Dispersion curves obtained from well logs provide crucial formation information around the borehole. An essential step in the inversion of such curves is to eliminate aliased modes, which can potentially interfere with true modes at higher frequencies. The traditional approach involves manually matching the inversion slowness values with the corresponding wave modes at each frequency, which is time consuming and labor intensive. In addition, the traditional method is applicable for the elimination of the aliased modes that are far from true modes, but it is ineffective in resolving aliased modes that approach or even intersect with the true modes. To address these limitations, a novel method is developed to transform aliased modes into true modes. The proposed method involves two key steps. First, the slownesses of individual modes are automatically picked by the density clustering algorithm. Second, based on the Riemann sheets selection of slowness, aliased modes are transformed into true modes by using the single valuedness of the wave-mode spectra and the correspondence between the amplitude spectrum and the slowness. In addition, a theoretical background is provided by introducing the concept of Riemann sheets to explain the origin of aliased modes in the computation. The proposed method can obtain comprehensive and precise dispersion curves, even in cases for which the true modes are interfered with or crossed by aliased modes. The accuracy of the method is validated by comparing the inversion results with the forward model’s dispersion curves. Furthermore, the robustness of the approach is determined by applying it to synthetic waveforms with noise and field waveforms.

Acoustic logging is essential for probing formations surrounding a borehole. However, conventional logging signal processing methods cannot process inhomogeneous formation structures. Inspired by seismic full waveform inversion (FWI), this study proposed a novel borehole imaging algorithm called borehole full-waveform inversion (BFWI) to probe the near-formation properties surrounding boreholes. To implement BFWI, we modified the original form of the elastic wave equations in cylindrical coordinates. We then derived the adjoint equations and gradient expressions in a pure form, avoiding the calculation of spatial derivatives and reducing the complexity of the program implementation. Simultaneously, we added a gradient preconditioning step to the BFWI to suppress the strong gradients generated by the guide waves. In addition, a multiscale inversion strategy was used to accommodate the initial model without prior structural information surrounding the borehole. The inversion results of the five test examples showed interface and velocity properties coincident with those of the true models, proving the validity and accuracy of the proposed algorithm. BFWI is not constrained by the formation structure and has wider applicability than traditional methods, showing considerable potential for near-borehole formation imaging.

The accurate measurement of mass and centroid is indispensable for accurate control of aircraft. In order to eliminate the influence of assembly error and product pose error on the measurement results, a multi-station measurement idea that can self-compensate for the geometric parameter error is proposed in this paper based on. At the same time, the kinematic model of the mechanical structure of the measurement equipment is established to prove the effectiveness of the structure in error compensation theoretically, and the experimental verification of the standard part and aircraft is carried out. The test results show that the measurement results using the idea of the multi-station compensation measurement are significantly better than those of the more common methods, with the mass measurement accuracy of 0.03% and the centroid error within ±0.15 mm, meeting the requirement of high precision measurement for the mass properties.

A time-varying moment of inertia (MOI) measurement model based on the torsion pendulum method is studied. According to the principle of dynamics, the motion equation of torsion pendulum with single degree of freedom is established by using the second Lagrange equation modeling, which indicates the time-varying MOI can be obtained by the instantaneous undamped natural frequency of torsional pendulum motion and the torsion bar coefficient. Among them, the instantaneous frequency is calculated by the instantaneous angular frequency and envelope signal of angular displacement using Hilbert transform. The torsion bar coefficient is calibrated by the standard weight with known MOI, and has been compensated by temperature. Based on air floating turntable, the torsion pendulum system is built to measure the MOI of fine sand in the funnel, which can minimize the effects of mechanical friction. The relative error of MOI between calculating the result and measuring the value is less than 0.65%, which verifies the effectiveness of the proposed time varying temperature compensation method.

Theoretical studies predict that a slow compressional wave propagating in a fluid-saturated porous medium can produce a coseismic electric field due to the electrokinetic effect, but the experimental proof is still lacking. Laboratory experiments are conducted to measure such a seismoelectric conversion inside a synthetic rock. Fluid pressure signals are recorded by using mini hydrophones 1.6 mm in diameter, and then electric field signals generated at the liquid-solid interface and inside the rock sample based on the seismoelectric effect are recorded by electrode arrays, respectively. The seismoelectric waves induced by fast and slow compressional waves can be clearly identified in the recorded electric signals and their attenuation properties are analyzed at an ultrasonic frequency, which confirms that the seismoelectric signals induced by fast/slow compressional waves are measurable in the experiments. To support our explanation of the experimental observation, theoretical simulations are conducted according to the experimental model and then compared with the recorded experimental data. The results find that the simulated wavefields are in excellent agreement with those signals measured in the measurements, which proves the theoretical prediction of the seismoelectric signal accompanying the slow compressional wave and suggests a feasible way for detecting the slow compressional wave property with seismoelectric conversions in field measurements.

