The effect of low pass filters in time domain electromagnetic soundings

In time domain electromagnetic, TDEM, it is a wide spread practice to model the current waveform (e.g. Asten, 1987), where as the effect of low pass filters (LP) in the receiver system is neglected. In this paper we show that when the TDEM method is used in hydrogeological and environmental investigations it is mandatory to include the filters in the forward and inverse modeling scheme to obtain avalid geological model for the survey site. IDEM systems is normally band-limited as input signals from amplitude modulated radio transmitters operating in the target bandwidth of the system often dominate the noise characteristic of the system over several decades in time. Band-limiting the input signals significantly improves the signal-to-noise ratio and thereby the depth of exploration. However, the LP filters distort the early and medium part of the decay curve due to damping of the high frequency content. The level of distortion depends on several factors such as cut-off frequency and subsurface resistivities. Therefore, the LP filters must be incorporated in the inversion scheme such that the calculated forward response is subject to the same filtering as the measured response. In this paper we (1) quantify the distortion of responses due to band-limitation for a number of typical resistivity models. The band-limitations are based on cut-off frequencies typically used in commercial instruments. (2) We demonstrate that distortion due to band-limitation can be modeled in the inversion scheme by including the LP filters in the forward response. This approach is tested on theoretical data.