Travel-time Curves Research Articles

New Methods for Data Stacking and P‐ and S‐wave Arrival Time Determination Using the Deep Moonquake Apollo Recordings

AbstractA new method of simultaneous time domain cross‐correlation of multiple components of motion at multiple seismic stations is developed for P‐ and S‐wave identification for determination of arrival times of deep moonquakes using Apollo Passive Seismic Experiment (APSE) data. Deep moonquakes occur in selenographically isolated clusters, allowing the stacking of a large number of moonquakes in each cluster to improve signal‐to‐noise ratio. The method developed here seeks to improve upon multichannel cross‐correlation (MCCC) of a single component of motion and multiple events used in past work by incorporating all components and stations simultaneously. This multicomponent multichannel cross‐correlation (MCMCCC) method not only maximizes the inherent correlation between the 12 components (three components of motion at each of four stations) across all events in a given cluster, but also establishes representative stacked traces of moonquakes within each cluster. Additionally, the MCMCCC method utilizes components and events with the highest data quality to guide the inclusion of lower quality components and events (or events with fewer components). A closure difference residual time is introduced to identify moonquake events with better data and/or close source locations, to further improve the signal‐to‐noise ratio of stacked traces. A multicomponent short‐term to long‐term average algorithm is also developed to objectively determine P‐ and S‐wave arrivals that best align with their travel time curves based on lunar velocity models, simultaneously for all 12 components of motion. Newly computed deep moonquake cluster stacks and some updated deep moonquake cluster locations are presented, which improve upon past results.

Surface Diffraction Noise Attenuation for Marine Seismic Data Processing With Mathematical Morphological Filtering

Surface diffraction noise (SDN) is a kind of coherent noise and pervasive in marine seismic data acquisition. This kind of noise is intractable during seismic data processing. We present a new method based on mesh scanning and mathematical morphological filtering (MMF) to attenuate the SDN in marine surveys. The idea of mesh scanning will be first put forward to get the energy-map of the survey. Compared with the traditional mesh scanning method, the energy-map computing formulas will be improved to protect the reflections from the recognition of SDNs, especially the shallow reflections. All the possible surface diffractors will be shown on the computed energy-map with strong energy. Then, a 2-D mathematical morphological closing operation will be innovatively used to get the upper envelope of the energy-map and reduce the number of the possible surface diffractors with a maximal value picking. After confirming all the possible surface diffractors on the energy-map, the travel-time curves of SDNs could be calculated easily according to the kinematic propagation law of seismic wave for all the related shot gathers and the MMF will be used to extract all the SDNs along the trajectories of them within appropriate time windows. Both a real 2-D and a real 3-D marine seismic data will be processed by the proposed method to show the positive effects in attenuating the SDNs.

Variations in P-wave Travel Times Based on a Digital Seismogram Dataset

The paper presents the parameters of digital seismograms of artificial seismic events observed in from 1967 to 1990. The data were provided to the authors from the archive of the Sadovsky Institute of Geosphere Dynamics (IDG RAS). Seismograms were obtained after reformatting digital records. The archived data were analyzed to obtain a posteriori information on P-wave travel times, determined from the moment of the first arrival on the record of the vertical component (Z) of the short-period channel. The practice has shown that combining these data is sufficient to compile a map of signal arrival times from seismic sources. We present the regional travel-time curve for three test sites and linear trends of P-wave travel times by calendar date, as well as an isochron map for one of the sites.

