Optimized Model Resolution Using Low Pass Filters In Tdem Soundings

In the time domain electromagnetic (TDEM) method it has been common practice to model the<br>current waveform of the transmitted signal, but the effect of the instrument transfer function has<br>been neglected. In Effersar et al. (1999) it was shown that inverting TDEM data without taking<br>into account the system transfer function in the forward modeling scheme lead to an erroneous<br>description of the shallow part on the geophysical/geological model. The degree of error is very<br>dependent of the near-surface resistivity of the model. High resistivity models leads to distinct<br>but localized errors at very early time gates, whereas the effect from models with low to<br>intermediate resistivities is “smeared” out over a larger time interval.<br>The system transfer function can be described as a combination of one or several low-pass (LP)<br>filters in the TDEM receiver. In this abstract we describe some side effects of having a bandlimited<br>receiver. It turns out that it is possible to actually enhance the model resolution to some<br>degree by band-limiting the receiver. This means that not only do the low-pass filters in the<br>receiver system suppress coherent noise from power lines etc., but they also enhance the model<br>resolution if the filters are of optimal designed.