Multiple Time Windows Research Articles

Intrinsic and scattering seismic wave attenuation in the Canary Islands

The Canary Islands volcanic complex is studied in terms of coda wave attenuation. A multiple lapse time window method, based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers, is applied to the available seismic data in order to obtain the intrinsic absorption (Qi−1) and the scattering attenuation (Qs−1) in the Canarian lithosphere. The analysis is performed for two hypocentral distance ranges: from 0 to 80 km and from 0 to 140 km. Results show that in both cases and for all the studied frequency bands (1–2, 2–4,6–8 and 8–10 Hz ) intrinsic absorption dominates. The low albedos found in the region indicate the low degree of heterogeneity in the Canarian lithosphere at the scale length of the studied frequencies. On the other hand the coda attenuation (Qc−1) calculated on the basis of the single‐scattering theory gives values near (Qi−1) for low frequencies and near the total attenuation (Qt−1) for high frequencies. The degree of frequency dependence of the attenuation parameters is strong in all cases. A correlation of the observed attenuation parameters with the geological evidence for a hotspot‐type archipelago and with other geophysical studies is suggested.

Seismic attenuation in underground mines: a comparative evaluation of methods and results

The goal of this study is to evaluate methods of estimating seismic attenuation in underground mining environments, and to relate the attenuation information to rock mass characteristics as determined from geomechanical data. Therefore, four methods — Spectral Ratios, Spectral Decay, Coda- Q, and Multiple Lapse Time Window Analysis (MLTWA) — of measuring attenuation are examined at three underground excavation sites. Each site utilizes microseismic monitoring systems `tuned' to different frequency bandwidths. Considering the MLTWA at the dominant corner frequencies for each site (Site 1, 2.1 to 4.3 kHz; Site 2 and Site 3a, 0.5 to 1.0 kHz; Site 3b, 0.1 to 0.2 kHz), the results indicate that over the entire frequency range examined at all study sites, the intrinsic S-wave Q ( Q β ) is nearly constant at approximately 180, and the scattering Q β follows a frequency dependence of Q β = Q o f 1.4 (where Q o=120). Interestingly, at Site 3 there is a crossover in the dominance of scattering to intrinsic attenuation, and this trend continues through to the Site 1 data. In order for the frequency-independent intrinsic Q β to be substantiated, it is speculated that a fractal distribution of heterogeneities in the rock mass is responsible for intrinsic attenuation (e.g., velocity anomalies, cracks, inclusions). Conversely, the increasing scattering Q β may be attributed to minimum characteristic underground opening dimensions of 3–5 m, causing decreasing wavelengths to encounter lower scattering due to these openings, and vice versa.

Estimation of the Intrinsic Absorption and Scattering Attenuation in Northeastern Venezuela (Southeastern Caribbean) Using Coda Waves

—Northeastern Venezuela has been studied in terms of coda wave attenuation using seismograms from local earthquakes recorded by a temporary short-period seismic network. The studied area has been separated into two subregions in order to investigate lateral variations in the attenuation parameters. Coda-Q −1 (Q c −1) has been obtained using the single-scattering theory. The contribution of the intrinsic absorption (Q i −1) and scattering (Q s −1) to total attenuation (Q t −1) has been estimated by means of a multiple lapse time window method, based on the hypothesis of multiple isotropic scattering with uniform distribution of scatterers. Results show significant spatial variations of attenuation the estimates for intermediate depth events and for shallow events present major differences. This fact may be related to different tectonic characteristics that may be due to the presence of the Lesser Antilles subduction zone, because the intermediate depth seismic zone may be coincident with the southern continuation of the subducting slab under the arc.

Temporal change in coda Q at Nevado Del Ruiz Volcano, Colombia

An analysis of coda Q ( Q c ), intrinsic Q ( Q i ), scattering Q ( Q s ) and total Q ( Q t ), was made at Nevado del Ruiz Volcano (NRV) for local earthquakes recorded during 1992–1994. Coda Q has been calculated for the first time at Nevado del Ruiz Volcano (NRV) using the single-scattering model proposed by Aki and Chouet (1975). Estimations of Q c were made for five frequency bands centred at 1.5, 3.0, 6.0, 12.0 and 24.0 Hz. The estimated Q c varies from around 60 at 1.5 Hz to around 980 at 24 Hz, depending on the frequency with Q c = 30 ± 1.2 F 1.03±0.001. in general, Q c values at NRV are rather similar to those obtained in other volcanic areas in the world. A steady trend in Q c -1 was found from January 1992 to January 1994. This variation is coincident with the stability in the volcanic activity since the last phreato-magmatic eruption on 1 September 1989. In the middle of April 1994, a new activity started at NRV and a small volcanic crisis occurred. Continuous increasing was observed in Q c -1 around two months before the crisis. This suggests a possibility that the temporal variation of Q c can be useful for monitoring the volcanic activity at NRV. Q i and Q s were separated from Q t by using a multiple-scattering model called “Multiple Lapse Time Window Analysis”, MLTWA (Fehler et al., 1992). Estimations for Q i , Q s and Q t were made for four frequency bands, 3.0, 6.0, 12.0 and 24.0 Hz. Q i ; values were found to be near to those of Q c for frequencies higher than 6.0 Hz. For low frequencies Q s values were near to Q c values. This can suggest that the intrinsic absorption could be the predominant attenuation phenomenon in the NRV area at frequencies higher than 6 Hz and the scattering phenomenon is predominant for low frequencies; therefore the change in Q c at high frequencies is considered to reflect variations in the inner conditions of the volcano, such as changes in temperature, movement of fluids, and so on.

