Effect of Stress on the Magnetic Memory of Induced Magnetic Anisotropy of Rocks and Its Mathematical Model

When re-heated to temperatures below the Curie temperature and subsequently cooled in a constant magnetic field (H _T), rock samples which contain magnetic minerals can acquire an induced magnetic anisotropy (IMA). As the result of acquiring the IMA, a constriction develops in the hysteresis loop of the magnetization of these rocks at the values of the magnetizing field close or equal to the HT. Thus the IMA is capable of retaining the information on the palaeointensity of the geomagnetic field, i.e., if IMA was created in a rock in the geomagnetic field in a past geological epoch, it preserves the information on the intensity of that field. Investigations have shown, that when IMA is created in a rock under external stress, the stress has an impact on the magnetic memory. Here we also deal with the issue of how stress affects the magnetic memory of IMA. A mathematical model for the effect of stress on magnetic memory phenomena related to induced magnetic anisotropy in rocks containing multidomain magnetite and titanomagnetite grains is proposed herewith. The effect of temperature on the magnetic memory of rocks is discussed also.     

Shear-wave splitting in a critical crust: III. Preliminary report of multi-variable measurements in active tectonics

This is a preliminary report on two sets of recent observations from a region of active tectonics that provide comparatively direct evidence for the critical state of the fluid-saturated microcracked crust. The first data set from crosshole seismics in a controlled source stress-monitoring site (SMS) shows that the crust of the Earth is highly compliant and responds to low-level changes of tectonic stress at substantial distances. The second set of data from earthquake seismograms shows that the seismically active Húsavík–Flatey Fault plane is pervaded by critically high pore-fluid pressures, which cause 90° flips in the polarisations of seismic shear waves. We suggest that both sets of observations confirm previous hypotheses for a compliant crack-critical (CCC) crust. This is a new understanding of low-level pre-fracturing deformation that has fundamental implications for a range of applications in solid earth geophysics. These applications range from monitoring hydrocarbon production with time-lapse seismics to monitoring tectonic stress in in situ rock and stress-forecasting the times and magnitudes of impending large earthquakes.     

Observation and analysis of frequency-dependent anisotropy from a multicomponent VSP at Bluebell-Altamont field, Utah

In this paper, we present results from the analysis of a multicomponent VSP from a fractured gas reservoir in the Bluebell-Altamont Field, Utah. Our analysis is focused on frequency-dependent anisotropy. The four-component shear-wave data are first band-pass filtered into different frequency bands and then rotated to the natural coordinates so that the fast and slow shear-waves are effectively separated. We find that the polarisations of the fast shear-waves are almost constant over the whole depth interval, and show no apparent variation with frequency. In contrast, the time delays between the split shear-waves decrease as the frequency increases. A linear regression is then applied to fit the time-delay variations in the target and we find that the gradients of linear fits to time delays show a decrease as frequency increases. Finally, we apply a time-frequency analysis method based on the wavelet transform with a Morlet wavelet to the data. The variation of shear-wave time delays with frequency is highlighted in the time-delay and frequency spectra. We also discuss two mechanisms giving rise to dispersion and frequency-dependent anisotropy, which are likely to explain the observation. These are scattering of seismic waves by preferentially aligned inhomogeneneities, such as fractures or fine layers, and fluid flow in porous rocks with micro-cracks and macro-fractures.     

Any P-wave kinematic algorithm for vertical transversely isotropic media can be ADAPTed to moderately anisotropic media of arbitrary symmetry type

Most of P-wave anisotropic kinematic algorithms (modeling, processing, and inversion) have been developed for the case of Transverse Isotropy (TI). Does it mean that when dealing with more complex symmetry types (Arbitrarily tilted TI, orthorhombic, monoclinic or even triclinic), all these algorithms are irrelevant? In fact, not at all. It has recently been demonstrated that in 2D geometry any qP-wave TI kinematic algorithm can be simply generalized to the case of monoclinic symmetry using the so-called Azimuthally Dependent Anisotropy Parameter Transformation (ADAPT), assuming moderate anisotropy. The extension of the technique to the case of arbitrary anisotropy type (triclinic) is achieved in this paper. The method is successfully checked for seismic modeling in a full 2D model with layers of contrasted anisotropy types and with arbitrary vertical and horizontal velocity variations (non-constant gradient). Typically, the approximate travel times using ADAPT differ from the exact travel times by a few milliseconds for total travel times of the order of a few seconds. Applications to seismic processing are also described. The simplicity of the procedure and the generality of the applicability of the ADAPT recipe are striking and very convenient for practical applications. They certainly deserve further analysis.     

