Study On Inertial Sensors In Oil Exploration

Petroleum is the important strategic resource. The enormous need of petroleum in modern life promote the oil exploration industry, as a result, seismic exploration, an important method of oil exploration, has advanced a lot in past decades in two aspects: the performance of the seismic instrument is greatly improved and many new methods and new technology have been employed in seismic data processing and interpreting.In seismic exploration, seismic sensors are used to receive and convert the vibration signals. As the front-end of a seismic instrument, seismic sensors affecting the quality of seismic data directly. As seismic exploration advances further, more strict requirements are imposed on seismic sensors. Seismic sensors are required to have ideal frequency response and adequate dynamic range, otherwise high quality seismic data can't be guaranteed even if the performance of electronic system is excellent.MEMS accelerometers and moving-coil geophones are widely used in modern land seismic exploration. These seismic sensors can be viewed as inertial sensors, moving-coil geophones have restricted the whole system increasingly because of its poor distortion performance. MEMS accelerometers, on the other hand, not only provide excellent linearity, but also provide direct digital output, inherent high vector fidelity, excellent low frequency response. MEMS accelerometers have become the mainstream of the seismic sensors. Aim at these developing trends, this thesis studies two aspect of seismic sensors, first is theoretic analysis of factors of moving-coil geophones' distortion and the methods to improve it. The other aspect mainly deal with MEMS accelerometers. Several schemes based on MEMS technology, include analog close loop system and digital close loop system, are proposed and discussed in detail. Linear model analysis are performed on these schemes, modeling and simulation are carried out to verify the analysis results. A 5th orderΣ—Δdigital sensor model is proposed at last. The second aspect is the emphasis of this thesis.Physics principle is the theory basis of seismic exploration, It is necessary to capture full image of the physical background for seismic sensor designing,therefore, Chapter 2 in this thesis introduces the basic principles of seismic exploration first. Basic characters of the seismic wave is discussed and definition of high resolution seismic exploration is provided afterwards, the progress of seismic instrument is also introduced. Finally, requirements imposed on seismic sensor for high resolution seismic exploration are concluded and directions for improving current moving-coil geophone are proposed. Chapter 3 begins with dynamics of the moving-coil geophone and discuss the following aspects: the meaning and selection guide of geophone's parameters, the effects of geophone grouping, geophone online testing employed in seismic data acquisition system which I took part in developing. The emphasis of this chapter is theoretic analysis of factors causing geophone distortion and their contributions. The using of overdamping geophone to reduce the mass deflection is proposed and discussed. Analysis results are verified by simulation in SIMULINK.Chapter 4 introduces background and principle of MEMS technology first, the basic principle and difference of two main kind of digital sensor provided by foreign company is also introduced. Fully description of principles and models(include dynamic model and electronic model) is provided then. Criterions for selecting mechanical parameters of digital sensor are presented for acceptable Brownian noise and a set of parameters is provided, various signal pick off circuits are discussed and compared. Finally, open loop system employed pick off circuit is studied, simulation and verification in PSPICE is carried out.In chapter 5, analysis and simulation of analog close loop system for seismic sensor are presented. A PID controller is introduced in the loop and criterions for selecting proportion gain and differential gain are derived. Both analysis and behavior simulation show the deflection of the proof mass is greatly reduced.However, analog close loop system is unstable because of the nonlinearity between the feedback force and deflection, that's why digital close loop system is adopted in practical MEMS seismic sensors. Chapter 6 is the an emphasis of this thesis where digital sensor based onΣ—Δmodulation is modeled and simulated. A 2th orderΣ—Δmodulator implemented by the sensing element are discussed in detail. Simulation in SIMULINK shows 2th orderΣ—Δmodulator can't meat the requirement of seismic exploration, additional electronic integrator is required to form high orderΣ—Δmodulation. It is revealed that 5th orderΣ—Δmodulation is required to suppress the quantization noise to negligible level. To point against the requirement to THD(total harmonic distortion), the formulas for THD calculation is derived, constraints to electronics parameters for THD that less than -100dB is calculated by these formulas. Modeling and simulation of the whole system are carried out, results are inspected and verified with theoretical analysis.Chapter 7 summarizes the main ideas in this thesis, which is analyze the requirement imposed on seismic sensor by seismic exploration, analyze and improve popular moving-coil geophones, study the MEMS accelerometers and proposed several scheme for seismic sensors, linearization methods are employed to analyze these schemes and modeling and simulation are used to verify the theoretic results. This chapter also pointed out the future...