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Ambient Noise (ambient + noise)
Selected AbstractsDS/CDMA throughput of a multi-hop sensor network in a Rayleigh fading underwater acoustic channelCONCURRENCY AND COMPUTATION: PRACTICE & EXPERIENCE, Issue 8 2007Choong Hock Mar Abstract Asynchronous half-duplex Direct-Sequence Code-Division Multiple-Access (DS/CDMA) is a suitable candidate for the MAC protocol design of underwater acoustic (UWA) sensor networks owing to its many attractive features. Our ad-hoc multi-hop network is infrastructureless in that it is without centralized base stations or power control. Hence, we develop an asynchronous distributed half-duplex control protocol to regulate between the transmitting and receiving phases of transmissions. Furthermore, multi-hop communications are very sensitive to the time variability of the received signal strength in the fading channel and the ambient noise dominated by snapping shrimp in harsh underwater environments, because a broken link in the multi-hop path is enough to disrupt communications and initiate new route searches. In our configuration, we use the Ad hoc On-demand Distance Vector (AODV) routing protocol optimized for UWA networks. Empirical studies show that we can model the channel as a slow-varying frequency non-selective Rayleigh fading channel. We theoretically analyze the throughput of our configuration by considering three salient features: the ability of the receiver to demodulate the data, the effect of our control protocol and the effect of disconnections on the generation of routing packets. The throughput under various operating conditions is then examined. It is observed that at optimal node separation, the throughput is improved by a factor of 10. Copyright © 2007 John Wiley & Sons, Ltd. [source] The Effect of Noise in the Emergency DepartmentACADEMIC EMERGENCY MEDICINE, Issue 7 2005Leslie S. Zun MD Abstract Background: It is hypothesized that high ambient noise in the emergency department (ED) adversely affects the ability of the examiner to hear heart and lung sounds. Objective: To determine the ability of various examiners to hear heart tones and lung sounds at the high end of loudness typically found in the ED setting. Methods: The study was divided into two parts. First, sound levels in the ED were measured over various times during the months of January through June 2001, using a sound level monitor. The second part of the study was the determination of the ability to hear heart and lung sounds on a young healthy volunteer using the same Littmann lightweight stethoscope at a predetermined ambient noise level of 90 dB. The results were entered into a database and analyzed using SPSS version 10 (Chicago, IL). Descriptive statistics, analysis of variance, frequencies, and correlation were calculated using this program. Results: Two hundred five sound measurements were taken in the ED during the study period in three locations at various hours. The mean noise level at the nursing station was 57.60 dB, with a minimum of 45.00 dB and a maximum of 70.00 dB. Four of the 104 test subjects (3.8%) were unable to hear the heart tones, and nine of the 104 (8.7%) were unable to hear the lung sounds. Fifty percent (27 of 54) of the test subjects reported diminished lung sounds and eight of 15 (53.3%) reported diminished heart sounds. No significant difference was found between hearing heart sounds and years of experience, age, professional position, and quality of the sound. Significant differences were found between hearing lung sounds and years of experience and professional position, but not with age, gender, and sound quality. Conclusions: This study demonstrated that most of the tested examiners have the ability to hear heart and lung sounds at the extreme of loudness found in one ED. [source] Passive seismic imaging with directive ambient noise: application to surface waves and the San Andreas Fault in Parkfield, CAGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2009Philippe Roux SUMMARY This study deals with surface waves extracted from microseismic noise in the (0.1,0.2 Hz) frequency band with passive seismic-correlation techniques. For directive noise, we explore the concept of passive seismic-noise tomography performed on three-component sensors from a dense seismic network. From the nine-component correlation tensor, a rotation algorithm is introduced that forces each station pair to re-align in the noise direction, a necessary condition to extract unbiased traveltime from passive seismic processing. After rotation is performed, the new correlation tensor exhibits a surface wave tensor from which Rayleigh and Love waves can be separately extracted for tomography inversion. Methodological aspects are presented and illustrated with group-speed maps for Rayleigh and Love waves and ellipticity measurements made on the San Andreas Fault in the Parkfield area, California, USA. [source] Crosswell seismic waveguide phenomenology of reservoir sands & shales at offsets >600 m, Liaohe Oil Field, NE ChinaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 1 2005P. C. Leary SUMMARY Crosswell seismic data recorded at 620,650 m offsets in an oil-bearing sand/shale reservoir formation at the Liaohe Oil Field, northeast China, provide robust evidence for waveguide action by low-velocity reservoir layers. Crosswell-section velocity models derived from survey-well sonic logs and further constrained by observed waveguide seismic wavegroup amplitudes and phases yield plausible evidence for interwell reservoir,sand continuity and discontinuity. A pair of back-to-back Liaohe crosswell vector-seismic surveys were conducted using a source well between two sensor wells at 650 and 620 m offsets along a 200-m-thick reservoir formation dipping 7° down-to-east between depths of 2.5 and 3 km. A downhole orbital vibrator generated