Dipole transverse wave far exploration logging sound source excitation method based on low-frequency Blackman window function

Abstract

The invention discloses a dipole transverse wave far exploration logging sound source excitation method based on a low-frequency Blackman window function, and the method comprises the steps: calculating a discrete value of the Blackman window function, and obtaining a parameter of the low-frequency Blackman window function; setting the duty ratio of a pulse width modulation control signal based on the parameters of the Blackman window function to obtain a pulse width modulation control signal based on the Blackman window function; the pulse width modulation control signal is amplified and filtered, higher harmonics of the pulse width modulation signal are filtered out, and a low-frequency Blackman window function signal is obtained; after the low-frequency Blackman window function signal is matched with impedance, the dipole sound source transducer is excited, and the transducer sends out a transverse wave signal; and processing the excitation signal by adopting a low-frequency Blackman window function so as to highlight a specific frequency component, thereby better identifying and analyzing the characteristics of the underground rock layer.

Description

Technical Field

[0001] The invention relates to the field of acoustic well logging, and in particular to a dipole shear wave long-range well logging sound source excitation method based on a low-frequency Blackman window function. Background Art

[0002] Dipole shear wave long-range detection logging is an important acoustic logging technology. It can increase the detection distance from several meters of traditional acoustic logging to tens of meters, thereby obtaining comprehensive formation information, and plays an important role in detecting fractures and determining formation structures. At present, there are some difficulties in the practical application of long-range detection technology. One of them is that the amplitude of the direct wave in the receiving wave train is too large, and the reflected wave is too weak relative to the direct wave, which will cause several problems: First, the reflected wave of the reflector near the well wall is easily submerged in the direct wave; Second, in order to make the direct wave not limited, the amplification factor of the instrument acquisition circuit cannot be too high, so that the reflected wave is easily submerged in the noise due to its small amplitude. Therefore, how to suppress the direct wave and enhance the reflected wave has become the focus of current shear wave long-range detection research. The direct wave excited by the dipole in the wellbore is a dispersed bending wave, and its excitation intensity is the largest at the Airy phase frequency and is extremely low below the cutoff frequency. Therefore, a low-frequency signal with a frequency band below the cutoff frequency can be selected as a dipole sound source, which can achieve the purpose of suppressing the direct wave and enhancing the relative amplitude of the reflected wave.

[0003] The traditional way of exciting a dipole transducer is to use a rectangular high-voltage pulse through a transformer to excite the transducer. When exciting a dipole piezoelectric ceramic transducer, this excitation method will have energy distributed in the Airy phase due to the spectrum leakage problem of the rectangular pulse, thereby generating a strong high-frequency signal and failing to suppress the direct wave well. Therefore, research on new excitation signals suitable for dipole shear wave long-range detection logging has become a research hotspot.

[0004] The traditional excitation mode of acoustic logging excitation is to directly use a high-voltage power supply to generate rectangular pulses, then increase the voltage of the pulses through a transformer, and finally drive the transducer. This method is simple to implement, but the frequency of the sound source is fixed, and the excitation signal cannot be complexly controlled, nor can it be dynamically adjusted according to the formation environment. Summary of the invention

[0005] In view of the above problems, the present invention provides a dipole shear wave long-range logging acoustic source excitation method based on a low-frequency Blackman window function, characterized in that the method comprises:

[0006] The host computer calculates the discrete value of the Blackman window function and transmits the parameters of the low-frequency Blackman window function to the control circuit;

[0007] The control circuit sets the pulse width modulation control logic based on the parameters of the low-frequency Blackman window function, and transmits a Blackman window function signal based on the pulse width modulation to the driving circuit;

[0008] The driving circuit amplifies and filters the Blackman window function pulse and outputs it to the matching network;

[0009] The low-frequency Blackman window function matches the impedance through a matching network and then excites the dipole sound source transducer, which then emits a shear wave signal.

[0010] The low-frequency Blackman window function has small side lobes and can suppress the direct wave in the dipole shear wave long-range detection logging when used as an excitation signal.

[0011] The driving circuit uses a second-order Butterworth filter to filter the Blackman window function signal, removes high-order harmonics, and obtains the required analog signal again.

