Seismic characterization of an abandoned mine site

The near-surface rock structure that covers an abandoned marl mine nearby the village of Montevecchia (Italy) was investigated through a combination of seismic surveys. The methods selected for these investigations were refraction seismics and multichannel analysis of surface waves (MASW). A sort of transillumination experiment was also attempted. All the methods were successful and gave complementary information. Refraction seismics was used to characterize the upper low velocity layer and the second layer of the near-surface structure. The MASW method was necessary to assess the existence of a velocity inversion revealing the presence of a low velocity layer between the 2nd layer and a 4th high velocity layer covering the upper mine gallery. The transillumination experiment validated the presence of the 4th layer and gave an estimate of the average velocity that represents a lower boundary for the P-wave velocity within this layer. Both the refraction and transillumination data were analysed to derive average estimates of attenuation level and rock quality factor.     

An Attempt to Integrate Reflection Seismics and Balanced Profiles

—Different techniques in Geophysics and Geology are used to derive the structure of the subsurface. They are based on different data sets, i.e., seismic and geological data, and a combination of these techniques should produce better earth models. The case study presented in this paper is based on data of the German Continental Reflection program (DEKORP) collected in the Münsterland basin and the Rhenish Massif located at the northern border of the Rhenohercynian fold and thrust belt of the Mid-European Variscides. In this study we present an attempt to integrate balanced profiles, i.e., structural geology, and reflection seismics. The integration is performed by synthetically modelling seismic waves according to the acquisition of the field data, where the velocity model is based on the balanced profile. The synthetic data are compared with the field observations. Differences between observed data and field data are either caused by velocity errors in the model or by errors in the balanced profile. Criteria are developed to interpret these differences in order to improve the joint model of geologists and geophysicists. The case study presented in this paper shows that the combination of balanced profiles and reflection seismics may lead to shortcuts in the determination of seismic velocities of the subsurface. These shortcuts can reduce processing times and processing costs of reflection seismic data.     

云南思茅—中甸地震剖面的地壳结构

云南思茅—中甸宽角反射/折射地震剖面切割松潘—甘孜、扬子和华南三个构造单元的部分区域. 我们利用初至波和壳内反射波走时层析成像获得地壳纵波速度结构. 在获得新的地壳速度结构模型基础上，利用地震散射成像思想和低叠加次数的叠前深度偏移方法重建了研究区的地壳、上地幔反射结构. 综合分析研究区地壳P波速度模型和壳内地震反射剖面发现：沿测线从北至南地壳厚度从约50 km减薄至35 km左右，地壳厚度的减薄量主要体现在下地壳，剖面北段下地壳厚度约为30 km，剖面南段下地壳厚度仅为15 km左右；上地幔顶部局部位置P波速度值偏低，一般为76～78 km/s，反映出云南地区是典型的构造活动区的特点.剖面沿线地壳内地震反射发育，其中莫霍强反射出现在景云桥下方；在景云桥弧形断裂带8～10 km深处出现宽约50 km的强反射带.     

Combination of common-midpoint-refraction seismics with the generalized reciprocal method

A useful method for increasing the signal/noise ratio of refracted waves is Common-Midpoint (CMP)-refraction seismics. With this technique the shallow underground can be described in detail using all information (amplitude, frequency, phase characteristics) of the wavetrain following the first break (first-break phase). Thus, the layering can be determined and faults, weak zones, and clefts can be identified. This paper deals with the optimization of CMP-refraction seismics used in combination with the Generalized Reciprocal Method (GRM). Theoretical studies show a close relationship of both methods to the kinematics of wave propagation. Velocities and optimum offsets determined by the GRM can be used directly in the partial Radon transformation in CMP-refraction seismics. The integration of refracted waves leads to an increase in the signal/noise ratio but simultaneously the integration boundaries must be restricted to deal only with selective parts of the investigated refractor. The result of this process is an intercept-time section which can be converted directly to a depth section using standard refraction seismic techniques. Another possibility of depth conversion is the transformation of this intercept-time section to a `pseudo-zero-offset section', known from reflection seismics. Thus, zero-offset sections can be migrated using wave-equation techniques such as Kirchhoff migration.     

