Multiple Filter Technique Research Articles

Evaluation of a tangential-flow multiple-filter technique for detection of Giardia lamblia cysts in water.

A system of tangential-flow filtration was evaluated for use in the detection of Giardia cysts in drinking water. This method was more sensitive in recovering cysts than a frequently used wound-orlon system of through-filtration.

Interstation surface wave analysis by Frequency-domain Wiener deconvolution and modal isolation

AbstractA new technique which combines frequency-domain Wiener filtering and modal isolation is developed for determining interstation phase velocities, group velocities, and attenuation coefficients for seismic surface waves. Frequency-domain Wiener filtering is more effective than time-domain Wiener filtering for the determination because it uses a smaller window lag which produces a smoother interstation Green's function. This leads to greater accuracy and stability when noise-contaminated data are analyzed. We optimize Wiener filtering in the frequency domain by applying two trapezoidal windows of different lags to the cross-correlation function between two stations and to the autocorrelation function of the first station, respectively. The windowed correlation functions are then transformed to the frequency domain. The interstation Green's function in this technique is the ratio of the smoothed cross-spectrum to the smoothed auto-spectrum of the first station. Frequency-domain Wiener filtering is equivalent to time-domain Wiener filtering when the same rectangular window is applied to both the correlation functions.Wiener filtering, however, cannot efficiently remove higher mode interference when the higher modes are superimposed on the fundamental mode in the correlation functions. To more thoroughly eliminate the effects of such interference, phase-matched filtering or time-variable filtering can be employed to isolate one particular mode at each of two stations. Frequency-domain Wiener deconvolution is then applied to calculate the Green's function. The interstation group velocites can be obtained by applying the multiple filter technique to the Green's function, and can be refined by phase-matched filtering. The amplitude and phase spectra of the Green's function are used to calculate attenuation coefficients and phase velocities, respectively, for the interstation medium.This new technique is compared with other methods by applying them to both noise-contaminated synthetic seismograms and real data. The proposed technique is found to be superior, particularly in period ranges where the signal-to-noise is low.

堆積層上における中・強震動の波群解析の試み

In order to make clear the effects of sedimentary layer on strong ground motion due to earthquakes, a tripartite array with strong-motion accelerographs of three orthogonal components has been deployed since 1982 in the campus of Tokyo Gakugei University, Koganei, Tokyo. The array is located at the western suburbs of the Tokyo Metropolis on Pleistocene sedimentary layer which reaches ca. 2000m depth overlying Pre-Tertiary basement. Digital accelerographs with sampling rate of 60Hz and with 12bits/word are installed on the array of 300-400m sides.Several events among all the data acquired by the present time are available for analyses; those are in the magnitude range of 5.7-7.3, in the depth range of 20-400km and in the range of epicentral distance of 47-340km. Although the maximum height and the phase of high frequency acceleration are strongly affected by variety of surficial portions (thickness_??_10m) underlying the foundations of seismographs, ground velocity and ground displacement of each station of the array show coherent signals with same amplitude but poorly discriminated phase lag each other up to the coda of the raw wave trains. However, filtrated waves of the raw data through some narrow pass bands exhibit clearly discriminated phase lags among the three stations throughout the wave trains right after the S-wave onset.Method of sums and the Fourier's analysis are adopted to derive apparent velocities of the later phases which are of considerable amplitude in ground displacement and ground velocity; the frequency range of the phases is 0.2-1.0Hz within which the velocity spectrum is in fairly high level. The results can be interpreted as surface waves excited in the thick sedimentary layer which covers the Tokyo area. Though the frequency dependence of phase velocity is clearly seen, it could not be confidently interpreted as a branch of unified single mode of surface wave. It is more likely to be interpreted as some branches of higher modes.A moving-window analysis with multiple filtering technique has given time-varying frequency spectrum of the strong-ground motion wave trains. It also shows a sequence of average group velocities between seismic foci and the array station. The results in conjunction with those of the phase velocity suggest laterally heterogeneous surface layer in and around the Tokyo area.

Determination of sediment shear properties by numerical modeling of ocean‐bottom interface wave

The importance of sediment shear properties for low‐frequency, shallow‐water propagation is well established. Whereas the compressional properties can be measured directly on collected samples, the shear speeds and attenuations have to be determined indirectly from in situ experiments. This is due to the fact that these properties are highly affected by the deterioration of the chemical and mechanical bindings due to depressurization and change of temperature in the core sampling process. The propagation characteristics of ocean‐bottom interface waves (Scholte waves) are almost entirely controlled by the shear properties, and these waves, therefore, form a convenient basis for the inversion process. Here, it is demonstrated how the dispersion characteristics, determined from experimental results by a multiple filtering technique, can be numerically modeled by a full wave field solution code [H. Schmidt and F. B. Jensen, J. Acoust. Soc. Am. 77, 813–825 (1985)], yielding the possibility of determining both shear speed and shear attenuation profiles in the upper sediment layers.