[0012] In a second aspect, the present invention provides an excitation circuit structure, comprising:

[0013] A power supply circuit, used for supplying power to the excitation circuit system;

[0014] The control circuit is used to receive the transmission parameters sent by the host computer, provide a pulse width modulation control signal for the drive circuit, and perform the operation of the adaptive algorithm;

[0015] A driving circuit for amplifying the power of a pulse width modulation control signal;

[0016] Matching network to achieve the best match between the driving circuit and the transducer.

[0017] The power supply circuit adopts a flyback topology flyback switching power supply design, including:

[0018] The input rectifier and filter circuit is used to rectify and filter the AC power at the input end to provide a DC voltage for the switching power supply;

[0019] PWM control circuit, used to adjust the duty cycle according to the voltage feedback of the power supply circuit, and output a pulse width modulation signal to control the switching of the power switch tube of the power supply circuit;

[0020] Power switch tube, used to convert DC voltage into high-frequency AC power, which is converted into the required output AC power through a high-frequency transformer;

[0021] The output rectifier and filter circuit is used to filter the output high-frequency AC voltage, eliminate the ripple voltage, and obtain the required DC voltage.

[0022] The driving circuit is an H-bridge circuit structure, comprising:

[0023] Optocouplers, used to electrically isolate digital circuits from analog circuits;

[0024] A driver chip used to drive the bridge arm of the H-bridge class D power amplifier circuit;

[0025] RCD snubber circuit, used to suppress the overvoltage applied to the H-bridge switch tube and the increase of turn-off loss;

[0026] The LC filter circuit is used to filter out the high-order harmonics of the PWM signal to obtain the required analog signal.

[0027] Among them, two driving chips are used to drive the two bridge arms of the H-bridge D-class power amplifier circuit respectively, so as to realize reciprocating excitation of the transducer in both positive and negative directions.

[0028] In the above method provided by the embodiment of the present invention, due to the wide main lobe of the Blackman window, the frequency recognition accuracy is the lowest, but its side lobes are small and the suppression effect on high-frequency components is good. Therefore, by using a low-frequency Blackman window function as an excitation signal, the direct wave can be better suppressed. Through the excitation circuit diagram provided by the present invention, a usable low-frequency Blackman window function excitation signal can be generated. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 is a graph of phase velocity, group velocity and excitation intensity of bending waves excited by a dipole acoustic source in a fast formation wellbore in one embodiment of the present invention;

[0030] Figure 2 is a window function excitation signal and a spectrum curve in one embodiment of the present invention, including a time domain waveform and b spectrum curve;

[0031] Figure 3 It is a waveform diagram of the direct wave obtained by the rectangular window and the Blackman window excitation signal in the fast formation wellbore in one embodiment of the present invention.

[0032] Figure 4 It is a circuit diagram of an excitation circuit of the sound source excitation method provided by the present invention;

[0033] Figure 5 It is a circuit diagram of a power supply circuit in an excitation circuit of a sound source excitation method provided by the present invention;

[0034] Figure 6 It is a principle diagram of using SPWM wave to replace the sine half wave provided by the present invention;

[0035] Figure 7 This is a schematic diagram of a conventional Class D power amplifier circuit;

[0036] Figure 8 is a circuit diagram of a driving circuit in an excitation circuit provided by the present invention;

[0037] Fig. 9 is a parameter diagram of a second-order Butterworth filter in one embodiment of the present invention;

[0038] Fig.10 is a simulation diagram of a transducer and a matching network diagram in one embodiment of the present invention;

[0039] Fig.11 is an equivalent circuit diagram of a transducer in one embodiment of the present invention;

[0040] Fig.12 is a simulation diagram of a matching network circuit in an excitation circuit in one embodiment of the present invention;

[0041] Fig.13 This is an oscilloscope screenshot of the Blackman window logic signal and the stimulus waveform. DETAILED DESCRIPTION

[0042] In order to make the purpose, technical solution and advantages of the embodiments of the present invention clearer, the technical solution in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] In acoustic logging technology, dipole shear wave long-range detection logging is an important acoustic logging technology, which can increase the detection distance from a few meters in traditional acoustic logging to tens of meters, thereby obtaining comprehensive formation information. In logging, direct waves are mainly transmitted through the mud or wellbore environment in the well, which is independent of the formation structure and is interference data during analysis. Therefore, in actual logging, direct waves need to be suppressed as much as possible.

