Presence Of Reflectors Research Articles

Enhancing the overall performance of the hybrid solar photovoltaic collector by open water cycle jet-cooling

In this paper, the electrical and thermal performance of hybrid photovoltaic (PV/T) system works with an active cooling open water cycle has been improved. Experimental tests were conducted on the (PV/T) of two cases (with and without reflectors) and the reference photovoltaic (PV) in Iraq - Samarra located at latitude (34.26°N) and longitude (43.89 °C). Several experiments were carried out during June 2021 with a (0.014 kg/s) mass flow rate. Results manifested that the maximum daily electrical, thermal and overall PV/T collector efficiency with reflectors were 10.2%, 82.3%, 92.5%, respectively, with the highest improvement in the electrical power (EP) of 69.4%. This technique has proven its effectiveness in the hot climate that Iraq enjoys (suffers), and that the water jets played their role in decreasing the temperature of the hybrid panel despite the increase in the concentration of solar radiation throughout the presence of reflectors.

Mapping the edge of subducted slabs in the lower mantle beneath southern Asia

SUMMARYWe investigate the presence of seismic structures in the Earth's mantle by searching for seismic signals that travel off the great circle path direction and are reflected or scattered off structures in the lower mantle. We focus on areas of current and past subduction beneath Eurasia by using events from Indonesia and Japan recorded at the broad-band stations in Germany, Morocco and Namibia. Applying seismic array techniques, we measure the direction and traveltime of the out-of-plane arrivals and backtrace them to their location of reflection/scattering. We backtrace the signals as P-to-P and S-to-P waves and extend the methodology to P-to-S waves. There seems to be a low number of reflection points in the regions beneath Eurasia in our study. Investigating possible causes, we find that the focal mechanism influences the presence of out-of-plane reflected waves. However, the potential coverage with our data set is large and should allow detection, but there may potentially be few seismically visible structures in the region. Most of our backtraced reflectors are located beneath southern Asia and are found shallower than 1500 km depth. They correlate well with the edges of prominent high velocity anomalies in tomographic inversions beneath southern Asia, which have been interpreted as remnants of fossil slabs of the subduction of the Tethys Oceans. We also observe few reflectors deeper than 1600 km that are located away from subducting regions and their positions coincide with the eastern edge of the African low velocity anomaly. These observations suggest that the presence of reflectors in the mid-lower mantle is not exclusively related to current or past subducting regions, but widespread throughout the mantle.

Low-cost Solutions for Velocimetry in Rotating and Opaque Fluid Experiments using Ultrasonic Time of Flight

Many common techniques for flow measurement, such as Particle Image Velocimetry (PIV) or Ultrasonic Doppler Velocimetry (UDV), rely on the presence of reflectors in the fluid. These methods fail to operate when e.g centrifugal or gravitational acceleration leads to a rarefaction of scatterers in the fluid, as for instance in rapidly rotating experiments. In this article we present two low-cost implementations for flow measurement based on the transit time (or Time of Flight) of acoustic waves, that do not require the presence of scatterers in the fluid. We compare our two implementations against UDV in a well controlled experiment with a simple oscillating flow and show we can achieve measurements in the sub-centimeter per second velocity range with an accuracy of sim 5-10%. We also perform measurements in a rotating experiment with a complex flow structure from which we extract the mean zonal flow, which is in good agreement with theoretical predictions.

Application of Pre-Stack Time Migration (PSTM) Using Kirchoff Method on Marine Seismic Data 2D in Sulawesi Sea Waters

To see the potential of weak zones, subduction zones and geological structures below sea level, research phases are needed based on geological and geophysical approaches. Geological approaches include regional geology and stratigraphy studies, while geophysical approaches include the depiction of underground structures using reflection seismic. The purpose of this study was to process 2D marine seismic data until the Pre-Stack Time Migration stage of the Kirchoff method and to interpret the underground surface structure of the Sulawesi Sea waters based on the Pre-Stack Time Migration using Kirchoff method in the Sulawesi Sea Water. This process can move the position of the reflector to the actual position, which is conducted by adding up the amplitude of a reflector point as long as a position is estimated at the actual location. The results of the interpretation of Pre-Stack Time Migration in the Line 22 crossing of the Sulawesi Sea waters have the presence of reflectors, Sulawesi ditch, faults and horizon layers of sedimentation. While the existence of low velocity zones can be analyzed by reading Root Mean Square (RMS) velocity in the cross section of this track, the lowest velocity zone distribution is found in CDP 7,350.8 with a depth of 11,860 ms to TWT depth of 10,967 ms having a constant velocity amounting to 2,291 m/s, and the estimated presence of weak rock structures.

