Seismic tomography studies of cover thickness and near-surface bedrock velocities

Geophysics ◽  
2006 ◽  
Vol 71 (6) ◽  
pp. U77-U84 ◽  
Author(s):  
B. Bergman ◽  
A. Tryggvason ◽  
C. Juhlin
Keyword(s):  
Nuclear Waste ◽  
Velocity Structure ◽  
Synthetic Data ◽  
General Situation ◽  
Seismic Profiles ◽  
Low Velocity ◽  
Near Surface ◽  
Reflection Seismic ◽  
Crystalline Bedrock ◽  
Cover Thickness

Reflection seismic imaging of the uppermost kilometer of crystalline bedrock is an important component in site surveys for locating potential storage sites for nuclear waste in Sweden. To obtain high-quality images, refraction statics are calculated using first-break traveltimes. These first-break picks may also be used to produce tomographic velocity images of the uppermost bedrock. In an earlier study, we presented a method applicable to data sets where the vast majority of shots are located in the bedrock below the glacial deposits, or cover, typical for northern latitudes. A by-product of this method was an estimate of the cover thickness from the receiver static that was introduced to sharpen the image. We now present a modified version of this method that is applicable for sources located in or on the cover, the general situation for nuclear waste site surveys. This modified methodalso solves for 3D velocity structure and static correctionssimultaneously in the inversion process. The static corrections can then be used to estimate the cover thickness. First, we test our tomography method on synthetic data withthe shot points in the bedrock below the cover. Next, we developa strategy for the case when the sources are within the cover. Themethod is then applied to field data from five crooked-line,high-resolution reflection seismic profiles ranging in lengthfrom 2 to [Formula: see text]. The crooked-line profiles make the study 2.5dimensional regarding bedrock velocities. The cover thicknessalong the profiles varies from 0 to [Formula: see text]. Estimated thickness ofthe cover agrees well with data from boreholes drilled near theprofiles. Low-velocity zones in the uppermost bedrock generallycorrelate with locations where reflections from the stackedsections project to the surface. Thus, the method is functional,both for imaging the uppermost bedrock velocities as well as for estimating the cover thickness.

scholarly journals Geophysical imaging of permafrost in the SW Svalbard – the result of two high arctic expeditions to Spitsbergen

2020 ◽  
Author(s):  
Mariusz Majdanski ◽  
Artur Marciniak ◽  
Bartosz Owoc ◽  
Wojciech Dobiński ◽  
Tomasz Wawrzyniak ◽  
...  
Keyword(s):  
Surface Waves ◽  
Active Layer ◽  
High Arctic ◽  
The Arctic ◽  
Seismic Profiles ◽  
Frozen Ground ◽  
Near Surface ◽  
Seismic Processing ◽  
Reflection Seismic ◽  
Sea Coast

<p>The Arctic regions are the place of the fastest observed climate change. One of the indicators of such evolution are changes occurring in the glaciers and the subsurface in the permafrost. The active layer of the permafrost as the shallowest one is well measured by multiple geophysical techniques and in-situ measurements.</p><p>Two high arctic expeditions have been organized to use seismic methods to recognize the shape of the permafrost in two seasons: with the unfrozen ground (October 2017) and frozen ground (April 2018). Two seismic profiles have been designed to visualize the shape of permafrost between the sea coast and the slope of the mountain, and at the front of a retreating glacier. For measurements, a stand-alone seismic stations has been used with accelerated weight drop with in-house modifications and timing system. Seismic profiles were acquired in a time-lapse manner and were supported with GPR and ERT measurements, and continuous temperature monitoring in shallow boreholes.</p><p>Joint interpretation of seismic and auxiliary data using Multichannel analysis of surface waves, First arrival travel-time tomography and Reflection imaging show clear seasonal changes affecting the active layer where P-wave velocities are changing from 3500 to 5200 m/s. This confirms the laboratory measurements showing doubling the seismic velocity of water-filled high-porosity rocks when frozen. The same laboratory study shows significant (>10%) increase of velocity in frozen low porosity rocks, that should be easily visible in seismic.</p><p>In the reflection seismic processing, the most critical part was a detailed front mute to eliminate refracted arrivals spoiling wide-angle near-surface reflections. Those long offset refractions were however used to estimate near-surface velocities further used in reflection processing. In the reflection seismic image, a horizontal reflection was traced at the depth of 120 m at the sea coast deepening to the depth of 300 m near the mountain.</p><p>Additionally, an optimal set of seismic parameters has been established, clearly showing a significantly higher signal to noise ratio in case of frozen ground conditions even with the snow cover. Moreover, logistics in the frozen conditions are much easier and a lack of surface waves recorded in the snow buried geophones makes the seismic processing simpler.</p><p>Acknowledgements               </p><p>This research was funded by the National Science Centre, Poland (NCN) Grant UMO-2015/21/B/ST10/02509.</p>

