Arrival Time Error Research Articles

Shear wave arrival time estimates correlate with local speckle pattern.

We present simulation and phantom studies demonstrating a strong correlation between errors in shear wave arrival time estimates and the lateral position of the local speckle pattern in targets with fully developed speckle. We hypothesize that the observed arrival time variations are largely due to the underlying speckle pattern, and call the effect speckle bias. Arrival time estimation is a key step in quantitative shear wave elastography, performed by tracking tissue motion via cross-correlation of RF ultrasound echoes or similar methods. Variations in scatterer strength and interference of echoes from scatterers within the tracking beam result in an echo that does not necessarily describe the average motion within the beam, but one favoring areas of constructive interference and strong scattering. A swept-receive image, formed by fixing the transmit beam and sweeping the receive aperture over the region of interest, is used to estimate the local speckle pattern. Metrics for the lateral position of the speckle are found to correlate strongly (r > 0.7) with the estimated shear wave arrival times both in simulations and in phantoms. Lateral weighting of the swept-receive pattern improved the correlation between arrival time estimates and speckle position. The simulations indicate that high RF echo correlation does not equate to an accurate shear wave arrival time estimate-a high correlation coefficient indicates that motion is being tracked with high precision, but the location tracked is uncertain within the tracking beam width. The presence of a strong on-axis speckle is seen to imply high RF correlation and low bias. The converse does not appear to be true-highly correlated RF echoes can still produce biased arrival time estimates. The shear wave arrival time bias is relatively stable with variations in shear wave amplitude and sign (-20 μm to 20 μm simulated) compared with the variation with different speckle realizations obtained along a given tracking vector. We show that the arrival time bias is weakly dependent on shear wave amplitude compared with the variation with axial position/ local speckle pattern. Apertures of f/3 to f/8 on transmit and f/2 and f/4 on receive were simulated. Arrival time error and correlation with speckle pattern are most strongly determined by the receive aperture.

PROPAGATION OF THE 2014 JANUARY 7 CME AND RESULTING GEOMAGNETIC NON-EVENT

On 2014 January 7 an X1.2 flare and coronal mass ejection (CME) with a radial speed ≈2500 km s−1 was observed from near an active region close to disk center. This led many forecasters to estimate a rapid arrival at Earth (≈36 hr) and predict a strong geomagnetic storm. However, only a glancing CME arrival was observed at Earth with a transit time of ≈49 hr and a KP geomagnetic index of only 3−. We study the interplanetary propagation of this CME using the ensemble Wang-Sheeley-Arge (WSA)–ENLIL+Cone model, that allows a sampling of CME parameter uncertainties. We explore a series of simulations to isolate the effects of the background solar wind solution, CME shape, tilt, location, size, and speed, and the results are compared with observed in situ arrivals at Venus, Earth, and Mars. Our results show that a tilted ellipsoid CME shape improves the initial real-time prediction to better reflect the observed in situ signatures and the geomagnetic storm strength. CME parameters from the Graduated Cylindrical Shell model used as input to WSA–ENLIL+Cone, along with a tilted ellipsoid cloud shape, improve the arrival-time error by 14.5, 18.7, 23.4 hr for Venus, Earth, and Mars respectively. These results highlight that CME orientation and directionality with respect to observatories play an important role in understanding the propagation of this CME, and for forecasting other glancing CME arrivals. This study also demonstrates the importance of three-dimensional CME fitting made possible by multiple viewpoint imaging.

Microseismic event location using an inverse method of joint P–S phase arrival difference and P-wave arrival difference in a borehole system

The accuracy of hypocenter location is the essential issue for microseismic monitoring, and is the basis for evaluating the effect of fracture. Although the signal obtained from a borehole monitoring system has a higher signal to noise ratio (SNR) than the surface system, a narrow monitoring aperture makes the location sensitive to noise and tends to be a misguided shape. In order to overcome this disadvantage and obtain a more accurate estimation of the source, we develop a ‘jointing method’, which combines the P–S phase arrival difference and P-wave arrival difference of each receiver pair (PSP) in the objective function. In the synthetic example, we compare the noise responses of three different location methods which are based on P-wave arrival time difference, P–S wave arrival time difference and the PSP method, respectively. This analysis shows that the P-wave arrival difference method is more sensitive to arrival time error than the others and the location results tend to be in a misleading line directed to the receivers. The P–S arrival difference method is more robust than the method using P-wave and its error distribution is perpendicular to the ray-path direction. The PSP method, as expected, is the most stable and accurate. When the P–S method and PSP method are applied to field data of a coal bed methane hydro-fracture process monitoring, the results indicate that the PSP method is preferable. The successful location with the PSP method proves that it is suitable for field data.

