmitter takes time to reach the deeper bodies). The responses can be modelled and interpreted by a geophysicist, but the comment about late channels being related in a general way to depth is fair. Hope this makes sense, sorry about the wall of text. A geophysicist would explain it better.