[0044] For dipole acoustic logging, the direct wave excited by the sound source in the well has a dispersion characteristic, and the phase velocity and group velocity change with the frequency. The excitation intensity of the bending wave is the largest near the Airy phase (i.e., the frequency where the group velocity has the lowest frequency), and gradually decreases to zero on both sides.

[0045] Among them, phase velocity refers to the speed at which a certain phase of a wave (such as a crest or trough) propagates in a medium, and group velocity refers to the speed at which the envelope of a wave packet (i.e. a wave group or pulse composed of waves of multiple different frequencies) propagates in a medium. Both are important parameters for analyzing formation structure and properties in acoustic logging.

[0046] like Figure 1As shown in the figure, the black curve is the phase velocity, the red curve is the group velocity, and the blue curve is the excitation intensity. The excitation intensity is the largest at the Airy phase frequency and is extremely low below the cutoff frequency. In order to suppress the direct wave, it is necessary to minimize the energy distribution of the excitation signal near the Airy phase. As for the reflected wave, the viscoelastic properties of the formation will attenuate the high-frequency components. The more low-frequency components, the longer the detection distance. Therefore, a low-frequency signal with a frequency band below the cutoff frequency can be selected as a dipole sound source to achieve the purpose of suppressing the direct wave and enhancing the relative amplitude of the reflected wave.

[0047] Therefore, the embodiment of the present invention selects a low-frequency Blackman window function as the excitation function, and the source function and spectrum function of the rectangular window and the Blackman function are as follows: Figure 2 As shown, the main lobe of the Blackman window is wide and has the lowest frequency recognition accuracy, but its side lobes are small and have a good suppression effect on high-frequency components, thereby being able to better suppress direct waves.

[0048] Rectangular Window:

[0049] w 0 (t) = U 0 (1)

[0050]

[0051] Blackman Window:

[0052] w(t)=0.42-0.5cos(2πt / T 0 )+0.08cos(4πt / T 0 )

[0053] F(ω)=0.42F 0 (ω)+0.25F 0 (ω-ω 0 )+0.25F 0 (ω+ω 0 )

[0054] +0.04F 0 (ω-2ω 0 )+0.04F 0 (ω+2ω 0 )

[0055] Where ω 0 =2π / T 0 , U 0 is the voltage peak, T 0 is the pulse duration, and 0≤t≤T 0 ; When t<0 or t>T 0 All function values ​​are 0.

[0056] Depend on Figure 2 It can be seen that the rectangular window function has high side lobes in the frequency domain and serious energy leakage in the high-frequency band. In contrast, the energy of the low-frequency Blackman window function is concentrated in the low-frequency band and there is basically no energy distribution in the high-frequency band where the Airy phase is located. Therefore, it can better suppress the direct wave and increase the relative amplitude of the reflected wave.

[0057] The traditional excitation mode of acoustic logging excitation is to directly use a high-voltage power supply to generate rectangular pulses, then increase the voltage of the pulses through a transformer, and finally drive the transducer. This method is simple to implement, but the frequency of the sound source is fixed, and the excitation signal cannot be complexly controlled, nor can it be dynamically adjusted according to the formation environment.

[0058] According to the above principle, the present invention provides a dipole shear wave far detection logging sound source excitation method based on a low-frequency Blackman window function, comprising the following steps:

[0059] 1. The host computer calculates the discrete value of the Blackman window function and transmits the parameters of the low-frequency Blackman window function to the control circuit;

[0060] Blackman Window:

[0061] w(n)=0.42-0.5cos(2πn / N-1)+0.08cos(4πn / N-1)

[0062] Where N is the length of the window and n is the index of the sampling point.

[0063] In a specific embodiment, for each sampling point n, the corresponding window function value w[n] is calculated to form a set of sampling values ​​of the Blackman window function, and the sampling values ​​w[n] of the Blackman window function are normalized so that their values ​​are between 0 and 1.