NextGen Surface Trajectory-Based Operations (STBO): Evaluating Conformance to a Four-dimensional Trajectory (4DT)

A pilot-in-the-loop simulation was conducted to assess the feasibility of a Surface Trajectory-Based Operations (STBO) concept [1]. The STBO concept was investigated from the pilot perspective by evaluating pilot conformance to a four-dimensional trajectory (4DT) while taxiing under manual control. Simulation conditions included a Current-Day Equipage condition; a verbal Speed-Advisory condition, which represented a near-term condition in which pilots were provided with verbal speed guidance from ATC, but not provided with any additional flight-deck tools by which to support schedule conformance; and two NextGen 4DT conditions in which the pilots were provided with flight deck displays to support conformance to the 4DT at two different levels, +/- 30sec (4DT30) and +/- 15sec (4DT15). In the NextGen 4DT conditions, pilots were presented with a graphical representation of the 4DT on an Airport Moving Map (AMM). Results showed that both the 4DT30 and 4DT15 conditions afforded more than 99% conformance to the 4DT across the entire route. An evaluation of the time of arrival (TOA) at the end-point revealed significantly less TOA error in the 4DT30 and 4DT15 conditions as compared to the Speed-Advisory condition. Although eye-tracking data revealed an increase in eyes-in time in the 4DT-display conditions, pilots rated this increase as acceptable. The results of this pilot-in-the-loop simulation demonstrated that the NextGen 4DT displays afforded high conformance with a substantial increase in predictability throughout the entire taxi route and at the runway queue. Pilots also rated that conforming to a speed profile, during taxi, was acceptable and safe.

Method for Rapid Judgment of Faulty Stations of Multilateration System

An airport surface multilateration system based on the time difference of arrival (TDOA) posi- tioning principle establishes equations to calculate the position of target through utilizing the TDOA of targets' answering signals to the ground receiving stations. The fault with any ground receiving station will result in a significant measuring error of signal's arrival time and have a lethal impact on the accuracy of positioning. A method for rapid judgment of station fault based on the smoothing filtering and judgment of deviation statistics is proposed. The target location will be firstly estimated through smoothing filtering of data, thus it is allowed to calculate the deviation of measuring position and the estimated position. When the deviation exceeds a certain threshold, it indicates that there is a large increase in the errors of single stations; then, the TDOA of each station can be calculated based on the estimated position, and the deviation can be calculated through comparison with the actually measured value of TDOA. For the stations with large deviation of TDOA as indicated by statistics, the station with the most deviations of large TDOA will be the station with problems. A simulation experiment for an airport indicates that the method can rapidly and effectively judge the faulty station.

Indoor TOA Error Measurement, Modeling, and Analysis

This paper presents a comprehensive investigation on the error characteristics of time-of-arrival (TOA) measurements obtained from positioning systems in indoor environments where rich multipath and nonline-of-sight propagation exist. By careful analysis of the measured data from three different sets of measurements, a model for the range errors is developed. The model has two components, one associated with the TOA measurement in the receiver and another associated with the delay excesses accumulated along the propagation path. By modeling multipath signals mathematically, and application of the central limit theorem, it is shown that statistical shape of the leading edge of a received pulse is essentially independent of the details of the scattering environment. Application of this statistical shape of the leading edge allows estimates of the statistical distribution of the TOA measurements to be calculated, either analytically or numerically from simulations. The second component of the delay excess is a model based on the number of walls along the path. Statistical performance based on this combined model is shown to be in good agreement with measured data collected from three different systems that have different RF frequencies, signal bandwidths, and TOA detection algorithms. The insights afforded by the theory assists in the design of more accurate positioning systems.