Identification of smart–1 array data by multifilter technique

Abstract The purpose of this study is to develop a concise multiple filter technique to analyze the nonstationary characteristics of seismographs. The display of instantaneous amplitude as well as particle locus provides simultaneous description of frequency and time domain characteristics for an accelerogram. Based on the orbit spectrum analysis of three‐dimensional seismic waves, the wave types can be identified through the study of SMART‐1 array data. The mathematical form of coherence has been proposed to examine the spatial variation of these identified seismic waves. Finally, the application of spatial variation of seismic waves for soil‐structure interaction is discussed.

Higher order accuracy in multiple filter technique

AbstractIt is known that there are systematic errors in the determinations of group velocity and amplitude spectrum by multiple filter technique (MFT) due to change of group velocity with frequency. Here, we obtain the relation of each of these systematic errors with the variations of group velocity, amplitude spectrum, and initial phase with frequency. This is achieved by expanding (in the theory of MFT) wavenumber, amplitude spectrum, and initial phase by Taylor series and retaining some of the higher order terms. It has been seen that the above variations lead measured group velocity by MFT to the group velocity of neighboring frequency toward which amplitude spectrum is increasing. Based on the relations obtained, two methods by MFT are proposed for improved determinations of group velocity, and amplitude and phase spectra of dispersive waves. The methods are tested on a theoretical seismogram.

Measurement of interstation phase and group velocities andQusing Wiener filtering

abstractA method for calculating interstation phase and group velocities and attenuation coefficients using a Wiener (least-squares) filtering technique is presented. The interstation Green's (or transfer) function is estimated from surface wave data from two stations laying along the same great circle path. The estimate is obtained from a Wiener filter which is constructed to estimate the signal recorded at the station further from the source from the signal recorded at the nearer station.The interstation group velocity is obtained by applying the multiple-filtering technique to the Green's function, and the interstation phase velocity from the phase spectrum of the Green's function. The amplitude spectrum of the Green's function is used to calculate average attenuation between the two stations. Using synthetic seismograms contaminated by noise, it is shown that the Q values calculated from the Green's function are significantly more stable and accurate than those obtained by taking spectral ratios.The method is particularly useful for paths involving short station separations and is applied to a surface wave path crossing the Iranian Plateau.

Observation and inversion of surface wave group velocities across central India

abstractSurface wave group velocities across central India are measured from computer plots of frequency-time analysis using multiple-filter technique. Periods range from 5 to 56 sec for fundamental mode Rayleigh wave, 5 to 16 sec for first higher mode Rayleigh wave, and 5 to 49 sec for fundamental mode Love wave. The wave paths from Koyna earthquakes to Bokaro cross purely through the Peninsular Plateau. The group velocities for these paths are found to be similar to those of surface waves recorded at Poona from earthquake which lie to the northeast of Bokaro in sub-Himalayan regions. From this, it is concluded that the shield structure of the peninsula extends to the sub-Himalayan regions to northeast of the Peninsular Plateu with a negligible part of sedimentary layer. Group velocities of fundamental mode Rayleigh and Love waves are also obtained for the paths from Koyna earthquakes to Shillong and are found to be similar to the above-mentioned group velocities except at very low periods where group velocities are affected by the small part of each path through thick sedimentary layer of Bengal Basin. Northern limit of this basin sedimentary layer has been identified.Mean and standard deviation of observed group velocity at each period are obtained for each of Rayleigh fundamental, Rayleigh first higher, and Love fundamental modes. The group velocities closely agree with those obtained earlier for the path between New Delhi and Kodaikanal and are also found to be similar to those observed in some shield regions.Monte Carlo inversion method is applied and crust and upper mantle structure of the Indian Shield across central India is obtained. Crustal structure obtained shows a marked resemblance to the results obtained earlier through body wave study.

