Apparent Azimuth Research Articles

The Use of Coring-Induced Petal Fractures in Coal to Supplement and Ground Truth the Interpretation of Image Logs

Resistivity and acoustic scanner image logs, in both the CSG and coal-mining industries, are the preferred means of determining azimuths of joints and cleat in coal. This paper indicates the need for care when interpreting cleat azimuths from image logs.Image logs of the bore wall often exhibit large fractures (joints) that intersect the entirety of the bore wall. They are visible as sinusoidal traces. Those fractures that have low height (cleat), and intersect the bore wall in one or two places, are represented by vertical to sub-vertical linear traces on an image log. These lineations can be represented on a stereo- or polar net as a point with a plunge and trend. Both joints and cleat generally have high dips (greater than 75°).Resistivity image logs detect both joints and cleat. Acoustic image logs often only record joints. The value of obtaining cleat, joint and horizontal stress azimuths, is in planning the optimal orientation of deviated in-seam (lateral) production wells.An image log of a cleat lineation records the bore wall intersection azimuth (BIAZ), that is an apparent azimuth, as well as the apparent dip (or plunge) of the lineation. The best way to determine true azimuth of cleat from lineations, on an image log, is from a statistical weighted mean of numerous BIAZ measurements. In some instances, cleat azimuth has been calculated as 90° to BIAZ. Some interpreters view the dispersion of azimuths as a natural feature, when the dispersion is an artefact arising from the intersection of cleat of the same azimuth, at varying distances from the centre of a bore wall.Petal fractures (PF) in coal, when combined with breakout information, can also be used to determine or ground truth joint/cleat azimuths of both large and small scale fractures. Bedding-plane observations of core also provides cleat/fracture information not obtainable from an image log.

Estimation of position and velocity for a low dynamic vehicle in near space using nonresolved photometric and astrometric data.

An estimation method for indirectly observable parameters for a typical low dynamic vehicle (LDV) is presented. The estimation method utilizes apparent magnitude, azimuth angle, and elevation angle to estimate the position and velocity of a typical LDV, such as a high altitude balloon (HAB). In order to validate the accuracy of the estimated parameters gained from an unscented Kalman filter, two sets of experiments are carried out to obtain the nonresolved photometric and astrometric data. In the experiments, a HAB launch is planned; models of the HAB dynamics and kinematics and observation models are built to use as time update and measurement update functions, respectively. When the HAB is launched, a ground-based optoelectronic detector is used to capture the object images, which are processed using aperture photometry technology to obtain the time-varying apparent magnitude of the HAB. Two sets of actual and estimated parameters are given to clearly indicate the parameter differences. Two sets of errors between the actual and estimated parameters are also given to show how the estimated position and velocity differ with respect to the observation time. The similar distribution curve results from the two scenarios, which agree within 3σ, verify that nonresolved photometric and astrometric data can be used to estimate the indirectly observable state parameters (position and velocity) for a typical LDV. This technique can be applied to small and dim space objects in the future.

The Measurement of Apparent Velocity and Azimuth using Adaptive Processing Techniques on Data from the Warramunga Seismic Array

Summary Adaptive processing of multichannel data can be achieved by cross-correlating the signal on each channel with a velocity and azimuth filtered composite signal in an iterative manner. In this way it is possible to accurately determine the arrival times of the wavefront at each sensor. This technique, which is currently used for the autematic analysis of seismic events recorded at the LASA and NORSAR arrays, has been used as the basis of a software system developed for processing data from the medium aperture Warramunga (WRA) array in northern Australia. This system has been developed for a small and relatively inexpensive computer installation, and permits the rapid, direct measurement of apparent velocity and azimuth of signals coherent across most or all of the array. The position of WRA relative to the Earth'sactive seismic zones, and in particular the belt north of Australia, results in multiple P wave signals from single events being frequently recorded. Emphasis was therefore placed on assessing the capabilities of adaptive processing for making measurements not only on the first arrival but also on its immediate coda. The effects of signal multiplicity on parameter measurement were studied, for both real and synthetic data, using a number of different processing variables such as convergence criteria, parts of waveform used and window positioning. The results show that several measurements must be made across a waveform in order to identify systematic drifts shown to result from interference effects. Experiments using inverse filtering were performed in an attempt to improve the resolution.

