| In the process of oilfield exploration and development,acoustic logging is a common logging method,but with the increase of complex wells,deviated wells,horizontal wells and ultra-deep wells,the conventional cable acoustic logging is no longer applicable.In view of this,it is of great engineering significance to study and develop high precision,low noise and small size through bit array acoustic logging tool which combined with the new logging means suitable for complex wells.Aimed at the characteristics of small space,downhole mass storage,battery power supply and wide signal frequency band(300Hz-30kHz)and the large signal dynamic range(176?V-1.76V)of multi-pole array acoustic logging,in this paper,the research of the acoustic logging signal acquisition circuit of the multi-pole array for through bit logging tools is carried out,the corresponding circuit design scheme is put forward,and the practical acquisition strip circuit is developed.The main research is as follows:1.In view of the frequency response characteristics of the acoustic receiving transducer,this paper analyzes the advantages and disadvantages of the two receiving circuits,which are respectively based on charge amplification and voltage amplification in detail.Finally,the instrument amplifier circuit is adopted as the design scheme of the preamp circuit according to the design requirements for digitizing the acoustic signals to the transducer nearby,considering the high temperature working environment underground.2.In order to better reduce the circuit design noise and improve the measurement dynamic range of the instrument,the paper carefully analyzes and compares the noise of different devices according to the theoretical analysis method for circuit noise.Then analysising the noise of signal conditioning circuit with ADI and TI noise analysis software lay the theoretical foundation for device selection and circuit design.Finally design and implement the corresponding signal conditioning circuit.3.According to the amplitude and frequency characteristics of the acoustic signals,the digitizing scheme of the acoustic signal is proposed,which is based on the 18-bit analog-to-digital conversion chip AD4007.In order to raises the acquisition board to higher integration,the daisy chain connection of serial data transmission is adopted between ADs,which greatly reduces the wiring on the PCB board and the connection between the boards.The acquisition control structure based on FPGA+off-chip SRAM is adopted,which make circuits simpler,reliability higher,controlmore flexible.Also the high-speed data transmission with the main control unit based on LVDS interface is realized.4.To verify the performance of the designed circuit,it is tested when input grounding and connecting sinusoidal signal under normal temperature and high temperature environments in the laboratory.The measured results show that the total power consumption of 12 channel acquisition circuit is less than 1.8W,the mean square root value of noise is less than 10?V,the dynamic range of signal measurement is98 dB,the area of single channel acquisition circuit is only 70mm×19mm,and the high temperature characteristic is up to 160 ?.The acquisition strip circuit developed in this paper has the characteristics of high precision,large dynamics,low power consumption,high temperature resistance and high integration.The main innovations of the work is that the area of the array acoustic receiving circuit is the smallest among the domestic similar instruments when the sampling accuracy is the highest and the recording dynamic range is the largest.In summary,the researchs of the paper have important application value,which can not only meet the requirements of the acquisition of multi-pole array acoustic logging signals for through bit,but also serve as a reference scheme for the acquisition of other array acoustic logging. |
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