过套管电阻率测井信号采集与处理方法研究

微弱信号检测技术是过套管电阻率测井仪器研制的关键技术难点。采用超低噪声前置放大器、过采样、取样积分/平均、数字相敏检波等技术,使微弱信号的检测精度达到30nV。设计的前置放大器采用了浮地设计,低输入阻抗的超低噪声放大器具有尽可能低的1/f噪声拐点和良好的低频噪声抑制性能。对微弱信号检测采用过采样技术,用高达16.384MHz的采样时钟对频率为0.1～10Hz范围内的信号进行采样,采用delta-sigma技术的A/D转换器,达到了24bit的采样精度、192dB的输出信噪比和32ksps的采样速率。过套管电阻率测井信号采集与处理技术已经应用到过套管电阻率模型机仪器研制中。室内刻度和现场实验结果证明,刻度电阻在0～100Ω范围变化时,测量精度在10%以内。

Through-casing Resistivity Logging Signal Acquisition and Processing Techniques

Weak signal detection is the key technique in developing through-casing resistivity logging tool.In this paper,ultra-low-noise preamplifier,oversampling method,sampling integration and sampling average method,digital phase-sensitive detection technique are applied in detecting logging signals and 30 nV is achieved.The designed preamplifier uses the technique shown in Fig.2 Ultra-low-noise preamplifier with low input resistance has 1/f noise inflection point as low as possible and better low frequency noice suppression ability.The oversampling method is used in weak signal detection,and 16.384 MHz sampling clock ia used to pick up the signals within the range of 0.1 Hz to 10 Hz.With the delta-sigma A/D convertor,the effective resolution is up to 24 bit,output signal-noise ratio 192 dB and sampling rate 32 ksps.The through-casing resistivity logging signal acquisition and processing technique has already been used in the development of the through-casing resistivity logging experiment model.The indoor calibration test and field experiment of the model with those weak signal detection techniques showed that its measurement accuracy is less than 10% when the range of calibration resistance is 0～100 Ω.