Aarhus University Seal / Aarhus Universitets segl

Surface Nuclear Magnetic Resonance

The group has recently developed a lightweight wireless surface NMR system, called APSU. The system is composed of a transmitter unit (ApsuTx), a power supply (ApsuPs), a control unit (ApsuMaster) and one or several receiver units (ApsuRx). The transmitter unit drives an untuned coil and the unit is powered from a low-weight generator. The NMR transmit pulses are controlled by ramping the transmitter current up and down in a triangular waveform. Each half-oscillation is independently designed and the system is capable of transmitting any arbitrary NMR pulse waveform or pulse sequences, e.g. on-resonance pulse, adiabatic pulse and numerically optimized modulation (NOM) in immediate succession. The peak current at the Larmor frequency in a 50 m by 50 m transmit coil is 105 A, corresponding to 30 m depth of investigation. The receiver units can each be connected to up to three coils and can be used to measure either NMR signals or remote reference noise. The receiver units connect wireslessly to the ApsuMaster control unit through WiFi. The system works in the central-coil configuration and has quasi-zero dead time.

magnetometer is needed to determine the Larmor frequency and for recording of fluctuations in the earth magnetic field during recording of a full NMR sounding. The NMR-sounding is followed up by a TEM sounding (WalkTEM), which provides the resistivity structure of the subsurface. The resistivity structure is used in the modeling and inversion of the NMR signals.


Key Specifications

The Apsu surface NMR system

  • Transmitter unit (ApsuTx) with full control of NMR pulses and pulse sequences (on-resonance, adiabatic, numerically optimized modulation)
  • Maximum output 105 A, 600 V
  • Powered from low weight generator
  • Quasi-zero dead time
  • Three-channel receiver units (ApsuRx), wireless connection with WiFi
  • Dual gain receiver channels sampled in 24-bit at 31.25 kHz
  • Central receiver coil configuration 9 m x 9 m, 12 turn receiver coils for NMR signal and remote noise measurements
  • GPS synchronization of all units (ApsuRx’s, ApsuTx)
  • Depth of investigation: ~30 m