A Comparison of Adaptive Processing Techniques with Nth Root Beam Forming Methods

Summary Small differences in slowness and azimuth for overlapping phases especially where the branches of the travel-time curve are triplicated must be resolved for a meaningful inversion of array data. A computer program package has been written in FORTRAN IV which enables a user to determine, automatically, the apparent azimuth and slowness of any portion of the seismic wavetrain recorded at various arrays if he has the raw data on digital tape and has access to any modern computer. These programs make use of two methods, (i) adaptive processing, and (ii) Nth root beam forming which have been compared to determine the apparent azimuth and slowness of the seismic wavelets. The former method is performed by cross correlating the signal on each channel with a velocity and azimuth filtered trace in an iterative manner until the convergence takes place. In the latter method the operation is done by delaying the various channels to align a group of arrivals with a particular velocity and azimuth; taking the Nth root of the signal; summing and then raising the result to the Nth power. The value of apparent velocity and azimuth which produces a maximum filtered signal is determined. Experiments with clean and noisy synthetic data have shown that the adaptive processing method is more successful for resolving small differences in apparent velocity and azimuth of overlapping wavelets. It also has an advantage that a set of residuals may easily be obtained from the analysis. The Nth root method is extremely powerful in enhancing the signal to noise ratio at the expense of signal distortion. The computation time for both methods is about the same.

Coordinate registration algorithms for over-the-horizon radar

Different from the other conventional radars, the over the horizon radar (OTHR) faces complicated nonlinear coordinate transform due to electromagnetic wave propagation and reflection in ionospheres. A significant problem is the phenomenon of multi-path propagation. Considering it, the coordinate registration algorithms of planar measurement model and spherical measurement model are respectively derived in detail. Noticeably, a new transforming expression of apparent azimuth and an integrated form of transforming expressions from measurement vector to ground state vector in coordinate registration algorithm of spherical measurement model are proposed. And then simulations are made to verify the correctness of the proposed algorithms and expression. Besides this, the transforming error rate of slant range, Doppler and apparent azimuth of the two kinds of models are given respectively. Then the quantitative analysis of error rate is also given. It can be drawn a conclusion that the coordinate registration algorithms of planar measurement model and spherical measurement model are both correct.

Self-consistent retrieval of source parameters using mantle waves

Abstract The large number of high-quality stations from global networks (e.g., IRIS, GEOSCOPE) allows for rapid and robust recovery of source parameters of large earthquakes. We present a self-consistent analysis of surface waves in terms of directivity parameters. The boxcar is often used to recover source parameters but is rarely a faithful representation of an event's time function. We demonstrate that more reliable estimates of directivity parameters can be obtained when we take into account the source time function (STF) of the event. A low-frequency STF is obtained from first (R1) and second (R2) orbit Rayleigh waves in the range 2 to 15 mHz, with a spectral synthesis and inversion method. We measure the differential amplitudes between data and 3D synthetics. We use the R1/R2 ratio of differential amplitudes to invert, in a single narrow-frequency band, for the event's apparent rupture azimuth and velocity. For the Mw 8.1 Chile 1995 event, data are consistent with unilateral southward propagation, with a velocity of about 2.4 km/sec. For the Mw 7.9 Mexico 1995 earthquake, the rupture propagated toward the NW at about 2.0 km/sec. Our low-frequency estimates agree with broadband studies of both events and can be retrieved rapidly on a routine basis after a large earthquake.

Effectiveness of interaural delays alone as cues during dynamic localization of virtual sources

The contribution of interaural time differences (ITDs) to the localization of virtual sound sources with and without head motion was examined. Listeners estimated the apparent azimuth, elevation, and distance of virtual sources presented over headphones. Stimuli (3-s, white noise) were synthesized from minimum-phase representations of nonindividualized head-related transfer functions (HRTFs); binaural magnitude spectra were derived from the minimum phase estimates and ITDs were represented as a pure delay. During dynamic conditions, listeners were encouraged to move their heads; head position was tracked and stimuli were synthesized in real time using a Convolvotron to simulate a stationary external sound source. Two synthesis conditions were tested: (1) both interaural level differences (ILDs) and ITDs correctly correlated with source location and head motion and (2) ITDs correct, no ILDs (flat magnitude spectrum). Head movements reduced azimuth confusions primarily when interaural cues were correctly correlated, although a smaller effect was also seen for ITDs alone. Externalization was generally poor for ITD-only conditions and was enhanced by head motion only for normal HRTFs. Overall, the data suggest that, while ITDs alone can provide a significant cue for azimuth, the errors most commonly associated with virtual sources are reduced by location-dependent magnitude cues.

