Neotectonic state of stress in the Jura Mountains

Abstract

For the determination of the tectonic stress field in the region of the Jura Mountains, three different relief techniques have been used in 18 test sites. In six test sites measurements have been carried out in horizontal boreholes using the doorstopper method. In the remaining test sites measurements were made in vertical boreholes by the doorstopper method (8), triaxial strain cell method (3) and borehole slotter (1). Additionally, analyses of borehole breakouts from six deep wells are presented.

Orientation of maximum horizontal stress clearly deviates from the uniform NW-SE orientated central and western European stress field. Local stress sources, which are probably related to active decollement tectonics in the Jura Mountains, may explain the deviation in stress orientations in comparison to the foreland. Normal stress magnitudes perpendicular to the trend of compressional structures in the Folded Jura are largest in the southeastern Upper Rhine Graben and its southwards prolongation into the Jura Mountains. Low stress magnitudes are characteristic for the eastern termination, intermediate ones for the southern foreland of the Jura Mountains.     

Palynology of Triassic–Jurassic boundary sections in northern Switzerland

A first palynostratigraphic scheme of Upper Triassic deposits in northern Switzerland was established based on spore-pollen associations and dinoflagellate cyst records from the upper part of the Upper Triassic Klettgau Formation and the lower part of the Lower Jurassic Staffelegg Formation. Drill cores from the Adlerberg region (Basel Tabular Jura) and from Weiach (northern part of Canton Zurich) as well as from an outcrop at the Chilchzimmersattel (Basel Folded Jura) were studied and five informal palynological associations are distinguished. These palynological associations correlate with palynological association of the Central European Epicontinental Basin and the Tethyan realm and provide a stratigraphic framework for the uppermost Triassic sediments in northern Switzerland. Throughout the uppermost Triassic to Jurassic palynological succession a remarkable prominence of Classopollis spp. is observed. Besides Classopollis spp. the three Rhaetian palynological associations A to C from the Upper Triassic Belchen Member include typical Rhaetian spore-pollen and dinoflagellate taxa (e.g., Rhaetipollis germanicus, Geopollis zwolinskae, Rhaetogonyaulax rhaetica, and Dapcodinium priscum). Association B differs from association A in a higher relative abundance of the sporomorph taxa Perinopollenites spp. and the consistent occurrence of Granuloperculatipollis rudis and Ricciisporites tuberculatus. Spore diversity is highest in the late Rhaetian palynological association C and includes Polypodiisporites polymicroforatus. A Rhaetian age for the Belchen Member is confirmed by palynological associations A–C, but there is no record of the latest Rhaetian and the earliest Jurassic. In contrast to the Rhaetian palynological associations the Early Jurassic associations W and D include Pinuspollenites spp., Trachysporites fuscus (in association W), and Ischyosporites variegatus. In the view of the end-Triassic mass extinction and contemporaneous environmental changes the described palynofloral succession represents the pre-extinction phase (associations A and B) including a distinct transgression, the extinction phase (association C) associated with a regression, and the post-extinction phase (association W).     

中国大陆科学钻主孔现今地应力状态

用钻孔崩落法确定了中国大陆科学钻探主钻孔5 047 m深度以上的现今地应力状态.由钻孔声波成像测井资料发现, 科学钻主钻孔在1 200 m深度以下出现了钻孔崩落现象.我们从1 216~5 047 m的深度范围内采集了143个钻孔成像测井图象资料, 对钻孔崩落椭圆长轴方位进行了统计, 结果表明崩落椭圆长轴平均方位为319.5°±3.5°, 最大水平主应力方位平均为49.5°±3.5°.利用崩落形状要素(崩落深度和崩落宽度) 以及岩石的内聚力和内摩擦角, 估算了1 269 m至5 047 m范围内52个深度上的最大和最小水平主应力的大小.结果表明, 在浅处1 216 m深度, 最大水平主应力为42 MPa, 最小水平主应力为30.3 MPa; 在深处5 000 mm深度, 最大水平主应力为160.5 MPa, 最小水平主应力为120 MPa; 地应力随深度近于线性增加.据岩石密度测井资料计算了各个深度上静负载应力.3个主应力的大小和方向反映出科学钻主孔位置的应力场处于走滑应力状态, 与临近地区地震震源机制解和其他方法得到的应力场一致.利用声发射法对岩心试件进行了声发射测量, 得到了最大水平主应力幅值, 并与崩落法测量结果进行了对比, 两者十分一致.      