The split delivery vehicle scheduling problem with time windows and grid network distances

We consider an extension of the Split Delivery Vehicle Routing Problem, whereby customers may have a time window for their delivery. A construction heuristic is developed which uses a look-ahead approach to solve the Split Delivery Vehicle Scheduling Problem with Time Windows. Two improvement heuristics are also described: one attempts to move customers between routes, while the other exchanges customers between routes. All three heuristics are implemented on a specifically developed data set and on some standard problems from the literature. Our heuristic implementations also consider the possibility of multiple time windows, grid network distances, non-linear delivery time functions and bounds on the size of any split deliveries.

Q β estimates from spectral ratios and multiple lapse time window analysis: results from an underground research laboratory in granite : A. J. Feustel & R. P. Young, Geophysical Research Letters, 21(14), 1994, pp 1503–1506

Q β estimates from spectral ratios and multiple lapse time window analysis: results from an underground research laboratory in granite : A. J. Feustel & R. P. Young, Geophysical Research Letters, 21(14), 1994, pp 1503–1506

Separation of intrinsic and scattering attenuation in southern California using TERRAscope data

A multiple lapse time window analysis was applied to three‐component broadband seismograms recorded at five TERRAscope stations in southern California to separate scattering and intrinsic attenuation. Seismic energies were integrated over three consecutive lapse time intervals: 0–15, 15–30, and 30–45 s (measured from the S arrival) for approximately 30 earthquakes with hypocentral distances of less than 70 km from each station. Using the fundamental separability of source, site, and path effects for coda waves, the integrated energies for different magnitude earthquakes were normalized to a common source size at each station, and the effect of near‐site amplification is removed. Subsequently, we constructed a group of geometric spreading‐corrected normalized energy‐distance curves for each station region over frequency bands 0.5–1, 1–2, 2–4, and 4–8 Hz for all five stations. Two more frequency bands, 8–16 and 16–32 Hz, were added at stations PAS and SVD, for which higher sample rate data were available. A theoretical model of body wave energy propagation in a randomly heterogeneous elastic medium was employed to interpret the observation. Two parameters describe the medium in this model. These are the scattering attenuation coefficient ηs and the intrinsic attenuation coefficient ηi. By assuming that scattering is isotropic and including all orders of multiple scattering, this model predicts the spatial and temporal distribution of seismic energy. A two‐step least squares fitting procedure was used to find the best fitting model parameters. The result shows the following: (1) the seismic albedo, B0 = ηs/(ηi + ηs), increases with decreasing frequency for all station regions. The difference in B0 among stations is frequency dependent, and the difference disappears at 6.0 Hz. (2) Significant differences exist for the scattering attenuation coefficient ηs and seismic albedo B0 among stations at lower frequencies. Stations on or close to the fault zone(s), such as PAS, PFO, and SVD, show a stronger scattering at frequencies of 0.75 and 1.5 Hz, whereas station ISA, located far away from any major active faults, shows the lowest scattering. (3) The coda Q−1 determined from the decay rate of coda waves lies, in general, between the intrinsic Q−1 and total Q−1, sometimes closer to the former than the latter, and sometimes opposite, depending on the frequency and station.

Qβ estimates from spectral ratios and multiple lapse time window Analysis: Results from an underground research laboratory in granite

An experiment to measure quality factor (Q) in‐situ was carried out at the Underground Research Laboratory in Manitoba, Canada using the Spectral Ratio technique and a form of the Mulitple Lapse Time Window Analysis technique. The volume of rock examined measures approximately 30 × 30 × 30 m at a depth of 420 m in Lac Du Bonnet Granite. The site encompases 16 triaxial accelerometers grouted in the bottom of boreholes drilled from access tunnel walls. The source for the seismic pulses was a mechanical hammer placed at the bottom of specially located boreholes. Results from the Spectral Ratio experiment gave S‐wave quality factor (Qβ) of 114 ± 18. Based upon the short duration of the first arrival pulses and low scattered wave energy in the signal coda, we conclude that the Spectral Ratio technique measured the intrinsic attenuation of the rock mass. Further analysis using the more recently developed Multiple Lapse Time Window Analysis gave results for intrinsic Qβ (Qi) of between 85 and 100. Conversely, values for scattering Qβ (Qs) were greater than 800. The high correlation for Qi values between the two methods help to confirm the nature and absolute values of the attenuation, and also support the use of the later technique where spectral ratios cannot be applied.

