جستجوی مقالات مرتبط با کلیدواژه « attenuation » در نشریات گروه « فیزیک »

Measurement of ionizing radiation in various fields, such as environmental safety, industrial detection processes, radiation protection and medical is very important. Radiation dosimetry plays an important role in determining the amount of energy absorbed and the effects of radiation. Recent optical fiber sensors have been shown to be radiation dosimeters. Our goal here is to investigate the effect of ionizing radiation on fiber optics. Thus, in this paper, the effect of electron beam radiation on the loss of light passing through the fiber optics after the end of radiation in the wavelength range of 1300-1550 nm is investigated. Optical fibers with doses of of 22 and 47 kGy were irradiated. All measurements were carried out at a temperature of 25 ± 2 ° C. The results show that the light dissipation of the fiber optically increases with the radiation dose and decreases with the passage of time after the end of radiation.

Seismology has an important role in identifying earth structure using seismic waves. The amplitude and frequency of these waves change when they pass into the earth due to anisotropy and heterogeneity. Seismic waves decay as they radiate away from their sources, partly for geometric reasons because their energy is distributed on an expanding wave front, and partly because their energy is absorbed by the material they travel through. The energy absorption depends on the material properties.
The amplitude of seismic waves decreases with increasing distance from earthquake, explosion, and impact sources. How this amplitude decrease, occurs, how rapidly it occurs, and how it depends on the frequency of the seismic waves is fundamentally important to the efforts to describe Earth structure and seismic sources.
Attenuation of seismic waves is expressed with inverse quality factor (Q-1) and helps us to understand the physical laws governing the propagation of seismic waves in the lithosphere.
The observed seismic-wave amplitudes usually decay exponentially with increasing travel distance after the correction for geometrical spreading. These decay rates are proportional to the Q−1 values which characterize the spatial attenuation of seismic waves.
The study area is located in Fariab region, south-east of Sanandaj-Sirjan metamorphic zone and adjacent to the Main Zagros Reverse Fault (MZRF). This rejoin is among the rare regions located in Sanandaj-Sirjan area which has high seismic activity. Seismicity in this area has a north east- south west trend. Depths of events are relatively low, in the range of less than 40 km.
The 28 Feb. 2006 earthquake with a magnitude of Mw=6.0 has taken place in this region. Despite the remarkable magnitude of this earthquake, no significant damage was reported, even for low strength buildings such as clay buildings. This phenomenon may indicate a significant attenuation of the elastic wave in the area. Aftershocks of this event have been recorded by a temporary network. The well-located aftershocks, with hypocentral distances less than 100 km, have been used for estimation of the P-wave quality factor.
The attenuation of P-waves has been estimated using waveforms of 431 well-located aftershocks. The attenuation of P-wave (QP-1) has been estimated at 5 frequency bands (1.5, 3, 6,12,24 Hz) using extended coda normalization method. The estimated QP values show highly frequency dependency. The frequency dependent relation for longitudinal waves in the study area has been derived as Qp=23f-0.78. The QP values have been estimated by using waveforms with two major directions to investigate the probably existed attenuation anisotropy. The frequency dependent relation in two directions of north west-south east and north east-south west were calculated as Qp=10f-0.94 and Qp=25f-0.75, respectively.
It has been observed that the quality factor value at the reference frequency of 1 Hz is smaller than 200, which is reported for seismically active regions. Therefore it could be concluded that this region is active in terms of seismicity and tectonics.
The small values of the quality factor which demonstrates the relatively high attenuation indicate high seismic activity of this region. This is not so compatible with what we expect for the earth’s crust structure in this area with the metamorphic zone. This high attenuation is probably due to the crushed zone affected by various earthquakes in this region

Seismic waves when crossing the Earth in heterogeneous and anisotropic environments, having interaction. Recognizing the impact of these factors on the seismograms help us to find out more information about interior of the Earth. Coda waves are the main reason for the random heterogeneities in earth. Local earthquakes in the northeast region of Iran with epicentral distance less than 200 km is used with magnitude range of 2 - 6 recorded in period 2006 to 2013. Finally, data for five stations which have (15441 earthquakes) were chosen. In this study, the attenuation parameters, Q, were estimated using the single scattering models. Aki and Chouet (1975) proposed a single backscattering model to explain the coda waves as a superposition of secondary waves from randomly distributed heterogeneities. The decrease of coda wave amplitude with lapse time at a particular frequency is due to energy attenuation and geometrical spreading, and is independent of earthquake source, path effect and site amplification (Aki, 1969). Generally, the Q factor increases with frequency (Mitchell, 1981) following the relation where Q0 is the quality factor at the reference frequency f0 (generally 1 Hz) and n is the frequency parameter, which is close to 1 and varies from region to region depending on the heterogeneity of the medium (Aki, 1980). This relation indicates that the attenuation of seismic waves with the passage of time (distance from source) is different for different frequencies. Hence, the seismic data are first bandpass-filtered to calculate the attenuation. In the present study, the attenuation of the S-coda wave (Figure 5) is calculated at seven central frequencies after getting bandpass-filtered using a Butterworth four pole filter as given in Table 1.The amplitude of the coda wave at lapse time t seconds from the origin time for a bandpass-filtered seismogram at central frequency f is related to the attenuation parameter Q by the following equation: Where C(f) is the coda source factor at frequency f, which is independent of time and radiation pattern, α is the geometrical spreading parameter and is equal to 1.0, 0.5 or 0.75 for body waves, surface waves or diffusive waves, respectively (Sato and Fehler, 1998), Qc(f) is the quality factor of coda waves. As coda waves are backscattered body waves, α= 1. Equation (1) can then be rewritten as: is determined from the slope (b) of a least-squares straight-line fit between versus t, using the relation Shows different steps involved in the computation of Qc (f) from the RMS values of amplitude with time. According to Rautian and Khalturin (1978), the above relation is valid for lapse times greater than twice the S-wave travel time for avoiding the data of the direct S-wave. Sato (1977) introduced the source receiver offset in a single scattering model so that the coda analysis begins after the arrival of the shear wave. In the present study, the time envelope for the coda decay observation is taken at twice the time of S-wave (2ts) from the origin time of the event. Q0 and n values indicate the average values for each station in the surroundings of the station. As an outcome, average of quality factors and frequency-dependents, is given by: In addition, to evaluate the variation in depth direction, we used the quality factor of 18 Coda windows from five to 90 seconds by 5 seconds step. Low values of Q0 in the initial Q-coda windows, indicating strong heterogeneity in the shallow layers of the Earth. The results in studying region have been compared with another zone in Iran (SSZ) (Figure 8).