Frequency Shift Method Research Articles

Monitoring of seismic attenuation change associated with vapor-liquid phase transition using time-lapse reflection seismic data in Kakkonda geothermal field, Japan

Seismic wave attenuation can potentially be used for the investigation of the fluid phase and flow properties in a fractured reservoir. However, seismic attenuation estimation is easily affected by various factors, such as tuning effects, source and receiver coupling, and effects from near-surface weathering layers. Time-lapse seismic attenuation estimation can avoid these problems because the above factors are canceled between surveys. We conducted time-lapse attenuation estimation using three reflection seismic surveys with different timings acquired in the Kakkonda geothermal field during a 12-day period spanning a well shut-in period. To investigate geothermal fluid changes associated with increased pore pressure, we propose an approach for estimating attenuation changes between time-lapse seismic surveys. Firstly, three seismic reflection sections were obtained from the corresponding time-lapse seismic data using the modified prestack time migration method, which can handle the uncertainties of migration velocity estimation in a highly fractured and structurally very complex medium, for which accurate velocity estimation is difficult to achieve using conventional methods. Then, time-lapse attenuation was evaluated for two of the three seismic reflection sections using the spectral ratio and the centroid frequency shift methods. Numerical modeling was conducted to verify our proposed procedure. Careful preprocessing was carried out to enhance the repeatability of time-lapse seismic data, especially on the frequency content, which is indirectly related to seismic attenuation. Finally, we demonstrated an increase of attenuation associated with an increase of pore pressure in and around the known super-shallow vapor-dominated reservoir during a short period (7 days) spanning from one day before the shut-in of producing well to a week after that. A possible cause of this short-period attenuation increase could be the vapor-liquid phase transition in the reservoir.

Impedansa sistema uzemljenja - opseg frekvencijske linearnosti

The measurement of grounding system impedance of in-service power facilities is a very complex issue since the power system frequency interference and harmonic frequencies may adversely affect the measurement. Recently, to overcome this problem the Frequency Shift Method (FSM) has been proposed. FSM requires that two of three test frequencies must be above power system frequency (in our case 50 Hz). However, due to the nonlinear dependence of the grounding system impedance versus frequency, the choice of test frequencies is not arbitrary. In this paper, this issue is experimentally examined based on the analysis of grounding system impedance linearity range. For this purpose, the grounding system impedance, for a sample of 16 substations (HV and MV), is examined. We find that the highest test frequency should be ≤80 Hz.

Pregled metoda za eliminisanje uticaja sistemske smetnje u aktivnom visokonaponskom postrojenju

In this paper, practical and conceptual problems related to the way of suppressing or eliminating the influence of interference (at a frequency of 50 Hz) in an active high-voltage power facility are discussed. The problem is relevant in the context of measuring the safety characteristics of the grounding system: grounding system impedance, touch voltage and step voltage. The paper gives an evolutionary overview of title methods. The review focuses on the characteristic problems and shortcomings of individual methods. Also, the basic characteristics of the author's FSM (Frequency Shift Method) method, which guarantees precise control of interference, are elaborated. In this sense, the FSM method is the basis for accurate and economical measurement of the safety characteristics of the grounding system.

Experimental Observation of Temperature Change Induced by Echo Train Length of Fast-Spin Echo at 3T MRI

This study aims to observe and estimate the temperature changes caused by the echo train lengths (ETLs) of fast-spin echo to ensure radiofrequency (RF)-related safety. RF-induced heating on ETL-stepped changes is evaluated using the proton resonance frequency shift (PRF) method and optic-fiber thermography (OFT). The increase in temperature caused by a clinical sequence was compared to that accumulated by the corresponding ETLs. The temperatures are similarly increased in ETLs from 10 to 30 (PRF: 0.109 ± 0.025 ℃, OFT: 0.079 ± 0.009 ℃), suggesting that a large number of ETLs could be utilized at practical sites. The RF-induced heating in the clinical setting (PRF: 0.775 ± 0.069 ℃, OFT: 0.510 ± 0.015 ℃) is comparable to the estimated temperature on the ETL-stepped changes (PRF: 0.780 ℃, OFT: 0.553 ℃). These enable accelerated acquisition with a large number of ETLs to be practically used and RF-induced safety to be thoroughly guaranteed.

