Design of an Eccentric Multi-Frequency NMR Downhole Device

We designed a nuclear magnetic resonance logging tool to provide evaluation data of formation previously not available from routine logs. ^[1] This instrument has an eccentric probe which generates a gradient field. It achieves the information of corresponding depth of investigation by sending RF pulses into the formation. Adjusting the resonance frequency of antenna achieves echo signals of different radial depth. The electronic circuit of this instrument includes Power amplifier, Q-switch, De-coupler, Preamplifier and Digital circuit. ^{[2] [3]} The output voltage of dual-full-bridge switch power amplifier which drives antenna load directly without high-power transformer is 2400V, as four times higher as the input DC voltage. Q-switch circuit which is used to discharge the energy of antenna controlled by high voltage MOSFETs reduces the Q value of the antenna after pulse emission, which quickly releases the energy and efficiently shortens the recovery time of the antenna. Additionally, de-coupler controlled by active MOS is adapted to protect the receiver. The preamplifier was designed based on low-noise principle in which the front-end amplifier is high-gain based on instrumental amplifier architecture. The programmable attenuation is employed to widen the dynamic range of the circuit. Digital Signal Processor (DSP) and Field Programmable Gate Array (FPGA) are embedded in main-controller to generate pulse sequences, control instruments and acquire signals, etc. The amplitudes and phases of echo signals are extracted with the digital phase sensitive detection (DPSD) algorithm. This instrument has been tested in Calibrator tank and acquires good echo trains.

Fig.1 The sensitive region corresponding to respective DOI (Depth of Investigation)

Fig.2 The functional block diagram of electronic circuit