Single Azimuth Research Articles

Soliton-comb solutions for fiber-based bottle microresonators.

We predict a stable generation of optical solitons and related low-repetition frequency combs in high-Q fiber-based bottle resonators of different shapes. The modal localization effect is controlled by the shaping of the fiber radius. The solitons consist of axial resonator modes, belong to a single azimuth modal number, and possess anomalously small velocities. They are remarkably similar to each other for different resonator shapes and strongly different from conventional azimuth soliton-comb states: instead of the modal dispersion, the soliton width and amplitude are controlled by the critical self-focusing power.

ScS shear-wave splitting in the lowermost mantle: Practical challenges and new global measurements

Many regions of the Earth's mantle are seismically anisotropic, including portions of the lowermost mantle, which may indicate deformation due to convective flow. The splitting of ScS phases, which reflect once off the core-mantle boundary (CMB), is commonly measured to identify lowermost mantle anisotropy, although some challenges exist. Here, we use global wavefield simulations to evaluate commonly used approaches to inferring a lowermost mantle contribution to ScS splitting. We show that due to effects of the CMB reflection, only the epicentral distance range between 60° and 70° is appropriate for ScS splitting measurements. For this distance range, splitting is diagnostic of deep mantle anisotropy if no upper mantle anisotropy is present; however, if ScS is also split due to upper mantle anisotropy, the reliable diagnosis of deep mantle anisotropy is challenging. Moreover, even in the case of a homogeneously anisotropic deep mantle region sampled from a single azimuth by multiple ScS waves with different source polarizations (in absence of upper mantle anisotropy), different apparent fast directions are produced. We suggest that ScS splitting should only be measured at "null" stations and conduct such an analysis worldwide. Our results indicate that seismic anisotropy is globally widespread in the deep mantle.

Converting a smartphone into an accurate solar compass.

We have developed an app, named Sunpass, that is able to convert every smartphone into a solar compass. Sunpass uses input data from the smartphone sensors, calculates the Sun position, and elaborates data to give the desired information. The azimuth values measured by a smartphone equipped with Sunpass show a typical accuracy of 0.5°, which is limited by camera aberrations and misalignment of both accelerometer and CCD camera of the smartphone. In this paper, we show that both accuracy and reliability in azimuth measurements can be improved by a specific calibration procedure and a dedicated mechanical tool. We obtained a remarkable accuracy better than 0.06° on the single azimuth measurements, which improves to 0.03° on the average of eight measurements.

2qth-Order cumulants based virtual array of a single acoustic vector sensor

A spatially co-located four-element single acoustic vector sensor offers numerous advantages over the scalar pressure sensor array such as compact size, ability to handle spatially undersampled systems, and the intrinsic two-dimensional directivity independent of the range, frequency, and bandwidth of the incoming signal. Despite all these advantages, a single acoustic vector sensor finds limited application in the field of the direction of arrival estimation as it can only resolve a maximum of two sound sources using classical second-order statistics-based methods. This paper proposes a 2qth-order circular cumulants based processing (q is an integer, and q≥1) by utilizing the concept of virtual array, to enhance the degree of freedom and hence to maximize the sources resolvability of a single acoustic vector sensor. Additionally, this paper derives a theoretical upper bound on the maximum number of the identifiable two-dimensional direction of arrivals of statistically independent, non-Gaussian sources by exploiting the proposed virtual array of a single acoustic vector sensor. The upper bound on the maximum number of the identifiable two-dimensional direction of arrivals by exploiting the proposed virtual array is found to be 3, 8, and 14, for q=1, 2, and 3, respectively, if both the elevations and azimuths are distinct for all the given statistically independent, non-Gaussian sources. Furthermore, the upper bound on the maximum number of the identifiable two-dimensional direction of arrivals by exploiting the proposed virtual array is found to be 2, 4, and 6, for q=1, 2, and 3, respectively, if all the given statistically independent, non-Gaussian sources are lying in a single azimuth (or elevation) plane with distinct elevations (or azimuths).