For precise measurement of the moment of inertia (MOI) of asymmetric objects, using an air bearing torsional pendulum, the nonlinearity of the torsion bar and the damping effect caused by the interaction between air and sample need to be included in the analysis. The instantaneous undamped natural frequency and instantaneous damping coefficient of the motion are obtained from the Hilbert transform of the angular displacement signal and are used for data fitting to calculate the MOI. Measurements for algorithm verification were made using a special sample with large windvanes to increase the damping effect. The nonlinear analysis of the MOI experimental results indicated a relative error of less than 0.1% compared with 0.3% for classical linear analysis of the data.

Acoustic logging while drilling (LWD) technology is extensively applied for petroleum prospecting. It utilizes the acoustic characteristics of stratum to deduce the formation properties such as porosity, permeability, and oil saturation, etc., which can help to detect the location of oil reservoirs. In acoustic logging process, the formation longitudinal (P) and transverse (S) wave velocity is the primary factor of interest. Nevertheless, an accurate identification of P/S wave velocity in different formations is hard to be performed due to the influence of the collar waves, which has become an urgent problem in LWD measurements. To study this issue more practically, a small-sized measurement system based on the propagation mechanism of collar waves is established in our laboratory. With this system, we design and perform multiple sets of monopole acoustic LWD experiments with different models. After recording the experimental data, the attenuations of inner and outer collar waves received along the axial direction are compared and analyzed. The result shows that the collar waves have an exponential decay in amplitude by the expansion of source-receiver distance, but the outer collar wave decreases more rapidly. We also conduct experiments at different source frequencies (20-400 kHz) to quantitatively describe the relationship between the amplitude and frequency. The result suggests that the inner/outer collar waves and P/S waves have a similar frequency response, and it's difficult to remove collar waves from the full waveforms of LWD signals by only changing source frequency. Therefore, we present a new data processing method for extracting the formation wave velocities according to the energy distribution of the componential waves, and then apply it in the analysis of two borehole measurements. The inner and outer collar waves are almost removed from the processed time domain waveforms, which leads to a successful verification of formation P/S wave velocities. These experimental results prove the feasibility and repeatability of our method, and propose an alternative method for the LWD data explanations.

The accurate measurement of aircraft mass properties, such as the mass, centroid, and moment of inertia (MOI), plays a key role in the precise control of aircraft. In order to obtain high-precision information on the parameters of the mass, centroid, and MOI of an aircraft using a single instrument, an integrated mass property measurement system was developed in this study by analyzing and comparing the latest technologies, especially the function-switching device, which switches the measurement states between the center of mass and the MOI. The purpose of mass property measurement was achieved through single clamping. In addition, the system has strong versatility and expansion and can be used with different tooling or adapter rings to measure the mass properties of aircraft with different shapes. In this paper, the main mechanical structure of the measurement system, the measurement method of relevant mass parameters, and the solution method of the transformation matrix are introduced, and the standard parts and the aircraft were verified experimentally. The test results showed that the mass measurement accuracy was 0.03%, the centroid measurement error was within ±0.2 mm, and the measurement accuracy of the MOI was within 0.2%, all of which meet the high-precision measurement requirements for the mass properties.

For periodic time-varying systems, a method of parameter identification based on the block-pulse function is presented. Firstly, the state-space equation of the system was expanded using the block-pulse function, then the recursion formula of the parameter identification of a time-varying system was obtained, according to the irrespective and orthogonal characteristics of the block-pulse function. This study provides a wide range of applications by saving time in calculation with a highly accurate method. The parameter identification was carried out by including the numerical simulation model of a three-degree freedom system and the vibration experiment results of an asymmetrical rotor system. The state space wavelet method and EMD method were compared cross-sectionally with the proposed method; this shows that the proposed method is accurate and effective, which makes it valuable in numerous applications. It also has a certain application value for several related projects.

