Time-distance Curve Research Articles

Model of time-distance curve of electromagnetic waves diffracted on a local feature in the georadar study of permafrost zone rock layers

In GPR (georadar) studies, one of the most popular procedures for determining electromagnetic waves propagation velocity in a rock mass is the selection of theoretical hyperbolic time-distance curves and subsequent comparison with the time-distance curve obtained from a GPR measurement. This procedure is based on the model of homogeneous medium, but nowadays the subject of GPR study is often inhomogeneous media, such as horizontally layered media characteristic of loose permafrost zone sediments. The paper presents the findings of studying the formation of hyperbolic time-distance curves of georadar impulses in a horizontally layered medium without taking into account the dispersion and absorption of electromagnetic waves. On the basis of geometrical optics laws, formulas were derived to calculate the shape of the hyperbolic lineup of georadar impulses reflected from a local feature in a multilayer frozen rock mass. On the example of a permafrost zone rock mass containing a layer of unfrozen rocks, the effect of the thicknesses of rock layers and their relative dielectric permittivity on the apparent dielectric permittivity resulting from the calculation of the theoretical hyperbolic time-distance curve was shown. The conditions under which it is impossible to determine the presence of a layer of unfrozen rocks from a hyperbolic time-distance curve are also presented. The established regularities were tested on synthetic georadar radargrams calculated in the gprMax software program. The findings of the theoretical studies were confirmed by the comparison with the results of the analysis of the georadar measurements computer simulation data in the gprMax system (the relative error was less than 0.5%).

An Optimum 1D Velocity Model for the Garhwal–Kumaun Himalaya Using Monte Carlo Style Inversion

Abstract A Monte Carlo based algorithm is developed to improve the 1D velocity inversion routines and minimize bias due to the choice of a starting model. Using this algorithm, a well-resolved six-layer minimum 1D velocity model, down to ∼24 km depth, is determined for the Garhwal–Kumaun Himalaya. A total of 4765 P- and 4724 S-phase travel times of local earthquakes recorded at 53 broadband seismic stations are used for this purpose. The travel time–distance curves from these carefully analyzed phase data of events are used to subsequently derive a prior 1D velocity model. Forward modeling of the travel time–distance curve yields an average Moho depth of ∼46 km and bulk crustal P- and S-wave velocity values of 7.60 and 4.47 km/s, respectively. To circumvent the subjectivity due to manual intervention in the inversion and automate the process, we propose a Monto Carlo style semirandom generation of initial trial velocity models, guided by the initial values derived from forward modeling. The estimated minimum 1D model reveals P- and S-wave velocities increasing from 5.17 to 6.85 km/s and 3.12 to 3.82 km/s, respectively, from the surface to a depth of 26 km. Subsequently, an optimum model is constructed for the region by incorporating the Moho layer in the minimum 1D model.

Breaking Operations of a Vacuum Test Interrupter Setup Using a Common Servo Drive With Belt Transmission

For interruption operations at higher voltage levels sulfur hexafluoride (SF6) is the dominating insulation and interruption medium. In medium to high voltage levels vacuum has advantages compared to SF6. In order to expand the operation voltage levels of vacuum circuit breakers (VCB), two approaches are possible. The first is the enlargement of the single contact gap and second is the series arrangement. In both cases the Time–Distance-Curve of the contact travel is one factor that influences the interruption performance of VCBs. The usage of a single gap for higher voltage levels requires an increased contact distance. In dependence of different interruption operations, like short circuit or capacitive interruption, a controlled Time–Distance-Curve could be useful. For experimental investigations a vacuum single break test setup using a brushless servo motor with a frequency converter is built. The electrical control of the drive enables parameter variations of the Time–Distance-Curve. The velocity, acceleration, deceleration, and final contact gap can be adjusted as required for testing and later in field application. For fine control a mechanical simulation considering the regulation loop between frequency converter and drive as well as the mechanical components is employed. Finally, tests of the setup for interruption operations are presented.

Research on Accuracy of Normal Moveout in Reflection Seismic Exploration by Cross Correlation Method

In this paper, we took the time-distance curve equation and the Normal Moveout (NMO) principle as breakthrough point. According to the forward seismic simulation model and the cross-correlation principle in signal processing, we have convincingly studied the accuracy of NMO in simulation experiment. Firstly, the time-distance curve equation of single-layer reflection and the characteristics of cross-correlation are simply analysed. Then, we build the seismic forward model by Tesseral software, and accurate results are obtained after the simulation experiment is carried out. Finally, we get the conclusion by analysing and summarizing these results. The conclusion shows that the method effectively demonstrates the feasibility of the cross-correlation method in testing the accuracy of NMO, and provides a new idea for analysing the accuracy of velocity picking, even the accuracy of the underground model during petroleum exploration.

