Modeling sideswipe in 2D oceanic seismic surveys from sonar data: Application to the Mariana arc

In two-dimensional (2D) marine seismic-reflection surveys, out-of-plane rough seafloor bathymetry can cause multiple ocean-bottom reflections that complicate the interpretation of shallow reflections. Although migration corrects for the in-plane position of reflectors, it cannot resolve the inherent ambiguity in their out-of-plane positions. We show how swath bathymetry, routinely collected in many such surveys, can be used to model out-of-plane seafloor reflections and prevent their misinterpretation as subsurface geology. We use both raw and gridded multi-beam bathymetry data to build images that represent seafloor reflections in migrated seismic data. Comparison of these images to the seismic sections reveals whether suspicious features are out-of-plane water bottom reflections or subsurface reflections. Multi-channel seismic surveys across the Marianas intra-oceanic arc system provide examples where rough seafloor topography produced reflections that were initially misinterpreted. We use our seafloor reflection modeling (SRM) approach to help distinguish a possible landslide from a volcanic cone, to help distinguish real from apparent fault-plane reflections bounding a sediment-filled basin, and to verify that a possible magma chamber reflection results from sub-surface structure, not seafloor sideswipe.     

Shallow seismic reflection profiles and geological structure in the Benton Hills, southeast Missouri

During late May and early June of 1993, we conducted two shallow, high-resolution seismic reflection surveys (Mini-Sosie method) across the southern escarpment of the Benton Hills segment of Crowleys Ridge. The reflection profiles imaged numerous post-late Cretaceous faults and folds. We believe these faults may represent a significant earthquake source zone.

The stratigraphy of the Benton Hills consists of a thin, less than about 130 m, sequence of mostly unconsolidated Cretaceous, Tertiary and Quaternary sediments which uncomfortably overlie a much thicker section of Paleozoic carbonate rocks. The survey did not resolve reflectors within the upper 75–100 ms of two-way travel time (about 60–100 m), which would include all of the Tertiary and Quaternary and most of the Cretaceous. However, the Paleozoic-Cretaceous unconformity (Pz) produced an excellent reflection, and locally a shallower reflector within the Cretaceous (K) was resolved. No coherent reflections below about 200 ms of two-way travel time were identified.

Numerous faults and folds, which clearly offset the Paleozoic-Cretaceous unconformity reflector, were imaged on both seismic reflection profiles. Many structures imaged by the reflection data are coincident with the surface mapped locations of faults within the Cretaceous and Tertiary succession. Two locations show important structures that are clearly complex fault zones. The English Hill fault zone, striking N30°–35°E, is present along Line 1 and is important because earlier workers indicated it has Pleistocene Loess faulted against Eocene sands. The Commerce fault zone striking N50°E, overlies a major regional basement geophysical lineament, and is present on both seismic lines at the southern margin of the escarpment.

The fault zones imaged by these surveys are 30 km from the area of intense microseismicity in the New Madrid seismic zone (NMSZ). If these are northeast and north-northeast oriented fault zones like those at Thebes Gap they are favorably oriented in the modern stress field to be reactivated as right-lateral strike slip faults. Currently, earthquake hazards assessments are most dependent upon historical seismicity, and there are little geological data available to evaluate the earthquake potential of fault zones outside of the NMSZ. We anticipate that future studies will provide evidence that seismicity has migrated between fault zones well beyond the middle Mississippi Valley. The potential earthquake hazards represented by faults outside the NMSZ may be significant.     

Study of subsurface features using seismic reflection data in Erbil area,Kurdistan Region,Iraq

The present study seeks to interpret seismic reflection data for an area measuring approximately about 1,450 km² within the Erbil plain in the Kurdistan Region of Iraq to produce subsurface geological pictures. Thirteen seismic lines were identified by the Iraqi Oil Exploration Company using dynamite sources and then processed; then, a seismic line, BH-15, of around 432.5 km total length was used in this study. Three horizons (reflectors) were selected and identified by tying them directly to Demir Dagh well no. 1, these are H1, a reflector that represents the near top of the Lower Fars Formation; H2, a reflector that represents the top of the Pila Spi Formation; and H3, a reflector that represents the top of the Shiranish Formation. The qualities of the reflectors range between fair and good. All the isochrone and depth maps and 3D view pictures of the reflectors show the same main subsurface structural features trending in a NW–SE direction, namely, Erbil trough, Sherawa trough, and the southeastern plunge of the Demir Dagh anticline. Ninety-five normal and reverse-type faults trending in a NW–SE direction were detected. Also, the maps show that the reflectors tend to increase in dipping and deepening towards the southeastern and eastern parts.     

