地层流体核磁测井仪扫频方法研究

Study on sweeping method of nuclear magnetic logging tool for formation fluid

井下地层流体核磁测井仪探头内部磁场强度会随环境温度变化而变化,从而导致核磁共振频率发生变化。确定工作环境下核磁共振频率,即发射天线的最佳高压脉冲发射频率,对获取信噪比较高的回波信号十分重要,也是流体核磁共振测井需要确定的参数之一。设计并实现了一种DSP和FPGA联合控制DDS芯片来完成频率快速准确切换的扫频电路来完成不同频率高压脉冲的发射及确定不同温度下核磁共振频率。实验表明,电路发射高压脉冲频率范围为4~5 MHz,能确定不同环境温度下最佳高压脉冲频率,频率精度为10kHz。

The magnetic fieldintensity of the probe in the borehole fluid magnetic logging tool varies with the ambient temperature, resulting in a change in the nuclear magnetic resonance frequency. Determining the preferred high-voltage pulse frequency for the transmitting antenna in the operating environment is important for obtaining a high signal to noise ratio of the echo signal and is one of the identified parameters required for fluid NMR logging. Designing and achieving a DSP and FPGA jointly control the DDS chip of the sweep circuit which can complete the frequency of fast and accurate switching and determine the nuclear magnetic resonance frequencies at different temperatures. Experiments show： The circuit emits high-voltage pulse frequency in the range of 4 MHz to 5 MHz and can determine the best high-voltage pulse frequency at different ambient temperatures with frequency accuracy of 10 kHz.