UPPER MANTLE SEISMIC VELOCITY OF BAGHDAD SEISMIC STATION USING TRAVEL TIMES-DISTANCE CURVES OF BODY WAVES

Travel time-distance curves are plotted using seismic events recorded by Q330 broadband seismic station in Baghdad. Three component STS-2 seismometers record 132, 97 events represented the first arrival of Pn, Sn, respectively for the period between 2010 to January 2015. The minimum and maximum hypocentral distance for these events are 129 and 650 Km, respectively, their magnitudes range between (3.5 – 6.4) on the Richter scale. The velocity of the upper mantle has been calculated using travel time-distance curve, the velocity values are 8.13,4.49 km/s for Pn, Sn, respectively. The standard deviation (SD) are 1.211, 1.556 and Correlation coefficient R2 = 0.9965, 0.9982 for Pn and Sn respectively, VP/VS-ratio beneath seismic Baghdad station is 1.807.

Curvature-Varying Hyperbolic Trace TFPF for Seismic Random Noise Attenuation

Time-frequency peak filtering (TFPF) is effective in suppressing random noise in seismic records. However, the signal may be attenuated at the same time, especially for the high-frequency signal. In this letter, we propose a curvature-varying hyperbolic trace TFPF to reduce the error. We sample the seismic record along the time–distance curve of the event. For the event area, the sampled signal in each sampling trace is approximately linear (low frequency) due to the correlation of the signal along the time–distance curve, which reduces the bias of the seismic wave estimation brought by TFPF. However, the curvatures of seismic events are various, so adopting traces with a single curvature to sample the whole seismic record is not reasonable. As the events with different curvatures are located in different areas in Radon domain, the events with almost the same curvature can be separated out by an inverse Radon transform of only part of the Radon data. The optimal hyperbolic sampling traces are chosen for each separated subrecord, and we filter the data along the traces by TFPF. The denoised signal is reconstructed by summing all of the processed subrecords. Compared with TFPF, our method gets a good performance in both seismic reflection event preservation and noise attenuation.

Application of the Mine Multi-Wave Seismograph in the Measurement of Coal Seam Washout Zone

In order to avoid the useless tunneling and the low mining rate, the mine seismograph measurement new technology is applied to measure the coal seam washout zone with long distance and high resolution. Based on the analyzing geophysics characteristic of the coal seam washout zone, this paper investigates the characteristics of time-distance curve of the measurement technology, and studies the application example of detecting the development and boundary scope of the old river-seam coal seam washout zone long-distance ahead of tunnel. The result shows that the measurement of the development and the boundary scope of the coal seam washout zone by using the mine multi-wave seismograph meet the practical requirement with small error and being consistent with the actual conditions during the process of mining.

DETERMINATION OF UNDERGROUND WATER CONTAMINATION ZONE AND WATER TABLE BY SEISMIC REFRACTION METHOD AT AL-AZIZIYAH CITY WASIT PROVINCE - SOUTHREN IRAQ

A seismic refraction investigation was carried out at Al-Aziziyah area for delineating water table and determining the velocity of different layers, as well as locating the contamination zones in under ground water which are resulted from agricultural activities. Nineteen seismic profiles were carried out in this study using ABEM TERRALOC seismic system. The length of each profile ranged from (33-115) m. with normal and reverse shooting. The recorded field data interpreted by delay time method using time distance curve as well as plus-minus method, mean-minus time method, ABC method and generalized reciprocal method. The velocities of the first layer range from (252-404) m/sec. and its average is (295.53) m/sec. while the velocities of the second layer ranges from (1335-1666) m/sec. and its average is (1494.21) m/sec. the velocities show no significant difference after interpreting by different methods. In this study the second layer represents the water table in the area with the depths range from (2.2- 5.5) m. Finally the contour map was drawn for the water table of the ground water, as well as a map differentiating the contaminated from non-contaminated zones.

Seismic refraction investigation of the subsurface structure at the southern part of Niger State College of Education, Minna, Nigeria

This paper discussed the seismic refraction survey conducted at the southern part of Niger State College of Education Minna, in which a three-channel seismograph was used for the survey. The profiles were marked at 100m intervals, while the profiles lines traversed 1000m (1km). The exact site surveyed covers a total area of about 500,000m2. From the data collected, time – distance curves for each shot points were drawn, the velocities of the underlying layers for each points were obtained and the depths to the refractor layer were computed. The results obtained gave an overview of the lateral variation in the lithological changes of the subsurface earth materials in the surveyed area. The basement surface varied in depth, from depths as shallow as 2.05m to a maximum of 10.13m. The rock materials identified in the surveyed area are chiefly sand, saturated clay, gravel, and granite. Keyword: Basement, aquifer, Seismograph, time-distance curve, and velocity.

