Design Of Transmitting Module For Oil-based Mud Electric Imaging Logging Instrument

With oil-based mud(OBM) is widely used in oil exploration in deep sea environment and special rock formation, oil-based mud electronic logging instrument which is developed from Extented range micro-resistivity imager develop rapidly.This logging technology provides accurate and clear resistivity image not only in oil-based mud but also in water-based mud logging. It is becoming one of the main technical means of the oil well logging. Signal transmitter as the excitation signal source of oil-base mud logging tool, is an important part of the instrument design.The principle of oil-based mud logging instrument is firstly introduced. And then according to the requirements of the logging tool, technical indicators of emission module is proposed.Finally, a whole design of hardware and software is put forward and verified.The main function of transmitter module is to generate the amplitude modulation(AM) signal.The frequency of carrier wave is 1MHz,the modulated wave is 2kHz and the modulation depth is 85%,so that the signal can penetrate the rock formation successfully. In consideration of the need of acquiring transmitter condition in well, a output signal detecting circuit is designed in project.it scquire and calculate amplitude of output voltage and current and send date to control and processing module, and as the basis of emission closed-loop control. The design mainly involves the direct digital frequency synthesis(DDS), digital-to-analogue conversion(DAC), differential band-pass filter, power amplificationand analog-digital conversion(ADC),etc.After having determined the scheme of hardware circuit,the whole system is modeled,a control system simulation is implemented to verify whether the response time of the designed system can meet the instantaneity and stability.In the final,software based on FPGA and DSP is completed.Compared with the previous transmitter, the transmitter module researched in this dissertation has the following characteristics: direct digital frequency synthesis based on the FPGA + DAC, improve the flexibility of the output signal. The LVDS differential transmission, differential amplifier circuit improve the anti-interference ability of the system and harmonic suppression ratio. Low output resistance, make the output more efficient.A series of experiment and field logging are presented in order to show the rationality and the correctness of the design of transmitter.The spectrum analysis result indicates that the harmonic suppression ratio of the excitation signal meets the demands of technical specification; output impedance experiment verifies the transmitter module has a strong capacity to drive load. closed-loop control ecperiment verifies the output amplitude can change according to the rock impedance.Finally the field logging experiments are illustrated to verify the principle correctness of the entire logging instrument.