用梯度和感应测井联合确定地层电阻率各向异性(英文)

实际地层可能出现 2种电阻率的宏观各向异性 :微观的统计平均效应和裂缝走向等因素的构造效应。三分量感应测井方法可能是确定地层电阻率各向异性的最好方法 ,但是 ,该仪器目前在中国还没有应用。已有的数据一般是梯度和双感应曲线 ,梯度和双感应曲线单独使用都难以揭示各向异性 ,但是 ,将二者联合使用则有可能。在反演模型中考虑了电阻率各向异性的存在 ,给出了梯度和双感应曲线联合求地层电阻率各向异性的方法。通过直井水平层的二维人工模型表明 ,用感应方法可确定地层的水平电阻率和厚度。而梯度方法的视电阻率可以近似看作水平电阻率和纵向电阻率的几何平均值 ,梯度方法的视厚度是各向异性参数和真实厚度的乘积。因而不能用梯度方法单独确定各向异性参数和真实厚度 2个参数中的任何 1个。然而 ,联合梯度和双感应的方法则可能确定 3个参数 :各向异性参数 ,水平电阻率和地层厚度。人工数据的例子表明了这种联合的可行性     

Shear-wave splitting and reservoir crack characterization: the Coso geothermal field

This paper aims to improve current understanding of the subsurface fracture system in the Coso geothermal field, located in east-central California. The Coso reservoir is in active economic development, so that knowledge of the subsurface fracture system is of vital importance for an accurate evaluation of its geothermal potential and day-to-day production. To detect the geometry and density of fracture systems we applied the shear-wave splitting technique to a large number of high-quality seismograms from local microearthquakes recorded by a permanent, 16-station, down-hole, 3-component seismic array running at 480 samples/s. The analysis of shear-wave splitting (seismic birefringence) provides parameters directly related to the strike of the subsurface fractures and their density (number of cracks per unit volume), and, consequently, is an important technique to outline zones of high permeability. Three major fracture directions N10–30W, N0–20E, and N40–50E, of which the first and the second are the most prominent, were identified from the seismograms recorded by the 16-station down-hole array. All orientations are consistent with the known strike of local sets of faults and fractures in local wells and at the surface, as well as with previous analyses of seismic anisotropy in the region. The high quality of the recordings has allowed us to launch an unprecedented investigation into the characteristics of the temporal variations in crack polarization and crack density in a producing geothermal environment. Preliminary results point to significant temporal changes in shear-wave time delays, probably influenced by temporal changes in crack density within a period of 5 years (1996–2000). They are tentatively interpreted as due to a local 3% increase in shear-wave velocity in the southwestern part of the field during 1999.     

裂隙参数对P波的影响及裂隙检测可行性数值研究

在油气勘探、工程探测等领域中，各向同性介质的背景上，因存在裂隙导致各向异性，描述裂隙的两个主要参数是裂隙密度和裂隙方位。这里以Hudson理论为基础，通过数值计算，分析了描述裂隙介质的两个主要参数对P波速度和衰减的影响，同时，分析了利用P波属性检测裂隙发育带的可行性。     

大地电磁在地震预报中的应用

论述了大地电磁（MT）地震前兆多参数综合分析方法和扩展膨胀各向异性（EDA）电性前兆识别。不同的MT参数以不用的形态特征和方式，从不同侧重面反映地球电导率结构特征，充分利用包含于各参数内的信息，对它们随时间的变化进行综合分析研究有利于更可靠地判断新出现的MT前兆。EDA电性前兆是由孕震期间深部原地应力场的变化引起的，可能出现在一个较大的地域范围内，因而该前光是一种具有很好应用前景的地震前兆，利用MT法进行监测较为有利。     

Upper mantle low anisotropy channels below the Pacific Plate

A new 3D anisotropic model has been obtained at a global scale by using a massive dataset of seismic surface waves. Though seismic heterogeneities are usually interpreted in terms of heterogeneous temperature field, a large part of lateral variations are also induced by seismic anisotropy of upper mantle minerals. New insight into convection processes can be gained by taking seismic anisotropy into account in the inversion procedure. The model is best resolved in the Pacific Plate, the largest and the most active tectonic plate. Superimposed on the large-scale radial (ξ parameter) and azimuthal anisotropy (of V_SV velocity) within and below the lithosphere, correlated with present or past Pacific Plate motions, are smaller-scale (<1000 km) lateral variations of anisotropy not predicted by plate tectonics. Channels of low anisotropy down to a depth of 200 km (hereafter referred to as LAC) are observed and are the best resolved anomalies: one east-west channel between Easter Island and the Tonga-Kermadec subduction zones (observed on both radial and azimuthal anisotropies) and a second one (only observed on azimuthal anisotropy) extending from the south-west Pacific up to south-east Hawaii, and passing through the Polynesia hotspot group for plate older than about 40 Ma. These features provide strong constraints on the decoupling between the plate and asthenosphere. They are presumably related to cracking within the Pacific Plate and/or to secondary convection below the rigid lithosphere, predicted by numerical and analog experiments. The existence and location of these LACs might be related to the current active volcanoes and hotspots (possibly plumes) in the Central Pacific. LACs, which are dividing the Pacific Plate into smaller units, might indicate a future reorganization of plates with ridge migrations in the Pacific Ocean.