Present-day stress field and deformation in eastern Austria

 We analysed fault plane solutions and borehole breakouts in the eastern part of the Eastern Alps and found a heterogeneous stress field which we interpret as a transition zone of three different stress provinces: the western European stress province with NW to NNW S_H orientation and mainly strike-slip faulting regime; the Adriatic stress province with a radial stress pattern and thrust faulting to strike-slip faulting regime; and the Dinaric-Pannonian stress province with NE S_H orientation and strike-slip faulting regime. The western Pannonian basin seems to be a part of the transition zone with WNW to NW S_H orientation. A stress regime stimulating strike-slip faulting prevails in the Eastern Alps. The south Bohemian basement spur as a major tectonic structure with a high rheological contrast to surrounding units has a strong influence on the stress field and exhibits the highest seismicity at its tip due to stress concentration. From a constructed vertical stress orientation profile we found stress decoupling of the Northern Calcareous Alps from the underlying European foreland. Both the Molasse and the Flysch-Helvetic zone are considered as candidates for decoupling horizons due to stress orientation observations and due to their rheological behaviour. From seismological and rheological data, we suggest a horizontal stress decoupling across the Eastern Alps caused by a weakened central Alpine lithosphere. Received: 3 July 1998 / Accepted: 4 April 1999     

Stress orientation to 5 km depth in the basement below Basel (Switzerland) from borehole failure analysis

A vertical profile of maximum horizontal principal stress, SHmax, orientation to 5 km depth was obtained beneath the Swiss city of Basel from observations of wellbore failure derived from ultrasonic televiewer images obtained in two 1 km distant near-vertical boreholes: a 2755 m exploration well (OT2) imaged from 2550 m to 2753 m across the granitic basement-sediment interface at 2649 m; and a 5 km deep borehole (BS1) imaged entirely within the granite from 2569 m to 4992 m. Stress-related wellbore failure in the form of breakouts or drilling-induced tension fractures (DITFs) occurs throughout the depth range of the logs with breakouts predominant. Within the granite, DITFs are intermittently present, and breakouts more or less continuously present over all but the uppermost 100 m where they are sparse. The mean SHmax orientations from DITFs is 151 ± 13° whereas breakouts yield 143 ± 14°, the combined value weighted for frequency of occurrence being N144°E ± 14°. No marked depth dependence in mean SHmax orientation averaged over several hundred meters depth intervals is evident. This mean SHmax orientation for the granite is consistent with the results of the inversion of populations of focal mechanism solutions of earthquakes occurring between depths of 10–15 km within regions immediately to the north and south of Basel, and with the T-axis of events occurring within the reservoir (Deichmann and Ernst, this volume). DITFs and breakouts identified in OT2 above and below the sediment-basement interface suggest that a change in SHmax orientation to N115°E ± 12° within the Rotliegendes sandstone occurs near its interface with the basement. The origin of the 20–30° change is uncertain, as is its lateral extent. The logs do not extend higher than 80 m above the interface, and so the data do not define whether a further change in stress orientation occurs at the evaporites. Near-surface measurements taken within 50 km of Basel suggest a mean orientation of N–S, albeit with large variability, as do the orientation of hydrofractures at depths up to 850 m within and above the evaporite layers and an active salt diapir, also within 50 km of Basel. Thus, the available evidence supports the notion that the orientation of SHmax above the evaporites is on average more N–S oriented and thus differs from the NW–SE inferred for the basement from the BS1/OS2 wellbore failure data and the earthquake data. Changes in stress orientation with depth can have significant practical consequences for the development of an EGS reservoir, and serve to emphasise the importance of obtaining estimates from within the target rock mass.     