Imaging beneath an opaque basaltic layer using densely sampled wide-angle OBS data1

A combined reflection/refraction (wide-angle) seismic survey was conducted on the continental shelf north-west of Britain, using a conventional streamer with an airgun source, and static ocean-bottom seismometers (OBS) to record wide-angle energy. The shallow structure down to a basaltic layer was reasonably well imaged on the stacked reflection section. The basalts, however, proved to be opaque to the conventional reflection method and prevented the imaging of deeper horizons, where an important velocity inversion was anticipated. This paper reports on the processing, modelling and interpretation of the densely sampled wide-angle OBS data that were coincident with the reflection profile. Eleven OBS instruments were deployed along a 75 km line and recorded signal from a powerful 149 litre (9100 in.³) airgun array fired every 50 m. Data processing was performed using a standard industrial reflection seismic software package prior to first-arrival picking. Processing steps included geometry definition, trace summation and display of the data using various scaling algorithms. An initial model was constructed from 1D velocity-time profiles digitized every 4 km along the stacked section. First arrival traveltime modelling rapidly converged to a detailed model of the structure of the top 5 km of the crust. Modelling revealed the existence of a buried low-velocity Mesozoic sedimentary basin, of a prominent basement horst and of a normal fault penetrating to the basement.     

A brief study of applications of the generalized reciprocal method and of some limitations of the method

An analysis of the generalized reciprocal method (GRM), developed by Palmer for the interpretation of seismic refraction investigations, has been carried out. The aim of the present study is to evaluate the usefulness of the method for geotechnical investigations in connection with engineering projects. Practical application of the GRM is the main object of this study rather than the theoretical/mathematical aspects of the method. The studies are partly based on the models and field examples presented by Palmer. For comparison, some other refraction interpretation methods and techniques have been employed, namely the ABC method, the ABEM correction method, the mean‐minus‐T method and Hales' method. The comparisons showed that the results, i.e. the depths and velocities determined by Palmer, are partly incorrect due to some errors and misinterpretations when analysing the data from field examples. Due to the limitations of the GRM, some of which are mentioned here, stated by Palmer in his various publications, and other shortcomings of the method (e.g. the erasing of valuable information), the GRM must be regarded as being of limited use for detailed and accurate interpretations of refraction seismics for engineering purposes.     

The structure of the crust in TESZ area by kriging interpolation

A precise 3D model of the crust is necessary to start any tectonic or geodynamic interpretation. It is also essential for seismic interpretations of structures lying below as well as for correct analysis of shallow structures using reflection seismics. During the last decades, a number of wide-angle refraction experiments were performed on the territory of central and eastern Europe (POLONAISE’97, CELEBRATION 2000, SUDETES 2003), resulting in many high quality 2D models. It is an interesting and complicated transition zone between Precambrian and Palaeozoic Platforms. This paper presents 3D model of the velocity distribution in the crust and upper mantle interpolated from 2D models of the structure along 33 profiles. The obtained model extends to a depth of 50 km and accurately describes the main features of the crustal structures of Poland and surrounding areas. Different interpolation techniques (Kriging, linear) are compared to assure maximum precision. The final model with estimated uncertainty is an interesting reference of the area for other studies.     

用地震反射法对玉溪盆地普渡河断裂的探测

通过在玉溪盆地用地震反射法探测普渡河隐伏断裂的实例, 阐述了在覆盖层很厚、 人口密集、 环境干扰强条件下的地震勘探方法技术. 本次勘探针对场地条件使用了大型可控震源和小检波距、 长排列、 高覆盖次数的观测系统. 获得的高信噪比时间剖面清晰地揭示了玉溪盆地内普渡河断裂的产状和规模. 勘探结果表明, 玉溪盆地覆盖层(N+Q)最大厚度超过820 m; 基岩面西陡东缓, 呈北北东走向的箕状分布; 普渡河断裂分两支穿过玉溪盆地西部, 主断裂为正断性质, 走向约N20°E, 倾向东, 视倾角75°—80°; 断裂规模由南向北逐渐变大, 基岩断距在盆地南部的大营街附近为40 m, 而在盆地北部的九龙池东部, 基岩断距增加到280 m左右, 穿过玉溪盆地西部的普渡河断裂错断了新近系上新统中上部地层.      

