NMR (Nuclear Magnetic Resonance) measurements on rock samples provide important information about the structure and the fluids of the pore space. Normally these measurements are done in fields of strong permanent magnets. However, in some cases it is important to do this in the weak earth’s magnetic field. Unfortunately the high laboratory noise level in combination with the very low NMR signal amplitudes complicates the NMR measurement and reduces the data quality. For this reason we have updated a former NMR instrument. The new hardware concept considers three unwanted noise signal types: 1) Mains noise (50/60 Hz), railway traffic (162/3 Hz) and harmonics, 2) wideband noise from switched power supply and electronic instruments, 3) low-frequency magnetic fields (1 Hz) from DC motors. Three noise reduction techniques were implemented for routinely use: a) Gradiometric receiver coils to suppress up to 97-99% of the lab noise, b) Remote Reference Receiver Loops to predict and eliminate residual noise in the time series, c) Method for compensating the influence of rapidly changing Larmor frequency while stacking. As long as the lab noise is moderate, these three methods provide a good NMR data quality as tests at different locations have demonstrated.


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