Mapping an aquitard breach using shear-wave seismic reflection

In multi-layered hydrostratigraphic systems, aquitard breaches caused by faulting or paleo-erosion can allow substantial quantities of water of differing quality to be exchanged between aquifers. Seismic reflection technology was used to map the extent and orientation of an aquitard breach connecting a shallow alluvial aquifer to the deeper semi-confined Memphis aquifer in southwestern Tennessee, USA. Geophysical well logs indicate the presence of the aquitard at borehole locations that define the beginning and end points on two seismic survey lines, which intersect at a borehole where the aquitard is absent. A SE–NW-oriented paleochannel, 350 m wide and approximately 35–40 m deep, is interpreted from the seismic reflection surveys. The paleochannel cuts through the aquitard and into the upper part of the Memphis aquifer, thus creating a hydraulic connection between the shallow unconfined and deeper, semi-confined aquifers. The results indicate the potential of the shear-wave seismic reflection methods to resolve shallow breaches through fine-grained aquitards given availability of sufficient well control.     

反射地震成像技术在新疆喀拉通克铜镍矿区的应用

新疆喀拉通克铜镍矿自1989年建成投产,已生产27年,属中度危机矿山。为探测该矿山深部资源,需要提供地质构造和隐伏含矿岩体的信息,为此开展了地震数值模拟和高精度反射地震研究。地震正演模拟结果为该区金属矿地震数据采集观测系统设计和实际地震资料的解释提供了依据,地震勘探获得的反射地震剖面分辨率高,成像清晰,能够对测区深部地质结构和构造及隐伏岩体进行可靠解释。地震剖面的地质解释得到了钻探验证,证实了深部含矿岩体的存在。该地震探测结果为在矿山寻找深部隐伏矿指提供了线索。     

High-resolution reflection seismic imaging of the upper crust at Laxemar, southeastern Sweden

A major cost in exploring the upper 1–2 km of crystalline crust with reflection seismics is the drilling required for explosive sources. By reducing the charge size to a minimum, shallow inexpensive shotholes can be drilled with handheld equipment. Here, we present results from a full-scale test using small charges for high-resolution seismic surveying over a nuclear waste disposal study site (not an actual site). Two 2–2.5-km-long crossing profiles were acquired in December 1999 with 10-m shot and geophone spacing in the Laxemar area, near Oskarshamn in southeastern Sweden. After standard processing, including dip moveout (DMO), several subhorizontal to moderately dipping reflections are imaged. Many of the dipping ones can be correlated to fracture zones observed in a ca. 1700-m-deep borehole where the profiles cross and/or to fracture zones mapped on the surface. The imaged fracture zones form a complex 3D pattern illustrating the necessity of having 3D control before interpreting seismic reflection data. Analyses of sonic and density logs from the borehole show that greenstones have significantly higher impedances than the more dominant granite found in the borehole (granite/greenstone reflection coefficient is +0.065). These greenstones may contribute to the reflectivity when associated with fracture zones. In some cases, where they are present as larger subhorizontal lenses, they may be the dominant source of reflectivity. A set of north-dipping (10°) reflectors at 3–3.5-km depth can be correlated to a similar set observed below the island of Ävrö about 3 km to the east.     

藏北羌塘盆地反射地震剖面与叠加速度研究

本文主要阐述藏北羌塘盆地龙尾错区块的二维反射地震试验的数据采集和处理的过程.分析两条剖面的反射特征,并对叠加剖面进行了初步解释:羌塘盆地中央隆起北侧存在大规模的深凹陷构造;盆地浅部不同方向的构造变形存在较大差异;出现较强的基底反射特征;中生代、古生代地层的反射特征差异较大.文中尝试对数据处理过程中的叠加速度变化进行研究,获得了与叠加剖面较一致的结果,并进一步对某探井的构造背景提出见解.     