On amplitudes of P near the shadow zone

51 short-period vertical North American and Canadian records of the Samoa earthquake 14 April 1957 were examined. Amplitudes of P were very large at the western stations (Δ < 84˚) and small at the stations to the east, 8 of which recorded P at distances greater than 100˚. There was a lack of stations at intermediate distances. At the greatest distance, 108˚.2, the P waves were not diffracted waves, for the phase had the same general character as at shorter distances; the period was 2 sec. From 70˚ to about 90˚ the Samoa P timedistance curve is steeper than the Jeffreys-Bullen curve. It had been found previously that from about 90˚ epicentral distance onwards the variation ot the P amplitude with distance was not always the same and also that the shadow zone seemed to begin at different distances. It is suggested that this be due to small regional differences of velocity in the deepest layer D" of the mantle.

Effective wave identification and interference analysis of the seismic reflection method in mines

Through discussion of the time-distance curve characteristics of the direct wave and from the front, side and rear of the reflection waves of the seismic reflection method for advanced exploration in mines, and analysis of several major interference waves in mines, the differences in time-distance curve, frequency, apparent velocity between the effective wave and interference wave in the seismic reflection method for advanced exploration are obtained. According to the differences, the effective wave is extracted and the interference wave is filtered and the system’s precision and accuracy is improved.

Equivalence between Duvall’s Law and the Time-Distance Curve

Duvall's law is shown to be equivalent to the phase time-distance curve. The two are connected through a simple transformation. Thus, like Duvall's law, the phase time-distance curve can be analytically inverted to obtain the radial sound speed. The use of the phase information of the time-distance technique makes the inversion of the time-distance Duvall law particularly advantageous over the traditional inversion of Duvall's law. The radial sound speed can also be obtained numerically through the radial limit of the global or local tomography of the Sun. With global tomography, one can take advantage of the higher order skips of wave packets to obtain local tomograms of any section of the Sun, including the interior of the farside, that currently cannot be directly observed.

Radiocarbon Age Constraints on Rates of Advance and Retreat of the Puget Lobe of the Cordilleran Ice Sheet during the Last Glaciation

Calibrated radiocarbon dates of organic matter below and above till of the last (Fraser) glaciation provide limiting ages that constrain the chronology and duration of the last advance–retreat cycle of the Puget Lobe in the central and southeastern Puget Lowland. Seven dates for wood near the top of a thick proglacial delta have a weighted mean age of 17,420 ± 90 cal yr B.P., which is the closest limiting age for arrival of the glacier near the latitude of Seattle. A time–distance curve constructed along a flowline extending south from southwestern British Columbia to the central Puget Lowland implies an average glacier advance rate of ca. 135 m/yr. The glacier terminus reached its southernmost limit ca. 16,950 yr ago and likely remained there for ca. 100 yr. In the vicinity of Seattle, where the glacier reached a maximum thickness of 1000 m, ice covered the landscape for ca. 1020 yr. Postglacial dates constraining the timing of ice retreat in the central lowland are as old as 16,420 cal yr B.P. and show that the terminus had retreated to the northern limit of the lowland within three to four centuries after the glacial maximum. The average rate of retreat was about twice the rate of advance and was enhanced by rapid calving recession along flowline sectors where the glacier front crossed deep proglacial lakes.

Acoustic Imaging and Subsurface Absorption Structure of Sunspots

Recently, Chang et al. (1997, hereafter Paper I) developed a new method, based on time-distance curves (Duvall et al. 1993), to construct a three-dimensional acoustic intensity image of the solar interior. Here we report results from another data set.Solar p-mode waves, which are continually generated and dissipated stochastically by the turbulent convection, are scattered and absorbed by local inhomogeneities. If we appropriately add p-mode waves observed at the surface, based on the knowledge of how the waves propagate (time-distance curves), such that acoustic signals emanating from a particular point at a particular time are collected in phase, we can reconstruct the p-mode amplitude at the target point at the target time. The scheme of adding signals from a target point in phase plays the same role as a lens in optics. Thus we call it the “computational acoustic lens” (Paper I). This imaging technique can be used to construct acoustic amplitudes at any point in the solar interior based on the time-distance curve between that depth and the surface, which can be computed from a standard solar model based on the ray theory. The time series of constructed a plitudes provides information of both the intensity and the phase of the wavetrain. The intensity can be used to study p-mode absorption regions, and the phase can be used to probe other properties of local inhomogeneities, such as flow and change in wave speed.