Comparative Performance Evaluation of RF Localization for Wireless Capsule Endoscopy Applications

This paper presents an assessment of the accuracy of cooperative localization of a wireless capsule endoscope (WCE) using radio frequency (RF) signals with particular emphasis on localization inside the small intestine. We derive the Cramer–Rao lower bound (CRLB) for cooperative location estimators using the received signal strength (RSS) or the time of arrival (TOA) of the RF signal. Our derivations are based on a three-dimension human body model, an existing model for RSS propagation from implanted organs to the body surface and a new TOA ranging error model for the effects of non-homogeneity of the human body on TOA of the RF signals. Using models for RSS and TOA errors, we first calculate the 3D CRLB bounds for cooperative localization of the WCE in three major digestive organs in the path of GI tract: the stomach, the small intestine and the large intestine. Then we analyze the performance of localization techniques on a typical path inside the small intestine. Our analysis includes the effects of the number of external sensors, the external sensor array topology, number of WCEs used in cooperation and the random variations in the transmitted power from the capsule.

Seismogenic magma intrusion before the 2010 eruption of Eyjafjallajökull volcano, Iceland

We present relatively relocated earthquake hypocentres for >1000 microearthquakes (ML < 3) that occurred during the 2 weeks immediately prior to the 2010 March 20 fissure eruption at Fimmvorðuhals on the flank of Eyjafjallajokull volcano in Iceland. Our hypocentre locations lie predominantly in horizontally separated clusters spread over an area of 10 km2 and approximately 4 km below sea level (5 km below the surface). Seismic activity in the final 4 d preceding the eruption extended to shallower levels <2 km below sea level and propagated to the surface at the Fimmvorðuhals eruption site on the day the eruption started. We demonstrate using synthetic data that the observed apparent ∼1 km vertical elongation of seismic clusters is predominantly an artefact caused by only small errors (0.01–0.02 s) in arrival time data. Where the signal-to-noise ratio was sufficiently good to make subsample arrival time picks by cross-correlation of both P- and S-wave arrivals, the mean depth of 103 events in an individual cluster were constrained to 3.84 ± 0.06 km. Epicentral locations are significantly less vulnerable to arrival time errors than are depths for the seismic monitoring network we used. Within clusters of typically 100 recorded earthquakes, most of the arrivals exhibit similar waveforms and identical patterns of P-wave first-motion polarities across the entire monitoring network. The clusters of similar events comprise repetitive sources in the same location with the same orientations of failure, probably on the same rupture plane. The epicentral clustering and similarity of source mechanisms suggest that much of the seismicity was generated at approximately static constrictions to magma flow in an inflating sill complex. These constrictions may act as a form of valve in the country rock, which ruptures when the melt pressure exceeds a critical level, then reseals after a pulse of melt has passed through. This would generate recurring similar source mechanisms on the same weak fault plane as the connection between segments of the sill system is repeatedly refractured in the same location. We infer that the magmatic intrusion causing most of the seismicity was likely to be a laterally inflating complex of sills at about 4 km depth, with seismogenic pinch-points occurring between aseismic compartments of the sills, or between adjacent magma lobes as they inflated. During the final 4 d preceding the eruption onset between 22:30 and 23:30 UTC on 2010 March 20, the seismicity suggests that melt progressed upwards to a depth of ∼2 km. This seismicity was probably caused by fracturing of the country rock at the margins of the propagating dyke. Subsequently, on the morning of the eruption a dyke propagated eastward from the region of precursory seismic activity to the Fimmvorðuhals eruption site.

Correlation of ultrasound speckle pattern and arrival time errors in shear wave elastography

Speckle results from coherent illumination and phase-sensitive detection of echoes from many sub-resolvable scatterers. Speckle provides a texture that allows ultrasonic tracking of tissue motion, which in turn enables the many varieties of ultrasound elastography. A complication is that the presence of speckle can induce a shift in the apparent phase or arrival time of a propagating shear wave. The correlation between shear wave arrival time and the speckle pattern measured with a stationary transmit beam and swept receive beams is demonstrated. Simulated transient shear waves generated in response to acoustic radiation force are tracked with transmit f-numbers of 2 to 4 and receive f-numbers of 2 to 8. The shear wave arrival time error and the lateral first moment of the swept-receive speckle pattern scaled by the shear wave speed are shown to be strongly, though not perfectly, correlated (r ~0.6). These arrival time errors are comparatively insensitive to the amplitude (over a range of −20 to 20 μm) and direction of propagation of the shear wave; e.g., arrival times for −20 and + 20 μm shear waves differed by &amp;lt;10% of the total arrival time error. Approaches for the suppression of speckle noise in shear wave elastography are discussed.