P and S reflection and P refraction: An integration for characterising shallow subsurface

Characterization of shallow structures was performed by using different approaches analysing both P- and S-wave seismic data with different resolution. The refraction tomography provided P and S velocity models of the first 80 m, while the reflection seismic processing gives a reasonable stacking velocity field until 300 m depth for both P- and S-wave data. So, we estimated the V_p/V_s ratio and an empirical relationship between the two velocities. We characterised the shallow layers using tomographic velocity models and the deeper layers using seismic images with different resolution. The seismic images were obtained by conventional CMP reflection seismic processing and by a novel multi-refractor imaging technique.     

Surface and downhole shear wave seismic methods for thick soil site investigations

Shear wave velocity–depth information is required for predicting the ground motion response to earthquakes in areas where significant soil cover exists over firm bedrock. Rather than estimating this critical parameter, it can be reliably measured using a suite of surface (non-invasive) and downhole (invasive) seismic methods. Shear wave velocities from surface measurements can be obtained using SH refraction techniques. Array lengths as large as 1000 m and depth of penetration to 250 m have been achieved in some areas. High resolution shear wave reflection techniques utilizing the common midpoint method can delineate the overburden-bedrock surface as well as reflecting boundaries within the overburden. Reflection data can also be used to obtain direct estimates of fundamental site periods from shear wave reflections without the requirement of measuring average shear wave velocity and total thickness of unconsolidated overburden above the bedrock surface. Accurate measurements of vertical shear wave velocities can be obtained using a seismic cone penetrometer in soft sediments, or with a well-locked geophone array in a borehole. Examples from thick soil sites in Canada demonstrate the type of shear wave velocity information that can be obtained with these geophysical techniques, and show how these data can be used to provide a first look at predicted ground motion response for thick soil sites.     

REFLECTION-WINDOW MAPPING OF SHALLOW BEDROCK*

A field study was undertaken to evaluate the effectiveness of the high frequency seismic reflection technique for mapping of shallow and irregular bedrock. Bedrock reflections were obtained using a hammer source with both in-line and common offset field layouts. The recording equipment included 12-channel enhancement seismographs, 28 Hz vertical geo-phones and a microcomputer. The latter increased the overall versatility of the seismic system. Field sites for this study are typical of the geological settings of the tin mining areas of Malaysia. The topographical ‘lows’ of the irregular bedrock control the localization of tin ore. The subsurface geology consists of a thin low velocity layer (± 300 m/s) overlying the compact overburden (± 1700 m/s) which in turn lies on bedrock. This paper discusses various criteria for designing an optimum window for obtaining usable reflections between the first arrival and the leading edge of the ground roll cone. Detailed mapping of the overburden and the bedrock interface by the reflection method can be useful in delineating areas for exploratory drilling and for optimum planning of mining operations.     

A direct method of calculating interval velocities and layer thicknesses from wide-angle seismic reflection times

Wide-angle reflections are now routinely recorded in high resolution explosion seismics to study the crustal structure. Use of Dix's hyperbolic approximation to the nonhyperbolic wide-angle reflection travel times causes major errors in the determination of interval velocities and layer thicknesses of a stack of horizontal velocity layers. Here we propose a layer stripping method to directly calculate the interval velocities and layer thicknesses in a vertically heterogeneous earth from the strong and reliable wide-angle reflected events. Synthetic reflection travel times, at wide-angle range, for a given velocity model, contaminated by some random errors, have been used to demonstrate the reliability of the algorithms to determine the interval velocities and thicknesses of various layers. The method has also been tested on two field examples along two deep seismic sounding (DSS) profiles with well identified wide-angle reflection travel times, which illustrates the practical feasibility of the proposed method.     

浅层地震反射和折射层析成像在海州—韩山断裂探测中的联合应用

海州—韩山断裂是连云港地区一条重要断裂, 属于海泗断裂带的西边界断裂。 断裂隐伏于较浅的覆盖层之下且控制了基岩岩性分界。 浅层地震反射法作为断层探测的首选方法, 对海州—韩山断裂进行探测时仅能识别出基岩顶面反射波, 难以实现对断裂的准确判别。 而折射层析成像法适用于速度横向差异大的区域, 可获取断层两盘岩性速度差异信息, 进而判定断层位置, 弥补反射法的不足。 本文在跨海州—韩山断裂同一位置上联合应用浅层地震反射和折射层析成像两种探测方法, 进行了钻孔联合剖面探测和合成地震记录验证。 研究表明, 浅层地震反射和折射层析成像两种方法联合探测海州—韩山断裂, 较单一方法可对断层实现更精准的定位, 获取更丰富的断层信息, 为类似地质条件下的断层探测提供了思路。     