Seismogeological Criteria for the Gas Potential of Aptian–Cenomanian Sediments in the North of West Siberia (by the Example of the Yubileinoe Field)1

Abstract —Seismogeological criteria for the gas potential of the Aptian–Cenomanian sediments of West Siberia are substantiated. The research was carried out by the method of integrated interpretation of seismic materials, well logs, well test results, and petrophysical studies. As a reference object we used the Yubileinoe oil and gas condensate field located in the Nadym–Pur interfluve, in which a unique gas deposit is concentrated in the Cenomanian reservoir PK1 and the Albian sand bed PK18. Based on the research results, seismogeologic criteria for gas prediction have been formulated. Massive Cenomanian gas pools are displayed in wave seismic fields by: (1) the presence of reflectors formed at the gas–water contacts in time sections and a decrease in the amplitude parameters of the reflector G confined to the roof of Cenomanian sequence; (2) an increase in the time thickness (ΔT), a decrease in the interval velocities (Vint), and a decrease in the amplitude–energy parameters of the seismic record in the Aptian–Cenomanian sequence. Aptian–Albian sheet gas pools are displayed in the wave fields by a sharp increase in the amplitudes of the seismic record, forming “bright spot” anomalies in the time sections.

Factors influencing irradiance of locally fabricated phototherapy devices in Jos, north-central Nigeria

Locally fabricated phototherapy devices (LFPDs) are widely used in Nigeria for the treatment of neonatal jaundice. Ours was a cross-sectional observational study of all LFPDs in major hospitals in Jos between January and March 2015. We evaluated a total of 24 LFPDs. The irradiance at the level of the baby was in the range of 2-23.9 µW/cm2/nm. Fourteen devices had the recommended irradiance of ≥10 µW/cm2/nm and none had irradiance in the intensive range. Decreasing distance from the baby, presence of reflectors and increasing number of flourecent tubes significantly contributed to higher irradiance. A combination of six tubes, presence of reflectors and a distance of 10 cm from the baby produced a mean irradiance of 23.40 µW/cm2/nm. The irradiance of LFPDs varies widely and can be improved by simple modifications.

Temporal patterns in road crossing behaviour in roe deer (Capreolus capreolus) at sites with wildlife warning reflectors.

Every year, there are millions of documented vehicle collisions involving cervids across Europe and North America. While temporal patterns in collision occurrence are relatively well described, few studies have targeted deer behaviour as a critical component of collision prevention. In this study, we investigated weekly and daily patterns in road crossing behaviour in roe deer. Using road crossing events and movement data obtained from GPS telemetry, we employed mixed-effect models to explain frequency and timing of crossings at five road segments by a number of predictors including traffic volume, deer movement activity and the presence of wildlife warning reflectors. We analysed 13,689 road crossing events by 32 study animals. Individual variation in crossing frequency was high but daily patterns in crossing events were highly consistent among animals. Variation in the intensity of movement activity on a daily and seasonal scale was the main driver of road crossing behaviour. The seasonal variation in crossing frequency reflected differences in movement activity throughout the reproductive cycle, while daily variation in the probability to cross exhibited a clear nocturnal emphasis and reflected crepuscular activity peaks. The frequency of road crossings increased as a function of road density in the home-range, while traffic volume only exerted marginal effects. Movement activity of roe deer in our study coincided with commuter traffic mainly in the early morning and late afternoon during winter and during periods of high spatial activity such as the rut. Both timing and frequency of crossing events remained unchanged in the presence of reflectors. Our results emphasise the importance of behavioural studies for understanding roe deer vehicle-collision patterns and thus provide important information for collision prevention. We suggest that mitigation of collision risk should focus on strategic seasonal measures and animal warning systems targeting drivers.

Introduction to this special section: Nonreflection seismic and inversion of surface waves and guided waves

Seismic imaging need not be synonymous with or rely on the presence of reflectors in the Earth. Much can be gleaned from the nonreflected wavefield. For example, direct waves map out smooth velocity variations in crosswell seismic tomography, wide-angle refracted waves play a crucial role in full waveform inversion (Hole et al., 2005), and surface waves provide unmatched sensitivity to near-surface shear-wave velocity structure. Guided waves exist in both cracks (Korneev, 2008) and boreholes, the latter referred to as tube waves. The famous Biot slow wave is itself a guided-wave phenomenon akin to a tube wave (Norris, 1987). Surface-wave dispersion has a long history in seismology and was the first seismic characteristic to be subjected to an automated inversion procedure (Dorman and Ewing, 1962).