Turning‐ray tomography

Geophysics ◽  
1995 ◽  
Vol 60 (6) ◽  
pp. 1917-1929 ◽  
Author(s):  
Joseph P. Stefani
Keyword(s):  
Velocity Structure ◽  
Initial Point ◽  
Real Data ◽  
Point Sources ◽  
Surface Velocity ◽  
Inversion Algorithm ◽  
Low Velocity ◽  
Linear Inversion ◽  
Near Surface ◽  
Velocity Inversion

Turning‐ray tomography is useful for estimating near‐surface velocity structure in areas where conventional refraction statics techniques fail because of poor data or lack of smooth refractor/velocity structure. This paper explores the accuracy and inherent smoothing of turning‐ray tomography in its capacity to estimate absolute near‐surface velocity and the statics times derived from these velocities, and the fidelity with which wavefields collapse to point diffractors when migrated through these estimated velocities. The method comprises nonlinear iterations of forward ray tracing through triangular cells linear in slowness squared, coupled with the LSQR linear inversion algorithm. It is applied to two synthetic finite‐ difference data sets of types that usually foil conventional refraction statics techniques. These models represent a complex hard‐rock overthrust structure with a low‐velocity zone and pinchouts, and a contemporaneous near‐shore marine trench filled with low‐ velocity unconsolidated deposits exhibiting no seismically apparent internal structure. In both cases velocities are estimated accurately to a depth of one‐ fifth the maximum offset, as are the associated statics times. Of equal importance, the velocities are sufficiently accurate to correctly focus synthetic wavefields back to their initial point sources, so migration/datuming applications can also use these velocities. The method is applied to a real data example from the Timbalier Trench in the Gulf of Mexico, which exhibits the same essential features as the marine trench synthetic model. The Timbalier velocity inversion is geologically reasonable and yields long and short wavelength statics that improve the CMP gathers and stack and that correctly align reflections to known well markers. Turning‐ray tomography estimates near‐surface velocities accurately enough for the three purposes of lithology interpretation, statics calculations, and wavefield focusing for shallow migration and datuming.

3D seismic traveltime tomography imaging of the shallow subsurface at the C O2 SINK project site, Ketzin, Germany

Geophysics ◽  
2009 ◽  
Vol 74 (1) ◽  
pp. G1-G15 ◽  
Author(s):  
Sawasdee Yordkayhun ◽  
Ari Tryggvason ◽  
Ben Norden ◽  
Christopher Juhlin ◽  
Björn Bergman
Keyword(s):  
Velocity Structure ◽  
Velocity Model ◽  
Geological Structure ◽  
Seismic Survey ◽  
Storage Site ◽  
Near Surface ◽  
Shallow Subsurface ◽  
Traveltime Tomography ◽  
Reflection Seismic ◽  
Reflection Data