Detection of Miocene saucer‐shaped sills (offshore Senegal) via integrated interpretation of seismic,magnetic and gravity data

The Atlantic margin offshore Senegal has been explored by seismic reflection and GRAV‐MAG surveys. High‐amplitude, laterally transgressive seismic reflectors are found to coincide with gravimetric and magnetic highs. Once seismic data are integrated with potential fields modelling, these reflectors can be safely interpreted as saucer‐shaped igneous sills, up to some hundreds of metres thick, some km wide. The occurrence of hydrothermal vent complexes and forced folds in the stratigraphic sequence above the sills constrain the intrusion age to the Miocene. Field observations and in‐situ magnetic susceptibility measurements of Oligocene–Miocene and Quaternary igneous rocks emplaced in coastal Senegal support this interpretation.     

用双椭圆方法确定反射点位置

在反射波地震勘探中使用动校正确定反射点位置用到很多假设,如勘探深度要远远大于炮检距;假定倾斜反射界面的反射点与水平反射界面反射点都位于炮检距的中点;近似认为倾斜反射界面的动校正量等于水平反射界面的动校正量;假定反射面倾角固定且较小等,上述假设一定会造成勘探误差。由于反射点位置和反射面倾角未知,所以理论上无法唯一地确定反射点位置。如果反射波传播的介质的波速一定,从炮点发出的地震波,经反射点后,在接收点被接收,其可能的反射点是椭圆的一部分,但还不能唯一确定反射点;再取炮点和另外一个接收点,其可能的反射点是另外一个椭圆的一部分。如果假定反射面是平面,可以是水平面,也可以是有固定倾角的倾斜平面,该平面在地震波射线平面内是一条直线,该直线一定是两椭圆的公切线。把两椭圆方程和切线方程联立,就可以求解出公切点位置,公切点位置就是反射点位置,这就是用双椭圆确定反射点位置的方法。通过建立模型对勘探方法进行了检验,证实了用双椭圆方法确定反射点位置的有效性。双椭圆方法有一个重要的副产品,就是在确定反射面位置的同时计算出反射面的视倾角。     

用双椭圆方法确定反射点位置

在反射波地震勘探中使用动校正确定反射点位置用到很多假设,如勘探深度要远远大于炮检距;假定倾斜反射界面的反射点与水平反射界面反射点都位于炮检距的中点;近似认为倾斜反射界面的动校正量等于水平反射界面的动校正量;假定反射面倾角固定且较小等,上述假设一定会造成勘探误差。由于反射点位置和反射面倾角未知,所以理论上无法唯一地确定反射点位置。如果反射波传播的介质的波速一定,从炮点发出的地震波,经反射点后,在接收点被接收,其可能的反射点是椭圆的一部分,但还不能唯一确定反射点;再取炮点和另外一个接收点,其可能的反射点是另外一个椭圆的一部分。如果假定反射面是平面,可以是水平面,也可以是有固定倾角的倾斜平面,该平面在地震波射线平面内是一条直线,该直线一定是两椭圆的公切线。把两椭圆方程和切线方程联立,就可以求解出公切点位置,公切点位置就是反射点位置,这就是用双椭圆确定反射点位置的方法。通过建立模型对勘探方法进行了检验,证实了用双椭圆方法确定反射点位置的有效性。双椭圆方法有一个重要的副产品,就是在确定反射面位置的同时计算出反射面的视倾角。     

中国大陆科学钻探孔区三维地震资料的初步解释

利用中国大陆科学钻探岩心、测井与VSP资料来标定地震反射体，大大提高了大陆科学钻探孔区地质构造解释的可信度。超高压变质带地壳波速特征主要表现在上地壳顶部的高速层，其厚度一般小于10km；其下方正常的中下地壳波速结构反映了中新生代地壳的拉张与伸展。孔区三维地震拱形反射由榴辉岩及切穿它们的晚期破碎带共同形成的。在CCSD主孔东南方深部重力高部位有多个这样的拱形反射，预示在地表陡倾榴辉岩的延伸方向还有多个一定规模的隐伏榴辉岩体。孔区广泛发育韧性剪切带，韧性剪切带产生强反射的主要机制是韧性剪切复合岩套，由糜棱岩和经剪切错动产生的表壳岩片互层组成。三维地震调查提供了三维的反射地震波场，使揭示孔区三维的地质构造成为可能。450ms以上的时间切片与地袁地质构造和地磁异常相关，而1000～1200ms的时间切片与深部重力异常相关，主要反映由隐伏榴辉岩体产生的反射波场．根据上述研究结果，编制了沿三维地震主剖面的推断地质构造图。     

