Abstract
Ground conductivity meters use electromagnetic fields for the mapping of geological variations, like the determination of water amount, depending on ground layers, which is important for the state analysis of embankments. The VLF band is contaminated by numerous natural and artificial electromagnetic interference signals. Prior to the determination of ground conductivity, the meter’s working frequency is not possible, due to the variable frequency of the interferences. Frequency management based on the analysis of the selected band using track-before-detect (TBD) algorithms, which allows dynamical frequency changes of the conductivity of the meter transmitting part, is proposed in the paper. Naive maximum value search, spatio-temporal TBD (ST-TBD), Viterbi TBD and a new algorithm that uses combined ST-TBD and Viterbi TBD are compared. Monte Carlo tests are provided for the numerical analysis of the properties for a single interference signal in the considered band, and a new approach based on combined ST-TBD and Viterbi algorithms shows the best performance. The considered algorithms process spectrogram data for the selected band, so DFT (Discrete Fourier Transform) could be applied for the computation of the spectrogram. Real–time properties, related to the latency, are discussed also, and it is shown that TBD algorithms are feasible for real applications.
Highlights
Ground conductivity meters allow mapping of geological variations, related to the subsurface using the electromagnetic inductive technique without electrodes or ground contact [1]
Tracking of the radio interference signal reduces the possibility of weak signal distortion related to the ground, which is important for conductivity metering
Three proposed TBD systems are compared using Monte Carlo tests and the proposed combined spatio-temporal TBD (ST-TBD), and the Viterbi TBD system gives the best quality of tracking
Summary
Ground conductivity meters allow mapping of geological variations, related to the subsurface using the electromagnetic inductive technique without electrodes or ground contact [1]. Such a technique allows large area surveys with high spatial resolution. Ground conductivity meters are used near to the ground level or far above the ground level (tens of meters) in airborne surveys using a plane or a helicopter This technique has been used over the last few decades, but alternative electromagnetic measurement techniques are available, like VLF (very low frequency) and TEM (transient electromagnetic method, alternately called time-domain EM (TDEM) or pulse EM (PEM)) [2]. The rapid control of embankment quality is necessary using airborne techniques mainly, because embankments are long, and direct access to them is limited, depending on weather conditions
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