A 3D reflection seismic survey was performed in 2005 at the Ketzin carbon dioxide [Formula: see text] pilot geological-storage site (the [Formula: see text] project) near Berlin, Germany, to image the geological structure of the site to depths of about [Formula: see text]. Because of the acquisition geometry, frequency limitations of the source, and artefacts of the data processing, detailed structures shallower than about [Formula: see text] were unclear. To obtain structural images of the shallow subsurface, we applied 3D traveltime tomography to data near the top of the Ketzin anticline, where faulting is present. Understanding the shallow subsurface structure is important for long-term monitoring aspects of the project after [Formula: see text] has been injected into a saline aquifer at about [Formula: see text] depth. We used a 3D traveltime tomography algorithm based on a combination ofsolving for 3D velocity structure and static corrections in the inversion process to account for artefacts in the velocity structure because of smearing effects from the unconsolidated cover. The resulting velocity model shows low velocities of [Formula: see text] in the uppermost shallow subsurface of the study area. The velocity reaches about [Formula: see text] at a depth of [Formula: see text]. This coincides approximately with the boundary between Quaternary units, which contain the near-surface freshwater reservoir and the Tertiary clay aquitard. Correlation of tomographic images with a similarity attribute slice at [Formula: see text] (about [Formula: see text] depth) indicates that at least one east-west striking fault zone observed in the reflection data might extend into the Tertiary unit. The more detailed images of the shallow subsurface from this study provided valuable information on this potentially risky area.

scholarly journals Reflexně seismický výzkum pozdně kenozoické zlomové tektoniky na vybraných lokalitách hornomoravského úvalu

2020 ◽  
Vol 27 (1-2) ◽  
Author(s):  
Ondřej Bábek ◽  
Zuzana Lenďáková ◽  
Tamás Tóth ◽  
Daniel Šimíček ◽  
Ondřej Koukal
Keyword(s):  
Low Cost ◽  
Seismic Facies ◽  
Gravity Gradients ◽  
Seismic Profiles ◽  
Low Velocity ◽  
Shallow Subsurface ◽  
Static Correction ◽  
Reflection Seismic ◽  
Weight Drop ◽  
Siliciclastic Sediments

We measured shallow reflection seismic profiles across the assumed faults in the Late Cenozoic (Pliocene – Holocene) Upper Morava Basin (UMB). The faults in the UMB are indicated by horst-and-graben morphology, differential thickness of Pliocene and Quaternary siliciclastic sediments, considerable gravity gradients a present-day seismicity. Four seismic lines, 380 to 860 m long (fixed geophone spread) were designed to cross the assumed faults at three sites, Mezice, Drahlov and Výšovice. The data were acquired by 24-channel ABEM Terraloc Mk-8 seismic system with PEG-40 accelerated weight drop source and processed by Sandmaier ReflexW and Halliburton Landmark ProMax® seismic processing software. The processing included application of filters (DC shift, scaled windowgain, bandpass frequency and muting), stacking using normal moveout constant velocity stack, additional application of subtrack-mean (dewow) filter, topographic correction and low velocity layer static correction. Distinct reflectors were detected up to 400 ms TWT, which corresponds to maximum depth of 280 and 350 m at 1400 and 1750 km.s-1 velocities, respectively. The observed reflection patterns were classified into three seismic facies, which were interpreted as crystalline rocks (Brunovistulicum) and/or well consolidated Paleozoic sedimentary rocks (SF1), unconsolidated Quaternary siliciclastic sediments (SF2) and semi-consolidated Neogene clays (SF3) based on the cores drilled in their close vicinity. Distinct faults were observed at the Drahlov and Výšovice 2 profile, which coincided with the observed topographic steps between the horsts and grabens. Presence of the fault at the Drahlov profile separating the Hněvotín Horst from the Lutín Graben was demonstrated by independent electrical resistivity tomography profile. On the other hand, another topographic step at the Mezice profile, between the Hněvotín Horst and Olomouc Graben, does not correspond to any seismic indication of a fault. The reflection seismic proved to be useful and relatively low-cost method to visualize the shallow subsurface geology in the Upper Morava Basin.