Incomplete data problems in wave equation imaging

We study reflection imaging as an incomplete data problem in frequency domain. It turns out that this amounts to inverting the Fourier transform using only frequencies outside some set. By numerical simulations we show the effect of this incompleteness on concrete reconstruction problems. We try to complete the data by analytic continuation. An explicit formula is obtained by an inversion formula for the exponential Radon transform. We discuss the application to medical ultrasound tomography and to seismic imaging. We describe an alternative method based on the presence of reflectors.

Stereographic imaging condition for wave-equation migration

Imaging under the single-scattering approximation consists of two steps: wavefield reconstruction of source and receiver wavefields from simulated and recorded data, respectively, and imaging from the extrapolated wavefields of locations where reflectors occur. Conventionally, the imaging condition indicates the presence of reflectors when propagation times of reflections in the source and receiver wavefields match. The main drawback of a conventional crosscorrelation imaging condition is that it ignores the local spatial coherence of reflection events and relies only on their propagation time. This leads to interference between unrelated events that occur at the same time. Sources of crosstalk include seismic events corresponding to different seismic experiments, propagation paths, types of reflections (primary or multiple), or wave modes (P or S). An alternative imaging condition operates on the same extrapolated wavefields, but crosscorrelation takes place in a higher-dimension domain where seismic events are separated based on their local space-time slope. Events are matched based on two parameters (time and local slope), thus justifying the name “stereographic” for this imaging condition. Stereographic imaging attenuates wavefield crosstalk and reduces imaging artifacts compared with conventional imaging. Applications of the stereographic imaging condition include simultaneous imaging of multiple seismic experiments, multiple attenuation in the imaging condition, and attenuation of crosstalk between multiple wavefield branches or multiple wave modes.

Transient transport equations for high-frequency power flow in heterogeneous cylindrical shells

We use some recent mathematical results obtained for the high-frequency asymptotics of hyperbolic partial differential equations to derive exact transient power flow equations for vibrations of randomly heterogeneous cylindrical shells. The theory shows that the angularly resolved energy densities of an heterogeneous, elastic medium satisfy transient transport equations at higher frequencies. The behaviour of solutions of such equations short of their diffusion limit—if any—is fundamentally different from that of the solution of a diffusion equation, although the latter one is often invoked in the analyses of high-frequency vibrations of elastic structures. A condition by which diffusion equations can be obtained from transport equations is the presence of reflectors or heterogeneities such that scattering mean free paths are short with respect to the characteristic dimensions of the structure. The diffusion limit is reached in this study taking account of scattering by random heterogeneities of the background medium at the scale of the wavelength. This approach fills the gap between transport theory and the diffusion approach in structural dynamics, and clarifies the range of validity of the latter. Our results can be extended to fully coupled dynamic equations for compression, shear and bending of Timoshenko beams or Mindlin plates.

A search for anomalies in the lowermost mantle using seismic bulletins

For deep events and epicentral distances around 75° the Bulletins of the International Seismological Centre (BISC) contain many secondary arrivals between P and PcP, the reflection from the core-mantle boundary (CMB), which cannot be explained by current, spherical symmetric Earth models. Assuming that these arrivals in the P-coda are reflections from P-velocity anomalies in the lower mantle, we calculate the associated reflection points (geographical coordinates and distance from the CMB) and map both strength (relative density of reflection points in bins in the lower mantle) and distance of the reflectors from the CMB on a global scale. The regions with these anomalous bins cover 3% of the 50% of the lowermost mantle that can be analysed with this data set. The most significant regions with anomalous bins (values in parentheses give a preliminary estimate of the distance of the reflector from the CMB) are under the North Atlantic Ridge (250 km), northern Central Siberia (230 km), Japan (180 km), the central Mid-Atlantic Ridge (310 and 260 km) and off the coast of Ecuador (340 km). The presence of reflectors in one of these regions has already been confirmed by detailed seismic studies using waveform data from arrays and networks. A slight (2–3%) P-velocity contrast is sufficient to produce measurable reflection amplitudes in the distance range of 70–80°. Ten percent of the lower mantle sampled by this study is covered by bins with no or very few secondary arrivals in the P-coda. The most significant regions with such a ‘normal’ lower mantle are under Central America, the Caroline and Marshall Islands in the eastern Pacific, the area of Hawaii and the Easter Islands and the northern Atlantic. Identification of the regions with anomalous bins serves to locate the areas of the lowermost mantle which should be analysed further by using the full information content of seismograms. The study of the travel time, the amplitude and the form of the wavefield, recorded preferably at seismic arrays/dense networks, will allow the determination of the anomalous velocity structure of the core-mantle transition zone in more detail. The most promising areas for such a study are given here, but further anomalies are likely to exist.