An estimation method for effective stress changes in a reservoir from 4D seismics data

Advances in seismics acquisition and processing and the widespread use of 4D seismics have made available reliable production‐induced subsurface deformation data in the form of overburden time‐shifts. Inversion of these data is now beginning to be used as an aid to the monitoring of a reservoir's effective stress. Past solutions to this inversion problem have relied upon analytic calculations for an unrealistically simplified subsurface, which can lead to uncertainties. To enhance the accuracy of this approach, a method based on transfer functions is proposed in which the function itself is calibrated using numerically generated overburden strain deformation calculated for a small select group of reference sources. This technique proves to be a good compromise between the faster but more accurate history match of the overburden strain using a geomechanical simulator and the slower, less accurate analytic method. Synthetic tests using a coupled geomechanical and fluid flow simulator for the South Arne field confirm the efficacy of the method. Application to measured time‐shifts from observed 4D seismics indicates compartmentalization in the Tor reservoir, more heterogeneity than is currently considered in the simulation model and moderate connectivity with the overlying Ekofisk formation.     

基于地震探测的盐源盆地构造特征及动力学意义

基于覆盖盐源盆地的短周期天然地震台阵和布设的一条人工地震测线所获得的地震数据,从中提取地震能量属性,并通过地震层析成像获得该地区的浅部地震速度结构,继而对短周期地震台阵一个月的噪声数据进行互相关得到经验格林函数,再通过时频分析获得相速度频散曲线,反演获得不同深度的S波速度分布。研究结果显示,盐源盆地地震特征主要分为三层:浅部为低速新生代沉积地层,P波速度为1.0—2.1 km/s,反射轴明显、连续性好,反射能量强,S波速度为中高南低,南部低速体与盐源断裂走向一致,新生代盆地整体呈南深北浅,沉积发育和构造形态受盐源断裂控制;中部为中低速三叠系地层,P波速度为3.5—4.5 km/s,反射轴不连续,反射能量较弱,S波速度逐渐变大,整体变化变小;深部为高速古生代地层,地震反射较为杂乱,反射能量弱;上地壳存在滑脱面,该界面为沉积盖层与结晶基底的分界面,且向浅部发育一系列断层,其中金河—箐河断裂为盐源盆地与康滇地块的分界线,这些断裂带也是盐源盆地地震频发的部位。      

The P-wave velocity structure of the lithosphere of the North China Craton——Results from the Wendeng-Alxa Left Banner deep seismic sounding profile

For the first time on the Chinese mainland, long-range wide-angle seismic reflection/refraction profiling technology has been applied to seismic wave phases from different depths and with different attributes within the various blocks of the North China Craton to characterize the structure of the crust and upper mantle lithosphere. By comparative analysis of the seismic wave phase characteristics in each block across a 1500-km-long east-west profile, we have identified conventional Pg, Pci, PmP and Pn phases in the crust, made a clear contrast between PL1 and PL2 waves belonging to two groups of lithospheric-scale phases, and produced a model of crust-mantle velocity structures and tectonic characteristics after one- and two-dimensional calculations and processing. The results show that the thickness of the crust and lithosphere gradually deepens from east to west along the profile. However, at the reflection/refraction interface, seismic waves in each group show obvious localized changes in each block. Also, the depth to the crystalline basement changes greatly, from as much as 7.8 km in the North China fault basin to only about 2 km beneath the Jiaodong Peninsula and Taihang-Lüliang area. The Moho morphology as a whole ranges from shallow in the east to deep in the west, with the deepest point in the Ordos Block at 47 km; in contrast, the North China Plain Block is uplifting. The L1 interface of the lithosphere is observed only to the west of Taihang Mountains, at a relatively slowly changing depth of about 80 km. The L2 interface varies from 75 to 160 km and shows a sharp deepening to the west of Taihang Mountains, forming a mutation belt.     

MINI-SOSIE FOR LAND SEISMOLOGY*

Mini-sosie consists in using a vibration-rammer as seismic source and changing the striking rate by varying the engine speed, resulting in a random impulse transmission. The recording instruments are made up of two seismic traces, two constant gain amplifiers and a two-channel sosie processor which performs the decoding in real time by using the actual transmission times supplied by a captor located on top of the vibration-rammer's plate. An idea of the possible penetration is given by the recording of a velocity survey. Other results obtained in seismic reflection and refraction are given.     