scholarly journals Structural analysis of S-wave seismics around an urban sinkhole: evidence of enhanced dissolution in a strike-slip fault zone

2017 ◽  
Vol 17 (12) ◽  
pp. 2335-2350 ◽  
Author(s):  
Sonja H. Wadas ◽  
David C. Tanner ◽  
Ulrich Polom ◽  
Charlotte M. Krawczyk
Keyword(s):  
Fault Zone ◽  
Fracture Network ◽  
Strike Slip ◽  
Strike Slip Fault ◽  
Normal Faults ◽  
S Wave ◽  
Seismic Profiles ◽  
Near Surface ◽  
Reflection Seismic ◽  
Enhanced Dissolution

Abstract. In November 2010, a large sinkhole opened up in the urban area of Schmalkalden, Germany. To determine the key factors which benefited the development of this collapse structure and therefore the dissolution, we carried out several shear-wave reflection-seismic profiles around the sinkhole. In the seismic sections we see evidence of the Mesozoic tectonic movement in the form of a NW–SE striking, dextral strike-slip fault, known as the Heßleser Fault, which faulted and fractured the subsurface below the town. The strike-slip faulting created a zone of small blocks ( < 100 m in size), around which steep-dipping normal faults, reverse faults and a dense fracture network serve as fluid pathways for the artesian-confined groundwater. The faults also acted as barriers for horizontal groundwater flow perpendicular to the fault planes. Instead groundwater flows along the faults which serve as conduits and forms cavities in the Permian deposits below ca. 60 m depth. Mass movements and the resulting cavities lead to the formation of sinkholes and dissolution-induced depressions. Since the processes are still ongoing, the occurrence of a new sinkhole cannot be ruled out. This case study demonstrates how S-wave seismics can characterize a sinkhole and, together with geological information, can be used to study the processes that result in sinkhole formation, such as a near-surface fault zone located in soluble rocks. The more complex the fault geometry and interaction between faults, the more prone an area is to sinkhole occurrence.

Multidisciplinary study of the hanging wall of the Kiirunavaara iron ore deposit, northern Sweden

Geophysics ◽  
2012 ◽  
Vol 77 (6) ◽  
pp. B269-B285 ◽  
Author(s):  
Mai-Britt Jensen ◽  
Artem Kashubin ◽  
Christopher Juhlin ◽  
Sten-Åke Elming
Keyword(s):  
Seismic Data ◽  
Hanging Wall ◽  
Low Velocity ◽  
Contact Zones ◽  
Near Surface ◽  
Reflection Seismic ◽  
Imaging Results ◽  
Fracture Development ◽  
Weakness Zones ◽  
The Town

Potential weakness zones due to mining-related fracture development under the town of Kiruna, Sweden, have been investigated by integration of seismic, gravity, and petrophysical data. Reflection seismic data were acquired along two subparallel 2D profiles within the residential area of the town. The profiles of [Formula: see text], each oriented approximately east–west, nearly perpendicular to the general strike of the local geology, crossed several contact zones between quartz-bearing porphyries, a sequence of interchanging sedimentary rocks (siltstone, sandstone, conglomerate, and agglomerate), and metabasalt. The resulting reflection seismic sections revealed a strong east-dipping reflectivity that is imaged down to approximately 1.5 km. The location and orientation of major features agree well between the profiles and with the surface geology and known contact zones between the different rock types. Our imaging results, supported by traveltime modelling, indicate that the contact zones dip 40°–50° to the east. The deepest and the weakest reflections are associated with a [Formula: see text] dipping structure that is presumably related to the Kiirunavaara iron mineralization. Tomographic inversion of refracted arrivals revealed a more detailed image of the velocity distribution in the upper 100–200 m along the profiles, enabling us to identify near-surface low velocity zones. These could be possible weakness zones developed along the lithological contacts and within the geologic units. The structural image obtained from the seismic data was used to constrain data inversion along a 28 km long east–northeast to west–southwest-oriented gravity profile. The resulting density model indicates that the quartz-bearing porphyry in the hanging wall of the Kiirunavaara mineralization can be separated into two blocks oriented parallel to the ore body. One block has an unexpected low density, which could be an indication of extensive fracturing and deformation.