Stress field in the central and northern parts of the Gulf of Suez area,Egypt from earthquake fault plane solutions

Earthquake focal mechanism solutions from 18 events in the central and northern parts of the Gulf of Suez with local magnitudes ranging from 2.8 to 5.2 and occurring between 1983 and 2004 are used to determine the type of motion and stress pattern of the region. Fault plane solutions show mostly normal component; pure normal faulting mechanisms and normal faulting with a strike-slip component. Only some mechanisms show pure strike-slip faulting. The fault planes strike in NW, WNW, NNE and ENE directions, in conformity with the geologically observed striking faults in the northern and central parts of the gulf. The principal stress orientation is also estimated by inverting the selected focal mechanism solutions. The results show that the northern part of the Gulf is subjected to NE–SW to NNE–SSW extension, with a horizontal σ₃ (plunge 3°) and subvertical σ₁ (plunge 80°). This means that the horizontal extensional stresses are still present in the central/northern Gulf of Suez.     

利用钻孔崩落数据再认识白鹤滩右岸地应力场特征

白鹤滩水电站是仅次于三峡水电站的第二大水电站,位于中国西南地区川滇菱形块体内的金沙江上.通常地壳应力状态是影响地下工程安全的重要地质因素,对地下硐室稳定性分析具有重要意义.在水电站右岸厂房建设过程中,为了水电站的长期安全运营,采用超声波井下电视录井测试系统对白鹤滩右岸厂房锚固洞内7处钻孔进行测试,基于钻孔崩落数据计算了...     

Borehole instability in high-porosity Berea sandstone and factors affecting dimensions and shape of fracture-like breakouts

Stress-induced breakouts in vertical boreholes are failure zones caused by excessive compressive stress concentration at the borehole wall along the springline of the least horizontal far-field stress. Wellbores are sometimes drilled into aquifers or oil reservoirs that are weak, poorly consolidated, and highly porous sandstone formations, which are often conducive to breakout formation. Breakouts are an expression of borehole instability and a potential source of sand production. On the other hand, the breakout phenomenon can be used advantageously in obtaining an estimate of the in situ stress condition. The average orientation of breakouts, as identified by borehole geophysical logging, is a reliable indicator of in situ stress directions. It has also been suggested that breakout dimensions could potentially be used as indicators of in situ stress magnitudes. The research reported here has concentrated on the unique type of breakouts observed for the first time in high-porosity Berea sandstone. Drilling experiments in rock blocks subjected to critical far-field true triaxial stress regimes, simulating in situ conditions, induced breakouts that were unlike the ‘dog-ear’ ones previously observed in granites, limestones, and low-porosity sandstones. The newly observed breakouts were thin, tabular, and very long, resembling fractures that counterintuitively extended perpendicular to the maximum principal stress. We found that a narrow zone ahead of a fracture-like breakout tip underwent apparent localized grain debonding and compaction. In the field, such zones have been termed ‘compaction bands’, and are a source of concern because in oil fields and aquifers they constitute curtains of low permeability that can impede the normal flow of oil or water. In order to determine whether a correlation exists between fracture-like breakouts and in situ stress, we conducted several series of tests in which the minimum horizontal and vertical stresses were held constant and the maximum horizontal stress (σ_H) was increased from test to test. These tests showed strong dependence of the breakout length on far-field stress, signaling that potentially the ability to assess fracture-like breakout length in the field could be used to estimate in situ stress magnitudes in conjunction with other indicators. Another series of tests revealed that breakout length increased substantially when borehole diameter was enlarged. This result suggested that in the field, where wellbore size is considerably larger, fracture-like breakout could extend to sizable distances, creating a sand production hazard. Two series of tests, one to evaluate the effect of drill-bit penetration rate, and the other to verify the drilling-fluid flow rate effect on breakout formation and dimensions yielded inconsistent results and showed no unique trends. Remarkably, fracture-like breakouts maintained a consistent narrow width of about 5–10 grain diameters, irrespective of the test conditions. This characteristic supports the suggestion that fracture-like breakouts are emptied compaction bands.     