[0064] 2. The control circuit sets the pulse width modulation control logic based on the discrete value of the low-frequency Blackman window function and transmits the PWM control signal to the drive circuit;

[0065] For each sampling point n of the Blackman window function, the duty cycle of the PWM signal is determined according to the corresponding w[n] value. For example, if w[n] = 0.7, the corresponding PWM signal is high for 70% of the time and low for 30% of the time in one cycle.

[0066] In a specific embodiment, the duty cycle value required by the Blackman window function is calculated before each cycle begins, and then assigned to a register, and the Blackman window function reference signal is compared with the high-frequency triangular carrier signal. When the amplitude of the Blackman window function is greater than the carrier signal, a high-level pulse is output; when the amplitude of the Blackman window function is less than the carrier signal, a low-level pulse is output, and the shape of the Blackman window function is simulated by controlling the width of the pulse. The PWM control signal is output to the drive circuit.

[0067] 3. The driving circuit amplifies the Blackman window function pulse and outputs it to the matching network;

[0068] In a specific embodiment, a power-amplified PWM signal is generated through the switching process of the switching device, and after the high-order harmonics are filtered out by the Butterworth filter circuit, the low-frequency original analog signal is restored.

[0069] 4. The generated low-frequency Blackman window function excites the dipole sound source transducer through the matching network to emit a shear wave signal.

[0070] In a specific embodiment, a low-frequency Blackman window function is used as an excitation signal to excite the dipole, convert the electrical signal into an acoustic wave signal, and emit a shear wave signal.

[0071] The present invention designs an excitation circuit system to generate a frequency-modulated excitation signal with adjustable parameters, and adopts a pulse width modulation method to realize the generation of a low-frequency Blackman window function, thereby realizing multiple mode excitations of the transducer and the excitation of the low-frequency Blackman window function.

[0072] In a specific embodiment, the entire excitation circuit system is composed of a power supply circuit, a control circuit, a drive circuit and a matching network. Figure 4 The power supply circuit mainly supplies power to the circuit system, the control circuit mainly receives the transmission parameters sent by the host computer, provides PWM control signals to the drive circuit, and performs adaptive algorithm calculations. The drive circuit mainly amplifies the power of the PWM control signal, and the matching network realizes the matching between the excitation circuit and the transducer.

[0073] Among them, the main function of the power supply circuit is to provide power for the entire circuit system, including the +400V high voltage DC required by the drive circuit, the +5V and +12V required by the drive circuit, and the +5V required by the control circuit.

[0074] In a specific embodiment, the power supply circuit adopts a flyback topology flyback switching power supply design, which can achieve higher efficiency and realize multi-channel output effect.

[0075] Generally speaking, a switching converter is a main circuit that uses semiconductor power devices as switches to change one form of power supply to another. A switching power supply is a circuit that uses an automatic control closed loop during the transition, has a stable output and has a protection function. It mainly uses energy storage elements, such as capacitors and inductors, and uses the continuous on and off state conversion of the switch tube to gradually convert energy.

[0076] In a specific embodiment, the basic structure of the switching power supply used in the present invention is as follows: Figure 5 As shown, it mainly consists of four parts: input rectification and filtering circuit, PWM control circuit, power switch tube and output rectification and filtering circuit.

[0077] Among them, the input rectification and filtering circuit is used to rectify and filter the AC power at the input end to provide a DC voltage for the switching power supply;

[0078] The PWM control circuit adjusts the duty cycle according to the voltage feedback of the power supply circuit, and outputs a pulse width modulation signal to control the switching of the power switch tube of the power supply circuit;

[0079] The power switch tube is used to convert DC voltage into high-frequency AC power, which is then converted into the required output AC power through a high-frequency transformer;

[0080] The output rectifier filter circuit is used to filter the output high-frequency AC voltage, eliminate the ripple voltage, and obtain the required DC voltage.

[0081] The main function of the control circuit is to receive the transmission parameters sent by the host computer, provide PWM control signals for the drive circuit, and perform adaptive algorithm calculations. In a specific embodiment, the control circuit is mainly composed of a DSP chip, a power conversion chip, a CAN transceiver, and an active crystal oscillator.