seismic correlation wavelets with frequency range 50,350 Hz and signal/noise ratio up to 5:1 over local downhole ambient noise. The sensor wells were instrumented with a mobile 12- to 16-level string of clamped vector-motion sensor modules at 5 m intervals. Using 5 m source depth increments, crosswell Surveys 1 and 2 cover source/sensor well intervals above and through the reservoir of, respectively, 600 m/600 m (13 000 vector traces in 9 common sensor fans) and 300 m/560 m (7000 vector traces in 7 common sensor fans). Survey 1 common sensor gathers show clear, consistent high-amplitude 20 ms waveletgroup lags behind the first-arrival traveltime envelope. Such arrivals are diagnostic of seismic low-velocity waveguides connecting the source and sensor wells. Observed Survey 1 retarded wavegroup depths tally with source and sensor depths in low-velocity layers identified in sonic well logs. Finite-difference acoustic model wavefields computed for waveguide acoustic layers constrained by well-log sonic velocity data match the observed waveguide traveltime and amplitude systematics. Model waveforms duplicate the observed m-scale and ms-scale sensitivity of waveguide spatio-temporal energy localization. Survey 2 crosswell data, in contrast, provide no comparable evidence for waveguide action despite a sensor-well sonic well log similar to that of Survey 1. Instead, acoustic wavefield modelling of Survey 2 data clearly favours an interpreted waveguide model with 10° downdip interrupted by a 75,100 m throw down-fault near the sensor well. The absence of clear waveguide arrivals is adequately explained by dispersal of waveguide energy at the fault discontinuity. Auxiliary well sonic velocity and lithologic logs confirm the model-implied 75,100 m of down-throw faulting near the sensor well. [source] Low-frequency passive seismic experiments in Abu Dhabi, United Arab Emirates: implications for hydrocarbon detectionGEOPHYSICAL PROSPECTING, Issue 5 2010Mohammed Y. Ali ABSTRACT Low-frequency passive seismic experiments utilizing arrays of 3-component broadband seismometers were conducted over two sites in the emirate of Abu Dhabi in the United Arab Emirates. The experiments were conducted in the vicinity of a producing oilfield and around a dry exploration well to better understand the characteristics and origins of microtremor signals (1,6 Hz), which had been reported as occurring exclusively above several hydrocarbon reservoirs in the region. The results of the experiments revealed that a strong correlation exists between the recorded ambient noise and observed meteorological and anthropogenic noises. In the frequency range of 0.15,0.4 Hz, the dominant feature is a double-frequency microseism peak generated by the non-linear interactions of storm induced surface waves in the Arabian Sea. We observed that the double-frequency microseism displays a high variability in spectral amplitude, with the strongest amplitude occurring when Cyclone Gonu was battering the eastern coast of Oman; this noise was present at both sites and so is not a hydrocarbon indicator. Moreover, this study found that very strong microtremor signals in the frequency range of 2,3 Hz were present in all of the locations surveyed, both within and outside of the reservoir boundary and surrounding the dry exploration well. This microtremor signal has no clear correlation with the microseism signals but significant variations in the characteristics of the signals were observed between daytime and nighttime recording periods that clearly correlate with human activity. High-resolution frequency-wavenumber (f - k) spectral analyses were performed on the recorded data to determine apparent velocities and azimuths of the wavefronts for the microseism and microtremor events. The f - k analyses confirmed that the double-frequency microseism originates from wave activity in the Arabian Sea, while the microtremor events have an azimuth pointing towards the nearest motorways, indicating that they are probably being excited by traffic noise. Results drawn from particle motion studies confirm these observations. The vertical-to-horizontal spectral ratios of the data acquired in both experiments show peaks around 2.5,3 Hz with no dependence on the presence or absence of subsurface hydrocarbons. Therefore, this method should not be used as a direct hydrocarbon indicator in these environments. Furthermore, the analyses provide no direct evidence to indicate that earthquakes are capable of stimulating the hydrocarbon reservoir in a way that could modify the spectral amplitude of the microtremor signal. [source] The feasibility of electromagnetic gradiometer measurementsGEOPHYSICAL PROSPECTING, Issue 3 2001Daniel Sattel The quantities measured in transient electromagnetic (TEM) surveys are usually either magnetic field components or their time derivatives. Alternatively it might be advantageous to measure the spatial derivatives of these quantities. Such gradiometer measurements are expected to have lower noise levels due to the negative interference of ambient noise recorded by the two receiver coils. Error propagation models are used to compare quantitatively the noise sensitivities of conventional and gradiometer TEM data. To achieve this, eigenvalue decomposition is applied on synthetic data to derive the parameter uncertainties of layered-earth models. The results indicate that near-surface gradient measurements give a superior definition of the shallow conductivity structure, provided noise levels are 20,40 times smaller than those recorded by conventional EM instruments. For a fixed-wing towed-bird gradiometer system to be feasible, a noise reduction factor of at least 50,100 is required. One field test showed that noise reduction factors in excess of 60 are achievable with gradiometer measurements. However, other collected data indicate that the effectiveness