The Staffelegg Formation: a new stratigraphic scheme for the Early Jurassic of northern Switzerland

The deposits of the Early Jurassic in northern Switzerland accumulated in the relatively slowly subsiding transition zone between the southwestern part of the Swabian basin and the eastern part of the Paris basin under fully marine conditions. Terrigenous fine-grained deposits dominate, but calcarenitic and phosphorit-rich strata are intercalated. The total thickness varies between 25 and 50 m. In the eastern and central parts of N Switzerland, sediments Sinemurian in age constitute about 90% of the total thickness. To the West, however, in the Mont Terri area, Pliensbachian and Toarcian deposits form 70% of the total thickness. Stratigraphic gaps occur on a local to regional scale throughout N Switzerland. Such hiatus comprise a subzone to a stage in time. With respect to lithology and fossil content, the Early Jurassic deposits in northern Switzerland are similar to those in SW Germany. Nonetheless, an exact stratigraphic correlation is hardly possible, particulary in the southern and southwestern Folded Jura where distinct facies changes occur over short distances. Revised existing and new litho- and biostratigraphic data form the base to refine the stratigraphic subdivision of the deposits that have been informally called “Lias”. The name Staffelegg Formation is suggested and defined as the mapping unit for the Early Jurassic. The Staffelegg Formation is introduced for Early Jurassic sediments in northern Switzerland between the Doubs River and Mt. Weissenstein in the west and the Randen Hills located north of the city of Schaffhausen in the east. The Staffelegg Formation starts within the Planorbis zone of the Hettangian. The upper boundary to the overlying Aalenian Opalinus-Ton is diachronous. The lithostratigraphic names previously in use have been replaced by new ones, in accordance within the rules of lithostratigraphic nomenclature. The Staffelegg Formation comprises 11 members and 9 beds. Several of these beds are important correlation horizons in terms of allostratigraphy. Some of them correspond to strata or erosional unconformities encountered in the Swabian realm, some of them can be correlated with strata in the Paris basin. The facies transition to the Paris basin is expressed by introduction of a corresponding lithostratigraphic unit.     

Analysis of overdeepened valleys using the digital elevation model of the bedrock surface of Northern Switzerland

Based on surface and borehole information, together with pre-existing regional and local interpretations, a 7,150 square kilometre Raster Digital Elevation Model (DEM) of the bedrock surface of northern Switzerland was constructed using a 25 m cell size. This model represents a further important step in the understanding of Quaternary sediment distribution and is open to a broad field of application and analysis, including hydrogeological, geotechnical and geophysical studies as well as research in the field of Pleistocene landscape evolution. An analysis of the overdeepened valleys in the whole model area and, more specifically in the Reuss area, shows that, in most cases, overdeepening is restricted to the areas covered by the Last Glaciation Maximum (LGM). However, at various locations relatively narrow overdeepened valleys outreach the tongue basins and the LGM ice shield limits. Therefore, an earlier and further-reaching glacial event has probably contributed significantly to the overdeepening of these valleys. No significant overdeepening has been identified downstream of Böttstein (Aare) and Kaiserstuhl (Rhine), although the ice extended considerably further downstream, at least during the most extensive glaciation. Except for the bedrock between Brugg and Böttstein, no overdeepened valleys are found significantly north of the outcrop of Mesozoic limestone of the Folded and Tabular Jura. A detailed analysis of the Reuss area shows that the Lake and Suhre valleys are separated from the Emmen—Gisikon Reuss valley basin by a significant bedrock barrier. The individual bedrock valleys are divided into several sub-basins, indicating a multiphase evolution of the valleys. Some of the swells or barriers separating the sub-basins coincide with known late LGM retreat stages. In the Suhre valley, an old fluvial valley floor with restricted overdeepened sections is documented.     

