Aperture Length Research Articles

Subbottom profiling using synthetic aperture processing

Using synthetic aperture processing, a chirp sonar generates reflection profiles of the seabed. Synthetic aperture processing is implemented by calculating the vertical change in the positions of acoustic transducers between transmission and using the offsets to time shift the acoustic data so that horizontal reflectors are summed coherently. The calculated changes in the positions of the acoustic transducers between transmission events are based on Doppler velocity log and inertial measurement unit measurements of vehicle motion. Imagery generated from reflection profiles of seabeds with various sediment types are used to quantity the improvement in the spatial resolution and the SNR of reflection profiles as a function of the length of the synthetic aperture. [Work supported by Office of Naval Research.]

Selecting parameters of discrete scanning systems for digital X-ray imaging

For discrete scanning systems of digital X-ray imaging that contain an array of detectors, optimum values of the aperture length for each detector in the array and the step of discrete displacement (scanning) of a tested object are found. The determined values ensure the maximum resolution of such systems in the scanning direction for a given testing performance.

From Micro to Meso: an exercise in determining hydraulic conductivity of fractured sandstone cores from detailed characterization of the fractures

SUMMARY Hydraulic conductivities of fractured sandstone bore cores of 0.1 m in diameter are calculated using detailed characterization of the fracture geometry parameters determined using a resin casting technique. The accuracy of the measurements was about 0.25‐1.25 μm with the image size used. The values of the effective fracture apertures vary between 10 μm and 50 μm. For modelling purposes the samples are sectioned serially, perpendicular to the flow direction along the cylinder axis. The hydraulic conductivity of individual slices is estimated by summing the contribution of the matrix (assumed uniform) and each fracture (depending on its length and aperture). Finally, the hydraulic conductivity of the bulk sample is estimated by a harmonic average in series along the flow path. Results of this geometrical upscaling compare favourably with actual conductivity measured in hydraulic and pneumatic experiments carried out prior to sectioning. This study shows that the determination of larger-scale conductivity can be achieved, based on the evaluation of fracture geometry parameters (e.g. fracture aperture, fracture width and fracture length), measured using an optical method, at least at the laboratory scale.

Evaluation of an autoregressive spectral estimator for modal wave number estimation in range-dependent shallow water waveguides

In shallow water, geoacoustic properties of the seabed can be inferred from knowledge of acoustic normal modes propagating in the waveguide. For range-varying waveguides, modal content varies locally in response to changes in the environment. The problem becomes estimating the local modal content of a propagating field. For acoustic fields measured on horizontal arrays, resolution of closely spaced modal eigenvalues depends on the data aperture length. Therefore, range variability in a waveguide can only be detected for range scales of order of the resolvability of the individual modes. When only short data apertures are available for modal estimation, high-resolution methods must be used. In this paper, a high-resolution autoregressive (AR) spectral estimation method for extracting the modal information from measurements of a continuous wave acoustic field made on a horizontal array is evaluated. Performance is discussed for estimation accuracy and resolution in the presence of noise. Results are compared to previous work characterizing high-resolution wave number estimators. The AR estimator performance is comparable to other high-resolution methods and does not require prior information about the number of propagating modes. In addition, range/wave number plots obtained using a sliding window technique yield a robust method for identifying propagating modes and any changes with range.

Shell utilization pattern of the hermit crab Clibanarius vittatus (Crustacea, Anomura) in an estuary at São Vicente, State of São Paulo, Brazil

We evaluated the gastropod shell utilization pattern of the hermit crab Clibanarius vittatus (Bosc, 1802) at Pescadores Beach in São Vicente, State of São Paulo, Brazil. Specimens were collected monthly from May 2001 through April 2003, in the intertidal zone at low tide. The crabs were weighed and their carapace shield length measured. All gastropod shells were identified and had their shell biometric parameters (total length and aperture length) measured (mm) and weighed (g). A total of 2,344 hermit crabs (644 males, 1,594 females, 45 ovigerous females and 61 individuals in intersex), using 13 species of gastropod shells, were collected. Stramonita haemastoma (Linnaeus, 1767), Cymatium parthenopeum (Von Salis, 1793) and Achatina fulica (Bowdich, 1822) comprised over 98% of all the shells. Male and intersex crabs were significantly larger than the females. This size difference strongly influenced the shell utilization pattern, principally in A. fulica, which has the largest shell size, that was only used by males and intersexual individuals of C. vittatus. Cymatium parthenopeum was the only shell species that showed a high determinant coefficient in all the biometric correlations evaluated. The high abundance of S. haemastoma shells and a strong correlation between crab size and shell aperture length established by a significant determination coefficient, indicated that C. vittatus uses this species as the principal resource for shell occupation at Pescadores Beach.

