Authors: Jakob Juul Larsen, Stine Søgaard Pedersen, Nikolaj Foged and Esben Auken

Abstract: The transient electromagnetic method (TEM) is widely used for mapping subsurface resistivity structures, but data are inevitably contaminated by noise from various sources. It is common practice to gate signals from TEM systems to reduce the amount of data and improve the signal-to-noise ratio (SNR). Gating acts as a filter and optimum gating will pass the TEM signal un-attenuated while suppressing noise. In systems based on analogue boxcar integrators, the gating corresponds to filtering with a square window. The frequency response of this window shape has large side lobes, which are often insufficient in attenuating noise, e.g., from radio signals in the very low frequency (VLF) 3–30 kHz band. Tapered gates have better side lobe suppression and attenuate noise better, but tapering with analogue boxcar integrators is difficult. We propose using many short boxcar gates and denoted sub-gates and combining the sub-gates into semi-tapered gates to improve noise rejection at late gates where low signal normally leads to poor SNR. The semi-tapering approach is analyzed and tested experimentally on data from a roving TEM system. We quantify the effect of semi-tapered gates by computing an improvement factor as the ratio between the standard error of data measured with boxcar gates and the standard error of data measured with semi-tapered gates. Data from a test survey in Gedved, Denmark, with 1825 measurements, gave mean improvement factors between 1.04 and 2.22 for the 10 late-time gates centred between 78.7 and 978.1 μs. After the inversion of the data, we found that semi-tapering increased the depth of investigation by about 20% for this specific survey. We conclude that the semi-tapered approach is a viable path towards increasing SNR in TEM systems based on analogue boxcar integrators.

Geoscientific Instrumentation, Methods and Data Systems, 10(1), 81-90