Joint TOA/AOA Positioning Scheme with IP-OFDM Systems

In this paper, a new positioning scheme with impulse postfix OFDM (IP-OFDM) systems is proposed for estimating the positional information of a user within a wireless local area network. In the proposed scheme, the channel information is estimated by the IP of the IP-OFDM symbol in the time domain, and then the round-trip time and the angle of signal arrival from a wireless terminal to an access point equipped with a uniform linear antenna array are investigated for time-of-arrival (TOA) and angle-of-arrival (AOA) estimation for a user. The position of the wireless terminal is finally estimated by combining the estimated TOA and AOA information. The performance of the proposed scheme is evaluated with computer simulations, and the error characteristics are analyzed with respect to the TOA error, and the AOA error influence on the positioning error. According to the simulation results, the positioning error of the proposed scheme can be reduced to some centimeters for high signal-to-noise ratio (SNR). The major influence on the positioning error comes from the TOA error in low SNR, while the AOA error has more influence on the positioning error in high SNR.

Optimal MLAT Tracking Based on Improved Simulation for Airport Surveillance

MLAT (Multilateration) is becoming one of the leading techniques in the airport surveillance. It locates the positions of aircrafts by the TDOAs (Time Difference of Arrivals) of signal from the aircraft to the different remote stations. In views of the influence from the geometric relation and the clock error among different stations, an improved simulation is proposed to estimate the measurement error for the optimal tracking. The model of simulation takes both the GDOP (Geometric Dilution of Precision) and TOA (Time of Arrival) error into consideration. Experiment results show that compared with the traditional simulation method, the improved simulation is more accurate and is able to be used in the practical application.

Ultra-Wideband TOA Estimation in the Presence of Clock Frequency Offset

The paper is concerned with the impact of clock frequency offsets on the accuracy of ranging systems based on time of arrival (TOA) measurements. It is shown that large TOA errors are incurred if the transmitter and receiver clocks are mistuned by more than just one part per million (ppm). This represents a serious obstacle to the use of commercial low-cost quartz oscillators, as they exhibit frequency drifts in the range of ± 10 ppm and more. A solution is to estimate first the transmitter clock frequency relative to the receiver's and then compensate for the difference by acting on the receiver clock tuning. An algorithm is proposed that estimates the transmitter clock frequency with an accuracy better than 0.1 ppm. Computer simulations indicate that its use in ranging systems makes TOA measurements as good as those obtained with perfectly synchronous clocks.

Joint Estimation of Propagation Delay Dispersion and Time of Arrival in a 40-nm CMOS Comparator Bank for Time-Based Receivers

This brief presents an approach to compensate time-of-arrival (ToA) estimation errors caused by the effect of signal depending propagation delays in receivers for ranging applications. As a straightforward reasoning, reducing the ToA error results in increased power consumption in the analog-front-end blocks of such receivers. In the proposed system topology, the asynchronous receiver's analog-front-end blocks contain an array of identical continuous-time comparators, which makes it suitable for a joint modeling approach and today's low-supply voltages. Attention is paid to tuning the nonstrobed comparator topology to make it suitable for this joint modeling. Using the comparator model in a least-squares-based algorithmic approach can reduce the ToA error (accuracy improvement) to nearly zero. A ToA systematic error reduction of 28 is faced for the ToA figure. Furthermore, energy consumption is more related to the activity of the ranging events, which makes this an interesting approach for ranging measurement rates of less than 1 MHz.

PULSE INTENSITY MODULATION AND THE TIMING STABILITY OF MILLISECOND PULSARS: A CASE STUDY OF PSR J1713+0747

Most millisecond pulsars, like essentially all other radio pulsars, show timing errors well in excess of what is expected from additive radiometer noise alone. We show that changes in amplitude, shape and pulse phase for the millisecond pulsar J1713+0747 cause this excess error. These changes appear to be uncorrelated from one pulse period to the next. The resulting time of arrival variations are correlated across a wide frequency range and are observed with different backend processors on different days, confirming that they are intrinsic in origin and not an instrumental effect or caused by strongly frequency dependent interstellar scattering. Centroids of single pulses show an rms phase variation \approx 40 microsec, which dominates the timing error and is the same phase jitter phenomenon long known in slower spinning, canonical pulsars. We show that the amplitude modulations of single pulses are modestly correlated with their arrival time fluctuations. We also demonstrate that single-pulse variations are completely consistent with arrival time variations of pulse profiles obtained by integrating N pulses such that the arrival time error decreases proportional to 1/\sqrt{N}. We investigate methods for correcting times of arrival for these pulse shape changes, including multi-component TOA fitting and principal component analysis. These techniques are not found to improve the timing precision of the observations. We conclude that when pulse shape changes dominate timing errors, the timing precision of PSR J1713+0747 can be only improved by averaging over a larger number of pulses.