scholarly journals High resolution reflection seismic profiling over the Tjellefonna fault in the Møre-Trøndelag Fault Complex, Norway

2012 ◽  
Vol 4 (1) ◽  
pp. 241-278 ◽  
Author(s):  
E. Lundberg ◽  
C. Juhlin ◽  
A. Nasuti
Keyword(s):  
P Wave ◽  
The South ◽  
Seismic Profiles ◽  
Surface Geometry ◽  
Secondary Fracture ◽  
Near Surface ◽  
North West ◽  
Reflection Seismic ◽  
North East ◽  
The North

Abstract. The Møre-Trøndelag Fault Complex (MTFC) is one of the most prominent fault zones of Norway, both onshore and offshore. In spite of its importance, very little is known of the deeper structure of the individual fault segments comprising the fault complex. Most seismic lines have been recorded offshore or focused on deeper structures. This paper presents results from two reflection seismic profiles, located on each side of the Tingvollfjord, acquired over the Tjellefonna fault in the south-eastern part of the MTFC. Possible kilometer scale vertical offsets reflecting, large scale north-west dipping normal faulting separating the high topography to the south-east from lower topography to the north-west have been proposed for the Tjellefonna fault. In this study, however, the Tjellefonna fault is interpreted to dip approximately 50–60° towards the south-east to depths of at least 1.4 km. Travel-time modeling of reflections associated with the fault was used to establish the geometry of the fault structure at depth and detailed analysis of first P-wave arrivals in shot-gathers together with resistivity profiles were used to define the near surface geometry of the fault zone. A continuation of the structure on the north-eastern side of the Tingvollfjord is suggested by correlation of an in strike direction P-S converted reflection (generated by a fracture zone) seen on the reflection data from that side of the Tingvollfjord. The reflection seismic data correlate well with resistivity profiles and recently published near surface geophysical data. A highly reflective package forming a gentle antiform structure was also identified on both seismic profiles. The structure may be an important boundary within the gneissic basement rocks of the Western Gneiss Region. The Fold Hinge Line is parallel with the Tjellefonna fault trace while the topographic lineament diverges, following secondary fracture zones towards north-east.

Mode misidentification in Rayleigh waves: Ellipticity as a cause and a cure

Geophysics ◽  
2013 ◽  
Vol 78 (4) ◽  
pp. EN17-EN28 ◽  
Author(s):  
Jacopo Boaga ◽  
Giorgio Cassiani ◽  
Claudio L. Strobbia ◽  
Giulio Vignoli
Keyword(s):  
Rayleigh Wave ◽  
Dispersion Curve ◽  
Velocity Structure ◽  
Vertical Component ◽  
Real Data ◽  
Smooth Transition ◽  
Correct Identification ◽  
Low Velocity ◽  
Low Frequencies ◽  
Near Surface