A study of radio sources using interplanetary scintillations at 111 MHz. Core-dominated sources

A complete sample of core-dominated radio sources has been studied using the interplanetary-scintillation method. In total, 72 sources were observed, with scintillations detected in 28 of them. The remaining sources have upper limits on their flux densities. Integrated flux densities are estimated for 24 sources. Cut-offs have been observed in the spectra of many sources. The thermal-electron densities have been estimated, assuming that these cut-offs are due to free-free absorption of the synchrotron radio emission.     

Investigation of the weathering layer using seismic refraction and high-resolution seismic reflection methods, NE of Riyadh city

Five seismic refraction and five high-resolution seismic reflection (HRSR) profiles were carried out in northeastern part of Riyadh city to investigate depth of the weathering layer. Results obtained from seismic refraction survey reveal the depths of weathering layer at 12, 25, 17, 12, and 16?m, respectively. On the other hand, HRSR stack sections illustrate the depths of weathering layer at 14, 28, 20, 13, and 18?m, respectively. The weathering layer is composed of alluvial sediments and gravel, which is underlain by a sequence of limestone and dolomite layer. Seismic results from site no. 2 have been found to be in good agreement with lithological information reported from the adjacent water well. The HRSR data generally reveal better signal-to-noise ratio and enhanced resolution compared to the refraction data. Although, the HRSR data failed in achieving high-quality common midpoint (CMP) stacking profile at site no. 3, it provide an improved image of the subsurface features than the refraction data, recognizing it as a potential seismic technique.     

A microearthquake survey of Bouvet Island, South Atlantic

A microearthquake survey carried out during the 1978–1979 Norwegian Antarctic Research Expedition indicates that only minor seismic activity is associated with the Bouvet volcano.     

一种地震共反射点道集数据的叠前反演方法

叠前地震资料含有地层的纵波、横波速度和密度等信息。利用叠前反演获得隐藏在地震数据中的这些基本参数后,即可揭示大量岩性及孔隙流体性质的信息。这里推导了平面波在层状弹性介质中传播的正演算子,提出了一种基于Brent正交搜索方向组的叠前三参数反演方法．该方法不需要求解庞大而复杂的导数矩阵。通过自适应退火因子和罚函数来处理约束条件,提高了算法的稳定性。将K—L变换引入到方向置换过程,有效防止了搜索方向组的线性相关。经理论模型和油田实际数据的反演结果表明,该反演方法是一种利用叠前地震数据进行储层预测的有效手段。     

Active tectonics of the Yizre'el valley, Israel, using high-resolution seismic reflection data

We present a series of high-resolution seismic reflection lines across the Yizre'el valley, which is the largest active depression in Israel, off the main trend of the Dead Sea rift. The new seismic reflection data is of excellent quality and shows that the valley is dissected into numerous small blocks, separated by active faults. The Yizre'el valley is found to consist of a series of half grabens, rather than a single half graben, or a symmetrical graben. The faults are generally vertical and appear to have a dominant strike-slip component, but some dip-slip is also evident. A marked zone of compression near Megido is associated with the intersection of the two largest faults in the valley, the Carmel fault and the Gideon fault. Variable trend of the faults reflects the complexity of the local geology along the boundary between the wide NW–SE trending Farah–Carmel fault zone and the E–W trending basins and ranges in the Lower Galilee. This tectonic complexity is likely to result from a highly variable stress pattern, modified by the structures inside it. Normal faulting in the valley occurred at an early stage of its development as a tectonic depression. However, strike-slip motion on the Carmel fault, and possibly also on some of the other faults, appears to have started together with the onset of normal faulting. Earthquake hazard in the area appears to be uniform as faults are distributed throughout the Yizre'el valley.     