Continuous time-varying Q-factor estimation method in the time-frequency domain

The Q-factor is an important physical parameter for characterizing the absorption and attenuation of seismic waves propagating in underground media, which is of great significance for improving the resolution of seismic data, oil and gas detection, and reservoir description. In this paper, the local centroid frequency is defined using shaping regularization and used to estimate the Q values of the formation. We propose a continuous time-varying Q-estimation method in the time-frequency domain according to the local centroid frequency, namely, the local centroid frequency shift (LCFS) method. This method can reasonably reduce the calculation error caused by the low accuracy of the time picking of the target formation in the traditional methods. The theoretical and real seismic data processing results show that the time-varying Q values can be accurately estimated using the LCFS method. Compared with the traditional Q-estimation methods, this method does not need to extract the top and bottom interfaces of the target formation; it can also obtain relatively reasonable Q values when there is no effective frequency spectrum information. Simultaneously, a reasonable inverse Q filtering result can be obtained using the continuous time-varying Q values.

Determination of the Attenuation Factor via Seismoacoustic Studies in Wells to Estimate the Distribution of Heterogeneities in Artificial Soils

Quality control by hidden works in artificial soils is a relevant problem in construction engineering. Seismoacoustic methods are widely used for its solution. In this work, seismoacoustic well logs in artificial soils manufactured by the jet grouting technology were processes to obtain additional data on the distribution of heterogeneities in an artificial soil. The spectral ratio and frequency shift methods were used to estimate the attenuation factor.

<i>Q</i>-factor determitation with seismoacoustis well logs for estimation of distribution of inhomogeneities in artificial grounds

Quality control in geotechnical engineering is becoming an important issue, especially when products are buried underground. Seismoacoustic methods are widely used for investigations in such structures. In this article seismoacoustic well logs from hidden structures (made by jet grouting technology) are being processed with the goal of obtaining additional information about distribution of heterogeneities in improves grounds. Two methods of estimation Q-factor are used: frequency shift method and amplitude ratios method.

Compositional variations along the route of Chang\u2019e-3 Yutu rover revealed by the lunar penetrating radar

Using the high-frequency lunar penetrating radar data obtained by the Chang’e-3 mission, we apply the frequency-shift method to calculate the decay rate of the electromagnetic wave in the regolith-like ejecta deposits of the Ziwei crater. The radar data are divided into segments according to the navigation points along the traverse route of the Yutu rover. For each segment, we calculate the bulk loss tangent of materials within the top ~ 50 ns of the radar data based on the frequency decreasing rate of the electromagnetic wave. The loss tangent varies from ~ 0.011–0.017 along the route of Yutu, and it is within the range of the measured loss tangent of Apollo regolith samples. Using the empirical relationship between loss tangent and TiO2 + FeO content derived from the Apollo lunar samples, we estimate the TiO2 + FeO content for the bulk regolith along the route of Yutu, which is ~ 23–30 wt.%. This value is comparable with that estimated using both orbital reflectance spectral data and in situ observation made by the Yutu rover. The loss tangent derived along the route of Yutu is larger than the average value of returned lunar samples, which is mainly caused by the larger content of TiO2 + FeO at the landing site compared to the global average. Variations of the TiO2 + FeO content along the route of Yutu are mainly due to the excavation of the Ziwei crater. The TiO2 + FeO content map derived by the radar has a much higher spatial resolution compared to orbital observation, testifying the feasibility of this technique for regional geology study.