Panoramic azimuthal Schlumberger Vertical Electrical Sounding for fracture orientation and anisotropy quantification

Vertical electrical sounding (VES) data acquired with the Schlumberger configuration is popularly used to image the electrical resistivity variation with depth at a single azimuth. Apart from the random subjective choice of the single azimuthal direction by the field geophysicists, important hydrological information such as fracture orientation and anisotropic coefficients needed for understanding resultant groundwater flow direction are by design lost in the process. Panoramic (0°–360°) azimuthal VES data were acquired at two data points at the Federal University of Technology, Akure (FUTA) at angular step of 15°, making a total of 24 data sets per data point. Each azimuthal VES data was inverted using equal number of layers in order to confirm the presence of anisotropy, quantify the anisotropic coefficients and image the orientation of fracture at a particular depth. Little to large apparent resistivity data and model suggested the presence of anisotropy which otherwise would have been lost in a single azimuthal survey. Elliptical fit of each layer azimuthal inverted resistivity was used to quantify the fracture orientation and coefficient of anisotropy with depth. From the results, it is established that anisotropy is present only at the near-surface: and the anisotropic coefficient increases from the surface to 7m. The result also showed the presence of an isotropic unit from 8m to the fresh basement. In agreement with existing published results on the geology of the area, the majority of the fractures trend North West and North East at stations 1 and 2 respectively. We hope that the methodology will foster detailed 3D panoramic imaging of the fracture network within and outside the study location, which will help in designing better groundwater management scheme and understanding resultant groundwater flow direction for contaminant and pollutant prevention and for flood control.

Multi-Azimuth Ground Penetrating Radar Surveys to Improve the Imaging of Complex Fractures

Ground Penetrating Radar (GPR) images are affected, to some degree, by the relative orientation of antennas and subsurface targets. This is particularly true not only for targets that show a significant directivity, but also for inclined planes, such as fractures and faults. Depending on the relative geometry between the antennas and the orientation of the target, radar waves can be preferentially scattered, which causes changes in the reflected signal amplitude. Therefore, traditional single polarization and single azimuth surveys may produce inadequate results. The work presented here examines the use of a multi-azimuth GPR survey to increase the imaging performance of inclined fractures, showing the shortcomings of single-profile surveying and highlighting the benefits that such a strategy has on detection and characterization.

Mapping and analysis of tectonic lineaments of Pachamalai hills, Tamil Nadu, India using geospatial technology

In the present study an updated tectonic lineament database for Pachamalai hill, a resource-rich hill of central Tamil Nadu state of India has been generated and analysed. Digital Elevation Model generated from CARTOSAT-1 satellite data have been the data source. Lineaments extracted from eight different azimuth angles were compiled together towards the generation of the lineament database, which has been subsequently analysed for their number, length, density and spatial distribution using ArcGIS software. In addition, their orientations were analysed using Rockworks software. Of the 561 lineaments of the study area, about 90% are very short and short lineaments. The total length of the lineaments is 680 sq. km and their density ranges from 0 to 3.4 km/sq. km. The diversely oriented lineaments of the hill reflect the multiple deformation events that have affected the region through geologic time. In about 30% of the hill, the lineament density is high to very high which implies higher degree of deformation, fracturing, shearing and permeability of rocks besides higher soil erodibility and groundwater yield. These high to very high density areas render themselves unsuitable for the construction of dams and reservoirs as the possibility of water leakages into the subsurface, slope and dam failures and rate of sedimentation would be higher. Further, the analysis of the lineaments clearly underlines the need to extract lineaments from different azimuth angles instead of the widely adopted practice of mapping lineaments from single azimuth angle.

Spaceborne SAR Tomography for Vertical Profile Retrieval of Forest Vegetation

Polarimetric SAR remote sensing has shown its potential for forest structural and biophysical parameter retrieval. Multibaseline polarimetric data in interferometric mode were acquired from space-borne C-band SAR system over Dudhwa National Park, India. Potential of multibaseline SAR tomography data was tested with six scenes of SAR data for forest height retrieval. Capon spectral estimation algorithm was implemented on PolInSAR data for vertical profile generation. Vertical profile generated from capon algorithm had shown different forest height values at different locations in range direction for single azimuth bin. Maximum height value recorded from the vertical profile was 45 m and the minimum height was 5 m. The algorithm was implemented for all the locations for which field data was collected for forest height measurement. Tomography based forest height was compared with field-measured forest height for 190 locations of the study area. Coefficient of determination and root mean square error obtained from the relationship between field and modelled forest heights were 0.91 and 3.91 m respectively. This study concluded that forest height with reliable accuracy can be retrieved with space-borne PolInSAR tomography.