A microearthquake survey in the Abu-Simbel area in Egypt

From the middle of January to the middle of March, 1981 a microearthquake survey was conducted jointly by Polish and Egyptian seismologists in the Abu-Simbel area, about 230 km upstream from the Aswan High Dam. A small seismic array composed of six stations and a magnetic-tape digital recording system was operated during the survey. Altogether 63 microearthquakes at epicentral distances of up to 50 km and 12 local earthquakes at distances of up to 200 km were recorded, but only 16 microearthquakes could be properly located. All of them originated either directly under the High Dam Reservoir or close to its shores, thus implying their association with its water level fluctuations.It was deduced that under the Abu-Simbel array a deep fracture zone or deep fault extending in the NNE—SSW direction, 1–3 km wide and several kilometers deep, must be present to explain the observed effects of heavy attenuation and scattering of seismic waves under the array. This fracture zone might be a deep expression of the western edge of the Nile Valley in the Abu-Simbel area.     

Regionalization of seismic hazard in Greece based on seismic sources

A semi-probabilistic approach to the seismic hazard assessment of Greece is presented. For this reason, a recent seismotectonic model for shallow and intermediate depth earthquake sources, based on historical as well as on instrumental data, was used. Different attenuation formulae were proposed for the macroseismic intensity and the strong ground motion parameters for the shallow and the intermediate focal depth shocks. The data were elaborated in terms of McGuire's computer program, which is based on the Cornell's method.A grid of equally spaced points at 20 km distance was made and the seismic hazard recurrence curves for various parameters of the seismic intensity was estimated for each point. Finally, seismic hazard maps for the area of Greece were compiled utilizing the entire range of recurrence curves. These maps depict areas of equal seismic hazard and for every area the analytical relations of the typeSI =f(Tm), whereSI is a seismic intensity parameter andTm is the mean return period, were determined.     

Fractals in deep crustal seismic reflection profiles

Application of a syntactic pattern recognition technique, seismic skeletonization, to deep crustal seismic reflection data allows attributes such as energies, lengths and dips to be associated with individual reflection events. Some of these attributes exhibit fractal properties, e.g. the relationship between seismic event lengths and their spatial distribution throughout the crust. This approach provides a new technique to analyse complex geometry on seismic reflection data.Dedicated to Professor William George Laidlaw on his SIXTIETH birthday     

浅层高分辨率反射地震试验研究：以雄安新区第四系为例

雄安新区城市规划建设对地下空间的高精度勘探提出了高要求,作为具有高分辨率的地面物探方法,反射波地震勘探技术具有良好应用前景。本文探究了厚覆盖区条件下的高精度有效反射地震探测技术,并利用反射纵波技术在雄安新区进行提高浅层分辨率的地震勘探方法试验。根据实际探测目标,通过对比分析不同检波器、震源类型、道距大小及接收排列长度等不同采集因素试验效果,探究已有条件下适宜的震源类型和最佳采集参数。结合实际资料处理和分层解释,试验研究表明,采用高频检波器、选择可控震源激发效果最佳,针对试验目标,结合小道距、适当排列接收等措施,可有效提高厚覆盖区50~300 m深度内反射纵波地震勘探的纵向分辨率,试验获得了丰富的有效波组信息,为内部地层精细划分和解释提供了依据。该方法可为类似环境下城市地下空间的精细探测提供有益参考借鉴。     

Landslide characterization using active and passive seismic imaging techniques: a case study from Kerala,India

The unusually intense precipitations of the 2018 monsoon triggered numerous landslides in the Western Ghats region, southwest of India. Although the landslides caused no casualties, significant damage to property and infrastructure was observed. We present, as a case study, the results of active and passive seismic prospecting at two of those landslides with the goal of characterizing them, in a first application of shallow seismic exploration to landslides in the region. Our deployments included both sites perturbed by the landslides and unperturbed slopes adjacent to them with the purpose of identifying possible structural differences between slopes that underwent landsliding from slopes that were not affected. We analyze seismic sections obtained using the multi-channel analysis of surface waves technique and compare the results with seismic noise analyzed using seismic interferometry. We show that different analyses give similar results. The lateral variations observed in the shear-wave velocity distribution below the different profiles are well correlated with dominant frequency determined from seismic noise horizontal-to-vertical spectral ratios. Our measurements were taken after the landslides occurred. In hindsight, our results suggest that soil thickness played a major role in the triggering of landslides.

     

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.