[0082] As the main component, the DSP chip realizes CAN communication (receiving the transmission parameters sent by the host computer), 4-way PWM logic control and real-time calculation functions;

[0083] The function of the power conversion chip is to convert the +5V provided by the power supply circuit into +3.3V and +1.9V for use by other chips on the board;

[0084] CAN transceiver realizes the connection between CAN bus and DSP chip;

[0085] Active crystal oscillator provides clock signal for DSP.

[0086] In a specific embodiment, the DSP chip receives the transmission parameters of the host computer through CAN communication and sets the value of the time base period register. This value determines how often the timer is reset, thereby determining the period of the PWM signal. In simple terms, the larger the value of the time base period register, the lower the frequency of the PWM signal and the longer the period; the smaller the value, the higher the frequency of the PWM signal and the shorter the period.

[0087] The Blackman window function reference signal is compared with the high-frequency carrier signal. When the amplitude of the Blackman window function is greater than the carrier signal, a high-level pulse is output; when the amplitude of the Blackman window function is less than the carrier signal, a low-level pulse is output. This method controls the duty cycle of the PWM signal.

[0088] Setting the dead time means adding a short delay time between the two complementary PWM signals, so that when one PWM signal is 0, the other signal does not immediately become 1, but waits for a while before becoming 1.

[0089] Generally speaking, high-power amplifiers, converters, and motors mostly have H-bridge or 3-phase bridge circuits composed of switching components such as MOSFET and IGBT. For a full-bridge circuit, the upper and lower arms of the bridge must not be turned on at the same time, which is equivalent to a short circuit in the power supply, and the instantaneous high current will burn the switching components. When the actual circuit is working, the PWM control signal often has a delay effect before reaching the switching element. For example, the optocoupler and driver chip have a delay time and cannot respond to the signal change immediately. This causes the switch tube to open early or close late, causing the switch tubes of the upper and lower arms to open at the same time, and the instantaneous high current to damage the components.

[0090] Setting the dead time can avoid the possibility of the upper and lower bridge arms being turned on at the same time due to signal delay in the actual circuit, thereby protecting the switch tube.

[0091] The main function of the driving circuit is to receive the logic signal output by the control circuit and drive the transducer after power amplification.

[0092] In a specific embodiment, the present invention uses an H-bridge class D power amplifier in a driving circuit to amplify the power of a signal.

[0093] Among them, the Class D amplifier is also called a digital amplifier. Its main working principle is that the input analog audio signal first generates a PWM signal through a comparator or a complex control chip, and then increases the driving ability after amplification to control the on or off of the switching components. Through the switching process of the switching device, a power-amplified PWM signal is generated. After filtering out high-order harmonics through the filter circuit, the low-frequency initial analog signal is restored, and finally a corresponding excitation signal is generated to drive the load, such as Figure 6 shown.

[0094] Class D amplifiers mainly control the on and off of the switch tube. When the switch tube is on, the saturation voltage drop is much smaller, and the power consumption is low, which is equivalent to directly loading the high-voltage power supply on both ends of the load. The circuit itself consumes very little power; when the switch tube is turned off, the switch tube is in the cut-off state and does not consume power. Therefore, in theory, the efficiency of Class D amplifiers can reach 100%, and even in practical applications, it can achieve an efficiency of more than 90%, which is an effective tool for achieving high-efficiency power drive.

[0095] In a specific embodiment, the driving circuit schematic diagram is as follows: Figure 7 The whole circuit is composed of an optocoupler, a driver chip, an H-bridge circuit, an RCD buffer circuit and an LC filter circuit.

[0096] The main function of the optocoupler is to electrically isolate the digital circuit from the analog circuit. Its signal is transmitted unidirectionally with high transmission efficiency, stable operation, and no contacts. It realizes electrical isolation between the input and output ends, has strong anti-interference ability, and the output signal has no effect on the input end, so that the DSP is free from interference from the driving circuit and can work stably.

[0097] For the H-bridge circuit composed of four MOSFET switching tubes in this article, two driver chips are used to drive the two bridge arms respectively to achieve reciprocating excitation of the transducer in both positive and negative directions.

[0098] The snubber circuit can suppress the overvoltage applied to the switch tube and the increase of the turn-off loss. When the switch tube is turned off, a surge voltage will occur between the collector and the emitter due to the energy stored in the inductor. At this time, the snubber capacitor can share part of the energy during the turn-off, thereby protecting the switch tube.