Indoor Positioning Using UWB-IR Signals in the Presence of Dense Multipath with Path Overlapping

This paper presents a method for positioning using ultra-wideband impulse radio (UWB-IR) signals that is robust in indoor environments characterized by dense multipath channel with path overlapping. Path overlapping effects arising from multipath in dense cluttered environments decrease the direct path resolution in time domain, and hence induce time-of-arrival (TOA) and angle-of-arrival (AOA) based positioning inaccuracy. To mitigate this problem, system design that is capable of resolving the closely-spaced multipath at low cost is of value. Our method yields the least-squares estimation of joint TOA and AOA with low computational cost. It is based on the spectral observation of beam forming, in which the path overlapping effect is mitigated using multipath-aided acquisition. The computational cost is reduced using in-band power spectrum and accurate initial estimations. The performance is verified both in IEEE 802.15.4a CM3 channel model and in a real environment. Simulation results in CM3 channel model show that the TOA and AOA estimation improvements on average are 2.4cm and 13.1° in the dense multipath scenario. Measurement in an indoor open hall environment shows that the AOA error decreases from 10.6° to 7.2°, and the TOA error decreases from 2.1cm to 1.9cm when the path overlapping effects are mitigated in the range of 21m.

Uncertainty estimation in dynamic contrast‐enhanced MRI

Using dynamic contrast-enhanced MRI (DCE-MRI), it is possible to estimate pharmacokinetic (PK) parameters that convey information about physiological properties, e.g., in tumors. In DCE-MRI, errors propagate in a nontrivial way to the PK parameters. We propose a method based on multivariate linear error propagation to calculate uncertainty maps for the PK parameters. Uncertainties in the PK parameters were investigated for the modified Kety model. The method was evaluated with Monte Carlo simulations and exemplified with in vivo brain tumor data. PK parameter uncertainties due to noise in dynamic data were accurately estimated. Noise with standard deviation up to 15% in the baseline signal and the baseline T1 map gave estimated uncertainties in good agreement with the Monte Carlo simulations. Good agreement was also found for up to 15% errors in the arterial input function amplitude. The method was less accurate for errors in the bolus arrival time with disagreements of 23%, 32%, and 29% for K(trans) , ve , and vp , respectively, when the standard deviation of the bolus arrival time error was 5.3 s. In conclusion, the proposed method provides efficient means for calculation of uncertainty maps, and it was applicable to a wide range of sources of uncertainty.

Web 기반의 실시간 수질오염사고대응예측시스템 개발

국내 하천의 유해물질로 인한 수질오염사고가 매년 약 25여건 발생하고 있다. 중 대규모 수질오염사고의 경우 신속한 방제작업뿐 만 아니라 취 정수장 등의 사전대응을 위해서 수리 및 독성모델을 이용한 도달시간 및 확산농도의 예측이 매우 중요하다. 현재 우리나라의 사고대응예측능력은 여전히 초보적 단계로 2008년 김천 유화공단 화재에서 페놀에 대한 도달시간 예측오류는 취 정수장의 사전대응에 혼란을 주었다. 최근, 4대강의 보 건설로 하천흐름 특성이 크게 변화되었기 때문에 화학물질 유출사고대응에 있어서도 보의 연계 운영을 통한 적절한 방제가 필요하다. 국립환경과학원에서는 4대강의 수리변화를 반영하여 보 댐의 연계운영의 시나리오분석을 통해 최적의 방제방안을 선정하는 수질오염사고대응예측시스템을 개발하였다. 이 시스템은 4대강 본류 구간의 수리모델이 매일 자동갱신되어 신속하게 도달시간과 확산농도의 예측이 가능하며, 상 하류 보의 연계운영을 고려한 시나리오 분석이 가능하다. 본 연구에서는 사고대응예측시스템의 특징을 소개하고 가상의 페놀유출사고 시나리오를 통해 시스템의 활용성을 평가하였다. In Korea, around 25 oil and chemical water pollution accidents take place every year. In case of middle or large chemical spill in river, not only field response but also forecasting for the arrival time and diffusion concentration is important. However, the nation's forecasting ability using the hydrodynamic and water quality model is still in its infancy. For example, when fire occurred at the petrochemical plant in Kimcheon in 2008, forecasting error of arrival time for phenol prevented intake plants from taking proper measures in advance. In particular, as river flow characteristics have greatly changed following the construction of the weirs in four major rivers, we need to take into account weir management for prompt water pollution accident response. To this end, National Institute of Environmental Research developed the Water pollution Accident Response Management System (WARMS) for four major rivers. This system is designed not only to forecast arrival time and diffusion concentration in a timely manner using its automatic update of EFDC hydro module, but also to conduct scenario modeling using weir-connected operation. This study introduces the WARMS characteristics and evaluates the performance of the system using a phenol-spill simulation.

