Suppression of very low frequency radio noise in transient electromagnetic data with semi-tapered gates

Abstract

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 analog 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 analog boxcar integrators is difficult. We propose using many short boxcar gates, denoted sub-gates, and combine 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 centered between 78.7 and 978.1 µs. After inversion of the data, we find that semi-tapering increases 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 analog boxcar integrators.

Evaluating the impact of correlated noise for time‐lapse transient electromagnetic (TEM) monitoring studies

“…Each time series had the same magnitude; however, the perturbation associated with the correlated noise sources is smaller than the random noise source, compare Figures 1 and 4. This effect arises due to the filtering effect of the gating, which suppresses specific noise frequencies (see, Larsen et al., 2021). It can be observed that the bias imparted on the data varies smoothly across the gate indices; however, the effect on the typical TEM decay is only evident at the later gate times, see Figure 4a.…”

“…Covariance matrices could also be used to optimize non-standard gate widths and shapes. For instance, Larsen et al (2021) focused on minimizing the error of each measurement gate. Still, future work could also focus on reducing the correlation between gates by designing gates to exclude specific long-wave sources.…”

“…Traditionally, boxcar gates, where all the samples within the gate interval are weighted equally, were used. However, modern systems typically use more exotic gate shapes (e.g., Larsen et al., 2021). Gating acts as a filter; the geometry and width of gates influence the frequency content retained within each gated measurement.…”

“…For instance, Larsen et al. (2021) highlighted how semi‐tapered gates can suppress specific frequencies. In this work, raw data is collected in 81 box car gates spanning from 10 μs to 1 ms. To avoid confusion in this work, the terms early and late gates are used to describe the timing on the decay curve, whereas earlier and later times are used to refer to the date and time of field data collection.…”

An Optimized and Hybrid Gating Scheme for the Suppression of Very Low Frequency Radios in Transient Electromagnetic Systems

“…However, the frequency response of the boxcar has large side lobes that can let the VLF radios in the measured signal pass through (Harris, 1978). If the TEM instrument allows for acquisition of many short boxcar gates during single transients, semi-tapered gates with improved frequency responses can be formed as linear combinations of the short gates, which leads to improved suppression of VLF radios (Larsen et al, 2021). New full-sampled digital receiver systems can employ any desired gate shape, e.g., a B-spline based gating scheme has been explored to compress the measured TEM data in the SkyTEM receiver system (Nyboe and Mai, 2017).…”