Crustal Stress in the northern North Sea as inferred from borehole breakouts and earthquake focal mechanisms

The regional stress field in the northern North Sea (offshore western Norway) has been studied through the acquisition and analysis of directions of maximum horizontal compression (_H) as extracted from borehole breakouts and from earthquake focal mechanism solutions.
The results indicate that the regional stress field is dominated by NW-SE compression, with good consistency between shallow borehole breakouts (2–5 km depth) and deeper earthquakes (10–25 km depth). The broad spatial consistency in stress direction indicates that the main stress field is related to factors of primarily plate tectonic origin, and the results are in good agreement with the western Europe trend found in earlier investigations.
The Tampen Spur region in the northern North Sea has been subjected to particularly complex deformation, with two dominating fault directions trending NW-SE and NE-SW. From Tampen Spur in the west to the Sogn graben in the east an anomalous stress field is indicated, with NE-SW oriented maximum horizontal compressions. This anomaly is clearly seen both in the borehole breakout data and in the earthquake data. Possible sources for this anomaly are discussed, and include postglacial uplift and/or lateral variations in the physical properties of the crust.     

Paleostress pattern and salt tectonics within a developing foreland basin (north-western Subhercynian Basin, northern Germany)

Analysing the paleostress field in sedimentary basins is important for understanding tectonic processes and the planning of drilling campaigns. The Subhercynian Basin of northern Germany is a perfect natural laboratory to study the paleostress field in a developing foreland basin. The simple layer-cake geometry of the basin-fill is dominated by several piercing and non-piercing salt structures. We derived the paleostress field from the orientation of fracture sets, faults, slickensides and stylolites. On a regional scale, the basin-fill is characterized by a horizontal compressional paleostress vector that is mainly NNE-SSW-oriented, which reflects the Late Cretaceous inversion phase in Central Europe. We show that the local paleostress field is distinctly perturbated due to the salt structures. Along the edge of the salt pillows, the maximum horizontal paleostress vector is deflected by up to 90° from the regional trend. In the case of the Elm salt pillow, it forms a radial pattern. Restoration of balanced cross-sections demonstrates at least 9 % of the shortening of the north-western part of the Subhercynian Basin was achieved by folding. The salt structures in the north-western Subhercynian Basin are the result of varying stress conditions. Initial extension in the Triassic caused first salt movements that prevailed during the Jurassic and Early Cretaceous. Most important is the Late Cretaceous contractional phase that shortened the diapirs and led to the formation of the salt pillows between diapirs due to detachment folding. We derive four main controlling factors for such salt-dominated contractional basins: (1) the wedge-shape basin-fill is the product of the dynamic load at the southern margin of the basin, (2) a basal salt layer fed the diapirs and acted as a detachment horizon during the later shortening, (3) detachment folding was the dominating deformation mechanism during contraction, and (4) the pre-existing diapirs controlled the position of the detachment folds.     

大兴安岭呼中火山岩区中生代古构造应力场初探

大兴安岭火山岩带北段呼中火山岩区,在中-晚侏罗世时期,最大主压应力轴方向为NW266°~316°,早白垩世最大主压应力轴为NE12°~50°,显示侏罗纪与白垩纪应力场的应力状态是截然不同的,但都处于拉张环境下。中-晚侏罗世由于NNE-SSW向的拉伸应力场的作用,在本区北部发生构造岩浆活动,形成了上黑龙江断陷盆地如NWW向展布的劲涛-蒙克山中基性火山喷发带,派生的NEE向张扭性应力使前中生代基底构造重新活动,形成大面积火山喷发。而早白垩世在NE-SW向拉伸应力场作用下,"额尔古纳地块"东部形成拗陷带,火山基底构造为NE-NNE向。      