Adaptation auditive et localisation

This paper presents the main results of an experiment on the possible role of adaptation in auditory localization. Measurements are performed in an anechoic room on the apparent azimuth of a brief 4-kHz tone, coming from the side (15 o ) of the listener, in the presence of a tone at the same frequency coming from the front. This frontal source is turned on first, for a certain amount of time, in order to induce some sort of auditory adaptation; next, a 50-ms tone is presented from the lateral source. It is found that, as the duration of the pre-exposure is increased, the apparent azimuth of the second sound varies under some conditions; in most cases, it is shifted towards the side of the subject, away from the actual position of the source. The same the of effect is demonstrated under earphones. In this case, the adapting signal is a diotic tone, and the adapted signal is a 50-ms dichotic tone with an interaural time difference of 70 μs. The summation of these tones leads to an interaural level difference, and therefore to a lateralized auditory image. The apparent location of this image also varies with adaptation, as in the fist experiment

Minimum audible angle for clicks with simulated echoes: Effects of azimuth and standard

Minimum audible angle (MAA) was measured in free field with a center position at 0°, and 25° or 50° on the right. Stimuli were 6-ms-wide band noise bursts presented as single bursts or noise pairs under precedence-effect conditions. In the single-noise condition, one burst was presented from the right or left. In precedence lead discrimination, a leading sound was presented from the left or right speaker and a lagging sound from a speaker at center. In precedence lag discrimination, the lagging sound was on the right or left and the leading sound was at center. The test stimulus was either presented in isolation or was preceded by a single standard noise at center. MAAs were lowest for the single condition, slightly higher for lead discrimination, and much higher for lag discrimination. Presence of a standard yielded a benefit in lag discrimination, especially at 0°. Thresholds at 50° were significantly higher than at 0° and 25°, especially in lag discrimination. The data can be described by a model in which judgments are based on apparent azimuth of the fused stimulus, and the lagging sound in a pair perceptually ‘‘pulls’’ the image along the azimuth dimension. [Work supported by NSF and NIH.]

Sound localization without interaural time or intensity differences

In a series of localization experiments, blindfolded listeners were asked to give the apparent azimuth and elevation coordinates of 250‐ms broadband noise bursts. The noise bursts were spectrally shaped so that the spectrum level in each critical band was set randomly (from trial to trial) within a 20‐dB range. In a reference condition, the sounds were transduced by miniature loudspeakers placed at 36 locations in an anechoic chamber. Performance in this free‐field condition agreed with previous results in the literature except for an increased frequency of frontback reversals, which is felt to be the result of the lack of a visual frame of reference. In a control condition, digital techniques were used to synthesize headphone‐presented stimuli that were nearly identical, as measured in listeners' ear canals, to the free‐field stimuli. Localization performance with these stimuli was virtually the same as in free field, thus verifying the adequacy of the simulation. The digital filters used to synthesize t...

Frequency dependent polarization analysis of high‐frequency seismograms

We present a multitaper algorithm to estimate the polarization of particle motion as a function of frequency from three‐component seismic data. This algorithm is based on a singular value decomposition of a matrix of eigenspectra at a given frequency. The right complex eigenvector corresonding to the largest singular value of the matrix has the same direction as the dominant polarization of seismic motion at that frequency. The elements of the polarization vector specify the relative amplitudes and phases of motion measured along the recorded components within a chosen frequency band. The width of this frequency band is determined by the time‐bandwidth product of the prolate spheroidal tapers used in the analysis. We manipulate the components of to determine the apparent azimuth and angle of incidence of seismic motion as a function of frequency. The orthogonality of the eigentapers allows one to calculate easily uncertainties in the estimated azimuth and angle of incidence. We apply this algorithm to data from the Anza Seismic Telemetered Array in the frequency band 0 ≤ ƒ ≤ 30 Hz. The polarization is not always a smooth function of frequency and can exhibit sharp jumps, suggesting the existence of scattered modes within the crustal waveguide and/or receiver site resonances.