Influence of the Initial Shock Speed Excited by Solar Flares on Shock Arrival Time Prediction

AbstractBy using the Hakamada‐Akasofu‐Fry (HAF) solar wind model, we simulated 48 solar flare events during Carrington Rotation 1981 to 1985 and compared the simulated shock arrival time with observations by satellites at 1 AU. It is found that the initial solar wind shock speed deduced from the metric Type II radio burst observations plays an important role in the shock arrival time prediction. The match between predicted and observed shock arrival times (SATs) was considerably improved by iteratively adjusting the initial shock speed. We obtained the adjustment factors as the function of the initial shock speed for different hemispheres by using the statistical analysis and applied them to adjust the initial shock speed driven by the solar flares which occurred in the following five Carrington rotations (1986~1990), and the mean absolute error of the shock arrival time was reduced from 16 (15) hours to 12 (11) hours for the flare occurred on the western (eastern) hemisphere. This indicates that significant improvements can be made for the shock arrival time prediction by adjusting the initial shock speed using the statistical functions.

ARRIVAL TIME CALCULATION FOR INTERPLANETARY CORONAL MASS EJECTIONS WITH CIRCULAR FRONTS AND APPLICATION TOSTEREOOBSERVATIONS OF THE 2009 FEBRUARY 13 ERUPTION

A goal of the NASA STEREO mission is to study the feasibility of forecasting the direction, arrival time and internal structure of solar coronal mass ejections (CMEs) from a vantage point outside the Sun-Earth line. Through a case study, we discuss the arrival time calculation of interplanetary CMEs (ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large elongations from the Sun in combination with different geometric assumptions about the ICME front shape (Fixed-\Phi (FP): a point and harmonic Mean (HM): a circle). These forecasting techniques use single-spacecraft imaging data and are based on the assumptions of constant velocity and direction. We show that for the slow (350 km/s) ICME on 2009 February 13-18, observed at quadrature by the two STEREO spacecraft, the results for the arrival time given by the HM approximation are more accurate by 12 hours than those for FP in comparison to in situ observations of solar wind plasma and magnetic field parameters by STEREO/IMPACT/PLASTIC, and by 6 hours for the arrival time at Venus Express (MAG). We propose that the improvement is directly related to the ICME front shape being more accurately described by HM for an ICME with a low inclination of its symmetry axis to the ecliptic. In this case the ICME has to be tracked to > 30{\deg} elongation to get arrival time errors < 5 hours. A newly derived formula for calculating arrival times with the HM method is also useful for a triangulation technique assuming the same geometry.

Estimation of the effect of nonisotropically distributed energy on the apparent arrival time in correlations

Correlations of random seismic noise are now widely used to retrieve the Green’s function between two points. Whereas this technique provides useful results in tomography and monitoring studies, it is mainly limited by an uneven distribution of noise sources. In that case, theoretical requirements are not completely fulfilled and we may wonder how reliable the reconstructed signals are, in particular for the purpose of estimating traveltime from correlations. This study finds a way to quantify effects of a nonisotropic noise field by estimating the arrival-time error resulting from a particular nonisotropic distribution of recorded wave intensity. Our study is based on a theoretical prediction of this bias and we successfully test the theory by comparing the theoretical expectation to real measurements from seismic-prospecting data. In particular, we distinguish between the effects of source distribution and the effects of medium heterogeneity between the sources and the region of receivers. We find relative errors in the order of a percent which may affect monitoring results, especially where smaller relative velocity variations (smaller than [Formula: see text] for some applications) are investigated. Finally, we see that correlation of coda waves helps mitigate the effects of a nonisotropic field, hence making the estimation of traveltime quite stable irrespective of the source distribution.