华北克拉通东部地壳与地幔盖层结构——长观测距深地震测深剖面结果

利用文登—阿拉善左旗长观测距地震宽角反射/折射剖面东段资料,辩识出4组地壳震相和3组地幔盖层震相.采用二维射线追踪走时反演和正演拟合交替计算方法,得到了包括鲁东隆起和华北裂陷盆地在内的地壳和地幔盖层二维速度结构.研究结果表明:华北裂陷盆地基底深达6km以上,研究区壳内界面C1埋深约15km,C2界面深约25km,Moho面平均埋深约35km.上地壳速度6.0~6.1km·s-1,且横向变化较大;中地壳速度相对均匀约为6.2~6.4km·s-1;下地壳速度为6.5~7.0km·s-1,速度梯度较大.地壳平均速度与隆起和坳陷构造相关.研究区岩石圈底界面一般为75~80km,西端接近太行隆起构造时深至90km左右,向西呈明显加深趋势,地壳厚度呈现相同的增厚特征.地幔盖层上部速度8.0~8.2km·s-1,具明显正梯度特征.岩石圈平均速度在郯庐断裂带附近显著偏低.PmP和PLP震相存在不同程度的复杂性,意味着在本地区Moho界面和岩石圈界面有较为复杂的结构,可能具有一定厚度或过渡带性质.结合其他研究结果认为,地幔盖层和下地壳速度梯度、界面性质差异与华北克拉通破坏相关,意味着破坏是一个渐变、缓慢和不均匀的过程.郯庐断裂带附近的低速应是其为软弱带的证据.     

上海地区地壳精细结构的综合地球物理探测研究

通过在上海地区开展深、浅地震反射、地震宽角反射/折射、高分辨地震折射和大地电磁测深等联合剖面探测, 获得了该地区近地表至Moho面的精细速度结构、电性结构和深浅构造关系.结果表明, 该地区地壳可划分为上、中、下三个组成部分.其中，上地壳厚为12～14 km，波速为57～59 km/s；中地壳厚度约为10 km，波速为59～62 km/s；下地壳厚为10～11 km, 波速为62～63 km/s，Moho面深度约为32 km.剖面浅部地质构造复杂，共解释出12条特征明显的断裂.其中，除3条断裂错断结晶基底（G界面）并向下延伸至上地壳底界面外，其他断裂均在深度3～5 km以上终止或收敛于G界面之上.此外，仅在剖面西侧基底下部约13～15 km埋深处发现一厚度在2 km左右的壳内高导层.所以，在综合各方面资料后分析认为，在剖面经过地区不存在发生大地震的深部构造条件，近地表所存在的活动断层是未来产生对该区有影响地震的震源区.     

SEISMIC REFRACTION AND SCREENING BY THIN HIGH-VELOCITY LAYERS *

The screening effect of thin, relatively shallow high-velocity layers often presents considerable problems in seismic exploration. Such layers prevent the greater part of the seismic energy from travelling to greater depths and introduce additional refraction arrivals, confusing the seismogram still further. In order to investigate both the screening and refractive properties of high-velocity layers, scale-model experiments have been made over a wide range of layer-thickness/ wavelength ratios (0.05 < d/λ < 2) for suitably chosen material contrasts. The results may be summarised as follows. Refraction arrivals from thin layers in the field may be recognised by their relatively rapid amplitude decay. Furthermore, the “echeloning”-effect observed for refraction first arrivals may be due to the presence of a (thin) layered structure. Since the apparent refraction velocity varies with d/λ when d/λ < 1, differences between vertical well-log velocities and velocities observed along the surface may be expected, making time/depth conversion using surface velocity data inaccurate. Transmission of elastic energy may be expected, if anywhere, only near the shotpoint, at small geophone offset, and for relatively thin screens (d/λ < 0.1). The transmitted signal shape is then independent of the layer thickness. This transmitted energy may be registered either in a reflection set-up with geophones near the shotpoint, or in long-distance refraction work. Three possibilities are offered for overcoming the screening effect of thin high-velocity layers: Use longer-wavelength signals Apply short-spread reflection shooting Apply long-distance refraction shooting The experimental results obtained in scale-model arrangements of such set-ups confirm the potentialities of these methods.