Analytic solutions are derived for the problem of scattering of obliquely incident plane waves from a rectangular crack in a fluid-saturated porous solid. The propagation direction and polarization direction of the incident waves can be arbitrary. The solutions are based on expanding the crack opening displacements in terms of Chebyshev functions. Explicit expressions for the frequency-dependent stress intensity factors and scattering far fields are obtained. The poroelastic effects play a key role in determining the crack opening mode by changing the effective normal stress applied on the crack surfaces. Consequently, the dynamic responses of the crack can be quite different from those of conventional dry crack in an elastic solid. The effects of scattered slow-P waves as well as wave-induced crack fluid pressure on the dynamic stress intensity factors are clarified, and a corner effect is observed in the scattering patterns for moderately high frequencies. The comparisons of synthetic seismograms with the elastic model show the slow-P waves can cause significant energy loss and waveform change. This study can be used for fracture analysis of porous media and nondestructive testing for individual hydraulic fractures in reservoirs.

To accurately detect the P-wave velocity is very important in acoustic logging while drilling (LWD) measurements, but it is hard to be performed from the recorded LWD signals because of the influence of strong collar waves. Therefore, we conduct experimental studies on the properties of acoustic LWD signals in the laboratory, and focus on the measurements of P wave velocity in this paper. Firstly, we measure the monopole inner and outer collar waves in an infinite fluid model, then quantitatively study the frequency response of the collar waves excited by different source frequency. The results show that both the inner collar wave and the outer collar wave become stronger at low frequency region, then become weaker at high frequency region, as the source frequency increases gradually. However, the rate of the change varies for the inner and outer collar waves. Secondly, we carry out experimental measurements of the monopole acoustic LWD in a small-size sandstone borehole model, and analyze the amplitude differences of the collar waves between these two models. The results indicate that the inner collar wave is the main contribution in the full waveform of LWD measurement. Finally, we propose a method for extracting P-wave velocity based on the distribution of collar wave energy, then verify the feasibility of the method with the experimental data. The results indicate that subtracting the inner collar wave from the full waveform of LWD signal can effectively reduce the influence of the collar wave on P-wave. Therefore, the P-wave velocity can be measured accurately after data processing, even if the detector is not grooved. Our results suggest a meaningful method for detecting the P-wave velocity in the field measurements.

When a seismic wave propagates in a fluid-saturated porous medium, a relative movement forms between the solid and the fluid and induces an electric current due to the electronic double layer. As a result, two kinds of seismoelectric coupling responses are generated in this procedure: the localized electric/magnetic field and interfacial electromagnetic wavefield. One important potential application of these two seismoelectric conversions is used for measuring formation P and S waves in well logging. Considering that the strong collar wave seriously affects the velocity measurements of formation P and S waves in current acoustic logging while drilling (LWD), the seismoelectric LWD method, which combines seismoelectric conversion and acoustic LWD technique, was suggested to be a novel method in oil and gas exploration. The collar wave cannot induce any seismoelectric signal on the metal collar since there is no double layer formed on a metal surface. In this paper, acoustic and seismoelectric LWD measurements are conducted in the laboratory. We build a scaled multipole acoustic LWD tool to conduct acoustic measurements in a water tank and a sandstone borehole model. We also build a multipole seismoelectric LWD tool and record the seismoelectric signals induced with the same acoustic source. Then, we compare the recorded acoustic and seismoelectric signals by using the experimental data. The result indicates that the apparent velocities of seismoelectric signals are equal to the formation P- and S-wave velocities and the collar waves do not induce any visible electric signal in the full waveforms. We further analyse the mechanism of seismoelectric LWD by a quantitative comparison of the amplitudes between the inner collar wave and outer collar wave. The results show that the amplitude of outer collar wave decreases significantly when it radiates out of the tool, so that the seismoelectric signals induced by collar waves are too weak to be distinguished in the full waveforms of seismoelectric LWD measurements. Thus, the formation P- and S-wave velocities are detected accurately from the recorded seismoelectric LWD data. These results verify the feasibility of the seismoelectric LWD method for measuring acoustic velocities of the borehole formation.