On-axis triplications in elastic orthorhombic media

SUMMARY Triplicated traveltime curve has three arrivals at a given distance with the bowtie shape in the traveltime-offset curve. The existence of the triplication can cause a lot of problems such as several arrivals for the same wave type, anomalous amplitudes near caustics, anomalous behaviour of rays near caustics, which leads to the structure imaging deviation and redundant signal in the inversion of the model parameters. Hence, triplication prediction becomes necessary when the medium is known. The research of the triplication in transversely isotropic medium with a vertical symmetry axis (VTI) has been well investigated and it has become clear that, apart from the point singularity case, the triplicated traveltime only occurs for S wave. On contrary to the VTI case, the triplication behaviour in the orthorhombic (ORT) medium has not been well focused due to the model complexity. In this paper, we derive the second-order coefficients of the slowness surface for two S waves in the vicinity of three symmetry axes and define the elliptic form function to examine the existence of the on-axis triplication in ORT model. The existence of the on-axis triplication is found by the sign of the defined curvature coefficients. Three ORT models are defined in the numerical examples to analyse the behaviour of the on-axis triplication. The plots of the group velocity surface in the vicinity of three symmetry axes are shown for different ORT models where different shapes: convex or the saddle-shaped (concave along one direction and convex along with another) indicates the existence of the on-axis triplication. We also show the traveltime plots (associated with the group velocity surface) to illustrate the effect of the on-axis triplication.

PKIKPBC ON THE TRAVELPATH NEVADA – “MIRNYY” STATION (ANTARCTICA)

The Geophysical Service of the Russian Academy of Sciences (Obninsk, Moscow Region) made it possible to process seismograms of “Mirnyy” station in Antarctica. Seismograms for the period 1961–1991 were viewed, when underground nuclear explosions were conducted at the Nevada Test Site of the United States. To calculate and construct a local travel time curve and a linear trend of the PKIKPBC body wave travel time on the Nevada Test Site (NTS) – “Mirnyy” seismic station (MIR), 120 seismograms from Kirnos seismograph (SVKM) were selected for the period May 13, 1966 – October 18, 1991: 60 for each site Pahute and Yucca of the Nevada Test Site (NTS). The article presents local travel-time curves and linear trends of the PKIKPBC wave on the travelpath NTS – MIR. Increase in time travel with years as in travelpaths Nevada Test Site – Borovoye [14]. Dependency of arrival times of the PKIKPBC wave on mb magnitude was detected. The obtained data can be used for comparison of results of PKIKPBC wave study from the Nevada Test Site on other stations, for examples, at “Novolazarevskaya” Antarctic station.

Adverse Effects of an Edge Diffractor in Seismic Reflection Interferometry

To understand steeply dipping events in seismic reflection interferometry (SRI), we derived an expression that describes the difference in travel time (Δτ) from a diffractor to two receivers in two dimensions. For a fixed receiver interval, the expression shows that Δτ is zero when the diffractor is at the midpoint of the paired receivers, increases with an apparent velocity of half the medium velocity as the diffractor moves toward either receiver, and remains constant for a diffractor located on the same side of both receivers. The horizontal portion of Δτ is slightly skewed during the normal moveout correction, yielding a maximum peak of the horizontally stacked trace at a slightly smaller time than Δτ. Accordingly, the diffracted waves have an apparent velocity slightly higher than half of the medium velocity in a horizontally stacked image. This conformed to virtual data for an elastic two-layer model with a vertical boundary. We then generalized the expression to three dimensions, in which listric travel time curves were predicted for an oblique edge diffractor, a vertex diffractor offline from the receiver pair, or a buried diffractor. Based on both two- and three-dimensional analyses of the edge diffractor, we tentatively interpreted the linear and listric dipping events observed in the passive SRI image across the Korean Peninsula to have been caused by diffractors near the intersection of the profile and geologic boundaries.

Seismic phase picking in China Seismic Array using a deep convolutional neuron network*

Seismic phase picking is the preliminary work of earthquake location and body-wave travel time tomography. Manual picking is considered as the most accurate way to access the arrival times but time consuming. Many automatic picking methods were proposed in the past decades, but their precisions are not as high as human experts especially for events with low ratio of signal to noise and later arrivals. As the increasing deployment of large seismic array, the existing methods can not meet the requirements of quick and accurate phase picking. In this study, we applied a phase picking algorithm developed on the base of deep convolutional neuron network (PickNet) to pick seismic phase arrivals in ChinArray-Phase III. The comparison of picking error of PickNet and the traditional method shows that PickNet is capable of picking more precise phases and can be applied in a large dense array. The raw picked travel-time data shows a large variation deviated from the traveltime curves. The absolute location residual is a key criteria for travel-time data selection. Besides, we proposed a flowchart to determine the accurate location of the single-station earthquake via dense seismic array and phase arrival picked by PickNet. This research expands the phase arrival dataset and improves the location accuracy of single-station earthquake.