[0099] PWM amplifiers need to filter PWM signals through low-pass filters to remove high-order harmonics and regain the required analog signal. Sometimes, filters can also be used to transform the impedance between the amplifier and the load to maximize the amplifier output power.

[0100] In a specific embodiment, the low-pass filter may be a second-order Butterworth filter. The Butterworth filter is characterized by having a maximally flat frequency response curve in the passband, with small fluctuations, and gradually decreasing to zero in the stopband. Figure 8 shown.

[0101] The Butterworth filter can be expressed as follows:

[0102]

[0103] Where n is the filter order, ωc is the cutoff frequency, and ωp is the passband edge frequency.

[0104] In the present invention, a second-order Butterworth filter is implemented using an LC filter circuit, and its calculation parameters are as follows:

[0105]

[0106] The power supply internal resistance R0 = 0.25Ω, the load R = 17Ω, the cut-off frequency fL = 20kHz, and the calculated L = 192.7uH, C = 0.33uF.

[0107] In a specific embodiment, the filter is simulated using Filter Solutions analog filter simulation software, such as Fig. 9 The transducer and matching network are simulated using Multisim circuit simulation software, as shown in Fig.10 The red curve at the bottom of the figure is the output signal of the signal source, and the curve at the top is the signal loaded at both ends of the load. It can be seen from the figure that the high-frequency components of the square wave signal with a main frequency of 13.3kHz are filtered out after passing through the filter, and it is approximately a sine wave signal.

[0108] The matching network realizes the matching between the excitation circuit and the transducer.

[0109] In order to maximize the power transfer between the power amplifier circuit and the load, impedance matching is often required between the circuit and the load. When the power amplifier circuit is used to drive the transducer, if the best match between the driving circuit and the transducer can be achieved, the maximum power output can be achieved. If a good match cannot be achieved, the energy of the circuit cannot be effectively transmitted to the transducer, the expected excitation effect cannot be achieved, and the transducer cannot radiate sound waves normally.

[0110] For the transducer, the matching is mainly to transform the impedance and achieve resonance. Usually the output impedance of the Class D amplifier is relatively small, with a large resistance component and a small reactance component. The equivalent circuit diagram of the transducer at resonance is as follows Fig.11 As shown in the figure, in the resonant state, the dynamic inductance L1 and the dynamic capacitance C1 cancel each other out, which is equivalent to the static capacitance C0 and the static resistance R1 being connected in parallel, and the overall is capacitive. Therefore, it is necessary to use appropriate inductance and resistance to transform the equivalent impedance of the transducer into a pure resistance and match it with the impedance of the power amplifier, so as to improve the output efficiency.

[0111] At the resonance point, the input impedance of the transducer is expressed as:

[0112]

[0113] Assuming that the internal resistance of the power amplifier circuit is purely resistive, according to the principle of maximum power output, the ideal impedance matching condition is:

[0114]

[0115] In a specific embodiment, according to the measurement result of the impedance analyzer, R1 = 17Ω, C0 = 189.7nF, f0 = 13.36kHz, it can be calculated that R0 = 15.98Ω, L0 = 52uH.

[0116] Use simulation software to simulate the transducer and matching network, such as Fig.12 The red curve at the bottom of the figure is the output signal of the signal source, and the curve at the top is the signal loaded at both ends of the load. It can be seen from the figure that after the input sine wave signal passes through the filter and impedance matching, the signal loaded at both ends of the transducer is also a sine wave, and the phase is the same as the input signal.

[0117] In a specific embodiment, the circuit provided by the present invention is used to first calculate and generate the discrete values ​​of the Blackman window function, and the host computer sends these parameters to the control circuit. The control circuit uses the amplitude values ​​of these Blackman window functions as the duty cycle of the PWM signal, amplifies the PWM signal and uses a low-pass filter to eliminate the high-frequency components in the PWM signal, extracts the average voltage signal that changes with the duty cycle, and after passing through the matching network, the output signal is a smooth signal that approximates the Blackman window function, which can be directly used to excite the acoustic transducer. Fig.13 As shown, Fig.13 This is a screenshot of the Blackman window logic signal and excitation waveform in an oscilloscope in one embodiment. It can be seen that in areas with dense high-level signals, the amplitude of the simulated Blackman window function is large, and a low-frequency Blackman window function excitation signal can be output by controlling the logic signal.

