SEISMIC REFRACTION MEASUREMENTS IN THE BALTIC SEA*

Seismic refraction measurements were made in two areas of the Baltic Sea in June 1967. The refraction data were obtained in the course of the transmission measurements program of Operation MILOC BALTIC 67. Three profile pairs were obtained, two in the area south of Öland Island, Sweden, the third to the east, north of the peninsula of Hel, Poland. The water depths vary from 60 to 90 meters between the areas. The receiving positions for the two profiles south of Oland Island are only 46 km apart but the structures differ markedly. The northern section, only 500 m thick, shows a rather thin sedimentary cover above 350 m of 3.7 km/sec material that in turn overlies 5.6 - 5.9 km/sec velocity material. The southern section, almost 2 km thick, has an equivalent amount of low velocity material, and a layer about 1 km thick having velocity 4.8 km/sec that overlies 6.0 km/sec velocity material. The eastern profile shows high velocity material, 5.6 km/sec, at 2.5 km depth. Correlation of the layers determined by seismic refraction with nearby geology suggests that the structural change south of Öland Island may represent the boundary of the Sarmatian Shield in this region.     

SEISMIC NOISE MEASUREMENTS IN A GEOTHERMAL AREA*

Generation of electrical power through the use of natural steam obtained from the Earth has the potential to be a significant source of pollution-free power. Successful future exploitation of geothermal power depends upon the development of cost-effective exploration techniques. A field experiment in the Imperial Valley of California indicates that there is a close empirical relationship between hot water deposits at depth and an anomalously high seismic background level at the surface. If this relationship proves to be a general one, it will serve as the basis of a simple and inexpensive method of geothermal exploration.     

IN SITU SEISMIC MEASUREMENTS OF CRACK ANISOTROPY IN THE CARBONIFEROUS LIMESTONE OF NORTHWEST ENGLAND*

Seismic velocity anisotropy is predicted for cracked rocks containing either a naturally-occurring or an artificially-induced preferred orientation of cracks. Methods developed for the study of velocity anisotropy in the uppermost mantle have been applied to the in situ measurement of P-wave velocity anisotropy in fractured Carboniferous Limestone in Northwest England. At three different sites, first-arrival travel-time data has been obtained using conventional refraction equipment and a weight-drop source. Velocity anisotropy of 15–29% is identified with directions of maximum and minimum velocities that can be broadly related to previously mapped orientations of joints.     

EXPLORING DEEP INTERFACES BY SEISMIC WIDE ANGLE MEASUREMENTS*

Amplitudes of the vertical ground motion are larger in the critical angle region than in the near vertical region, especially amplitudes coming from deeper boundaries or interfaces with small velocity contrast. Four basic boundary models are used to derive specific travel time patterns for first order and higher order interfaces. The transition from reflected waves to diving waves in the supercritical angle region is shown for the different models. Examples from wide angle surveys in Bavaria and in the Buchara region in the USSR have many characteristic similarities. Travel time patterns and asymptotic velocities are almost identical. The most important boundaries of the continental crust - the Mohorov? I? discontinuity and the top of the crystalline basement - correspond to two boundary models mentioned before. Amplitude and frequency investigations have been used to derive items of the M-discontinuity. It is certainly a gradient zone and seems to have a lenticular structure with material of stepwise different velocities.     

SOIL CONSOLIDATION BEHAVIOR ASSESSED BY SEISMIC VELOCITY MEASUREMENTS*

Large gravity platforms are often used as alternatives to the more conventional pilesupported structures in hydrocarbon exploitation. A gravity structure, as opposed to the piled structure, is sitting on the sea floor by virtue of its weight and base width; as such it poses considerable problems for the site investigation engineer. One such problem is the calculation of the settlement of the structure and its time history; these depend upon the permeability and compressibility of the soil and its drainage conditions. The required data are usually obtained by sampling for subsequent laboratory testing. The collection of an undisturbed sample is beset by problems so that the consolidation behavior of the foundation material can only be inadequately assessed by laboratory testing. However, a series of laboratory consolidation experiments during which seismic velocities have been measured on the sample as consolidation proceeds shows that it is possible to reconstruct the stress-strain and time-dependent curves from the seismic data, once the initial void ratio and permeability of the soil are known. This leads the way to an in situ technique for predicting settlement using a combination of geophysical techniques (electrical resistivity and seismic velocities) to obtain the required engineering properties.     

