Wide-angle Reflector Research Articles

Efficient Anomalous Reflector Design Using Array Antenna Scattering Synthesis

A perfect anomalous reflector is designed by employing the receiving and scattering array antenna theory to optimize the scattering characteristics of a planar reflecting surface. For periodic reflectors with a supercell consisting of a relatively few reactively loaded radiating elements, reflection amplitudes into propagating Floquet modes are controlled in an algebraic optimization of the load reactances, avoiding brute-force optimization via electromagnetic simulations. Wide-angle reflectors are numerically designed and compared with the conventional reflectarray designs, showing that the proposed design approach achieves reflection efficiencies higher than the linear reflection phase gradient method in a computationally efficient manner.

TE Transmittance properties of one-dimensional symmetric quinary photonic crystals

In this work, a symmetric one dimensional quinary photonic crystal structure is proposed. The unit cell of the proposed structure is composed of materials chosen in such a way that the refractive index of the central layer is the highest among the two neighbouring layers from both sides. Basically, the transmittance of the S-polarized transverse electric () mode is obtained numerically using the transfer matrix method. The effect of several parameters such as the number of periods, the layer thickness and the angle of incidence of radiation on transmittance of the structure is investigated. The effect of changing thickness and refractive index of the central layer on the transmittance is found to play the major rule in tuning the photonic bandgap. The proposed structure is expected to be a good candidate for applications such as antireflection coatings used in solar cells, wide-angle reflectors and pressure or temperature sensors.

Upper crustal structure of an active volcano from refraction/reflection tomography, Montserrat, Lesser Antilles

To better understand the volcanic phenomena acting on Montserrat, the SEA-CALIPSO seismic experiment (Seismic Experiment with Airgun-source – Caribbean Andesitic Lava Island Precision Seismo-geodetic Observatory) was conducted in 2007 December with the aim of imaging the upper crust and the magmatic system feeding the active Soufriére Hills Volcano. The 3-D survey covered an area of about 50 × 40 km and involved the deployment of 247 land stations and ocean-bottom seismometers (OBSs). A subset of the data, recorded by four OBSs and four land stations on a southeast to northwest line, has been analysed, and traveltimes have been inverted to obtain a 2-D seismic velocity model through the island. Inverted phases include crustal and sediment P waves and wide-angle reflections. The resulting velocity model reveals the presence of a high velocity body (3.5–5.5 km s-1) beneath the island, with highest velocities beneath the Soufriére and Centre Hills, corresponding primarily to the cores of these volcanic edifices, built of a pile of andesite lava domes and subsequent intrusions. In the offshore region, velocities in the surficial sediment layer vary from 1.5 to 3.0 km s-1, consistent with a mainly calcareous and volcaniclastic composition. A wide-angle reflector is observed at a depth of ∼1200 m below the seabed, and appears to deepen beneath the island. The upper crust beneath this reflector has velocities of 4.0–6.0 km s-1 and is inferred to correspond to plutonic and hypabyssal rocks and sedimentary material of the old arc. The high velocity region beneath the island, extends into the crust to a depth of at least 5 km, and is believed to be caused by an intrusive complex, possibly of intermediate composition. A low velocity zone, as would be expected in the presence of an active magma chamber, was not observed perhaps due to the limited resolution beneath ∼5 km depth. Our results so far provide the first wide-angle seismic constraints on the upper crustal structure of the island to a depth of 10 km, and will help understanding the processes that drive volcanism at Montserrat and other island arc volcanoes.

Collisional tectonics of the Baltic Shield in the northern Gulf of Bothnia from seismic data of the BABEL project

