Arrival Time Fluctuations Research Articles

東北地方北部で観測される遠地P波の振幅及び走時の空間的変動

東北地方北部で観測される遠地P波の振幅及び走時の空間的変動

Measurements of ultrasonic pulse arrival time differences produced by abdominal wall specimens

The influence of propagation medium inhomogeneities on pulsatile ultrasonic fields has been investigated experimentally. The study employed a special curved transducer to produce a hemispherical wave pulse and a linear array to measure the resulting field along a line in a plane. Translation of the array in the elevation direction yielded data over a two-dimensional aperture. Time delay across the aperture was calculated by adding delay differences obtained by cross-correlating signals on adjacent elements and noting the position of the cross-correlation peaks. Received waveforms were shifted an amount given by the difference between the actual arrival time and a calculated geometric delay to isolate arrival time differences due to propagation path inhomogeneities. Waveform and time delay difference plots as well as histograms and statistics derived from them for propagation through a water path and for propagation through five specimens of human abdominal wall indicate that arrival time fluctuations in the presence of human abdominal wall specimens are significantly greater than for a water path and that degradation in focusing through human abdominal wall can be expected in ultrasonic imaging systems that operate in the low megahertz range and employ a relatively large aperture.

A matched filter network for estimating pulse arrival times

Pulse arrival times are often close to the arrival times of pulses at nearby positions. A method that estimates arrival times from a series of closely related receptions has been developed. It combines the matched filter with the minimum path algorithm of network programming. In an environment where noise is present, the arrival time estimates using traditional matched filter processing degrade, owing to arrival time jumps equal to a period of the carrier. Results obtained from experimental and simulated pulses demonstrate that the method removes the jumps while retaining smaller-scale arrival time fluctuations that are of interest. >

Measurements of ultrasonic pulse arrival time differences produced by abdominal wall specimens

The influence of propagation medium inhomogeneities on pulsatile ultrasonic fields has been investigated experimentally. The study employed a special curved transducer to produce a hemispherical wave pulse and a linear array to measure the resulting field along a line in a plane. Translation of the array in the elevation direction yielded data over a two‐dimensional aperture. Time delay across the aperture was calculated by adding delay differences obtained by crosscorrelating signals on adjacent elements and noting the position of the crosscorrelation peaks. Received waveforms were shifted an amount given by the difference between the actual arrival time and a calculated geometric delay to isolate arrival time differences due to propagation path inhomogeneities. Waveform and time delay difference plots, as well as histograms and statistics derived from them for propagation through a water path and for propagation through five specimens of human abdominal wall, indicate that arrival time fluctuations in the presence of human abdominal wall specimens are significantly greater than for a water path, and that degradation in focusing through human abdominal wall can be expected in ultrasonic imaging systems that operate in the low megahertz range and employ a relatively large aperture.

Small‐scale structure in the lithosphere and asthenosphere deduced from arrival time and amplitude fluctuations at NORSAR

We analyze the pattern of phase and amplitude variations of seismic waves across the NORSAR array on a statistical basis in order to determine the statistical distribution of heterogeneities under NORSAR. Important observables that have been analyzed in the past are the phase (or travel time) and log amplitude variances and the transverse coherence functions (TCFs) of phase and amplitude fluctuations. We propose and develop the theory and methods of using other observables to reduce the degree of nouniqueness and increase the spatial resolution of the analysis. Most important are the angular coherence functions (ACFs), which characterize quantitatively the change in the pattern of fluctuations across the array from one incoming angle (or beam) to another and which have a different sensitivity to the depth distribution of heterogeneities than the TCFs. A combination of the ACFs and TCFs allows estimation of the power spectra of the P wave speed variations under the array as a function of depth. We use data for phase fluctuations from 104 incident beams and amplitude fluctuations from 185 beams with 2‐Hz center frequency at NORSAR to calculate the three ACFs and three TCFs (of phase, log amplitude, and their cross coherence). The measured rms travel time fluctuation is 0.135 s, and the rms log amplitude fluctuation is 0.41. The half‐coherence widths of the ACFs are 3° for log amplitude and 9° for phase. The half‐coherence widths of the TCFs are 18 km for phase and less than the minimum separation between the elements of the array for log amplitude. In order to account for these features of the data, we adopt a two‐overlapping‐layer model for lithospheric and asthenospheric heterogeneities underneath NORSAR, with spectra that are band‐limited between the wavelengths of 5.5 and 110 km. Our best model has an upper layer with a flat power spectrum extending from the surface to about 200 km, and a lower layer with a K−4 power spectrum extending from 15 to 250 km. The latter spectrum corresponds to an exponential correlation function with scale larger than the observation aperture (110 km). The rms P wave speed variations the in the range 1–4%. The small scale heterogeneities may be attributed to clustered cracks or intrusions; the larger‐scale wavespeed heterogeneities are temperature or compositional heterogeneities that may be related to chemical differentiation, or dynamical processes in the boundary layer of mantle convection.