Solution of equation of extreme streamline with free flowing of a torrential stream behind rectangular pipe

The article discusses the methods of modeling two-dimensional in terms of turbulent flows. Their difference and advantages are described. The equations of flow motion in the plane of the hodograph are given, as well as the velocity and depth of the flow in the entire region of its flow. Next, a new method is proposed, the purpose of which is to solve a system of differential equations. Using a differential equation connecting the physical cavity of the flow stream and the plane of the travel time curve, a simple model of the flow of a steady, vortex-free water flow in a wide horizontal, smooth outlet channel is proposed. The flow model in the vicinity of the outlet from the pressureless pipe uses the fact that the resistance forces of the flow to the bottom of the outlet channel are small compared with the forces of inertia. The obtained solutions allow us to calculate the flow parameters with an accuracy sufficient for the design of hydraulic structures of the drainage system from the upper pool to the lower one under roads and railways. This model allows you to identify the basis of the laws of the real spreading of the flow and on its basis there is an opportunity for further building models taking into account the resistance forces in order to increase the correspondence of the mathematical model to the real flow.

Recording of Local Earthquakes under the Udina Volcanoes (Kamchatka) by Regional and Temporary Seismic Stations. Refinement of the Velocity Model by Reversing the Travel-Time Curve from a Deep Source

Currently, the Udina volcanoes (Kamchatka, Klyuchevskaya Volcano Group) are considered extinct. However, at the end of 2017, seismic activation was recorded for the first time since instrumental observations beneath them have been carried out at the middle−upper crustal level. To improve the accuracy in determining the coordinates of hypocenters in the vicinity Udina volcanoes, work was carried out to refine the local velocity model for this area. The velocity structure of the setting was refined using proprietary approaches to determining the coordinates of earthquake hypocenters and reversing the travel-time curves from deep sources. Seismic wave propagation velocities were obtained from data on earthquakes of the Udina seismic activation 2017−2019. The results of the Kamchatka regional seismic network and four additional temporary stations installed specifically to study the Udina seismogenic region were used. A one-dimensional velocity model was obtained for depths up to 40 km: the set of layers in which the longitudinal wave velocity varies linearly at a constant ratio VP/VS = 1.85. The model differs significantly from the generalized baseline curve obtained for the northern part of the Klyuchevskaya Volcano Group based on DSS data DSS. The largest differences in velocities are observed in the upper crust (at a depth of up to 4 km) and in the region of the Conrad discontinuity at a depth of ∼20 km In the depth range of 20−25 km, a reduced-velocity layer was revealed.

ФОРМА СЕЙСМИЧЕСКОГО ГОРИЗОНТА НА ВРЕМЕННОМ РАЗРЕЗЕ И ГОДОГРАФА ОБЩЕЙ ТОЧКИ ВЗРЫВА ОТРАЖЕННОЙ ВОЛНЫ В ПЕРЕХОДНОЙ ЗОНЕ ДЛЯ СЛУЧАЯ, КОГДА ПОКРЫВАЮЩАЯ ЦЕЛЕВОЙ ГОРИЗОНТ СРЕДА ОСЛОЖНЕНА НАРУШЕНИЕМ

The paper is devoted to study of reflection pattern (seismic horizon) and CSP travel time curve of reflected wave when the base of seismic oscillations receivers is within transition zone, i.e. the portion of base is in one flank while the other part is already in the next flank. The equation has been derived for travel time curve of zero-offset survey, i.e. reflection pattern) for the case when target horizon cover is complicated by fault. The other equation derived is the equation of traveltime curve of CSP of reflected wave, which upgoing rays are refracted on the fault line. For this, we have used the equation of traveltime curve of normal reflections refracted on the fault line. Based on derived formulae we have calculated traveltime curves of CSP for various models of environment. Analysis of these models has shown that while passing through seismic waves receiver base over the faulted zone the travel time curve is divided into two cuts with differing curvature and various time offsets relative to each other depending on velocity section difference from the fault line leading to the impression that these two cuts are not the sections of the same travel time curve.