The relative performance ofmband alternative measures of elastic energy in estimating source size and explosion yield

AbstractA comparison has been made of the relative scatter of classical mb and alternative measures of P-wave energy from underground nuclear explosions at test sites in East Kazakh, USSR. The scatter of the energy measures is observed in teleseismic arrays of short-period Global Digital Seismic Network (GDSN) stations and the local broadband array at Graefenburg, Federal Republic of Germany. Four measures of A in log(A/T), spectral magnitudes, peak velocity, rms coda, and integrated velocity-squared are compared. The measures are constructed to be in equivalent units of the flux rate of radiated elastic energy. All measures are assumed to have the same slope in a linear regression of log(yield) versus log(measure). Three independent tests were made of the stability of the yield estimators: the scatter of the measures using: (1) Graefenburg array data; (2) GDSN data normalized to a reference station; and (3) GDSN data normalized to a reference event. The differences among the standard deviations are small (≦ 0.1 mb units), making it difficult using a small data base to conclude whether the performance of one estimator is significantly better than another. The relative order in the performance of the yield estimators, however, is preserved in each of the three tests. The coda measure is the most stable, followed by the spectral and time-domain A/T measures. The relations observed at Graefenburg between (1) the amplitude of direct P versus P coda, (2) the apparent azimuth of direct P, and (3) complexity, suggest that amplitude variations across an array are a product of scattering along the entire ray path as well as scattering, focusing, and defocusing localized in the lithosphere beneath the source and receiver sites.

Analysis of earthquakes in the distance range 40–70° and inferred lower mantle structure

About fifty earthquakes in the distance range 40–70° and azimuthal range 45–120° from the Celebes, Philippines, Mariana and Kurile Island regions, and recorded at Gauribidanur seismic array in southern India, were used in the present study. Measurements on slowness and apparent azimuths were made on the first 30 s of the short period P-wave trains using an adaptive processing technique. Analysis of this data set has revealed no strong evidence for any triplications in the travel-time curve over the ranges in question. The P-wave velocity increases continuously with an almost uniform gradient below 1000 km depth range and is in very close agreement with the JB model. Almost all the observed slowness values of the events were anomalously low and consistent suggesting that they are caused by some azimuthal dependent structure near the array.

Simulation of the Localization of Distant Sound Sources by Earphones

Subjects made forced choices to either side of a reference direction (0°, 30°, 60°, 75°) to locate the image of pure tones presented through earphones. Various combinations of interaural intensity differences (IIDs) and interaural time differences (ITDs) were used, including some combinations that do not occur in nature. Psychometric functions confirmed the expected apparent azimuth of IID/ITD combinations based on free-field diffraction theory and microphone measurements. The relative dominance of the intensive and temporal cues was examined at each frequency (500, 1000, 8000 Hz). The shape of the psychometric plane for the bidimensional matrix of IID/ITD combinations centered around 30° azimuth showed possible nonlinearities in the trading relation for 1 kHz. Data were also collected from the same subjects using the same signals presented by means of distant loudspeakers in an open field. When the free-field data were compared with the data from earphone simulated sounds, they showed similar angular acuities, being approximately the size of those reported in the literature. Equations are presented for computing the IID/ITD combinations necessary at any frequency for any apparent frontal azimuth. [This research was partially supported by the Office of Naval Research and Columbia University.]

