Feasibility of Seismic Characterization of Multiple Fracture Sets
Estimation of parameters of multiple fracture sets is often required for successful exploration and development of naturally fractured reservoirs. The goal of this paper is to determine the maximum number of fracture sets of a certain rheological type which, in principle, can be resolved from seismic data. The main underlying assumption is that an estimate of the complete effective stiffness tensor has been obtained, for example, from multiazimuth, multicomponent surface seismic and vertical seismic profiling (VSP) data. Although typically only a subset of the stiffness elements (or some of their combinations) may be available, this study helps to establish the limits of seismic fracture-detection algorithms. The number of uniquely resolvable fracture systems depends on the anisotropy of the host rock and the rheology and orientation of the fractures. Somewhat surprisingly, it is possible to characterize fewer vertical fracture sets than dipping ones, even though in the latter case the fracture dip has to be found from the data. For the simplest, rotationally invariant fractures embedded in either isotropic or transversely isotropic with a vertical symmetry axis (VTI) host rock, the stiffness tensor can be inverted for up to two vertical or four dipping fracture sets. In contrast, only one fracture set of the most general (microcorrugated) type, regardless of its orientation, is constrained by the effective stiffnesses. These results can be used to guide the development of seismic fracture-characterization algorithms that should address important practical issues of data acquisition, processing, and inversion for particular fracture models.
Seismic signatures of two orthogonal sets of vertical microcorrugated fractures
Conventional fracture-characterization techniques operate with the idealized model of penny-shaped (rotationally invariant) cracks and ignore the roughness (microcorrugation) of fracture surfaces. Here, we develop analytic solutions based on the linear-slip theory to examine wave propagation through an effective triclinic medium that contains two microcorrugated, vertical, orthogonal fracture sets in isotropic background rock. The compliance matrix off-diagonal elements associated with the corrugation cause the deviation of the polarization vectors of the vertically traveling S-waves from the horizontal plane. Also, the shear-wave splitting coefficient at vertical incidence becomes sensitive to fluid saturation, especially for tight, low-porosity host rock. In contrast to the model with two orthogonal sets of penny-shaped cracks, the NMO (normal-moveout) ellipses of all three reflection modes (P, S1, S2) are rotated with respect to the fracture strike directions. Another unusual property of the fast shear wave S1, which can help to distinguish between models with one and two microcorrugated fracture sets, is the misalignment of the semimajor axis of its NMO ellipse and the polarization vector at vertical incidence. The model analyzed here may adequately describe the orthogonal fracture sets at Weyburn Field in Canada, where the axes of the P-wave NMO ellipse deviate from the S1-wave polarization direction. Our results can be used to identify the underlying physical model and, potentially, estimate the combinations of fracture parameters constrained by wide-azimuth, multicomponent seismic data.
The Analysis on Fire Smoke Exhausting Robot Vibration Based on Wavelet Transform
Belt relaxation, which affects a lot on operation reliability of the fire smoke exhausting robot, is hard to detect. In this paper, a new method for belt relaxation feature recognition based on continuous wavelet transform (CWT) is proposed, and a vibration model which simplified the robot as a 2-dof forced vibration system under harmonic excitation is established. The vibration acceleration signal has been collected using the IEPE sensor and data acquisition card, experimental results verified the accuracy of the vibration model, and weak impact signal caused by the belt relaxation was distinguished, that testify the practicability of the CWT method in belt relaxation feature recognition.