SUBSURFACE GRAVITY MEASUREMENTS AND DENSITY MAPPING IN THE MAILARAM COPPER MINES,ANDHRA PRADESH,INDIA*

Gravity measurements were made in the Mailaram copper mines, Andhra Pradesh. The observations were distributed between the two shafts situated about 220 m apart and in the three levels up to a maximum depth of 100 m. Assuming a normal free-air gradient, average densities for the three layers were determined as 2.631, 2.604, and 2.823 g cm^-3, respectively. Upon incorporating the weighted mean density values from measurements on samples, the free-air gradients were found to be 0.315 mGal m^-1 for the second layer (i.e. between the first and second levels) and 0.2978 mGal m^-1 for the third layer (i.e. between the second and third levels). The density variation map obtained from the gravity data, the deduced anomalies, the weighted mean density values from measurements on rock samples, and the varying free-air coefficients all suggest correspondence with the concentration of ore lodes.     

重大工程地震安全性评价中活动断裂分段的准则

活动断裂分段在重大工程地震安全性评价中具有特别重要的意义,但无论是方法本身还是实际应用都存在着有待于进一步研究和解决的问题,这些问题不解决,可能造成不良的后果。利用国际上公认的分段研究工作最深入、段落划分最可靠的３条活动断裂的分段依据,分析和评价了所使用的方法和标志以及它们的不确定性,结合中国活动断裂的特殊性和重大工程地震安全性评价对活动断裂的特殊要求,提出４个分段准则     

中国长线工程场地地震安全性评价工作中的活动构造问题

长线工程场地一般都得穿经活动构造带,因而活动构造研究在该类场地的地震安全性评价工作中尤为重要。为此,文中对长线工程场地对地震安全性评价工作的要求和活动构造的研究现状进行了总结和分析,在此基础上,提出了长线场地地震安全性评价工作中活动构造研究宜开展的研究内容、技术指标、技术路径和成果提交等工作要点。最后,对长线场地震安全性评价工作中活动构造研究目前存在的一些问题进行了初步的分析和探讨     

MULTICOVER MEASUREMENTS IN REFRACTION SHOOTING*

Multicover measurements in refraction shooting are comparable to long-spread reflection shooting. Of course, spread length, distance of traces and offset may be larger than in reflection shooting and depend on the, depth, the velocity and the dipping of the refractors to be detected. For later processing an equation for the refractor velocity is derived in case of flat and steep dipping refractors. The depth and the angle of dip will be computed from delay times. An outlook to digital processing is given.     

MEASUREMENTS OF EARTH ATTENUATION FROM DOWNHOLE AND SURFACE SEISMIC RECORDINGS*

Frequency-dependent attenuation of compressional waves within the earth has been estimated in the vicinity of wells from

• 1 spectral power ratios of the coherent events in separate time gates on the seismic section
• 2 matching a broadband synthetic trace with seismic data at the well, and
• 3 determining the operator that transforms one down(up) going pulse recorded in the well into another recorded at a deeper (shallower) level.

The accuracy of estimation of all three methods was insufficient to estimate attenuation over small depth intervals, and it was not possible to distinguish between the contribution due to internal multiples and that of genuine absorption with much confidence. Spectral ratios from (1) showed a smoother variation with frequency—and one more consistent with other estimates—when they were compensated for the spectra of the reflectivities over the time gates employed, but they did not provide more than a broad indication of attenuation over a substantial depth interval. Approach (2) was hampered by the restricted durations over which synthetic trace and seismic data can be reliably matched; approach (3) gave the best results. Here matching is a much more powerful tool than the spectral-ratio techniques that are commonly applied since it can yield the form of the attenuation operator, i.e., both its amplitude and phase response, together with properly defined measures of its accuracy, while at the same time it minimizes the influence of noise and local interference effects at each recording level. For seismic target depths where internal multiple activity was low the logarithms of the amplitude responses of the estimated attenuation operators decreased approximately linearly with frequency and the phase responses showed no significant dispersion. Application of approach (3) to downgoing and upgoing waves estimated from a vertical seismic profile revealed the importance of changes in frequency-dependent geophone coupling and their effect on values of Q determined from downgoing pulses only.     

物理探测手段在地震安全性评价中的具体运用

结合3个工程实例,对地球物理探测手段在地震安全性评价中的应用进行分析和探讨,以便为今后地震安全性评价工作开展活断层探测提供参考。浅层人工地震法和高密度电法以其观测结果直观、高效、方便、经济等特点成为活断层物理探测手段首选。两种手段对于探测环境以及精度要求可以优势互补,在活断层探测结果的判断方面可以相互验证。     

OFFSET CONTINUATION FOR SEISMIC STACKING*

Wave equation migration techniques have shown the limits of traditional stacking methods with data from tectonically complicated areas. An improved stack can be obtained utilizing the dip-moveout correction technique based on offset continuation. The properties and the limits of the algorithms used are summarized briefly. Several synthetic and real data examples are shown and compared with the results obtained using conventional processing in order to show the focusing effects and the strong improvement in signal-to-noise ratios, both at the stacked and migrated section level. The possibility of exploiting this technique to transform multiple coverage into increased spatial resolution is illustrated with examples.     