Separation of scattering attenuation and intrinsic absorption in Japan using the multiple lapse time window analysis of full seismogram envelope

Two seismic wave attenuation factors, scattering attenuation (Qs−1) and intrinsic absorption (Qi−1), can be determined from estimates of two parameters, the reciprocal of extinction length Le−1 ≡ (Qs−1 + Qi−1) ·ω/v and seismic albedo B0≡ Qs−1 / (Qs−1 + Qi−1), where ω and v are angular frequency and seismic wave velocity, respectively. These parameters are measured using the multiple lapse time window analysis method over frequency bands of 1–2, 2–4, and 4–8 Hz from 16 stations located throughout Japan. In the analysis, time‐integrated seismic wave energy from earthquakes having hypocentral distances less than 120 km and focal depths less than 40 km are compared with simulations of multiple isotropic scattering based on the assumption of spatial uniformity of Qs−1 and Qi−1. The modified coda normalization method is used to correct for sources and site effects, where the spatially non uniform distribution of the coda wave energy is considered. The Le−1 estimates do not have a distinct frequency dependence, but the Be estimates do: 0.2–0.7 for 1–2 Hz and 0.1–0.3 for 4–8 Hz. The results mean that Qs−1 is similar to Qi−1 for 1–2 Hz but is 3–5 times smaller for 4–8 Hz and that the frequency dependence of Qs−1 is larger than Qi−1. Analyses using different components of motion and different window lengths give nearly the same results. Coda attenuation (Qc−1) estimates based on the single scattering approximation are nearly equal to intrinsic absorption Qi−1 rather than to total attenuation (Qs−1 + Qi−1) for Japan.

A Labeling Algorithm for the Navigation of Automated Guided Vehicles

Material handling is an important component of most automated manufacturing systems. AGVs are commonly employed for this function. Efficient use of the AGV system requires proper routing and scheduling of vehicular traffic. This problem is modeled as a shortest path problem with multiple time windows on arcs and at nodes of a network. A polynomial-time labeling algorithm has been developed. The algorithm has complexity O(D2logdD), where D is the total number of time windows in the problem. The data required for the model is easy to maintain.

Neural processing of temporal gaps in the inferior colliculus of the awake young and old CBA mouse

The ability to detect brief silent intervals or gaps in sound forms one important property of temporal resolution. Psychoacoustical evidence suggests such fundamental processing correlates with more complex perceptions, such as speech perception in noise. Moreover, studies have also implicated deficits in temporal resolution as a possible cause for the difficulties observed in presbycusis. The current study compared neuronal response properties and minimal gap thresholds (MGT) in the young (2–4 month) and old (≳24 month) CBA mouse model of presbycusis. The following measures were obtained from over 40 single units in each age group; intensity functions for 100-ms tone and noise bursts, spontaneous rate, Q10 bandwidth, and gap functions. Neural correlates of gap detection were studied by imbedding a silent gap (widths from 0.5 to 20 ms) between two noise bursts presented at 30 dB above unit threshold. MGT was quantified by comparing spike counts in multiple time windows before, during, and after the gap. Comparison between young and old response patterns indicate: (1) PST histograms displayed numerous patterns including; onset, onset-sustained, on-off, primarylike, buildup, and off responses, (2) a wide range of spontaneous rates were encountered from 0 to 60 spikes/s, (3) MGT varied with response type regardless of age, and (4) the lowest MGTs (1–2 ms) observed were found both in young and old animals, but the frequency of occurrence was lower in the older animals. [Work supported by NIH-NIA P01 AG09524-01A1.]

Latency and time-dependent exposure in a case-control study

Detailed historical data are elicited often from subjects in retrospective studies, yielding time-dependent measures of exposures. Investigation of a hypothesized period of latency can be made by examining disease/exposure relationships in multiple time windows, either along the age or time-before diagnosis axes. We suggest splitting the data into many time intervals and separately fitting regression models tothe available data in each interval. Covariances between estimated coefficients from different intervals are empirically estimated, and used for assessing variability of specified functions of the time-specific coefficients. Alternative methods of interval formation and their consequences are discussed. We apply these methods to a French case-control study of oral contraceptive use and cervical cancer incidence, and compare the results to those of standard analyses.

Use of “time windows” to investigate lung cancer latency intervals at an ontario steel plant

This paper describes an application of the time windows method to an examination of the temporal pattern of lung cancer risk among steel workers. Case-control methodology was utilized. The cases were 36 men who had died of lung cancer and the controls were 289 men who had died of any other cause. The number of years of employment in the steel pouring area was used as a surrogate measure of exposure. The data were examined by contingency table analysis and by logistic regression, which permitted adjustment for exposures in multiple time windows and the use of continuous, rather than categorical, measures of exposure. It was found that lung cancer risk was associated with exposures occurring between 18 and 30 years before death. It cannot yet be determined whether this time course reflects a biological response or the temporal pattern of exposure to an as yet unidentified toxic agent.