Estimation of microfracture porosity in deep carbonate reservoirs based on 3D rock-physics templates

Carbonate reservoirs in the S area of the Tarim Basin (China) are ultradeep hydrocarbon resources, with low porosity, complex fracture systems, and dissolved pores. Microfracturing is a key factor of reservoir connectivity and storage space. We have performed measurements on limestone samples, under different confining pressures, and we used the self-consistent approximation model and the Biot-Rayleigh theory of double porosity to study the microfractures. We have computed the fluid properties (mainly oil) as a function of temperature and pressure. Using the dependence of seismic [Formula: see text] on the microfractures, a multiscale 3D rock-physics template (RPT) is built, based on the attenuation, P-wave impedance, and phase velocity ratio. We estimate the ultrasonic and seismic attenuation with the spectral-ratio method and the improved frequency-shift method, respectively. Then, calibration of the RPTs is performed at ultrasonic and seismic frequencies. We use the RPTs to estimate the total and microfracture porosities. The results indicate that the total porosity is low and the microfracture porosity is relatively high, which is consistent with the well log data and actual oil production reports. This work presents a method for identification of deep carbonate reservoirs by using the microfracture porosity estimated from the 3D RPT, which could be exploited in oil and gas exploration.

Toroidal dipole bound states in the continuum metasurfaces for terahertz nanofilm sensing.

A novel terahertz nanofilm sensor consisting of toroidal dipole bound states in the continuum (TD-BIC) inspired Fano resonance metasurface is proposed and investigated, which exhibits both the TD character and BIC feature. When the mirror symmetry of the unit cell was broken, the TD resonance was excited and demonstrated by anti-aligned magnetic dipoles and calculated scattering powers and the BIC mode was verified with the quality factor satisfying the inverse square law. Combined with the amplitude difference referencing technique, the TD-BIC inspired Fano resonance was utilized for nanofilm sensing at THz frequencies for the first time. Simulation results show that the amplitude difference can be easily observed by comparing the resonance frequency shift under difference thicknesses of germanium overlayer. Moreover, by coating with a 40 nm-thick analyte overlayer, the sensitivity of amplitude difference can achieve 0.32/RIU, which is a significant value and more suitable for sensing nanofilm analytes than the traditional frequency shift method. These advantages make our proposed structure have potential applications in sensing nanofilm analytes.

Q-factor estimation using bisection algorithm with power spectrum

The centroid frequency shift (CFS) method is a widely used [Formula: see text] estimation approach. However, the CFS approach assumes that the amplitude spectrum of a source wavelet has a particular shape, which can cause systematic error in [Formula: see text] estimation. Moreover, the amplitude spectrum at high and low frequencies is susceptible to random noise, which can reduce the robustness of [Formula: see text] estimation using the CFS method. To improve the accuracy and robustness of [Formula: see text] estimation, we have developed a [Formula: see text] extraction method using the bisection algorithm based on the centroid frequency shift of power spectrum (BPCFS). In the BPCFS approach, we first obtain the source and the received wavelet. Then, we calculate the centroid frequency of the attenuated wavelet and that of the received wavelet from the power spectrum. Based on the obtained centroid frequencies, we establish an equation containing only one variable — the [Formula: see text] factor. Introducing the Jeffrey divergence to measure the attenuation of the power spectrum, we prove that this equation has only one root when [Formula: see text] is greater than zero. The root of this equation, which is the desired [Formula: see text] factor, is obtained through the bisection algorithm; we do not make any assumption about the shape of the amplitude spectrum. The noise-free numerical tests indicate that the BPCFS gives more accurate results than the CFS, which demonstrates that the shape of the wavelet spectrum has little effect on the [Formula: see text] estimation accuracy for BPCFS. Gaussian random and blue noise tests also show that the stability of BPCFS is better than that of CFS. The frequency band selection for BPCFS is also more flexible when the amplitude spectrum of the seismic wavelet is band limited. The application to real vertical seismic profile data further demonstrate the effectiveness and feasibility of the new method.