Contemporary state of stress and neotectonic deformation in the Carpathian-Pannonian region

The Carpathian-Pannonian region has been characterized by a two-stage tectonic evolution since the beginning of the Neogene. During early and mid-Miocene the lateral eastward escape of the Pannonian Fragment caused thrusting and folding in the Outer Carpathian Mountains, south- and westward directed subduction of the Eurasian lithosphere, calc-alkaline volcanism along the Inner Carpathian bend, and localized fault-controlled subsidence of basins in the Pannonian region. This style of tectonic deformation ceased by the end of the mid-Miocene. The neotectonic stage began in the late Miocene. It is characterized by differential regional subsidence with maximum rates in the west and east and minimum rates in the centre of the Pannonian Basin. Further characteristics of the central Pannonian Basin are the anomalous high heat flow values, the thin crust and lithosphere. The neotectonic stress field has been determined by in situ stress measurements, the analyses of borehole breakouts and fault-plane solutions of earthquakes. In situ stress measurements by the doorstopper and triaxial strain cell methods indicate high compressional stresses in the western Pannonian Basin with its maximum in WNW-ESE to NW-SE direction. In contrast to its western part, the central Pannonian Basin shows tensional stresses near the Earth's surface with maximum tension in the same WNW-ESE direction. Borehole breakout data indicate a general WNW-ESE orientation of maximum horizontal stress in the western part of the Pannonian Basin and in the eastern part as well, whereas in the central Pannonian Basin this direction is the preferred azimuth of minimum horizontal stress. It is suggested that the neotectonic deformations and stresses in the Carpathian-Pannonian region have a sublithospheric origin. Asthenospheric convection with an upwelling mantle flow below the centre of the Pannonian Basin and downwelling flows along the cold lithospheric roots below the eastern Carpathians and the Alps seems to be most plausible. Accordingly, the relative uplift of the central Pannonian Basin, the high heat flow and also the tensional stresses are explained as the surface expression of an upstreaming branch of a localized convection cell below the Pannonian Basin.     

Wellbore breakouts of the main borehole of Chinese Continental Scientific Drilling (CCSD) and determination of the present tectonic stress state

The Sulu-Dabie high-pressure (HP)-ultrahigh-pressure (UHP) metamorphic belt as the product of subduction-collision between the northern China plate and Yangtze plate underwent a process of formation and evolution from deep subduction→exhumation→extension→slow uplift. The study of its modern tectonic stress field has great significance for a complete understanding of the process of formation and evolution of the HP-UHP metamorphic belt, especially the exhumation and uplift of the belt. Wellbore breakouts are the most visual tectonic phenomenon which can characterize the modern stress action in the main borehole of Chinese Continental Scientific Drilling (CCSD). Ultrasonic borehole televiewer reflection wave data show that wellbore breakouts began to occur at 1216 m depth of the main borehole. A total of 143 borehole televiewer images were collected from 1216 to 5118 m depth (hole completion depth). After data processing and statistics, the average azimuth of the long dimension of the wellbore breakout obtained was 319.5° ± 3.5°, indicating that the average azimuth of the maximum horizontal principal stress causing wellbore breakout initiation was 49.5° ± 3.5°. The maximum and minimum horizontal principal stress values at 52 depths in the interval of 1269 to 5047 m were estimated using the elements of wellbore shapes (wellbore depth and width), combined with the cohesive strength and internal frictional angle of the rock obtained by rock mechanical tests on samples, and the static load stresses at corresponding depths were calculated according to the rock density logging data. The results indicate that: the maximum and minimum horizontal principal stresses are 41.4 and 25.3 MPa at 1269 m depth respectively and 164.7 and 122 MPa at 5047 m depth respectively; the maximum vertical stress is 141.3 MPa at 5047 m depth with a density of 2.8 g/cm³; the in-situ stresses increase nearly linearly with depth. The magnitudes and directions of the three principal stresses reflect that the regional stress field around the CCSD main borehole is mainly in a strike-slip state, which is consistent with the basic features of the regional stress field determined using other methods.     