SUMMARY Densely spaced wide-angle data from stations recording airgun shots along BABEL profile 2 in the northern Gulf of Bothnia were analysed, modelled, and compared with the normal-incidence recordings. Profile 2 extends from the metasedimentary and metavolcanic Proterozoic Central Svecofennian Province into the volcanic Northern Svecofennian Province of magmatic arc affinity and crosses the Proterozoic-Archaean boundary of the northern Baltic Shield as defined from isotope studies. We used a 2-D traveltime inversion to derive P- and S-wave velocity-depth models and a distribution of Poisson’s ratios along the profile. A series of north-dipping wide-angle reflectors characterize the middle and lower crust in the southern part of the model, coinciding with similar events in previous multichannel wide-angle data and the CMP recordings. Several subhorizontal upper to mid-crustal wide-angle reflectors and a well-resolved Moho boundary at 42 to 45 km depth coincide with reflective zones or changes in reflectivity patterns of the CMP data. P-wave velocities increase from 5.7 km s-’ at the near-surface to 6.4 and 6.6 km s-l in the mid-crust and up to 7.6 km s-l in the lowermost crust. P, velocities reach 8.0 to 8.2 km s-’ below the Moho in the northern part of the model but are less than 8.0 km s-’ in the southern part. Poisson’s ratios, calculated from the P- and S-wave models, range from 0.21 to 0.25 in the uppermost crust to about 0.30 in the lower crust and upper mantle. Our velocity and Poisson’s ratio models indicate that the upper crust contains predominantly felsic, quartz-rich rocks. The increase of P-wave velocity and Poisson’s ratio in the mid- to lower crust is explained by increasingly mafic composition. We propose a geological model across the northern Gulf of Bothnia that combines our velocity-depth model with results from the seismic normal-incidence experiment, previous geophysical information, and geological surface observations. The model consists of a possibly multiple Proterozoic north-dipping subduction zone with remnants of oceanic crust and zones of imbricated metasediments and mafics on top. There is evidence for a metasedimentary fore-arc basin in the middle crust and a layer of mafic to ultramafic cumulates at the base or below parts of the lower crust. South-dipping mid- and lower crustal zones of high reflectivity in the northern part of profile 2 indicate that Archaean crust underlies the Northern Svecofennian Province. The major features of this Proterozoic collision zone, such as the suggested subduction slab, zones of crustal reflectors dipping parallel to the slab, a metasedimentary basin, and mafic to ultramafic cumulates between the subduction slab and overlaying Moho, are well-preserved analogues of recent oceaniccontinental collision and subduction zones.

Seismic structure of the Central Metasedimentary Belt, southern Grenville Province

Crust and upper-mantle structure interpreted from wide-angle seismic data along a 260 km profile across the Central Metasedimentary Belt of the southern Grenville Province in Ontario and New York State shows (i) relatively high average crustal and uppermost mantle velocities of 6.8 and 8.3 km/s, respectively; (ii) east-dipping reflectors extending to 24 km depth in the Central Metasedimentary Belt; (iii) weak lateral velocity variations beneath 5 km; (iv) a mid-crustal boundary at 27 km depth; and (v) a depth to Moho of 43–46 km. The wide-angle model is generally consistent with the vertical-incidence reflectivity of an intersecting Lithoprobe reflection line. The mid-crustal boundary correlates with a crustal detachment zone in the Lithoprobe data and the depth extent of east-dipping wide-angle reflectors. Regional structure and aeromagnetic anomaly trends support the southwest continuity of Grenville terranes and their boundaries from the wide-angle profile to two reflection lines in Lake Ontario. A zone of wide-angle reflectors with an average apparent eastward dip of 13° has a surface projection that correlates spatially with the boundary between the Elzevir and Frontenac terranes of the Central Metasedimentary Belt and resembles reflection images of a crustal-scale shear zone beneath Lake Ontario. A high-velocity upper-crustal anomaly beneath the Elzevir–Frontenac boundary zone is positioned in the hanging wall associated with the concentrated zone of wide-angle reflectors. The high-velocity anomaly is coincident with a gravity high and increased metamorphic grade, suggesting northwest transport of mid-crustal rocks by thrust faulting consistent with the mapped geology. The seismic data suggest (i) a reflective, crustal-scale structure has accommodated northwest-directed tectonic transport within the Central Metasedimentary Belt; (ii) this structure continues southwest from the exposed Central Metasedimentary Belt to at least southern Lake Ontario; and (iii) crustal reflectivity and complexity within the eastern Central Metasedimentary Belt is similar to that observed at the Grenville Front and the western Central Metasedimentary Belt boundary.