SEISMIC SOURCES FOR SHALLOW REFLECTION SURVEYING*

A standard seismic reflection profile was shot along a disused railway track at Onley, near Rugby, U. K. Four different seismic sources including explosives, the propane/oxygen gas-gun, the Bolt airgun, and the borehole sparker were used and compared with each other in terms of output energy, penetration and resolution. The results indicated that the resolution of the borehole airgun and the gas-gun was slightly higher than that possible with gelignite. Both these sources had an output energy which was equivalent to 30 g of gelignite. The borehole sparker was only useful for obtaining seismic information on the nearsurface weathered layer, since its output power (1 kJ) was very limited. However, McCann and McCann (1982) used a high-power sparker source (14 kJ) on the nearby Grand Union Canal for a wide angle seismic reflection survey and achieved a maximum penetration of 250 m, which is comparable with the results obtained on land with the seismic sources mentioned above. The seismic reflection profile, which was interpreted in the light of borehole information in the area and the results of McCann and McCann (1982), successfully identified the surface of the Palaeozoic rocks. The problem of detecting the presence of thin, high-velocity layers in a seismic refraction survey without the availability of other information to calibrate the seismic section is also highlighted.     

RESISTIVITY MEASUREMENTS IN VALLEYS WITH ELLIPTIC CROSS-SECTION*

In this paper an idealized valley of a semi-elliptic cross-section is considered. For a Schlumberger configuration on the axis, sets of master curves are calculated for the ratio of semi-axis a/b= 1, 2, 3, 4, 5 in corresponding to various resistivity ratios of surrounding rocks and valley sediments. For small resistivity ratios, these model curves have the shape of three-layer curves for horizontal bedding and are often equivalent to them within the accuracy of measurements. The axial ratio a/b considerably affects the depth determination of valley sediments. In the special case of a circular cross-section (a/b= 1) the influence of the position of the electrodes on the sounding curve is studied in more detail. The application of the master curves in practice shows that the influence of the specific shape of the valley on soundings should not be neglected. In general, the valleys have a greater “true” depth than can be seen from the interpretation of a sounding by master curves corresponding to the horizontal bedding.     

MAGNETO-TELLURIC MEASUREMENTS IN THE NORTH GERMAN BASIN*

Magneto-telluric measurements were carried out at a group of eight sites, from Braunschweig to Lübeck, to determine the resistivity values associated with the North German conductivity anomaly. The data were analyzed for scalar and tensor apparent resistivities over the period range 5–2000 seconds. The results in general indicate very conductive (?I Ω-m) sediments overlying a resistive basement which is strongly lineated. Interpreted basement depth (?6 km) is compatible with published seismic refraction data. Principal conductivity axes in the basement appear to be directly related to trends of salt domes and major troughs. A major change in axis direction occurs near the center of the North German anomaly.     

VERTICAL SEISMIC PROFILING IN COAL*

The intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program. Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k-plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening. Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free-surface multiple reflections in the recorded wavetrains.     

COMPARISON OF “AQUAFLEX” WITH CONVENTIONAL EXPLOSIVE FOR SEISMIC REFRACTION MEASUREMENTS AT SEA*

A seismic refraction line fired using conventional explosive charges was repeated using a seismic cord, “Aquaflex”. It was found that travel times and frequencies were the same for both kinds of source and that amplitudes from “Aquaflex” were comparable with those produced using conventional explosives more than ten times the weight. This advantage carries with it gains in cost and efficiency and reduces damage to fishing.     

INTERPRETATION GRAPH FOR VERTICAL TABULAR CONDUCTORS IN E.M. IN-LINE DIP-ANGLE MEASUREMENTS*

A graphical interpretation method is presented by which separately the depth and the product of conductivity and thickness of a vertical plate-shaped conducting orebody can be determined. The method is based on the result of model experiments.     

临汾地震安全性评价的极值理论方法

本文根据极值理论建立了烈度超越给定值Ｉｊ的平均重现期公式，根据最大熵原理建立了未来Ｔ年烈度超越给定值Ｉｊ的概率和烈度超越给定值Ｉｊ的平均重现期之间的关系式。利用１０２２─１９９３年的地震资料，计算了临汾（３６．１０°Ｎ，１１１．５０°Ｅ）周围４级以上地震在临汾造成的烈度，绘制了未来１００年临汾的烈度超越概率曲线（地震危险性曲线）。结果表明，计算场地烈度的极值理论方法可作为地震危险性分析的综合概率法的补充和验证。