Age, Growth and Shell Morphometrics of the Gastropod Monodonta dama (Neritidae:Prosobranchia) from the Gulf of Aqaba, Red Sea

A total of 360 of the midlittoral gastropod Monodonta dama (Phillipi, 1848) from the rocky shores in the Gulf of Aqaba, Red Sea, were studied for age, growth and shell morphometrics. The results show that M. dama may survive for five years. The mean observed length at each of the five age cohorts was 0.605, 0.977, 1.328, 1.568 and 1.934 cm. Theoretically, using Von Bertalaffy`s and Richard`s growth formulae, the maximum length of this snail may reach 2.772 and 2.210 cm, respectively. The annual observed increase in length ranges from 0.240 to 0.372 cm. Theoretically, the increase ranges from 0.180 cm in old snails to 0.472 cm in young ones using the VBGF and from 0.134 to 0.260 cm using RGF. The relationships of the shell length to shell width, aperture length and aperture width are linear. The ratios of the shell length to shell width, aperture length and aperture width increase with age. This may indicate that this snail become more elongate with age.Moreover, the ratio of the shell width to aperture width increases with length. Shell and dry body weights averaged 705 and 475 mg, respectively. The relationships of the shell length to shell and dry body weights are curvilinear. However, the relationships of the shell length to shell width, aperture length and aperture width are linear. The relative coefficient of condition (Kn) which measures the progression of body weight and length was 0.334.

Methods and apparatus for ultrasonic compound imaging

Methods and apparatus for achieving a compound ultrasound imaging mode are disclosed. The methods include a step of firing a plurality of ultrasound beams at a single location, receiving first and second echoes, and combining the first and second echoes to form a composite scan line. The following beam parameters may be varied: transmission focus depth, transmission aperture, transmission frequency, and transmission burst length. Echoes may vary in one or both of receive bandwidth and receive center frequency. In one embodiment, transmission focus depth, transmission aperture, and receive center frequency all differ between the beams. A weighting step may be performed before or after the summing step. The apparatus may include vector memory, a compound logic processor for weighting and summing the vectors, and memory for storing the summed vectors.

Multiple Phase Difference Method for Real-Time SAR Autofocus

This paper deals with the SAR autofocus problem, with the aim to implement the proposed autofocus algorithm in real-time. The crux of the SAR autofocus problem is to estimate the phase error e (·) based on the uncompensated SAR raw signal, and subsequently eliminate the phase error from the SAR data. The existing autofocus algorithms are mostly performed off-line as part of the post-processing step. This is due to the fact that the conventional autofocus methods require the entire synthetic aperture length for phase error estimation and hence, they are time-consuming and not practical for real-time implementation. The proposed algorithm, called the Multiple Phase Difference (MPD) Autofocus, utilises the maximum likelihood (ML) principle to solve the phase estimation problem. It is shown that the ML solution can essentially achieve better performance with minimum computational burden by employing the sub-aperture approach.