An evaluation of the influence of measurement geometry on the uncertainties of photometric model results based on the laboratory measurements of particulate surfaces

Sunlight reflected by particulate surfaces carries important information about its physical properties. Modeling the reflectance of different types of particulate samples is an attractive field of study, so estimating the favorable measurement geometry for accurate inversion of photometric model parameters is necessary. This research examines the distribution of the bidirectional reflectance factor (BRF) with different particle sizes by multi-angular reflectance. Two types of particulate samples (one with low reflectance and the other with moderate reflectance) with particle sizes of 0.3, 0.45 and 0.9mm were measured over a wide viewing range under the assumption of left-to-right symmetry of the BRF. Based on these measurements, we computed the reflectance of particulate surfaces by a photometric model and analyzed the influence of measurement geometry (different combinations of incident zenith angle, viewing zenith angle and azimuth angle) on the inverted parameters and the results modeled by the best-fit parameters. The results show that by using the measurements in the single azimuth (including the principal plane) to invert the model parameters, the difference between the modeled results and measured results will exceed the reflectance change caused by the samples' particle size; this difference is also found when we used the combined measurements at two different incident zenith angles. Including the measurements in the principal plane, an increase in the number of azimuth angles will improve the match between the modeled results and measurements. Our results also confirm that the single-scattering albedo is the only model parameter that could be empirically used to determine the particle sizes of our samples over a wide range of measurement directions. This study proposes several favorable combinations of measurement geometry and also appears to provide a promising empirical reference for the particulate surfaces similar to ours in future laboratory experiments.

Resolution enhancement for Doppler beam sharpening imaging

The efficient scan moving target indication (MTI) mode is implemented in the wide-area ground MTI systems for the purpose of wide-area surveillance. However, because of the scanning movement of the radar antenna, it is difficult to acquire enough coherent pulses in a single azimuth direction in the Doppler beam sharpening (DBS), so the sharpening ratio is greatly limited. To mitigate this problem, a combined super-resolution algorithm with aperture extrapolation for DBS imaging is proposed to enhance the sharpening ratio in this study. In this algorithm, aperture extrapolation technique is utilised to increase the data length in azimuth direction and the amplitude and phase estimation of a sinusoid, which can acquire accurate spectral estimation with much lower side lobes and narrower spectral peaks, is applied to replace the fast Fourier transform to perform the Doppler analysis. In this way, the sharpening ratio in the new algorithm could be enhanced by one time. Experimental results on simulation data and real data verify the effectiveness of the new super-resolution algorithm, and demonstrate that the proposed algorithm can provide sharp and clear scene information with lower side lobes and narrower peaks.

Azimuthal τ-panalysis in anisotropic media

SUMMARY For the purpose of transversely isotropic (TI) normal moveout (NMO) correction, we propose analysis of plane wave transformed azimuthal gathers, interactively using a single azimuth data at a time and a new delay time equation (developed in this paper), which is a function of two parameters at each azimuth. Results from independently estimated multi-azimuth gathers, then, can be combined to estimate stiffness or Thomsen coefficients. Azimuthal τ–p analysis also avoids numerical ray tracing, resulting in a rapid algorithm. We demonstrate the applicability of our method using a set of P-wave synthetic seismograms from a multilayered medium, consisting of isotropic and HTI layers. Azimuth-dependent anisotropy parameters are derived by delay time fitting and NMO correction. The reflections from the bottom interface of an isotropic layer with an anisotropic overburden show apparent anisotropic traveltime behaviour, which is easily accounted for by our layer-stripping based azimuthal NMO analysis. Unlike the previous approximate HTI NMO correction equation, this equation performs better NMO correction for the HTI medium and is also applicable to the VTI medium. Presence of only two reduced parameters in the equation helps the anisotropic parameter estimation become less ambiguous.

The 2005 and 2006 DANDELIONS NO2 and aerosol intercomparison campaigns

Dutch Aerosol and Nitrogen Dioxide Experiments for Validation of OMI and SCIAMACHY (DANDELIONS) is a project that encompasses validation of spaceborne measurements of NO2 by the Ozone Monitoring Instrument (OMI) and Scanning Imaging Absorption Spectrometer for Atmospheric Cartography (SCIAMACHY), and of aerosol by OMI and Advanced Along‐Track Scanning Radiometer (AATSR), using an extensive set of ground‐based and balloon measurements over the polluted area of the Netherlands. We present an extensive data set of ground‐based, balloon, and satellite data on NO2, aerosols, and ozone obtained from two campaigns within the project, held during May–June 2005 and September 2006. We have used these data for first validation of OMI NO2, and the data are available through the Aura Validation Data Center website for use in other validation efforts. In this paper we describe the available data, and the methods and instruments used, including the National Institute of Public Health and the Environment (RIVM) NO2 lidar. We show that NO2 from Multi‐Axis Differential Optical Absorption Spectroscopy (MAX‐DOAS) compares well with in situ measurements. We show that different MAX‐DOAS instruments, operating simultaneously during the campaign, give very similar results. We also provide unique information on the spatial homogeneity and the vertical and temporal variability of NO2, showing that during a number of days, the NO2 columns derived from measurements in different directions varied significantly, which implies that, under polluted conditions, measurements in one single azimuth direction are not always representative for the averaged field that the satellite observes. In addition, we show that there is good agreement between tropospheric NO2 from OMI and MAX‐DOAS, and also between total NO2 from OMI and direct‐sun observations. Observations of the aerosol optical thickness (AOT) show that values derived with three ground‐based instruments correspond well with each other, and with aerosol optical thicknesses observed by OMI.