Measuring the Solar Internal Rotation Using Time-Distance Helioseismology. I. The Forward Approach

view Abstract Citations (11) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Measuring the Solar Internal Rotation Using Time-Distance Helioseismology. I. The Forward Approach D'Silva, Sydney Abstract We set up the ray equations for acoustic-gravity waves in spherical geometry, in the presence of rotation or any horizontal subsurface flows. Rotation lifts the degeneracy and splits the single time-distance curve for the nonrotating Sun (D'Silva & Duvall 1995) into a family of closely spaced curves, of which the prograde and retrograde curves are at the extreme ends. We calculate time-distance curves (for travel times as measured by Duvall and coworkers for a variety of synthetic radial rotation profiles, including solid-body rotation and the known equatorial differential rotation. The travel time and distance differences between the pro grade and retrograde rays show specific signatures of the rotation profile. Hence, the forward problem to measure rotation is studied in detail. Publication: The Astrophysical Journal Pub Date: May 1996 DOI: 10.1086/177169 Bibcode: 1996ApJ...462..519D Keywords: SUN: INTERIOR; SUN: OSCILLATIONS; SUN: ROTATION full text sources ADS |

A method to interpret the time-distance curve of refraction waves for shallow layers near the shore : Shen, Jianping, 1981. J. Shandong Coll. Oceanol., 11(4):77–88. (In Chinese, English abstract.)

A method to interpret the time-distance curve of refraction waves for shallow layers near the shore : Shen, Jianping, 1981. J. Shandong Coll. Oceanol., 11(4):77–88. (In Chinese, English abstract.)

Crustal structure in the North Cascade Mountains of Washington and British Columbia from seismic refraction measurements

abstract Seismic waves generated by large chemical explosions at Greenbush Lake, British Columbia, were recorded at 15 stations along two profiles which extend from the shot point across the North Cascade Mountains of British Columbia and Washington into western Washington. T = Δ/6.13 expresses the time-distance curve for P g . The equation T = 6.21 + Δ/7.91 expresses the P n arrival time along a line of azimuth of 210.4° out to a distance of 519 km and the equation T = 6.85 + Δ/8.06 expresses the P n arrival time along a line of azimuth of 223.0° out to a distance of 540 km. Calculations, which assume a mantle velocity of 7.96 km/sec, indicate that crustal thickness increases toward the south and decreases toward the west. The average crustal thickness along azimuth 210.4° is 32 km and along 223.0° is 29 km. Amplitude data have large scatter but suggest a Q value near 384 for rocks of the upper mantle beneath the North Cascade Mountains.

Reply by author to discussion by Lyle V. A. Sendlein

Mr. Sendlein’s discussion suggests two errors in the author’s paper. These were errors in the plotting of the data and not in the data itself. In reply to Mr. Sendlein, the following three items will be discussed: 1. The method of determination of velocities from the time distance curve; 2. Reliability of velocities determined; and 3. The interpretation of the time‐distance curve for station 87+00.

Reflected ultrasound in the assessment of mitral valve disease.

An ultrasound beam directed into a medium of uniform acoustical impedance will travel in a straight line, but a portion of the waves will be reflected at the interface of a medium of different acoustical impedance. A time-distance curve of motion has been obtained from the heart by recording the echoes returned to a send-receive ultrasound transducer applied in the third, fourth, or fifth left interspace. A characteristic curve having two anterior peaks during each cardiac cycle was obtained in the 25 normal patients studied. One peak follows the P wave of the electrocardiogram and is not seen in atrial fibrillation. The other peak of motion toward the anterior chest is followed by rapid regression posteriorly during ventricular diastole. The velocity of the motion was 85 to 160 mm./sec. in normal subjects. In contrast, the records from 90 patients with mitral stenosis demonstrated a distinctive, abnormal pattern having a plateautype configuration with a velocity of posterior motion of only 2 to 35 mm./sec. The configuration of the curve changed toward normal after successful mitral valvulotomy but none of the 35 patients who were studied before and after surgery has developed a calculated postoperative velocity exceeding 70 mm./sec. Twelve patients with "pure" mitral regurgitation have had records similar to the normal, with velocities in or above the normal range. None of the 135 other patients studied, representing a variety of conditions, demonstrated a curve of the type found in mitral stenosis.

P波走時曲線の勾配について

P波走時曲線の勾配について