The surface wave method is a popular tool for geotechnical characterization because it supplies a cost-effective testing procedure capable of retrieving the shear wave velocity structure of the near-surface. Several acquisition and processing approaches have been developed to infer the Rayleigh wave dispersion curve which is then inverted. Typically, in active testing, single-component vertical receivers are used. In most cases, the inversion is carried out assuming that the experimental dispersion curve corresponds to a single mode, mostly the fundamental Rayleigh mode, unless clear evidence dictates the existence of a more complex response, e.g., in presence of low-velocity layers and inversely dispersive sites. A correct identification of the modes is essential to avoid serious errors. Here we consider the typical case of higher-mode misidentification known as “osculation” (“kissing”), where the energy peak shifts at low frequencies from the fundamental to the first higher mode. This jump occurs, with a continuous smooth transition, around a well-defined frequency where the two modes get very close to each other. Osculation happens generally in presence of strong velocity contrasts, typically with a fast bedrock underlying loose sediments. The practical limitations of the acquired active data affect the spectral and modal resolution, making it often impossible to identify the presence of two modes. In some cases, modes have a very close root and cannot be separated at the osculation point. In such cases, mode misidentification can create a large overestimation of the bedrock velocity and a large error on its depth. We examine the subsoil conditions that can generate this unwanted condition, and the common field acquisition procedures that can contribute to producing data having such deceptive Rayleigh dispersion characteristics. This mode misidentification depends strongly on the usual approach of measuring only the vertical component of ground motion, as the mode osculation is linked to the Rayleigh wave ellipticity polarization, and therefore we conclude that multicomponent data, using also horizontal receivers, can help discern the multimodal nature of surface waves. Finally, we introduce a priori detectors of subsoil conditions, based on passive microtremor measurements, that can act as warnings against the presence of mode osculation, and relate these detectors to the frequencies at which dispersion curves can be misidentified. Theoretical results are confirmed by real data acquisition tests.

scholarly journals Local CMP stacking improvement technique for better traceability of separate reflection horizon

2021 ◽  
Author(s):  
А.А. Дробинский ◽  
О.А. Жуковская
Keyword(s):  
Software Development ◽  
Seismic Data ◽  
Model Building ◽  
Synthetic Data ◽  
Signal Amplitude ◽  
Fine Tuning ◽  
Near Surface ◽  
Reflection Seismic ◽  
Geological Situation ◽  
High Flexibility