Theoretic studies show that there are two kinds of seismoelectric signals that can be induced with acoustic waves. First are electric and magnetic fields induced by the propagation of a seismic wave in a homogeneous porous medium; second are EM waves generated at an interface or through a heterogeneous medium. The seismoelectric signal induced inside the porous medium is a localized and stationary electric signal. When an acoustic wave propagates across an interface, a radiating electromagnetic (EM) wave can be generated due to the different properties of the interface media. A layered model is built with Lucite block and epoxy-glued sand. Five sets of electrodes are buried in the glued sand to measure the electric signals in the sand generated by a plan acoustic transducer. Experiments show the seismoelectric EM wave can be induced by a seismic wave at an interface and received by an electrode at the different location. When an acoustic wave propagates inside a porous medium, it induces a localized and stationary electric field. The induced electric signal can be recorded only at the area where the acoustic wave arrives. It does not propagate in the porous medium. Its apparent velocity is the acoustic wave velocity.

Thorough understanding of seismic signatures in cracked rocks is essential to estimate rock physical properties. Wave-induced fluid flow (or diffusion), scattering and Biot's global flow are three major mechanisms in controlling frequency-dependent attenuation and dispersion. To shed light on how those mechanisms and their interference affect the anisotropic features in cracked porous rocks, we develop an analytic model to estimate the angle-dependent attenuation and dispersion in such media. The most noteworthy feature of the model is that it is developed upon the exact solution of the problem of elastic wave scattered by a crack at oblique incidence. In particular, the poroelastic spring condition is applied to describe the influences of crack thickness and crack-filling fluid elasticity on wave scattering. Regardless of its complexity, we have showed that the model agrees with many benchmark theories under corresponding conditions, demonstrating its reasonability. It is found that the key factors that dominate anisotropic attenuation and dispersion are different in separating frequency regimes. At diffusion-dominated frequencies, the frequency-dependent anisotropic properties are mainly determined by the normal stress on the crack faces. In contrast, in Rayleigh scattering regime, they are greatly determined by the applied shear stress. At higher frequencies (Mie scattering regime), affected by the wave reflections between the crack edges, the frequency-dependent anisotropy becomes complex. The angle-dependent velocity can largely deviate from elliptic-shaped profile. As a result, the material properties cannot be described within the framework of the transversely isotropic medium model. Moreover, it is found that the attenuation is sensitive to the fluid compressibility and crack thickness, showing evidences that it is possible to invert fluid saturation and permeability from seismic attenuation. We also conclude that using a simple linear superposition of the attenuations due to wave-induced fluid flow and elastic scattering from their corresponding equivalent medium models will leads to an overestimation of the total attenuation. Our results demonstrate it is necessary to account for the mechanism interference to allow for an adequate estimation of the intrinsic attenuation of cracked porous rocks.

Wave-induced fluid flow (WIFF) between cracks and micropores is one of the major mechanisms causing attenuation and dispersion within seismic frequency ranges. Previous non-interaction-approximation (NIA) models often assume that the distribution of cracks is dilute, neglecting the influences of interacting cracks on dispersion and attenuation. To overcome this restriction, we have investigated the interaction between coplanar cracks and their influences on seismic dispersion and attenuation. First, a scattering problem for a longitudinal (P) wave normally impinging on a plane with equally distributed coplanar cracks in a porous medium is solved using an integral transform approach. Then, based on the solution, an effective wavenumber is derived for P-wave propagation in a porous material with coplanar cracks. It is found that the magnitude of dispersion and attenuation can significantly increase when the spacing between adjacent cracks decreases even if the crack density is unchanged. Moreover, frequency-dependent asymptotic behavior of inverse quality factor is also different from that of the NIA models at frequencies lower than the WIFF relaxation frequency. Specifically, the inverse quality factor scales with the square root of the frequency at low frequencies. When the spacing between adjacent cracks is large, an additional frequency-dependent scale occurs at relatively higher frequencies (but still lower than the WIFF relaxation frequency) with inverse quality factor scales with the first power of frequency. When the spacing becomes much larger so that the interaction between the adjacent cracks is negligible, the present model exactly reduces to an NIA model for the distribution of aligned slit cracks and the first power scale can maintain attenuation within low frequencies.