Rheological implications of the strong seismic reflector in the Kakkonda geothermal field, Japan

We re-processed the seismic reflection survey data of the Kakkonda geothermal field. The pre-stack migration delineates a strong and continuous reflector between 1800- and 2800-m depth, below which formations are not reflective. Earthquake data exhibit seismicity in the upper crust. The lower boundary of seismogenic layer is interpreted as the brittle–ductile transition. The thermal structure is thought to be the major factor controlling its depth. We compared the strong reflector with the thermal and rheological structure from drillholes. The depth of the reflector corresponds to the top of the highly–very highly fractured zone observed from formation microscanner imagery (FMI) logging in the Miocene formations. The density of fracture in the Kakkonda granite is very low, suggesting that granite corresponds to the nonreflective zone. The temperature–depth profile of well WD-1a shows that the temperature at the highly–very highly fractured zone is about 350 °C. This corresponds to a hydrothermal convection zone filled with two-phase geothermal fluid. The cut-off depth of seismicity that indicates the brittle–ductile transition lies at the isotherm of 300–350 °C near the reflector. We conclude that the strong seismic reflector is a strong contrast in acoustic impedance at the top of the fractured layer. The fractured layer could be a decoupling plane caused by different tectonic behaviors between the upper brittle and the lower ductile layers or a dehydration front by thermal diffusion. The similarity between the strong reflector and K-horizon, the strong reflector, found in southern Tuscany, Italy suggests that the P-wave reflector at the top of highly fractured zone at the brittle–ductile transition be common in areas with magmatic activity.     

A method for avoiding artifacts in the migration of deep seismic reflection data

Conventional wave-equation-based migration of deep seismic reflection data can produce severe artifacts, which appear as broad circular arcs or “smiles”, due to the existence of apparent truncations of reflections on the stack section arising from poor signal penetration, changes in orientation of the acquisition profile, and the existence of strong overlying lateral velocity variations. These artifacts limit the interpretation of deep seismic profiles, because they obscure weak reflections and reflection truncations that may, e.g., indicate the presence of subsurface faults. Here I present a new migration algorithm, in which each sample of the stack is migrated to a short linear segment whose position and dip are determined by its original position on the stack, an estimate of the local apparent dip at that point, and a user-specified migration velocity. No subjective interpretation of reflections on the stack section is required, and the algorithm produces no arc-like migration artifacts. The degree of lateral smearing can be easily controlled, allowing reflection truncations to be revealed. In practice, the algorithm is most effectively applied to data that have been coherency-filtered to remove low amplitude noise, which would otherwise be preserved.     

Dipping basement reflectors along volcanic passive margins—new insight using data from the Kerguelen Plateau

Multichannel seismic reflection data from the Southern Kerguelen Plateau show many dipping basement reflectors associated with volcanic flows. These reflectors are quite similar in their shape to seaward-dipping basement reflectors observed along volcanic passive margins. On the Kerguelen Plateau the sources are updip of the basement reflectors, in the presently extinct and eroded volcanoes. We suggest that the same source/reflector geometry may also apply to the seaward-dipping basement reflectors observed along passive margins. We interpret these reflectors to be the result of volcanism on the passive margin which flowed in all directions into the newly created ocean basin at an early spreading stage.     

Progress report on rhenish massif seismic experiment

This paper presents some data and results from a seismic refraction experiment, completed mainly in 1979 in the Rhenish Massif, Federal Republic of Germany and extending through Luxembourg and Belgium into the Paris Basin in France.Velocity-depth functions have been derived for each record section independently, based on the assumption that velocity varies only with depth: these models are being improved upon by time-term and ray-tracing methods capable of handling laterally varying velocity structures and by calculating synthetic seismograms.The P_g phase which is observed very clearly on all record sections represents a refracted wave, with velocity generally > 6 km/s, from depths below 1.5–5.5 km. Along the 600 km long main profile one intracrustal reflection can usually be recognized, while from the three shorter crossing profiles in the massif two intracrustal reflectors can always be seen. Beneath much of the main profile the crust-mantle boundary is either a first order discontinuity or thin (< 1 km) transition zone at ~30 km depth. However, beneath the Ardennes and West Eifel there is a 6–12 km thick transition zone before a velocity of 8.1 km/s is reached at ~36 km depth. Beneath the crossing profiles, there is generally a transition zone < 3 km thick between crust and mantle. In some cases, there can be recognized at the top of the mantle a thin high velocity layer which is underlain by a low velocity layer which, in turn, is underlain by a reflector 4–11 km below the crust-mantle boundary.     