Assessment of the Efficiency of Armenian Seismological Networks

For successful prediction of strong earthquakes, seismic risk zoning, and earthquake-resistant construction, it is necessary above all to have a sufficiently clear understanding of the hypocentral distribution of earthquakes in the area in question. In order to determine the position of earthquake foci with high accuracy, it is necessary to have an observation system capable of this. The modern seismological network in the Republic of Armenia (RA) essentially consists of two independent networks that barely interact with each other. Moreover, each network uses its own data processing methods (for determining hypocenters and travel-time curves). One network belongs to the National Seismic Protection Service under the Ministry of Emergencies of the Republic of Armenia (NSPS RA) and consists of 16 stations; the other is subordinate to the Institute of Geology of RA National Academy of Sciences (IG NAS RA) and hosts 14 seismic stations. Both networks are set up unevenly in the territory of Armenia, and each has its own disadvantages. The IG NAS RA network is sparser (with only 14 stations), and its stations are located only in northwestern Armenia. Therefore, it cannot accurately localize the hypocenters of earthquakes throughout the republic. The NSPS RA network consists of 16 stations. They are also located unevenly across Armenia, but at the same time, they encompass the southeastern part of the country. Calculations have been performed and maps compiled of the distribution of minimum energy classes and maximum errors in determining hypocentral coordinates of earthquakes recorded both separately by the IGN NAS RA and NSPS RA networks and by the integrated seismic network. It is shown that when the networks are combined, localization of earthquake hypocenters improves somewhat, but the territory between northwestern and southeastern Armenia remains virtually uncovered. This is because, at present, the stations of both existing networks, on the one hand, duplicate each other in some cases, while on the other, they are located unevenly across the territory. To obtain more accurate and reliable information, it is better to have a single network with be uniformly placed throughout the republic.

Seismic Data Separation for Simultaneous Pulse Source Operation

The article is devoted to multisource technology—a new direction in geophysics that provides high seismic performance due to the simultaneous use of several seismic sources. The main problem when working with multiple sources is separation of the interfering wave fields. To solve this, we propose a hyperbolic median filtering method, which uses the hyperbolicity of the travel-time curve of reflected waves in a common midpoint gather. Additionally, an inversion algorithm is used to adjust signal amplitudes after separation. This approach demonstrates good results with synthetic data.

Occurrence of “precursors” of PKP-waves in the layered radial-symmetric Earth

It is generally accepted that PKP‑waves precursors, which are observed on a real data ahead of PKP‑waves, are explained by scattering on small-scale inhomogeneities in the lower mantle. In this paper, a stable analytical solution (without interference) was obtained for the wave field of longitudinal waves in a layered (discrete) ball of planetary size. The calculations of the total wave field, rays and travel-time curves of longitudinal waves for the spherical model of the Earth AK135 with a carrier frequency of 1 hertz are presented. The analytical solution showed that at angles smaller than 145 degrees ahead of the PKP‑waves, low-amplitude waves appear, with a higher frequency of about 1,3 hertz. Indeed, these high-frequency oscillations have the form characteristic for waves scattered at a certain object. The ray pattern and the travel-time graph show that these high-frequency oscillations are due to exclusively to the spherical geometry of the Earth. This could be explained by the interference of refracted and reflected longitudinal waves in the bottom of a discrete outer core. This field propagates even further towards smaller angles due to the interference of diffraction waves.

