Monitoring overburden layer changes and fault movements from time-lapse seismic data on the Valhall Field

A method developed by Røste et al. , which discriminates between layer thickness and velocity changes, is tested on pre-stack time-lapse seismic ocean bottom cable (OBC) data from the Valhall Field. A key parameter in this discrimination process is the dilation factor, α, which is the relative velocity change divided by the relative thickness change within a given layer. The high quality and good repeatability of the OBC data enables us to estimate α with a reasonable accuracy for α-values between 0 and −5. For α-values below −5, complementary information, like for instance geomechanics is required. For the top reservoir horizon we estimate a maximum subsidence of 0.50 m ± 0.29 m and a corresponding velocity decrease for the sequence from the seabed to the top reservoir of 2.0 m s⁻¹± 0.40 m s⁻¹.
Time-lapse distortion patterns with characteristic time-shift versus offset signatures are observed. The positions and vertical extents of the distortion zones are determined from ray path considerations and modelling. The distortion zones correlate with buried faults, indicating that a (time-lapse) distortion zone might be produced by a localized slip in a fault zone. We present an extended method which allows for vertical (in addition to lateral) variations in the relative thickness and velocity changes. This method can be viewed as a simplified version of time-lapse tomography, but involving fewer unknown parameters, giving more stability to the estimated changes in thickness and velocity. Using this technique, we are able to estimate α for positions with localized time-lapse distortions.     

Comprehensive Multi-Level Exploration of Buried Active Faults： an Example of the Yinchuan Buried Active Fault

The paper introduces the steps and methods of multi-approach,multi-level exploration of buried faults in thick Quaternary sediment regions by taking the test exploration of the Yinchuan active fault as example.Based on the comprehensive analyses of previous data,we choose the Xinqushao Village of Xingqing District of Yinchuan City as the test site for the comprehensive exploration.Firstly,we adopted shallow seismic investigation with group intervals of 10m,5m and 1m to gradually trace layer by layer the master fault of the Yinchuan buried fault from a deep depth to a shallow depth where drilling could be used.Then,with composite geological profile drilling,we determined the precise location and dip angle of the fault.The drilling show the buried depth of the upper offset point is 8.3m.Finally,large-scale trenching revealed that the actual buried depth of the upper offset point of the fault is 1.5m from the ground surface and there are paleoearthquake events of 5 stages.Combined with the preliminary result of corresponding sample age,we conclude the Yinchuan buried fault is a mid to late Holocene active fault.     

高分辨率浅层人工地震勘探探测隐伏断裂实例

利用高分辨率浅层地震勘探反射波法探测了福建某火电厂场址内的隐伏断裂。结果表明断裂显示清楚,为一走向NE40~55°,上断点埋深为11 m和31 m,断距2~4 m的正断层,为工程避让提供较可靠的依据。     

雷达图像中的转换构造信息及其应用

低序次断裂是地壳变形的直接产物，具有丰富的构造变形信息，经过转换的低序次构造。形式多样，图案复杂，是构造研究的重要对象，也是地表稳定性研究的重要基础。利用香港地区航空雷达图像资料，提取其中低序次构造的弱信息，建立转换变形模型，以分析区内不稳定地表的构造背景。区内北东向断裂是主要构造，其右行剪切形成一系列低序次R面，P面，R^1面构造，在雷达图像上表现为线性弱信息并且与微地貌发育有关。利用线性构造增强，提取技术和低序次构造组合分析方法，结合野外地质调查，确定马鞍山地区EW向的张性破裂是NE向张剪断裂的转换，是造成马鞍山滑坡的重要地质因素。     

内蒙古白乃庙金矿断裂构造控矿研究

白乃庙金矿由石英脉型和蚀变岩型金矿组成，并受断裂及其破碎带控制，研究了白乃庙金矿断裂构造与成矿关系，断裂构造趋势面与找矿及矿体在断裂中的富集规律，并对成矿远景进行了预测。     

Reactivated strike–slip faults: examples from north Cornwall, UK

Several strike–slip faults at Crackington Haven, UK show evidence of right-lateral movement with tip cracks and dilatational jogs, which have been reactivated by left-lateral strike–slip movement. Evidence for reactivation includes two slickenside striae on a single fault surface, two groups of tip cracks with different orientations and very low displacement gradients or negative (left-lateral) displacements at fault tips.

Evidence for the relative age of the two strike–slip movements is (1) the first formed tip cracks associated with right-lateral slip are deformed, whereas the tip cracks formed during left-lateral slip show no deformation; (2) some of the tip cracks associated with right-lateral movement show left-lateral reactivation; and (3) left-lateral displacement is commonly recorded at the tips of dominantly right-lateral faults.

The orientation of the tip cracks to the main fault is 30–70° clockwise for right-lateral slip, and 20–40° counter-clockwise for left-lateral slip. The structure formed by this process of strike–slip reactivation is termed a “tree structure” because it is similar to a tree with branches. The angular difference between these two groups of tip cracks could be interpreted as due to different stress distribution (e.g., transtensional/transpressional, near-field or far-field stress), different fracture modes or fractures utilizing pre-existing planes of weakness.

Most of the d–x profiles have similar patterns, which show low or negative displacement at the segment fault tips. Although the d–x profiles are complicated by fault segments and reactivation, they provide clear evidence for reactivation. Profiles that experienced two opposite slip movements show various shapes depending on the amount of displacement and the slip sequence. For a larger slip followed by a smaller slip with opposite sense, the profile would be expected to record very low or reverse displacement at fault tips due to late-stage tip propagation. Whereas for a smaller slip followed by larger slip with opposite sense, the d–x profile would be flatter with no reverse displacement at the tips. Reactivation also decreases the ratio of d_max/L since for an original right-lateral fault, left lateral reactivation will reduce the net displacement (d_max) along a fault and increase the fault length (L).

Finally we compare Crackington Haven faults with these in the Atacama system of northern Chile. The Salar Grande Fault (SGF) formed as a left-lateral fault with large displacement in its central region. Later right-lateral reactivation is preserved at the fault tips and at the smaller sub-parallel Cerro Chuculay Fault. These faults resemble those seen at Crackington Haven.     

Pore fluid pressure effects in anisotropic rocks: Mechanisms of induced seismicity and weak faults

Many observations and studies indicate that pore fluid pressure in the crustal rocks plays an important role in deformation, faulting, and earthquake processes. Conventional models of pore pressure effects often assume isotropic porous rocks and yield the nondeviatoric pressure effects which seem insufficient to explain diverse phenomena related to pore pressure variation, such as fluid-extraction induced seismicity and crustal weak faults. We derive the anisotropic effective stress law especially for transversely-isotropic and orthotropic rocks, and propose that the deviatoric effects of pore fluid pressure in anisotropic rocks not only affect rock effective strength but also cause variation of shear stresses. Such shear stress variations induced by either pore pressure buildup or pore pressure decline may lead to faulting instability and trigger earthquakes, and provide mechanisms for the failure of crustal weak faults with low level of shear stresses. We believe that the deviatoric effects of pore fluid pressure in anisotropic rocks are of wide application in studies of earthquake precursors and aftershocks, oil and gas reservoir characterization, enhanced oil recovery, and hydraulic fracturing.