A robust Q estimation scheme for adaptively handling asymmetric wavelet spectrum variations in strongly attenuating media

The central frequency shift technique for estimating wave attenuation in seismic exploration assumes a quasi-symmetric amplitude spectrum and has its limitations in low quality factor ([Formula: see text]) regions. The asymmetry of the wavelet amplitude spectrum becomes more pronounced during wave propagation, so using a constant parameter [Formula: see text] (the asymmetry index estimated) in the modified frequency-weighted exponential formula method to estimate [Formula: see text] still leads to errors and can degrade the results of inverse [Formula: see text]-filtering. We have derived a new [Formula: see text]-estimation scheme that does not require constant parameter fitting that also works for strongly asymmetric receiver spectra. It is based on forming a synthetic wavelet as the geometric mean of the source spectrum (approximated by the near-source receiver spectrum) and the subsequent attenuated (receiver) wavelet spectrum. The changing centroid frequency and variance of this new wavelet as a function of traveltime can automatically adapt to a changing asymmetric spectrum caused by attenuation. It yields more stable and accurate results, especially in low-[Formula: see text] regions. This adopted approach is successfully applied to synthetic data and vertical seismic profile field survey data and proves to be superior to previous frequency-shift methods.

Flat optical frequency comb generation based on a dual-parallel Mach-Zehnder modulator and a single recirculation frequency shift loop.

To generate a flat optical frequency comb (OFC), a new scheme based on a dual-parallel Mach-Zehnder modulator and a single recirculation frequency shift loop is proposed and analyzed. Compared with the traditional single loop recirculation frequency shift method, the quantity of comb lines is doubled, and the comb flatness is better when the number of cycles is the same. The theoretical analysis model is established, and the simulation results show that a 111-line OFC with frequency spacing of 10 GHz, flatness of 1.32 dB, and optical signal to noise ratio of 27.4 dB can be obtained by adopting the proposed scheme.

Closed-loop capacitive readout based on mode localization effect in weakly-coupled RLC circuits

Accurate detection of capacitance change is an essential requirement for capacitive sensors development. This paper presents a closed-loop circuit of weakly-coupled RLC resonators for high-resolution capacitive readout. Mode localization effect in the weakly-coupled RLC resonators is studied and the comparison shows superiority in linearity by a closed-loop method. The experimental results validate a capacitive readout resolution of 37.5fF and a 216 times improvement of sensitivity as compared with the frequency shift method. This work indicates that the proposed closed-loop configuration has a better performance than the open loop method, and is less expensive, more compact and much easier for practical application to detect capacitance variance based on mode localization effect in coupled RLC resonators, which makes it a promising method for developing ultra-sensitive capacitive sensors.

A Solution to the Parameter Selection and Current Static Error Issues With Frequency Shift Islanding Detection Methods

Frequency shift islanding detection methods have been widely used in inverter-based distributed generations. Two representatives of such methods, Sandia frequency shift (SFS) and reactive current perturbation (RCP) methods, are investigated in this article. The investigation reveals two main issues with these two methods, i.e., parameter selection scheme for positive feedback gain and current static error. The current parameter selection scheme, in particular, is considered unreliable and inconvenient in this article, for it has not considered the weakening effect of PI regulators on positive feedback and the positive feedback gain is actually obtained by trial and error, which makes it difficult to use such methods well in field environments. In view of these issues, this article proposes a solution that contains an improved method to eliminate current static error and a reliable and convenient parameter selection scheme based on system stability analyses. The final simulations and experiments verify the good performance of the improved method and the parameter selection scheme.

Research on smooth switching and islanding detection technology for ER system

For the energy router (ER) switching on/off frequently to select the optimal route, this study mainly solves the problem of on/off-grid switching and islanding detection of grid-connected inverters on the AC side of ERs. A control strategy based on disturbance observer is proposed to suppress the disturbance of the output current to the output voltage. At the same time, the improved pre-synchronisation method is used to achieve fast synchronisation of voltage amplitude and phase. Then, an improved frequency-shift method is proposed for islanding and non-detection zone detection for droop control grid-connected inverters. Finally, the proposed methods are verified by simulation and experiments.