一种基于测井信息的山前挤压构造区地应力分析新方法

中国西部前陆盆地主要以挤压构造应力为主,地表与地下构造异常复杂。在油气的勘探与开发过程中既需要精细的地应力研究与预测,也需要简单易行、经济实用的近似方法。本文通过对稳定构造地区与挤压构造区地球物理测井信息的深度域特征进行分析和对比,提出了一种基于测井信息来计算最大岩石主应力的近似方法:统计模型法。应用该方法对塔里木盆地库车山前挤压应力区进行了实际计算,其结果与实验结果相符,还分析了岩石的最大主应力方向,据此对山前挤压区地应力分布与油气藏分布的关系进行了探索。     

Stress evaluation in offshore regions of Norway

A review has been undertaken of all available information on crustal stresses in Norwegian continental shelf and North Sea. The data have principally come from the results of borehole breakouts, supplemented by focal mechanisms. A simple model of a soft linear inclusion was developed to explain the rapid changes in stress orientation in the Central Graben area. This example indicates that the stress field is affected locally by the geological or structural environment.     

一种研究山前挤压构造区地应力的新方法

通过对稳定沉降地区与山前挤压构造区的地球物理测井信息的深度域特征进行理论分析和对比,提出了一种基于测井信息计算最大主地应力的统计模型法。应用该方法对塔里木盆地库车地区地应力进行了实际计算,其结果与实验结果相符,同时利用椭圆井眼法确定最大主应力方向。最后,对山前挤压构造区的地应力分布与油气藏分布关系进行了探索。     

淮南矿区深部地应力场特征研究

利用水压致裂法和套孔应力解除法对淮南矿区-500～-1 000 m深度范围内的12个矿井进行了地应力测量,取得了19个有效测点的数据。数据分析表明：（1）淮南矿区深部地应力场以水平应力为主,构造应力占绝对优势,属于典型的构造应力场类型;（2）地应力量级属高应力水平;（3）垂直主应力、最大及最小水平主应力随深度的增加均呈增大趋势;（4）侧压系数 随深度增大而减小,地应力场有从大地动应力场型向准静水压力场型过渡的趋势;（5）侧压比 的回归曲线分布于Hoek-Brown内外包线带内,且变化趋势与Hoek-Brown曲线相似,但与Hoek-Brown曲线相比,回归曲线 值偏小;（6）最大水平主应力与最小水平主应力的比值范围为1.12～2.02,其中68.42%的测点分布在1.67～2.02之间;（7）最大水平主应力方位为NEE－EW向。最后探讨了淮南矿区地应力与地质构造的关系,分析认为,矿区最大主应力方向与构造运动密切相关,矿区现今构造应力场最大主应力方位与实测最大水平主应力方位大致吻合。     

潞安矿区井下地应力测量及分布规律研究

在山西潞安矿区的13个煤矿采用小孔径水压致裂地应力测量装置,完成了60个测点的原岩应力测量工作。实测数据表明：潞安矿区55%的测点最大水平主应力大于垂直主应力,由于受埋藏深度与地质构造影响,矿区内各矿地应力值差别较大,但地应力大小总体上属于中等地应力值;潞安矿区最大水平主应力方向从南到北变化较大,构造应力场呈现出多变的形态。在实测数据的基础上,绘制了潞安矿区地应力分布图;分析了地应力随埋藏深度的变化规律、平均水平主应力与垂直主应力的比值与埋藏深度的关系。煤矿井下地应力测量为井田开拓、巷道布置与支护设计等工程实践提供了可靠的基础参数,提高了工程设计的科学性、合理性与可靠性。