Genetic Algorithm-based Design Optimisation of Aperture-coupled Rectangular Microstrip Antenna

The technique of feeding a microstrip antenna with a microstrip line through an aperture is gathering a lot of interest in communication and radar systems used for defence applications. This is due to the fact that this feeding technique has many adjustable parameters in the form of aperture length, width, and stub parameters. This paper presents a study on the effect of aperture dimensions and stub length on the voltage-standing wave ratio bandwidth of an aperturecoupled microstrip antenna. Without any additional matching network, optimal impedance matching of the antenna has been achieved using genetic algorithm. In the algorithm, each chromosome consists of three binary encoded genes, one for aperture length, second one for aperture width and the last one for stub length. Finally, the algorithm determines the parameters of the antenna that provides the minimum value of the average reflection coefficient. The investigation is made at different microwave frequency ranges and it extends up to Ku band. The results show that the percentage of fractional bandwidth improves for higher design frequencies and has a maximum of 13.2 per cent for centre frequency of 17.5 GHz. Experimental results are presented for a particular range of frequency and the accuracy of the analysis is briefly discussed.

Neutralization of Ion Beams for Reduction of Charging Damage in Plasma Etching

Neutral beams were proposed for plasma etching in order to reduce damage due to charging. We analyzed the efficiency of neutralization in a system proposed recently where a beam of ions was neutralized in the gas phase and in a set of narrow tubes. We studied surface neutralization as well as collisions of ions in the gas. In the case of collisions of ions with aperture walls, efficiency of neutralization was assumed to be 100%. We also investigated the influence of various parameters such as aperture length, tube diameter and initial ion energy on the efficiency of neutralization using a Monte Carlo code for ion motion simulation. In our calculations we used a well-established set of cross sections for argon.

Resolution of High-frequency Rayleigh-wave Data

It is essential to understand the potential resolving power of surface wave data in interpreting inverted vertical (layered 1D) and horizontal (‘pseudo-2D’) shear (S)-wave velocity models. The model resolution matrix indicates that the S-wave velocity model of a layered earth model can be perfectly resolved in a least-squares sense if error-free data are inverted. However, we show that errors in real data introduce a smear matrix in the least-squares solution, reducing the resolution of an S-wave velocity model. A field test shows how accurate dispersion curves can resolve a low velocity layer of about 25% contrast at [Formula: see text] depth below a stiff overburden. The horizontal resolution of surface wave surveys is limited to the aperture (length) of the recording array. In phase velocity measurements, the layered model estimate represents an average over this length. Any lateral discontinuities introduce a systematic error into the inversion, leading to acquisition geometry-dependent estimated models. The multichannel analysis of surface waves in roll-along mode allows construction of ‘pseudo 2D-sections’ by aligning 1D models at each spread midpoint. However, images from field data between forward and reverse can be inconsistent. We apply a generalized inversion to these ‘pseudo 2D-sections’ in the horizontal direction, which removes these inconsistencies and provides an improvement in horizontal resolution, allowing a more objective geological appraisal.

Beam Noise Characteristics

Beam-noise spectrum levels were measured with high-resolution horizontal arrays of 22 to 100 wavelengths long at frequencies between 50 and 320 Hz in the Mediterranean Sea. Results indicate that the low-frequency noise field has two major components one of which is due to the dynamic and temporally variable distant shipping. Beam noise level differences of 10 to 20 dB were observed between the resolved distant shipping and the uncorrelated background noise. The measured distributions of beam-noise-intensity fluctuations were found to be non-Gaussian. The beam-noise-intensity distribution was found to depend on aperture length, frequency, bandwidth, integration time, and steering angle. The shapes of the beam-noise-intensity cumulative distribution functions were variable and qualitatively described by nonhomogeneous compound Poisson process as shown with a simulation of the shipping-induced noise field. Temporal characteristics of beam-noise-intensity fluctuations were found to have noise level fades at 30% probability below the median noise level with durations of 1 h for observations over 10 h.

