Elastic full-waveform inversion of vertical seismic profile data acquired with distributed acoustic sensors

Distributed acoustic sensing (DAS) is a rapidly developing technology particularly useful for the acquisition of vertical seismic profile (VSP) surveys. DAS data are increasingly used for seismic imaging, but not for estimating rock properties. We have developed a workflow for estimating elastic properties of the subsurface using full-waveform inversion (FWI) of DAS VSP data. Whereas conventional borehole geophones usually measure three components of particle velocity, DAS measures a single quantity, which is an approximation of the strain or strain rate along the fiber. Standard FWI algorithms are developed for particle velocity data, and hence their application to DAS data requires conversion of these data to particle velocity along the fiber. This conversion can be accomplished by a specially designed filter. Field measurements show that the conversion result is close to vertical particle velocity as measured by geophones. Elastic time-domain FWI of a synthetic multioffset VSP data set for a vertical well shows that the inversion of the vertical component alone is sufficient to recover elastic properties of the subsurface. Application of the proposed workflow to a multioffset DAS data set acquired at the CO2CRC Otway Project site in Victoria, Australia, reveals salient subhorizontal layering consistent with the known geology of the site. The inverted VP model at the well location matches the upscaled VP log with a correlation coefficient of 0.85

Dynamic seismic signatures of saturated porous rocks containing two orthogonal sets of fractures: Theory versus numerical simulations

The dispersion and attenuation of seismicwaves are potentially important attributes for the noninvasive detection and characterization of fracture networks. A primary mechanism for these phenomena is wave-induced fluid flow (WIFF), which can take place between fractures and their embedding background (FB-WIFF), as well as within connected fractures (FF-WIFF). In this work, we propose a theoretical approach to quantify seismic dispersion and attenuation related to these two manifestations of WIFF in saturated porous rocks permeated by two orthogonal sets of fractures. The methodology is based on existing theoretical models for rocks with aligned fractures, and we consider three types of fracture geometries, namely, periodic planar fractures, randomly spaced planar fractures and penny-shaped cracks. Synthetic 2-D rock samples with different degrees of fracture intersections are then explored by considering both the proposed theoretical approach and a numerical upscaling procedure that provides the effective seismic properties of generic heterogeneous porous media. The results show that the theoretical predictions are in overall good agreement with the numerical simulations, in terms of both the stiffness coefficients and the anisotropic properties. For the seismic dispersion and attenuation caused by FB-WIFF, the theoretical model for penny-shaped cracks matches the numerical simulations best, whereas for representing the effects due to FF-WIFF the periodic planar fractures model turns out to be the most suitable one. The proposed theoretical approach is easy to apply and is applicable not only to 2-D but also to 3-D fracture systems. Hence, it has the potential to constitute a useful framework for the seismic characterization of fractured reservoirs, especially in the presence of intersecting fractures

Effect of grain-scale gas patches on the seismic properties of double porosity rocks

Time-lapse ultrasonic measurements constitute a tool to establish and calibrate rock physics models for surface seismic monitoring of partially saturated rocks. This workflow requires one to take into account seismic dispersion caused by frequency-dependent wave-induced fluid flow. We develop a theory of squirt flow in rocks saturated with a viscoelastic material containing isolated gas patches between compliant intergranular contacts. This model is valid for the entire frequency range, from seismic to ultrasonic. In the limit of full saturation the derived equations reduce to the Gassmann equations in the low-frequency regime and traditional squirt theory in the high-frequency regime. The model prediction of ultrasonic velocities versus saturation matches with experimental observations.</jats:p

Kinematic and dynamic characteristics of seismic pulses in porous-fractured rocks with effective spatial wave dispersion

This work concerns with seismic pulses propagation in porous-fractured rocks. Such geological media could possess strong macroscopic spatial dispersion of seismic waves. Results of physical modelling are compared with theoretical estimations of group velocities based on linear slip model of fluid-saturated fracture. They are in good agreement. Computer 3D full waveform simulation of compressional pulses reflection on the boundary of isotropic and transverse isotropic media with horizontal symmetry axis provide a validation that use of group velocities instead of phase ones raises accuracy of the Rueger's approximate equations when concerning utilized in practice seismic signals

Seismic velocity changes caused by an overburden stress

An increase in seismic velocity with depth is a common rock property, one that can be encountered practically everywhere. Overburden pressure increases vertical stress, producing a nonlinear elastic response. Application of a conventional nonlinear theory to this problem leads to transverse isotropy, with explicit relationships between nonlinear constants and elastic anisotropy parameters. These relationships can be used in velocity "depth trend" removal and in computing offset-dependent corrections for stacking and migration. Assumptions about small static stress and the use of linearized solutions for its evaluation are invalid for overburden problems - more accurate approximations are required. Realistic tomography models should account for elastic anisotropy as a basic feature. Our theory gives an accurate fit to well and stacking velocity data for the Los Angeles Basin. Overburden stress is a likely cause of shear-wave generation by underground explosions

Optimal bounds for attenuation of elastic waves in porous fluid-saturated media

Explicit expressions for bounds on the effective bulk and shear moduli of mixture of an elastic solid and Newtonian fluid are derived. Since in frequency domain the shear modulus of the Newtonian fluid is complex valued, the effective mixture moduli are, in general, also complex valued and, hence, the bounds are curves in the complex plane. From the general expressions for bounds of effective moduli of viscoelastic mixtures, it is shown that effective bulk and shear moduli of such mixtures must lie between the real axis and a semicircle in the upper half-plane connecting formal lower and upper Hashin-Shtrikman bounds of the mixture of the solid and inviscid fluid of the same compressibility as the Newtonian fluid. Furthermore, it is shown that the bounds on the effective complex bulk and shear moduli of the mixture are optimal; that is, the moduli corresponding to any point on the bounding curves can be attained by the Hashin sphere assemblage penetrated by a random distribution of thin cracks. The results are applicable to a variety of solid/fluid mixtures such as fluid-saturated porous materials and particle suspensions

E1: Seismic survey design technology based on 3D finite-difference wave field modeling

Seismic prediction of complex hydrocarbon reservoirs is a complicated task. Present approaches to this problem often give wrong results. Usage of 3D full waveform seismic computer modelling allows us to improve all stages of seismic prospecting in such regions. It is presented a case study of computer modeling application to Eastern Siberia reservoirs. Aspects of seismo-geological model construction are discussed. It is shown that analysis of synthetic seismograms enables choosing of appropriate data acquisition system, processing graph and interpretation techniques