Crust and upper mantle velocity structure of the Intermontane belt, southern Canadian Cordillera

As part of the Lithoprobe Southern Cordillera transect, seismic refraction data were recorded along a 330 km long strike profile in the Intermontane belt. An iterative combination of two-dimensional traveltime inversion and amplitude forward modelling was used to interpret crust and upper mantle P-wave velocity structure. This region is characterized by (i) a thin near-surface layer with large variations in velocity between 2.8 and 5.4 km/s, and low-velocity regions that correlate well with surface expressions of Tertiary sedimentary and volcanic rocks; (ii) an upper and middle crust with low average velocity gradient, possibly a weak low-velocity zone, and lateral velocity variations between 6.0 and 6.4 km/s; (iii) a distinctive lower crust characterized by significantly higher average velocities relative to midcrustal values beginning at 23 km depth, approximately 8 km thick with average velocities of 6.5 and 6.7 km/s at top and base; (iv) a depth to Moho, as defined by wide-angle reflections, that averages 33 km with variations up to 2 km; and (v) a Moho transition zone of depth extent 1–3 km, below which lies the upper mantle with velocities decreasing from 7.9 km/s in the south to 7.7 km/s in the north. Where the refraction line obliquely crosses a Lithoprobe deep seismic-reflection profile, good agreement is obtained between the interpreted reflection section and the derived velocity structure model. In particular, depths to wide-angle reflectors in the upper crust agree with depths to prominent reflection events, and Moho depths agree within 1 km. From this comparison, the upper and middle crust probably comprise the upper part of the Quesnellia terrane. The lower crust from the refraction interpretation does not show the division into two components, parautochthonous and cratonic North America, that is inferred from the reflection data, indicating that their physical properties are not significantly different within the resolution of the refraction data. Based on these interpretations, the lower lithosphere of Quesnellia is absent and presumably was recycled in the mantle. At a depth of ~ 16 km below the Moho, an upper mantle reflector may represent the base of the present lithosphere.

Estimation of interval velocities within the Earth's crust

Experiments to assess reliable interval velocities within the earth's crust have been carried out: 1. (1) using an enlarged spread length (e.g., 23 km for the Urach U1 profile, 2. (2) moving source and fixed geophone arrays up to 80 km in length (e.g., DEKORP 2-S) and 3. (3) using expanded spread configurations up to 100 km in length (e.g., DEKORP 4). The choice as to which of these methods will give the best and most reliable velocity-depth data depends on the differentiation and the tectonic style of the crust which determine whether continuous near-vertical and/or quasi-continuous wide-angle reflectors from the intracrustal boundaries develop. Method 1 has been applied in an area of young tectonothermal activity (Miocene volcanism). By means of the excellent data quality with phase correlations to depths of 20 km in the lower crust it has provided the most detailed velocity structure. On the other hand, with increasing tectonothermal age of the crust, a general decrease in reflector length is observed and method 3 seems to provide the most reliable velocity-depth information in this situation, although only in a limited area.

Crustal structure beneath exposed accreted terranes of Southern Alaska

Summary. The crustal structure beneath the exposed terranes of southern Alaska has been explored using coincident seismic refraction and reflection profiling. A wide-angle reflector at 8-9 km depth, at the base of an inferred low-velocity zone, underlies the Peninsular and Chugach terranes, appears to truncate their boundary, and may represent a horizontal decollement beneath the terranes. The crust beneath the Chugach terrane is characterized by a series of north-dipping paired layers having low and high velocities that may represent subducted slices of oceanic crust and mantle. This layered series may continue northward under the Peninsular terrane. Earthquake locations in the Wrangell Benioff zone indicate that at least the upper two low-high velocity layer pairs are tectonically inactive and that they appear to have been accreted to the base of the continental crust. The refraction data suggest that the Contact fault between two similar terranes, the Chugach and Prince William terranes, is a deeply penetrating feature that separates lower crust (deeper than 10 km) with paired dipping reflectors, from crust without such reflectors.

A new design method for phase-corrected reflectors at microwave frequencies

A method is given for the design of a class of wide-angle reflectors in which the aberrations are reduced by coating the reflector surface with a dielectric. At microwave frequencies this dielectric may take the form of an array of metal plates or waveguides and the constraint imposed by this form greatly simplifies the analysis. The path of the feed point is chosen so that only selected rays from it are equally phased. It is then found that both of the reflector's profiles and the path of the feed point can be described by a single parameter. The residual aberrations of several cylindrical systems, including the corrected parabola and circle, are analysed to find a reflector for which the scanning arc is the circle with centre at the vertex of the reflector. A refocusing procedure, which is found to be necessary, produces this scanning arc. Experimental results in agreement with the theory are given. It is shown that the design principle can be used to programme a step-wise procedure for the design of corrected surfaces using nonconstrained natural dielectric coatings.