A new system for acquiring and processing the three orthogonal components (Hx, Hy, and Hz) of the magnetic field in the Very Low Frequency (VLF) band (10 kHz to 30 kHz) and the Low Frequency (LF) Band (30 kHz to 300 kHz) has been developed at the Geological Survey of Sweden (SGU). The hardware is based on commercially available Analog to Digital Converters (ADCs) and an embedded computer for data recording and preliminary in-flight data processing. Presently the ADCs sample all three magnetic components simultaneously and synchronously with a 24 bit resolution at a rate of 200 000 samples per second (S/s), but they can be upgraded to 1 MS/s ADCs. All data processing is carried out in software developed at SGU and is separated into two parts: In-flight processing and post-flight processing. The former is based on Fast Fourier Transforms (FFTs) of the samples signals and it is a fast and simple approach to synchronize these data with other recorded geophysical data, as well as to carry out quality control and view preliminary results within the hour after the survey aircraft has landed. The latter processing is more demanding in terms of time and computational recourses, but in return it yields higher quality data. Here, all data processing is carried out in the time domain, beginning with data filtration by Finite Impulse Response (FIR) band-pass filters designed to match known transmitter frequencies. The filtered data is interpolated using cubic B-splines to obtain higher resolution representations of the sampled waveforms from which average amplitudes and phases can be calculated. Both the filtration and the interpolation are performed on a Graphics Processing Unit (GPU), which significantly boosts the performance compared to conventional Central Processing Units (CPUs). Average signal amplitude and phases are calculated in typically 250 ms long segments of the three magnetic components, with the strongest component chosen as the phase reference. The transmitter pair with the best signal strength, situated at an appropriate angle with respect to each other and the survey aircraft, is used to calculate quantities of geophysical interest, such as tipper values and apparent resistivity. The new VLF-LF system will be put to survey use in the Swedish summer of 2013.


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