Automated static and moveout corrections of high‐resolution seismic data from the Baltic Sea

ABSTRACTHigh‐frequency multichannel seismic systems provide detailed images of the shallow marine subsurface. In order to exploit the redundancy inherent in such data optimally, traveltime corrections need to account for normal moveout and static effects due to vertical source and receiver variations. Misalignment of reflections in common‐midpoint gathers will significantly lower the frequency content in the final stack, making this correction particularly important for very high‐frequency seismic data. Traditionally, normal moveout correction involves labour‐intensive picking of stacking velocities, while static corrections can be, by some techniques, performed automatically. In this paper, we present a high‐frequency seismic case study from the Baltic Sea, using seismic image matching as a novel, fully automated technique to perform joint moveout and static corrections. Our multichannel test profiles were acquired offshore Rügen island for wind farm development. Owing to the regular passage of up to 1.5 m high ocean waves during data acquisition, these boomer profiles suffer from strong static effects. We perform joint normal moveout and static corrections by defining the nearest common offset section as a fixed reference frame and minimizing its difference in traveltime with respect to all available common offset sections. Time shifts are computed independent of a pre‐defined traveltime curve, using the normalized cross‐correlation as a measure of data similarity while penalizing irregular displacements by a regularization term. Time shifts are converted to stacking velocities based on the traditional hyperbolic traveltime equation. Our results are compared with those derived by conventional manual velocity analysis and subsequent trim static corrections. We find that image matching produces stacks of similar quality and stacking velocity models of similar to slightly better quality compared with the conventionally derived ones, revealing the potential of this technique to automatize and significantly speed up this first part of the seismic processing chain.

Joint refraction traveltime tomography and migration for multilayer near-surface imaging

In near-surface velocity structure estimation, first-arrival traveltime tomography tends to produce a smooth velocity model. If the shallow structures include a weathering layer over high-velocity bedrock, first-arrival traveltime tomography may fail to recover the sharp interface. However, with the same traveltime data, refraction traveltime migration proves to be an effective tool for accurately mapping the refractor. The approach downward continues the refraction traveltime curves and produces an image (position) of the refractor for a given overburden velocity model. We first assess the validity of the refraction traveltime migration method and analyze its uncertainties with a simple model. We then develop a multilayer refraction traveltime migration method and apply the migration image to constrain traveltime tomographic inversion by imposing discontinuities at the refraction interfaces in model regularization. In each subsequent iteration, the shape of the migrated refractors and the velocity model are simultaneously updated. The synthetic tests indicate that the joint inversion method performs better than the conventional first-arrival traveltime tomography method with Tikhonov regularization and the delay-time method in reconstructing near-surface models with high-velocity contrasts. In application to field data, this method produces a more accurately resolved velocity model, which improves the quality of common midpoint stacking by making long-wavelength static corrections.

Uses of seismic refraction tomography in investigation of groundwater potential in Kaduna millennium city, Kaduna Nigeria

Seismic refraction tomography is unique in the sense that shot are taken at every receiver position along the spread. In that way, it becomes possible to conduct both forward and reverse shooting simultaneously along the profile lines. Therefore, every point within the subsurface along the profile lines is probed by both forward and reversed shots. This leads to what one might call the scanning of the subsurface. The data were processed with ReflexW software and a conventional forwardand reversed travel time curve was plotted for each profile lines. Thereafter joint inversion of the travel time picks was carried out. This resulted in the generation of an iterative model, which was used to generate tomographic sections. The result of seismic refraction tomography showed lateral and vertical variation in velocity along most of the profile lines, thereby giving a true image of velocity distribution, both vertically and laterally within the subsurface. Refraction Tomography gave accurate depth and thickness of the overburden along the profile and to delineate aquifer potentialin the basement topography. The aquifer is estimated to be around 45m to 60m along the entire profile line

Probabilistic simulation of advection-reaction-dispersion equation using random lattice Boltzmann method

Mass transfer is subject to numerous sources of uncertainties due to scarcity of observational data. In this research, a numerical procedure was developed for the probabilistic study of a two-dimensional advection-dispersion problem, while considering chemical reactions. Innovatively, the lattice Boltzmann method was coupled with the concept of random field theory for the probabilistic simulations. The effects of various coefficients of variations (COV) and a number of autocorrelation distances were considered for the stochastic parameters, including dispersion coefficient, pore velocity, and the reaction term. The results indicated that the introduced probabilistic framework can be employed to effectively describe the effects of uncertainties in parameters related to the advection-dispersion equation. Moreover, it was deduced that the mass travel time and the time-concentration curves were influenced significantly by the variations of COV and autocorrelation distance for pore velocity. Interestingly, the mass transfer in the transverse direction increased (through the dispersion phenomenon) with a rise in the values of COV for longitudinal pore velocity. However, different values of COV and autocorrelation distances for the dispersion coefficient and the reaction term caused small alterations in the mass travel time and time-concentration curve.