NIZUSE: a deep seismic reflection line in north-eastern Germany

Out of a dense network of seismic reflection lines for hydrocarbon exploration in the North-east German Basin, several lines were recorded to 12 s TWT to obtain information about the structure of the crust and the crust-mantle transition. One of these profiles is presented here. This stretches for 110 km in a NNE direction between Neustrelitz and the island of Usedom. It reaches from the External Variscides in the south across the North German Massif into the Rügen-Pomorze Terrane in the Baltic Sea. Below Cenozoic-Mesozoic-Paleozoic cover with clear reflections down to base Zechstein, the reflectivity varies considerably with depth and also laterally. The Paleozoic and Precambrian sediments and basement are generally void of reflections, but the lower crust and the Moho show strong reflections. To the north the reflectivity decreases, and the Moho depth increases to beyond the bottom of the record section at 12 s. There are no direct indications for deep-reaching faults such as the Trans-European Fault in the north. The North German Massif acted as a ramp towards the Variscan Orogeny, similar to the London-Brabant Massif further west.     

用宽频带高分辨率地震分析河道砂体层序地层特征

松辽盆地油气勘探开发的主要目标是大型河流三角洲沉积的河道砂体,河道砂体单层厚度普遍较小,河道改道频繁,地层对比特征不明显,缺乏明显的对比标志。为用地震资料描述河道砂体,近几年我们研制出宽频带高分辨率地震技术,地震频带宽度达5~360 Hz,视主频达150 Hz,分辨率大幅提高。怎样以层序地层的观点用宽频带高分辨率地震描述河道砂体是一项迫切的任务。应用正演模型技术,分析了薄层河道砂体的宽频带高分辨率地震特征,认识到河道砂体两侧河床的角度不整合斜反射是识别河道砂体的重要层序地层标志。本文通过井震结合,解释了松辽盆地杏树岗3D地震工区宽频带高分辨率地震剖面,识别出单一河道和复合河道两种河道类型,分析了扶余油层上部宽频带高分辨率地震的层序地层特征,通过x70井扶余油层层序五下凹型河道砂体的剖面和平面特征分析,依据地震地层学的观点,认为该河道砂体具有网状河三角洲分流河道特征。     

共反射面叠加在实际地震资料处理中的应用

共反射面叠加方法运用3个地震波场参数来描述地下反射面元时距关系,不对地下反射界面形状做任何假设,并且是一种不依赖于宏观速度模型的反射波成像方法。共反射面叠加利用了邻近多个共反射点道集的相似性,提高了资料的覆盖次数,从而压制了随机噪声,增强反射波同相轴的连续性,大幅提高了地震资料的信噪比。通过在实际地震资料处理中的应用,证明了该方法的这些特性。     

Seismic velocities from the Yaquina forearc basin off Peru: evidence for free gas within the gas hydrate stability zone

Multichannel seismic (MCS) data from the Yaquina forearc basin off Peru reveal a complex distribution of gas and gas hydrate related reflections. Lateral variations of the reflection pattern at the assumed base of the gas hydrate stability zone in terms of continuity, amplitude, and signal attenuation underneath are observed, as well as the possible occurrence of paleo-bottom simulating reflectors (BSRs). Phase reversed reflections above the bottom simulating reflector point to free gas within the gas hydrate stability zone (GHSZ). To constrain the interpretation of the observed reflection pattern we calculated the velocity distribution along the MCS line from high-resolution ocean bottom hydrophone recordings with two independent methods. Heat flux values estimated on the basis of the velocity-depth functions increase with decreasing amplitude of the BSR and peak near chemoherms. These results suggest a model of the Yaquina Basin where free gas is trapped under parts of the BSR, and within the GHSZ, particularly under the seafloor and under an erosional unconformity. The hypothesis of a paleo-BSR that reflects the uplift of the base of the hydrate stability zone caused by the deposition of a particular sediment sequence is supported by the estimated heat flux values.     