В последние годы всё больше объектов сейсморазведочных работ относится к проблемным территориям, характеризующимся неблагоприятными поверхностными условиями и сложной геологической обстановкой. Получение качественных сейсмических изображений при обработке в таких случаях входит в число приоритетных направлений современной сейсморазведки. Одним из путей решения проблем ухудшения прослеживаемости сейсмических горизонтов в сложных условиях является оптимизированное суммирование общей средней точки (ОСТ), учитывающее качество входных сейсмических данных. Цель исследования. Настоящая работа посвящена созданию и тестированию гибкой, универсальной методики оптимизации суммирования ОСТ на конечной стадии полевой или камеральной обработки сейсмических данных метода отражённых волн общей глубинной точки (МОВ-ОГТ 2D/3D) для улучшения прослеживаемости отражающих горизонтов. При создании такой методики важным требованием являлась возможность реализации в существующем программном обеспечении (ПО), в том числе, отечественном. Методы исследования. Для исследования влияния сложных геологических объектов на распределение энергии в выборках ОСТ было выполнено построение иллюминационной модели по целевому горизонту, расположенному под эрозионным врезом. Оценка влияния рассеивающих аномалий верхней части разреза (ВЧР) проводилась с помощью двумерного лучевого моделирования с получение синтетических сейсмограмм ОПВ по горизонтально-слоистому модельному разрезу, содержащему участок палеокарста. Чтобы оценить потенциал применения предлагаемой методики были генерированы синтетические данные, содержащие сильные помехи различной природы, а также зону падения амплитуды полезного сигнала. По этим данным были разными способами получены и оценены суммарные трассы. Предлагаемая методика также была опробована на реальных данных метода общей глубинной точки (МОГТ-2D). Оценка результатов работы различных вариантов суммирования выполнялась визуально, а также количественно (с помощью атрибутного анализа). Результаты работы. Выполненное исследование показало недостаточную эффективность стандартного суммирования ОСТ для сложных сейсмических данных. Польза от применения существующих методик улучшения суммирования ОСТ очевидна, но они имеют недостатки: нарушение естественной динамики волновой картины, невозможность локального применения, необходимость реализации в специальном ПО. Предлагаемая авторами методика оптимизации суммирования даёт высокую гибкость и маневренность работы и позволяет справиться с вышеуказанными проблемами. Свободный выбор критериев взвешивания интервалов трасс на основе анализа пользовательского набора атрибутов открывает широкие возможности тонкой настройки процедуры, вводит интерпретационную составляющую в процесс оптимизации суммирования, делая его более осмысленным в геолого-геофизическом отношении. Предлагаемая методика не требует написания и опробования нового специального ПО и может быть реализована в уже имеющихся сейсмических пакетах, включая и российские программные комплексы Today increasingly more objects of prospecting seismology belong to problem areas characterized by unfavorable surface conditions and complex geological situation. Acquiring of high-quality seismic images by processing in these cases is a part of priority directions of modern prospecting seismology. One of the way to overcome the problem of seismic horizon traceability worsening in hard conditions is optimized CMP stacking, considering the quality of input seismic data. Aim. This work is devoted to generation and examination of flexible universal technique of optimized stacking on the last stage of field or final processing of 2D/3D reflection seismic data for seismic horizon traceability improvement. Creating this technique assumed important condition of embodiment ability in existing software (including Russian). Methods. Illumination model building was performed for target horizon, located beneath the erosive cut for studying of complex geological objects influence on energy distribution in CMP gathers. Scattering superficial anomalies influence was estimated by means of 2D ray tracing and synthetic shot records generation on horizontally layered model sectionconsisting near surface ancient karst spot. Synthetic data with different kinds of noise and signal amplitude decay zone war generated to appreciate implementation potential of introducing technique. Stacked traces were obtained and evaluated on this data with different methods. The introduced technique was tested on real 2D seismic data too. Evaluation of results of different kinds of stacking was performed by sight and with quantitative (attribute) analysis. Results. Performed research showed insufficient efficiency convenience CMP stacking for complex seismic data. The advantages of existing CMP stacking improvement methods are obvious but there are drawbacks too: natural wave field dynamic violation, disability of local implementation, need of special software development. Offered technique of stacking optimization gives high flexibility and mobility in work and allow overcoming the aforementioned problems. Easy choice of trace range weighting criteria based on customer attribute set analysis gives wide opportunities of fine-tuning for this procedure, bringing in interpretation term of stack optimization process and making it more sensible in geological-geophysical relation. This technique need not new software development and testing, it could be embodied in existing seismic software suites, including Russian complexes

Refractor imaging using an automated wavefront reconstruction method

Geophysics ◽  
1992 ◽  
Vol 57 (3) ◽  
pp. 378-385 ◽  
Author(s):  
David F. Aldridge ◽  
Douglas W. Oldenburg
Keyword(s):  
Velocity Structure ◽  
Directional Derivative ◽  
Seismic Refraction ◽  
Synthetic Data ◽  
Reconstruction Method ◽  
Borehole Data ◽  
Data Set ◽  
Near Surface ◽  
Arrival Times ◽  
Subsequent Step

The classical wavefront method for interpreting seismic refraction arrival times is implemented on a digital computer. Modern finite‐difference propagation algorithms are used to downward continue recorded refraction arrival times through a near‐surface heterogeneous velocity structure. Two such subsurface traveltime fields need to be reconstructed from the arrivals observed on a forward and reverse geophone spread. The locus of a shallow refracting horizon is then defined by a simple imaging condition involving the reciprocal time (the traveltime between source positions at either end of the spread). Refractor velocity is estimated in a subsequent step by calculating the directional derivative of the reconstructed subsurface wavefronts along the imaged interface. The principle limitation of the technique arises from imprecise knowledge of the overburden velocity distribution. This velocity information must be obtained from uphole times, direct and reflected arrivals, shallow refractions, and borehole data. Analysis of synthetic data examples indicates that the technique can accurately image both synclinal and anticlinal structures. Finally, the method is tested, apparently successfully, on a shallow refraction data‐set acquired at an archeological site in western Crete.

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