ПРИМЕНЕНИЕ ПСЕВДОВОЛНОВЫХ ТРАНСФОРМАНТ В ИМПУЛЬСНОЙ ЭЛЕКТРОРАЗВЕДКЕ ДЛЯ ВЫДЕЛЕНИЯ НАКЛОННЫХ ГЕОЛОГИЧЕСКИХ ГРАНИЦ

Актуальность. Метод нестационарных электромагнитных зондирований или зондирований становлением поля является востребованным инструментом при поиске и разведке месторождений различного типа полезных ископаемых. Современные задачи электроразведки требуют использования установок с большим числом источников и приёмников. Для подобного типа установок задача интерпретации полученных данных является весьма ресурсоемкой. Отсюда вытекает острая необходимость развития методов обработки и интерпретации данных, полученных в результате импульсных электромагнитных зондирований. Одним из возможных путей такого развития могут послужить трансформации данных зондирований становлением в пространство решений волнового уравнения. Цель: применить метод волновой трансформации к данным зондирований становлением, зарегистрированным над проводящими средами с наклонной границей в многоразносных установках, для получения новых качественных оценок исследуемого геоэлектрического разреза. Методика исследования базируется на новых численных алгоритмах для устойчивого преобразования данных зондирований становлением в волновую область. Исходные данные для отладки процедур трансформации (расчётные кривые становления поля) были получены с использованием программного комплекса MODEM3D, который предназначен для прямого моделирования переходных процессов диффузии электромагнитных полей. Результаты. Были получены псевдоволновые трансформанты сигналов нестационарных электромагнитных зондирований в средах с наклонной границей, и построены их годографы. Полученные наборы трасформаций позволяют выделить линию выхода наклонного контакта на дневную поверхность. Установлено, что перекрывающие наносы ослабляют максимумы псевдоволновых трансформаций и снижают возможность точной локализации линии выхода наклонной границы. Рассмотрено поведение тороидальных вихревых токов в зависимости от времени для модели геологической среды с наклонной границей.

HISTORICAL SEISMIC RECORDS OF EXPLOSIONS, CARRIED OUT TO CREATE THE MUD DAM IN MEDEO

The author studied characteristics of explosions in Medeo (October 21, 1966, April 14, 1967 and November 20, 1973) implemented for construction of the dam, information on macro-seismic occurrences of these explosions in Almaty city as well as seismic effect in comparison with strong industrial and research explosions during the construction of dams in Central Asia. A travel-time curve of main regional phases for close and regional distances was described; it was constructed on the basis of historical archived seismograms as well as a composite travel-time curve for the region of Northern Tien Shan near Almaty. There was a comparison drawn with the travel-time curve, constructed on records of a strong Kambarata explosion on December 22, 2009, in Kyrgyzstan. The obtained results may be used to improve localization of seismic events and modeling of early warning system in Almaty.

USSR PEACEFUL NUCLEAR TESTS, GEOPHYSICAL OBSERVATORY “BOROVOYE”

The paper presents a summary of corrected data on the Soviet Peaceful Nuclear Explosions (abbreviated as PNEs) with the author's corrections, taking into account the information on the updated coordinates of the works of the authors Makei, Fujita, etc. As a result, the article presents the calculated epicentral distances for PNEs, travel-time curves and azimuths from the Geophysical Observatory “Borovoe” to the geographical coordinates of the PNEs, which were registered in digital format during the monitoring conducted by Expedition № 4 of the Special Sector of the Institute of Physics of the Earth of the USSR Academy of Sciences since 1966.