Building velocity models for depth imaging in the presence of short-wavelength velocity variations: a North Sea case study

This case study encapsulates several issues that are highly relevant to the latest model building workflows for depth imaging. Firstly, because several datasets of different vintages and most importantly, different acquisition azimuths are input, multi-azimuth tomography is required. This is an increasing trend as more surveys are acquired with wide- and rich-azimuth technology. Secondly, because the chalk shows widespread strong velocity heterogeneity, there are many locations that nicely illustrate how the different azimuths actually contribute to solving short-wavelength velocity variations – much more so than any single azimuth could have done. In order to achieve this, the velocity picking needs to be dense and all the non-hyperbolic trends must be modelled by means of full multi-offset picking. Finally, because of the large velocity contrasts with both the chalk and salt layers, the model building needs to use a hybrid grid tomography technique. More routinely, the model is calibrated with a number of wells distributed across the model building area and VTI anisotropy is incorporated for several layers.

Image-ray tracing for joint 3D seismic velocity estimation and time-to-depth conversion

Seismic time migration is known for its ability to generate well-focused and interpretable images, based on a velocity field specified in the time domain. A fundamental requirement of this time-migration velocity field is that lateral variations are small. In the case of 3D time migration for symmetric elementary waves (e.g., primary PP reflections/diffractions, for which the incident and departing elementary waves at the reflection/diffraction point are pressure [P] waves), the time-migration velocity is a function depending on four variables: three coordinates specifying a trace point location in the time-migration domain and one angle, the so-called migration azimuth. Based on a time-migration velocity field available for a single azimuth, we have developed a method providing an image-ray transformation between the time-migration domain and the depth domain. The transformation is obtained by a process in which image rays and isotropic depth-domain velocity parameters for their propagation are esti-mated simultaneously. The depth-domain velocity field and image-ray transformation generated by the process have useful applications. The estimated velocity field can be used, for example, as an initial macrovelocity model for depth migration and tomographic inversion. The image-ray transformation provides a basis for time-to-depth conversion of a complete time-migrated seismic data set or horizons interpreted in the time-migration domain. This time-to-depth conversion can be performed without the need of an a priori known velocity model in the depth domain. Our approach has similarities as well as differences compared with a recently published method based on knowledge of time-migration velocity fields for at least three migration azimuths. We show that it is sufficient, as a minimum, to give as input a time-migration velocity field for one azimuth only. A practical consequence of this simplified input is that the image-ray transformation and its corresponding depth-domain velocity field can be generated more easily.

Multi-azimuth towed streamer 3D seismic in the Nile Delta, Egypt – processing solutions

Multi-azimuth or wide azimuth seismic is not a new technology, and has been with us for many years in the form of land, and ocean bottom surveys. The literature is rich with examples of how high-fold multi-azimuth data can produce stunning improvements over their single azimuth 3D equivalents (Rogno et al., 1999, Keggin et al., 2002, Gaus and Hegna, 2003, Arntsen and Thompson, 2003, Riou et al., 2005, Manley et al., 2006, Michell et al., 2006). We know from theory and case histories that multi-azimuth data will lead to improved signal to noise, improved multiple attenuation and improved illumination. However, because of approximations in current processing technology, the processing of multi-azimuth data will leave errors in the final imaged results. Simple stacking of the data, though surprisingly robust makes assumptions about data consistency between surveys and will likely not result in the most optimal image. This paper shows how multi-azimuth (MAZ) towed streamer data is processed in the Nile Delta, looks at some of the issues highlighted above and discusses the initial processing sequence to improve the combined subsurface image.

Seven-element ground skirt monopole ESPAR antenna design from a genetic algorithm and the finite element method

The design of an optimized electronically steerable passive array radiator (ESPAR) antenna is presented. A genetic algorithm using a finite element based cost function optimized the antenna's structure and loading conditions for maximal main lobe gain in a single azimuth direction. Simulated gain results of 7.3 dBi at 2.4 GHz were attained along the antenna's elemental axis. The optimized antenna was fabricated and tested with the corresponding experimental gain better than 8 dBi. The 0.7 dB error between simulated and measured gain was constant for numerous structures and therefore did not affect the optimization. The optimized antenna reduced average main lobe elevation by 15.3/spl deg/ to just 9.7/spl deg/ above the horizontal.