[0118] In another embodiment, the suppression effect of the direct wave achieved by the method provided by the embodiment of the present invention is tested in a simulated formation model. The finite difference method is used to simulate the implementation effect of the window function excitation signal in the wellbore formation model. The formation has a well diameter of 0.2 m, a shear wave velocity of 2300 m / s, a longitudinal wave velocity of 4000 m / s, and a density of 2500 kg / m 3 The excitation signals are rectangular pulse signals and Blackman window function signals, which excite the dipole piezoelectric ceramic transducer and obtain the direct wave signal in the wellbore. Figure 3 As shown. Figure 3 It can be seen that the amplitude of the direct wave excited by the Blackman window function excitation signal is much smaller than that of the direct wave excited by the rectangular pulse excitation signal, thereby achieving the effect of suppressing the direct wave.

[0119] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit the present invention. Although the present invention is described in detail with reference to the embodiments, it should be understood by those skilled in the art that any modification or equivalent replacement of the technical solutions of the present invention does not depart from the spirit and scope of the technical solutions of the present invention and should be included in the scope of the claims of the present invention.

Claims

1. A dipole shear wave far-reaching logging sound source excitation method based on a low-frequency Blackman window function, characterized in that: The method comprises: Calculate the discrete value of the Blackman window function to obtain the parameters of the low-frequency Blackman window function; Setting a pulse width modulation control signal duty cycle based on the parameters of the Blackman window function to obtain a pulse width modulation control signal based on the Blackman window function; Amplifying and filtering the pulse width modulation control signal, filtering out the high-order harmonics of the pulse width modulation signal, and obtaining a low-frequency Blackman window function signal; The low-frequency Blackman window function signal matches the impedance and excites the dipole sound source transducer, which then emits a shear wave signal.

2. The sound source excitation method according to claim 1, characterized in that: The filtering adopts a second-order Butterworth filter to filter the Blackman window function signal, removes the high-order harmonics, and obtains the required low-frequency Blackman window function analog signal.

3. An excitation circuit, characterized in that: include: A power supply circuit, used for supplying power to the excitation circuit system; The control circuit is used to receive the transmission parameters sent by the host computer, provide the pulse width modulation control signal for the driving circuit, and perform the operation of the adaptive algorithm; A driving circuit for power amplifying and filtering a pulse width modulation control signal; The matching network is used to achieve impedance matching between the driving circuit and the dipole sound source transducer, so that the energy of the Blackman window function signal is transmitted to the dipole sound source transducer.

4. The excitation circuit according to claim 3, characterized in that: The power supply circuit adopts a flyback topology flyback switching power supply design, including: The input rectifier and filter circuit is used to rectify and filter the AC power at the input end to provide a DC voltage for the switching power supply; PWM control circuit, used to adjust the duty cycle according to the voltage feedback of the power supply circuit, and output a pulse width modulation signal to control the switching of the power switch tube of the power supply circuit; Power switch tube, used to convert DC voltage into high-frequency AC power, which is converted into the required output AC power through a high-frequency transformer; The output rectifier and filter circuit is used to filter the output high-frequency AC voltage, eliminate the ripple voltage, and obtain the required DC voltage.

5. The excitation circuit according to claim 3, characterized in that: The driving circuit is an H-bridge circuit structure, comprising: Optocouplers are used to electrically isolate digital circuits from analog circuits so that the control circuit chip is free from interference from the drive circuit and can work stably; A driver chip used to drive the bridge arm of the H-bridge class D power amplifier circuit; RCD snubber circuit, used to suppress the overvoltage applied to the H-bridge switch tube and the increase of turn-off loss; LC filter circuit is used to filter out the high-order harmonics of pulse width modulation signals.

6. The driving circuit according to claim 5, characterized in that: The driving circuit adopts two driving chips to respectively drive the two bridge arms of the H-bridge D-class power amplifier circuit to realize reciprocating excitation of the transducer in both positive and negative directions.

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