A High Duty Cycle Transmitter Unit for Steady-State Surface NMR Instruments
Abstract. Groundwater measurements using surface nuclear magnetic resonance (NMR) have been notoriously challenged by a poor signal-to-noise ratio (SNR), but a new steady-state methodology based on long, high duty cycle, phase-locked pulse trains have demonstrated huge SNR increases. The hardware requirements for transmitters for steady-state surface NMR are significantly increased compared to transmitters for standard surface NMR use, due to the need for very high pulse-to-pulse stability over long survey times and the increased thermal load caused by a much higher duty cycle. Further, the increased SNR leads to increased production rates, which necessitates lightweight equipment, that can be carried easily between many field sites during surveys. Here we demonstrate a novel steady-state surface NMR transmitter with a maximum 93 A peak current. The stability of the transmitter is evaluated on 10 minutes, 10 % duty cycle, pulse trains containing pulses of either 5 ms, 10 ms, 20 ms, or 40 ms duration, and low or high current. We observe less than 150 ns pulse-to-pulse timing jitter and amplitude variations below 0.4 % between pulses for all pulse durations and currents. During tests, we observed no temperature effects on the timing and current stability. We have designed a customized heatsink, which reduces the transmitter weight by 30 % and size by 16 % without compromising safe thermal operating conditions. We evaluate the capacitor bank size and current stability and demonstrate that a 10 mF capacitor bank is an appropriate trade-off with insignificant current drooping in measurements. The extensive analysis and verification demonstrate that the transmitter generates highly stable pulse trains resulting in high SNR signals.
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