The dynamic permeability and Electrokinetic Coupling Coefficient (ECC) are key parameters to characterize fluid flow and coupling between elastic and electromagnetic wavefields in porous rocks. Based on the model of capillary tube bundles for porous media, we derive the analytical expressions of the dynamic permeability and the ECC. By comparing with the approximate expressions in previous studies, we examine the validity and applicability of the analytical expressions proposed in this study. Besides, we further propose the simplified expressions for the dynamic permeability and the ECC. On this basis, we theoretically analyze the influences of the pore geometry and pore-size distribution on the dynamic permeability, the ECC and the seismoelectric logging wavefields. The results show that the influence of the pore geometry is ignorable, while it is over-valuated by previous studies. The influence of the pore-size distribution is significant. With the expansion of the pore-size distribution, the critical frequency of the permeability decreases but that of the ECC increases significantly, and the amplitudes of Stoneley waves and the accompanied electric field also increase significantly.

为了认识测量信号所对应的力学类型及相应类型的钻铤波在钻铤内的分布,该文将理论模拟的随钻声波测井波形与实验波形进行对比.与前人单独考虑压电效应或井孔传播效应不同,该文模拟单极随钻声波测井响应时,将发射器、接收器、光滑钻铤和井孔结构作为一个整体,采用有限元法计算模拟了电压源激励下接收器记录的声压信号和位移信号.将模拟的声压波形与电压信号进行比较,发现二者的钻铤波与斯通利波相对幅值相差较大,而模拟的径向位移波形更接近电压信号.进一步比较理论波形与小模型井内实验测量到的电压信号,证实电压信号更接近位移信号而与声压信号差异明显.这表明当钻铤光滑时,单极随钻声波测井换能器感知的主要是径向位移信号.研究还表明声压信号中的钻铤波能量主要集中在钻铤内壁,径向位移信号中的钻铤波能量主要集中在钻铤外壁.

To develop the new logging while drilling (LWD) method with seismoelectric effect is one of the current focused topics in geophysical exploration. To further understand the coupling mechanism between the elastic field and electromagnetic field in LWD measurements, seismoelectric LWD measurements are conducted in a scaled sandstone borehole with multipole sources (monopole/dipole/quadrupole). The time domain waveforms of seismoelectric LWD signals are recorded by a high resolution experimental set-up in the laboratory. They are analyzed and compared with the signals measured from the acoustic LWD method. The comparison shows that the relative amplitudes of the seismoelectric signals induced by collar waves decrease significantly in the full waveforms of seismoelectric LWD, so that the formation wave velocities can be obtained accurately from the logging data. This confirms the theoretical prediction that the seismoelectric LWD can effectively restrain the influence of collar waves on formation waves. The coupling and attenuation characteristics of seismoelectric signals induced by collar waves are further studied based on the experimental data. Then a quantitative analysis on the mechanism of seismoelectric LWD technique is presented in this paper by combining the seismoelectric LWD measurements in a Lucite borehole and the acoustic LWD measurements in the water tank without borehole. This work is also helpful for data interpretation of theoretical simulations.

Seismoelectric measurements are conducted with a synthetic porous rock sample to model an ocean exploration. Two kinds of seismoelectric coupling signals, that is, the interfacial EM wave signal and the coseismic electric signal, have been recorded by the electrodes buried inside a rock sample instead of those located in the fluid or in the solid region near the interface as performed in previous works. These seismoelectric signals are clearly observed and identified with a high signal-to-noise ratio. The characteristics of the measured interfacial EM wave and coseismic electric signals are analysed with the experimental data. We also simulate the seismoelectric conversion fields and make a comparison between the measured and simulated seismoelectric signals. The result shows that the simulated and measured signals match well for both the interfacial EM wave and the coseismic electric fields accompanying the fast P wave. Our results also show that the amplitudes of seismoelectric signals are in the order of tens to hundreds of microvolts with our experimental system. This confirms that the seismoelectric signals are measurable in the interior of the rocks with current measurement techniques, suggesting the seismoelectric measurement to be a potential method for studying characteristics of the material beneath the seafloor.

沉积地层"速度—深度"线性模型仅考虑了沉积物压实作用,无法适应沉积地层厚度达到一定程度时,或者其他构造因素起作用时,速度随深度的增长率减小的情况.因此,笔者比较了指数公式、幂函数和二次多项式的数学性质,并应用于地震剖面速度谱"时间—层速度"关系的拟合,以拟合优度大小为依据,在不同区域采用二次多项式和幂函数分别拟合,称为地震剖面沉积地层多公式拟合时深转换方案.该方案可应对各种构造环境中沉积地层的时深转换工作,可适用范围较广.