叠前时间偏移成像技术及其应用

零炮检距剖面是地震反射成像过程中的重要成果。通过弯曲反射界面模型,分析了共中心点(CMP)道集与共反射点(CRP)道集的差别,认为当反射界面倾斜时,常规处理中的CMP叠加不能得到准确的零偏移距剖面,而沿CRP道集叠加的叠前时间偏移能得到准确的零偏移距剖面,另外,还介绍了叠前时间偏移的实现方法和步骤,包括预处理、偏移速度场的建立和偏移后CRP道集的处理等,并结合实际资料进行了效果分析。     

Topographic migration of GPR data: Examples from Chad and Mongolia

Most ground-penetrating radar (GPR) measurements are performed on nearly flat areas. If strongly dipping reflections and/or diffractions are present in the GPR data, a classical migration-processing step is needed in order to determine the geometries of shallow structures. Nevertheless, a standard migration routine is not suitable for GPR data collected on areas showing a variable and large topographic relief. To take into account topographic variations, the GPR data are, in general, corrected by applying static shifts instead of using an appropriate topographic migration that would place the reflectors at their correct locations with the right dip angle. In this article, we present an overview of Kirchhoff's migration and show the importance of topographic migration in the case where the depth of the target structures is of the same order as the relief variations. Examples of synthetic and real GPR data are shown to illustrate the efficiency of the topographic migration.     

Application of revised ray tracing migration to imagine lateral variations of seismic fabric corresponding to different tectonic styles in the northern Apennines

Migration of zero-offset seismic sections of deep crust can be done with methods based on ray tracing. We modify the classical ray tracing migration method (RTM), introducing a consistency check to control whether back-propagated rays satisfy the condition of strict normal incidence at the migrated reflector. A synthetic test shows the effectiveness of the method; in particular the control of normal incidence allows elimination of physically inconsistent reflectors from the migrated section. Then RTM is applied to a crustal seismic profile acquired in central Italy, using a velocity model obtained from wide-angle data that reproduces the gross structures of the Apenninic crust. The lateral variation of the seismic fabric shown from the migrated section reveals the presence of coexisting extensional and compressional tectonic regimes. Kinematic diffraction modelling gives additional information about both the distribution of seismic velocities and major active geodynamic processes in the upper lithosphere. The migrated section supports the subdivision of northern Apennines in two tectonic regions: a stretched upper plate (Tuscany and northern Thyrrenian), supported by a rise of the asthenosphere, and a downwarped lower plate (Adria), subducted below the mountain belt.     

中国大陆科学钻探孔区反射地震剖面的数值模拟与分析

本初步分析了苏鲁超高压变质带大陆科学钻探孔区的地震波速度结构，在此基础上对在东海孔区三维反射地震勘探过井孔的NNW-SSE主剖面和该周围地区的DH99二维反射地震剖面进行了数值模拟分析。结果表明，数值理论模拟结果与三维反射主剖面地震波到时吻合良好，模拟法能实现有效的时深转换，也表明数值模拟中采用的速度结构是合理的。东海钻井孔区实施反射地震勘测，所得地震记录剖面图是单一P波记录，在超高压变质岩区反射地震记录图中S波的影响甚微。DH99二维反射地震剖面的模拟结果表明东海孔区2000米深度以上的密度，波速结构不随深度加大而变化；而且苏鲁超高压变质岩区地表浅层的地震波速和密度较高，是高速高密地区。岩心结果反映的反射地震阻抗差大的地段，数值模拟的理论剖面上强反射面与孔区反射地震剖面图中的强反射体基本吻合．它意味着在非沉积层的结晶岩区，反射地震勘测手段是研究地下复杂结构的有效方法。数值模拟方法可以进一步提取地震反射记录信息，是利用地震反射方法研究地球构造的一种有效的方法。     

频率测深的拟地震解释法

电磁波和弹性波的复反射函数无论从数学表达形式上,还是从物理意义上都有一定的相似性。通过对拟地震解释法中反射系数q_n的求取,将频率测深曲线转换成v_p－t₀曲线及时间剖面,并对均匀介质及2层、3层、4层断面的理论模型和实例做了正反演计算。结果表明,频率测深拟地震解释法作为一种新的解释手段具有较好的应用前景。