Crustal characteristics from short-period P waves

Abstract Thirty-four teleseismic events, recorded on the deep horizontal sediments of central Alberta, using one fixed and one movable station, have been analyzed as a test of the Haskell matrix formulation applied to short period P waves. Only limited agreement is obtained between averaged experimental vertical-horizontal spectral ratio curves and those calculated theoretically using known layer thicknesses and velocities. Scattering in the crust and upper mantle is indicated by large transverse amplitudes including distict phases and by lower coherency for smaller epicentral distances where the travel path is confined to the crust and upper mantle. Anomalous SV/P ratios are believed to contribute to the difficulties. A study of 20 events in the azimuth range 285° to 310° indicates an apparent azimuth approximately 18° more northerly than the true azimuth. Localized dips of approximately 15° on the crustal boundaries are required to explain this deviation. It is concluded that this region for which the sediments are horizontally layered does not fulfill the requirements of the Haskell matrix theory due to scattering and anomalous PS conversions in the crust and upper mantle.

Corrections to apparent azimuths and travel-time gradients for a dipping Mohorovičić discontinuity

abstract When a layer is overlying a half-space with a tilted interface, the refraction of seismic waves emerging from the half-space generally results in their reorientation away from the vertical plane of incidence, with the following consequences: A general formulation for the computation of the apparent azimuth and the angle of incidence at the free surface is obtained. The numerical corrections to the observed dT/dΔ and apparent azimuth, for a number of combinations of various dip angles and velocity contrasts, are tabulated. The tables may be helpful when the first and second derivatives of the travel time data are to be analyzed. Their comparison with observation can also be useful for making inferences about the underlying crustal structure. The latter approach has been applied to the P arrivals across the Tonto Forest Seismological Observatory (TFSO) array, Arizona. On the basis of these observations, the Mohorovičić discontinuity under the observatory appears to be dipping locally as much as eight degrees in the N70° ± 5°E direction.

Interaural intensity difference limen.

Three kinds of intensity-difference thresholds were compared for the purpose of determining interaural-binaural and interaural-monaural relations. Five subjects were tested at 250, 2000, and 6000 Hz, using a variation of the Békésy technique. Response to 27 interaural intensity combinations were measured at each frequency. Monaural and binaural conditions were included. Each of 24 combinations was measured under 0° and 180° interaural phase conditions, and all were measured at 50, 35, and 20 dB HL (ISO). Results indicate that interaural-difference thresholds decrease as a function of level, as might be predicted. These thresholds vary as a function of the frequency and apparent azimuth of the fused tone, but are relatively insensitive to variations in interaural phase. Interaural intensity-difference thresholds differ significantly from both monaural and binaural difference thresholds.

Interaural Intensity Difference Limen

Comparisons were made between three kinds of intensity difference thresholds for the purpose of determining interaural-binaural and interaural-monaural relations. A variation of the Békésy technique was developed for use in these experiments. Five subjects were tested at 250, 2000, and 6000 Hz. At each frequency, responses to 27 intensity combinations were measured. Simple monaural and binaural conditions were included. In addition, each of 24 of the combinations was measured under 0° and 180° interaural phase conditions. Maximum and minimum intensities were 50 and 20 dB HL (ISO). Results indicate that interaural DL's decrease as a function of level, as might be predicted. These thresholds vary as a function of frequency and apparent azimuth of the fused tone, but are relatively insensitive to variations in interaural phase. Interaural intensity-difference thresholds are more properly compared with monaural than with binaural DL's.

An instantaneous direct-reading radiogoniometer

The paper describes a visual direct-reading radiogoniometer capable of indicating the apparent azimuth of arrival of wave-trains the durations of which need not exceed 0.001 sec. The special properties of the device, which comprises essentially a combination of directional aerials with a cathode-ray oscillograph, are discussed. Amongst these properties is that of giving simultaneous bearings on two or more stations transmitting simultaneously on the same wave-length. A typical installation is described, and specimens of observations on the distribution in azimuth of received atmospheric disturbances are shown. A possible solution of the problem of navigational beacons is suggested.

An instantaneous direct-reading radiogoniometer

The paper describes a visual direct-reading radiogoniometer capable of indicating the apparent azimuth of arrival of wave-trains the durations of which need not exceed 0-001 sec. The special properties of the device, which comprises essentially a combination of directional aerials with a cathode-ray oscillograph, are discussed. Amongst these properties is that of giving simultaneous bearings on two or more stations transmitting simultaneously on the same wave-length. A typical installation is described, and specimens of observations on the distribution in azimuth of received atmospheric disturbances are shown. A possible solution of the problem of navigational beacons is suggested.