Aesthetic Proportions of the Nasal Aperture in 3 Different Racial Groups of Men

To define baseline aesthetic dimensions of the nasal aperture in 3 different racial groups. Healthy volunteers from 3 different racial groups (15 white, 15 Chinese, and 15 Indian men) were enrolled in the study at the Department of Otolaryngology, University of Dundee, Dundee, Scotland. Those with a history of nasal or facial surgery or trauma were excluded from the study. Images were obtained and stored in a digital format. The dimensions of nasal aperture were defined by the length of the columella at the narrowest point, the width of the columella at the narrowest point, the length of the nasal aperture at the maximum length, the width of the nasal aperture at the maximum width, and the width of the alar cartilage base. There was no significant difference in the length or the width of the columella for the 3 racial groups. There was no significant difference in the length of the nasal aperture between the Chinese and the white groups. The nasal aperture was longer in the Indian group compared with the other 2 groups (P<.002). The nasal aperture at the maximum width was narrower in the Chinese group compared with the other groups (P<.002); there was no significant difference between the white and Indian groups. The nasal alar width was slightly narrower at the alar base in the Chinese group compared with other racial groups (P<.001). The aesthetic dimensions of the nasal aperture differ between racial groups. The nasal aperture and the alar base were narrower in the Chinese group, and the nasal aperture was longer in the Indian group. The aesthetic surgeon should ideally have an understanding of these ethnic variations.

Tear‐film lipid layer morphology and corneal sensation in the development of blinking in neonates and infants

The aim of the study was to evaluate the role of lipid layer thickness and corneal sensation in the development of blinking in neonates. The study group comprised sixty-four neonates and infants (mean age 27.5 +/- 15 (sd) weeks, range 3.4-52) whose mothers were attending a general practice healthy baby clinic. Spontaneous eye-blink activity was determined from digital videographic recordings; tear film lipid layer morphology wasexamined using interference patterns produced by the Keeler Tearscopetrade mark Plus over a five-point grading scale (higher grades are associated with thick and stable lipid films); corneal sensation threshold was assessed with the Non-Contact Corneal Aesthesiometer (NCCA), using the eye-blink response as an objective indication that the cooling stimulus had been felt; palpebral aperture dimensions were measured using calibrated digital still images of the eye in the primary position. The overall mean spontaneous blink-rate was found to be 3.6 (+/- 0.3) blinks min(-1), and the mean interblink time was 21.6 (+/- 2.8) s. The lowest blink-rates were observed in the 0-17-week age group (average 2 blinks min(-1)). The blink-rate showed a highly significant correlation with age (r = 0.46, P < 0.01). The overall mean lipid layer grading was 3.6 (+/- 0.2 SE) arbitrary units. Higher grades were found in the newborn and the mean grading score reduced with age (P < 0.01). The mean sensation threshold to blink (TTB) was 0.69 (0.04 SE) mbar, which did not differ from a control group of older subjects (P > 0.05). There was a rapid increase in palpebral aperture length and width from birth to 1 year old, with surface area increasing by 50% over the same period. We concluded that the low rate of spontaneous eye blink activity in neonates is associated with a thick stable lipid layer that may be a function of a small palpebral aperture. Furthermore, neonates appear to have the capacity to detect ocular surface cooling, which is a major trigger for spontaneous blinking.

Single step fabrication of microlens arrays with hybrid HfO2-SiO2 sol-gel glass on conventional lens surface

We demonstrate a novel method for fabricating glass microlens (arrays) with single step on conventional lens surface. In this method, the glass microlens can be achieved by only one step with sol gel glass material. The microlens aperture and focus length can be controlled easily and uniformly. The fabricated sample shows good focusing property. This work will be useful to improve the performance of compound eyes optical system such as camera, telescope, 3D integral imaging and so on.

Spatial correlation properties of focused partially coherent light

We investigate the spatial coherence properties in the focal region of a converging, spatially partially coherent wave field. In particular, we find that, depending on the effective coherence length of the field in the aperture, the longitudinal and transverse coherence lengths in the focal region can be either larger or smaller than the corresponding width of the intensity distribution. Also, the correlation function is shown to exhibit phase singularities.

Walk-Off Compensated UV generation in Lithium Tetraborate crystals

As high as 21 mJ of energy at 288.5 nm is produced for the first time in Lithium Tetraborate crystals utilizing Type-I twin crystal walk-off compensated second harmonic generation. In the process it is shown that unless one uses the walk-off compensated configuration, the effect of walk-off degrades the conversion efficiency of the second harmonic generation even when the so-called aperture length is much greater than the actual length of the crystal as well as the crystal has an appreciably low value of walk-off angle.