of noise reduction can be hampered by the spatial variability of noise such as that encountered in built-up areas. Synthetic data calculated for a vertical plate model confirm the limited depth of detection of vertical gradient data but also indicate some spatial derivatives which offer better lateral resolution than conventional EM data. This high sensitivity to the near-surface conductivity structure suggests the application of EM gradiometers in areas such as environmental and archaeological mapping. [source] A comparison of three noise reduction procedures applied to bird vocal signalsJOURNAL OF FIELD ORNITHOLOGY, Issue 3 2007Myron C. Baker ABSTRACT Recordings of avian vocal signals in natural habitats include ambient noise. Often this background noise corrupts across all frequencies and is of substantial amplitude. Reducing this ambient noise to prepare vocal signals for playback stimuli or to remove habitat-specific noise signatures prior to analyzing a signal's acoustic characteristics can be useful. We conducted experimental evaluations of three noise reduction procedures to determine their effectiveness. We embedded two bird vocalizations ("clean" signals) in four kinds of natural noise, resulting in eight noise-signal combinations. We then applied three noise reduction procedures (Noise Profile, Band Pass, and Noise Estimate) to each of the embedded signals and compared the recovered signals to the original (clean) signals. Noise Profile filtering was effective in reducing noise and returning fairly high-quality signals from even severe levels of masking noise. The other two noise reduction procedures did not perform as well. For the two most corrupting maskers, however, Noise Profile filtering also altered the signal properties by reducing signal amplitude at those frequencies containing high levels of noise. Apart from this loss of amplitude, the quantitative features of the filtered signals were similar to those of the original model sounds. We conclude that Noise Profile filtering produces good results for cases where noise is approximately constant over the signal duration and the signal intensity exceeds noise intensity over the frequencies of interest. SINOPSIS La grabación de sonidos de aves en hábitats naturales incluye ruidos ambientales. A menudo este ruido es de amplitud sustancial y afecta todas las frecuencias. Antes de analizar una señal vocal, podría ser útil reducir este ruido ambiental, bien para preparar vocalizaciones grabadas para provocar respuestas de aves como para remover ruidos asociados a hábitats. Llevamos a cabo una serie de experimentos con tres procedimientos de reducción de ruido para determinar su efectividad. Insertamos dos vocalizaciones de aves (señales "limpias") en cuatro tipos de ruidos naturales, obteniendo como resultado ocho combinaciones de señales con ruido. Posteriormente, aplicamos tres procedimientos de reducción de ruido ("Perfil de Ruido,""Paso de Banda" y "Estimaciones de Ruido") a cada una de las señales insertadas y comparamos las señales recuperadas con los sonidos originales (limpios). La filtración tipo "Perfil de Ruido" resultó efectiva para reducir el ruido y producir señales de razonablemente buena calidad, aún en situaciones de ruido severo. Los otros dos tratamientos no funcionaron tan bien. Para los dos tipos de ruido con mayores efectos, el "Perfil de Ruido" alteró las propiedades de la señal y redujo la amplitud de la misma, en aquellas frecuencias que contenían altos niveles de ruido. Además de la pérdida de amplitud, los elementos cuantitativos de las señales filtradas fueron similares a las de los modelos sonoros originales. Concluimos que el procedimiento de filtrado "Perfil de Ruido" produce buenos resultados para casos en donde el ruido es de duración aproximadamente constante a lo largo de la señal y cuando la intensidad de la señal excede la intensidad del ruido sobre las frecuencias de interés. [source] Effect of anthropogenic low-frequency noise on the foraging ecology of Balaenoptera whalesANIMAL CONSERVATION, Issue 1 2001Donald A. Croll The human contribution to ambient noise in the ocean has increased over the past 50 years, and is dominated by low-frequency (LF) sound (frequencies <1000 Hz) from shipping, oil and gas development, defence-related and research activities. Mysticete whales, including six endangered species, may be at risk from this noise pollution because all species produce and probably perceive low-frequency sound. We conducted a manipulative field experiment to test the effects of loud, LF noise on foraging fin blue (B. musculus) and (Balaenoptera physalus) whales off San Nicolas Island, California. Naive observers used a combination of attached tracking devices, ship-based surveys, aerial surveys, photo-identification and passive monitoring of vocal behaviour to examine the behaviour and distribution of whales when a loud LF source (US Navy SURTASS LFA) was and was not transmitting. During transmission, 12-30% of the estimated received levels of LFA of whales in the study area exceeded 140 dB re 1 ,Pa. However, whales continued to be seen foraging in the region. Overall, whale encounter rates and diving behaviour appeared to be more strongly linked to changes in prey abundance associated with ceanographic parameters than to LF sound transmissions. In some cases, whale vocal behaviour was significantly different between experimental and non-experimental periods. However, these differences were not consistent and did not appear to be related to LF sound transmissions. At the spatial and temporal scales examined, we found no obvious responses of whales to a loud, anthropogenic, LF sound. We suggest that the cumulative effects of anthropogenic LF noise over larger temporal and spatial scales than examined here may be a more important consideration for management agencies. [source] | |||||||||||||