Acoustic propagation of AFAR: Theory and experiment

We examine the statistics of the second and fourth moments of the acoustic field measured at the Azores Fixed Acoustic Range (AFAR). The results are from pulse transmissions during 1973 and 1975 of several acoustic frequencies over two volume‐refracted paths in different scattering regimes: a 2.9‐km geometric and a 35‐km partially saturated path. The Garrett‐Munk internal wave model is used to specify the index‐of‐refraction fluctuations; model parameters are entirely determined by environmental observations. Predictions based upon the path‐integral formulation [Sound Transmission through a Fluctuating Ocean, edited by S. M. Flatté(1979)] compare favorably with the acoustic observations. The arrival time (phase) fluctuations are predominantly geometric. However at 35 km, microray effects in the phase can be identified. The 2.9‐km amplitude behavior appears geometric although comparison is hampered by a short time series. At 35 km, log‐amplitude and intensity fluctuations are due largely to microray effects; the power spectra of log‐amplitude are described by a single parameter model where the parameter is related to the intensity decorrelation caused by the microrays. [Work supported by ONR, Code 425UA.]

Systematic monitoring of millisecond travel time variations near Palmdale, California

Repeated measurements of P wave travel time changes have been completed near the San Andreas fault, 25 km west of Palmdale, California. Spanning a 2‐year period from June 1974, these observations were made on 0.1‐ to 8.5‐km base lines and have a worst case accuracy of ±4 ms. The observation area was centered around Bouquet Reservoir where the P wave source, a marine air gun, was secured. Our data were taken approximately every 5 days during June to July 1974, daily during November 1974 to January 1975, and every 2 hours during March to June 1976. Although the area has undergone regional strain at the rate of ∼2·10−7 yr−1 during this period, no net P wave travel time change greater than 1 part in 103 accumulated. We observed instead systematic and continuous arrival time fluctuations that lasted from hours to days and ranged from 1 to 10 ms (Δt/t ≃ 10−3‐10−2). Earth tides and temperature variations, which can produce some signal (Δt/t ∼ 10−3) at diurnal and semidiurnal periods, are at most partially responsible for the observations. We interpret the results in terms of P wave velocity variations in a waterfilled cracked medium in which velocity changes are due to the opening of microcracks during incremental strain. The time scale of anomalous velocity behavior is constrained to be less than the diffusion time of water into newly opened cracks. In this regime little prospect exists of detecting long‐term earthquake precursors.

Fluctuations in the Reflections of Low-Frequency Acoustic Pulses from the Ocean Surface

Thirteen-msec pulses of acoustic energy at 200, 300, 400, 500, 600 and 700 Hz were reflected from the ocean surface at normal incidence. Both the source and the receiver were fixed upon the bottom. The ocean surface was measured by a surface float, and power spectra of the surface displacements and slopes were calculated. The reflections were investigated for both arrival time and intensity fluctuations. These were arranged in a time sequence and analyzed for their power spectra. Curves will be presented to show that: (1) both the arrival time and the intensity spectra resemble the surface spectra, (2) both the arrival time and intensity spectra tend to broaden and become more sensitive to the surface slopes and/or shorter surface wave lengths as the insonifying energy spreads over the ocean surface.