Complex dispersion in simple fractured media

Previous studies noted situations with simple networks of fractured media where dispersion was complex. Certain network geometries provided principal directions of spreading at angles to the direction of mean flow and occasions where transverse spreading greatly exceeded longitudinal spreading. This article is concerned with understanding this complex dispersion in the context of more variable networks and to evaluate the scaling between dispersion and characteristics of the network. The approach involves modeling flow and particle transport through networks consisting of equally spaced finite fractures. Simulation trials examined how changes to the orientation of the overall network, fracture aperture, fracture connectivity, and fracture length influenced mass transport. With these geometries, particle spreading was due to a grid orientation effect and variability related to apertures and connectivity. The grid orientation effect inherently gives rise to the complex dispersion described in previous work. Spreading caused by variability in aperture and connectivity produces the more familiar, elliptical pattern of spreading. In the various model trials we created situations where either or both of these spreading mechanisms could dominate. When both grid orientation and heterogeneity effects were comparable, particle swarms took on a curious hybrid shape reflecting the outcome of two different spreading mechanisms. With sufficient heterogeneity the grid orientation effect was swamped and disappeared. As the block size increases so does transverse dispersion. Further studies are necessary to investigate whether the grid effects associated with the simplified fracture networks can be extrapolated to real fractured media.

Numerical analysis of rectangular microstrip patch over ground plane with rectangular aperture

In this paper, a rigorous full-wave analysis of rectangular microstrip patches over ground planes with rectangular apertures in substrates containing isotropic and anisotropic materials is presented. The dyadic Green's functions of the problem are efficiently determined in the vector Fourier transform domain. The integral equations for the unknown patch current and aperture field are solved numerically by applying the Galerkin method of moments. The TM set of modes issued from the magnetic wall cavity model are used to expand the unknown current on the patch. Also, the same basis functions are used for approximating the aperture field in accordance with the concept of complementary electromagnetic structures. The validity of the solution is tested by comparison of the computed results with experimental data. Numerical results show that changes in aperture length can drastically shift the resonant frequency. The aperture width, on the other hand, can be used for a fine adjustment of the operating frequency. Other results also indicate that dielectric anisotropy effect is especially significant when the size of the aperture is similar to that of the patch. Copyright © 2004 John Wiley & Sons, Ltd.

Shell-size selection by intertidal sympatric hermit crabs

This study evaluated selection for shell size by three species of tropical intertidal hermit crabs, Clibanarius antillensis, C. sclopetarius, and C. vittatus, from species of shells which are frequently used in nature. Crab size and weight were strongly and significantly related to all measured parameters of the selected shells. The strength of these relationships (r2 values) depended neither on the crab nor on the shell variables taken into account. The relationships between crab size and the dimensions of the selected shells showed higher r2 values than the corresponding relationships with the shells that the crabs had occupied when they were collected (0.482–0.903 in comparison to 0.091–0.652, respectively), indicating that the crabs were occupying sub-optimal shells in nature. Negative allometry was frequently found in the relationships between crab and shell variables, indicating that large crabs select and use proportionally lighter shells than do small crabs. This negative allometry was stronger for the shells used in nature (except for C. antillensis), i.e. larger crabs tended to select heavier shells in the laboratory than in nature. Different allometric relationships were also recorded among the dimensions of shells used in nature and those selected by the hermit crabs in free-access experiments: as shell length increased, the selected shells were heavier and had larger apertures than the shells used in nature. The relationships between crab size and the length and weight of the selected shells did not depend on the species of crab or species of shell, but only on crab size. Therefore, analyses using these variables can be performed without taking the species of crab or shell into account, i.e. data from different crab or shell species can be pooled. The influence of crab and/or shell species was recorded only in the models fitted for aperture length and width, variables which were more related to shell architecture than did shell length or weight. In contrast, if crab weight is used as an independent variable, different crab or shell species can be analyzed together independently of the particular shell parameter. This indicates that crab weight may be less susceptible than crab shield length to shell morphological constraints. Finally, the results indicate that the preferred size of a given shell type chosen by a given hermit crab will depend more on crab size or weight, than on the crab or shell species under consideration, i.e. crab shell-size relationships are not species specific.