Uticaj podzemnih kablova na merenje impedanse sistema uzemljenja primenom FSM koncepta

Ground impedance measurement in medium voltage substations, designed to operate in a predominantly cable network, is a complex research task. Namely, the inability to consider all the factors that can affect the measured value, in a situation where one deenergized cable line is used to form the Fall-of-Potential (FOP) measurement scheme, is directly reflected to an adequate assessment of safety risks. This paper proposes an alternative measurement method so called Frequency Shift Method (FSM), which inherently guarantees "better" measurement than any of the conventional methods. By providing insight into the behavior of the active and reactive ground impedance components, and at the same time taking into account the topological structure of the existing cable network, the application of the FSM concept in the FOP measurement scheme points out to alternative paths of test current distribution, and in that way pointing to the illogicality of the obtained measurement results. Experimental verification was conducted under real operating conditions, and can serve as a guide for solving similar problems in practice.

Recent technological advancements in thermometry.

Thermometry is the key factor for achieving successful thermal therapy. Although invasive thermometry with a probe has been used for more than four decades, this method can only detect the local temperature within the probing volume. Noninvasive temperature imaging using a tomographic technique is ideal for monitoring hot-spot formation in the human body. Among various techniques, such as X-ray computed tomography, microwave tomography, echo sonography, and magnetic resonance (MR) imaging, the proton resonance frequency shift method of MR thermometry is the only method currently available for clinical practice because its temperature sensitivity is consistent in most aqueous tissues and can be easily observed using common clinical scanners. New techniques are being proposed to improve the robustness of this method against tissue motion. MR techniques for fat thermometry were also developed based on relaxation times. One of the latest non-MR techniques to attract attention is photoacoustic imaging.

Seismic Attenuation Estimation Using the Centroid Frequency Shift and Divergence

Quality factor $Q$ can be used for hydrocarbon detection and reservoir characterization. The commonly used methods for $Q$ estimation include the logarithm spectral ratio (LSR) method, the centroid frequency shift (CFS) method, and the peak frequency shift (PFS) method. However, all these methods are existing limitations respectively. The LSR method is sensitive to random noise. The CFS and PFS methods are lack of applicability owing to their assumptions of the source wavelet type. To overcome these limitations, we derived a new formula without any wavelet assumption. This formula establishes a relationship between the $Q$ factor and the spectral centroid downshift concisely through the Jeffery divergence. The proposed method is named the generalized centroid frequency shift (GCFS) method. Compared with the conventional methods, the proposed method can achieve higher accuracy and better antinoise performance. In addition, the proposed method is suitable for various source wavelet types. Synthetic and field examples demonstrate the effectiveness and potential of the proposed method.

Multinuclear absolute magnetic resonance thermometry

Non-invasive measurement of absolute temperature is important for proper characterization of various pathologies and for evaluation of thermal dose during interventional procedures. The proton (hydrogen nucleus) magnetic resonance (MR) frequency shift method can be used to map relative temperature changes. However, spatiotemporal variations in the main magnetic field and the lack of local internal frequency reference challenge the determination of absolute temperature. Here, we introduce a multinuclear method for absolute MR thermometry, based on the fact that the hydrogen and sodium nuclei exhibit a unique and distinct characteristic frequency dependence with temperature and with electrolyte concentration. A one-to-one mapping between the precession frequency difference of the two nuclei and absolute temperature is demonstrated. Proof-of-concept experiments were conducted in aqueous solutions with different NaCl concentrations, in agarose gel samples, and in freshly excised ex vivo mouse tissues. One-dimensional chemical shift imaging experiments also demonstrated excellent agreement with infrared measurements.