Short-period surface waves across the Iberian Peninsula area

A large number of records of near earthquakes, for the period 1950–1975, have been revised at Toledo Observatory (I.G.C.), looking for clear trains of surface waves. All records used correspond to the Wiechert, WWSS or HGLP seismometers of this station. Multiple filtering techniques have been applied to the selected seismograms to obtain Love and Rayleigh fundamental mode dispersion curves. As most of the paths are of mixed continental and oceanic structure, the first interpretation has been made considering an average model for each set of similar seismic paths. However, for some regions, the separation of the two structures (pure continental and pure oceanic) has been attempted.

Optimum filter for surface-wave group-velocity determination

abstract An exponentially decaying function in time domain has been used as a filter for group-velocity determination with multiple filter technique. The bandwidth of the optimum filter is obtained minimizing time duration of the filtered signal. Sufficient resolution for practical purposes is obtained with bandwidths proportional to the center frequencies.

Estimations of Q -1 from Seismic Rayleigh Waves

The specific attenuation factor, Q(_γ)(^-1), has been estimated from seismic Rayleigh waves in the frequency range 0.015-0.11 Hz. The 95% confidence limits determine a narrow region around all estimates. The observational data consists of digitised Rayleigh wave traces from film chips of the long period vertical component instruments of the WWSSN stations. Events used are nuclear explosions in Movaya Zemlya, the Lop Nor region of China (Southern Sinkiang Province) and the Aleutian Islands. The group velocity and spectral amplitudes are obtained for each seismogram using an improved multiple filter technique Q(^-1)(_ γ) is estimated by a least squares regression fit to the subsequent amplitude distance plots. Values of Q(^-1)(_ γ) are generally larger when determined from Novaya Zemlya (.004) than for the Lop Nor test site (.003). The largest values of (.009) are found at low frequencies (0.02 Hz), implying a zone of high dissipation in the upper mantle sampled by these frequencies alone.-1The observed values of Q(^-1)*_ Q(^-1)(_ γ) are directly inverted using an extended Monte-Carlo technique - Hedgehog. This successfully inverted the data from Novaya Zemlya revealing a region of high dissipation coincident with the low velocity zone, although low velocity is not assumed. The inversion model shows Q(^-1)(_a) = .002, Q(^-1)(_β) = .0045 for the uppermost 120 km and Q(^-1)(_a) = .007, Q(^-1)(_β) = .015 (Q(_a) = 140, Q(_β) = 65) in the absorption zone below 120 km.

Filtering of Dispersed Wavetrains

Summary The filtering problem with dispersed signals is examined and an equation relating the duration of the filtered signal to the bandwidth of the filter and the dispersiveness of the dispersing system is established. This equation is applied in two domains: one, the optimization of the multiple filter technique, a method used for a time-frequency analysis of a signal; and two, the choice of parameters in a ‘time variable filter’, a technique used to isolate from a signal a wavetrain related to a dispersion curve. We give an application of this ‘time variable filter’ technique to isolate the fundamental mode of Rayleigh waves from a record of an earthquake 650 km deep.

Optimization of Filter Bandwidth in Spectral Analysis of Wavetrains

An equation relating the duration of a dispersed impulse to the bandwidth, and to the dispersiveness of the dispersing system, is used to obtain a filter bandwidth variation with frequency which will maximize the temporal resolution of the multiple filter technique used for the analysis of seismograms. Computational results based on a synthetic seismogram are given.

Response envelope spectrum and interpretation of strong earthquake ground motion

abstract A multiple filter technique is developed using a single-degree-of-freedom lightly-damped oscillator as a narrow band-pass filter. It provides for a physically simple approach to the analysis of accelerograph records from the point of view of structural engineering. The analysis of several typical accelerograph records indicates that a significant portion of strong earthquake ground motion consists of surface waves. It is concluded that the duration of intense shaking will be determined predominantly by the dispersion properties of the ground.

Great Circle Rayleigh and Love Wave Dispersion from 100 to 900 Seconds

Rayleigh and Love wave phase velocities for great circle paths from ordinary seismograms of World Wide Standard Seismographic Network /WWSSN/ stations

A technique for the analysis of transient seismic signals

Abstract An analytical method, called the “multiple filter technique,” is shown to be a fast and efficient means of studying multi-mode dispersed signals. Amplitudes and phases, as functions of period and velocity, are determined from the output of a set of narrow-band digital filters. The group velocities and other dispersion parameters determined by this technique are concordant with theoretical values when the method is tested with synthetic seismograms. It can recover broader portions of the dispersion present in ordinary seismic recordings compared to the classical peak and trough method. A simple diagnostic diagram is introduced in order to study the time and frequency resolution permitted by this analytic technique.