AzimMod: Method to honor channel directionality when building 3-D petrophysical models

Object-based (or Boolean) methods are used to create channels, bars, splays, or other features to model the distribution of facies in a reservoir. Geologic objects and their facies subsequently are used as templates to control the distribution of petrophysical and lithologic properties. Channels do not flow in a single direction or azimuth, but may meander or have different azimuths, so properties within them should not be aligned in a single direction or azimuth. Hence, standard geostatistical modeling methods, which assume a single azimuth, are not appropriate. This paper describes a process to assign petrophysical properties with continuity aligned parallel to the orientation of channels or bodies, regardless of their varying azimuths. The general process (1) transforms the coordinate system to one in which a channel is straight and can be modeled with geostatistical methods using a constant azimuth, (2) relates the geostatistical calculations to objects and facies, and (3) returns the petrophysical properties to the original, variable-azimuth coordinate system.

Array measurements of phases used in receiver-function calculations: Importance of scattering

AbstractReceiver-function methods utilize three-component teleseismic P coda to determine structure beneath seismographs. Most methods treat coda complexity as mode conversions and reflections off subhorizontal interfaces beneath the station, such as the Moho. This assumption is evaluated beneath a broadband array in the Kopet Dagh foothills, at Geyokcha, Turkmenistan, by applying array processing techniques to P coda waves at frequencies near 1 Hz. Beneath the Geyokcha array, horizontal components show signals that deviate significantly in slowness and bearing from predictions of a plane-layered Earth. Large phases arrive at the same time as the predicted Ps conversion from the Moho yet systematically come from a single azimuth (from NNE) and travel at phase velocities of 3 to 5 km/sec. The coda waves appear to be P-to-S converted waves from a near-surface scatterer. An inversion for scatterer location is developed and applied to 29 events. Most coda energy comes from a scatterer ∼16 km NNE of the array. This location is coincident with the range-bounding faults that separate Kopet Dagh mountains from adjacent basins. Thus, array analysis shows that signals potentially interpreted as Moho conversions are instead scattered from a near-surface structure. Although these results suggest caution is necessary in interpreting single-station receiver functions, particularly at high frequencies, they also demonstrate the power of array sampling in resolving lateral structure variations.

Remote sensing of surface hemispherical reflectance (albedo) using pointable multispectral imaging spectroradiometers

Several techniques that use multiple off-nadir view angles taken in a single azimuth plane (called a string of data) to estimate hemispherical reflectance have been developed. The range of view angles that is required in order to obtain accurate inferences of hemispherical reflectance using string techniques was studied as applicable to the Moderate Resolution Imaging Spectroradiometer (MODIS) and the High Resolution Imaging Spectroradiometer (HIRIS). Full and half strings out to 60°, 45°, 30°, and 15° were tested on ground data of a wide range of cover types and sun angles. Both a visible and near infrared band (AVHRR Bands 1 and 2) were tested using a knowledge-based system called VEG. Using string techniques that calculate a single coefficient from an internal spectral data base, the results showed small errors for the full string ± 60° (less than 4% error), HIRIS like data (less than 5% error), and MODIS like data (less than 7% error). Much larger errors occur in those cases where long extrapolations must be calculated (e.g., strings to only 15° and 30°). Systems that view fore and aft out to 60° would be desirable to minimize errors, assuming atmospheric corrections could be made at these angles. HIRIS (views 60° in forward and 30° in aft directions) is a good compromise by viewing out to 60° in only one direction and increases the error by approximately 2% or less. MODIS (full string ± 45°) still has a relatively small error of approximately 5% or less. Sensor systems capable of only viewing in the fore or aft directions (half strings) as opposed to both directions (full strings) greatly increase the error.

A Modeling of Atmospheric Gravity Waves and Wave Drag Generated by Isotropic and Anisotropic Terrain

Abstract Momentum deposition by orographically generated atmospheric gravity waves has been incorporated into a general circulation model of the troposphere and lower stratosphere by McFarlane, using a parameterization of real terrain and the waves that terrain would generate. Similar modeling will no doubt follow for extension to greater heights, where the effects of momentum deposition are expected to be greater. MeFarlane's parameterization treated an upward flux of momentum directed into a single azimuth (namely, that opposed to the surface wind) and tacitly assumed a Boussinesq approximation and horizontally isotropic terrain. These limitations are probed here and, to some extent, removed. The analysis is generalized from the Boussinesq to the “incompressible” approximation, with consequences that can be important in some circumstances. It is found appropriate to represent the momentum flux from isotropic terrain by two azimuths rather than one, these two being at angles of 32.5° or more from the sin...