In this abstract, we propose a new anisotropic acoustic wave equation for 2D TTI media. This is the extension of our previous work on VTI media to TTI media. Similar to VTI case, we follow the same procedures as Zhou et al. (2006), by the introduction of an auxiliary function, the original fourth-order differential equation becomes a coupled system of lower-order differential equations. However, unlike VTI case, a cross-derivative term has been added to each of the coupled system of the hyperbolic differential equations because of its TTI characteristics. Of these two equations, one equation plays a key role in governing the propagation of the wavefront, with the other equation compensating for the loss of anisotropy for TTI media not only in the lateral but also in the depth directions. The new anisotropic acoustic wave equation has the obvious physical meaning and is much easier to implement. Impulse responses for both modeling and migration have been tested to show the validation of the proposed anisotropic acoustic wave equation.

An Anisotropic Acoustic Wave Equation For Modeling And Migration In 2D TTI Media

v and sediment specific surface. The S velocity increases with hydrate saturation owing to an increase in skeletal shear stiffness, particularly when hydrate saturation exceeds Shyd≈ 0.4. At very high hydrate saturations, the small strain shear stiffness is determined by the presence of hydrates and becomes insensitive to changes in effective stress. The P velocity increases with hydrate saturation due to the increases in both the shear modulus of the skeleton and the bulk modulus of pore‐filling phases during fluid‐to‐hydrate conversion. Small‐strain Poisson’s ratio varies from 0.5 in soft sediments lacking hydrates to 0.25 in stiff sediments (i.e., subject to high vertical effective stress or having high Shyd). At Shyd ≥ 0.5, hydrate hinders expansion and the loss of sediment stiffness during reduction of vertical effective stress, meaning that hydrate‐rich natural sediments obtained through pressure coring should retain their in situ fabric for some time after core retrieval if the cores are maintained within the hydrate stability field.

/pdf/parametric-study-of-the-physical-properties-of-hydrate-2uf3s06adi.pdf

Parametric study of the physical properties of hydrate‐bearing sand, silt, and clay sediments: 2. Small‐strain mechanical properties

Fracture networks and hydrate distribution at Hydrate Ridge, Oregon

Gas hydrates are a potential energy resource and hazard for drilling and infrastructure, yet estimates of global volume vary by over three orders of magnitude. Hydrates are electrically resistive compared to water saturated sediment and so electromagnetic methods provide an additional tool to seismic surveys and drilling for determining hydrate saturations. A marine electromagnetic survey was carried out at Hydrate Ridge, Oregon, USA, with the aim of testing the use of controlled source electromagnetic (CSEM) and magnetotelluric (MT) methods to map gas hydrate and free gas below the gas hydrate stability zone. A 2-D CSEM inversion supports the scenario deduced from previous seismic and drilling results, which indicate two mechanisms of hydrate emplacement: a transport-dominated and reaction-dominated regime. A prominent resistive region of 2.5–4 ?m at a depth of about 130 mbsf, near the seismic bottom simulating reflector (BSR), suggests that 27 to 46 per cent of the bulk volume is filled with hydrate, depending on whether Archie's Law or the Hashin-Strikman bounds are used. This is representative of a reaction-dominated regime for hydrate emplacement, and where a significant low velocity zone exists based on a seismic tomography inversion, suggests large quantities of free gas below the BSR. Electrical resistivity logging while drilling (LWD) data show general agreement with the CSEM inversion model except for a CSEM-derived resistive region at seismic horizon A, known to transport free gas into the gas hydrate stability zone. Inversion of MT data collected simultaneously during the CSEM survey provides a complimentary low-resolution image of the shallow sediments and shows folding in the accretionary complex sediments similar to that imaged by a tomographic seismic velocity model.

/pdf/a-marine-electromagnetic-survey-to-detect-gas-hydrate-at-1zxr1i2pg0.pdf

A marine electromagnetic survey to detect gas hydrate at Hydrate Ridge, Oregon

Characterizing the Effects of Lean Zones and Shale Distribution in Steam-Assisted-Gravity-Drainage Recovery Performance

The principal objective of our work is to develop a technique for prestack depth migration in tilted transversely isotropic (TTI) media in which the axis of symmetry is not vertical and may be spatially varying. Such models are required to image seismic data in geologically complex regions such as the Canadian Foothills. We have developed a 2D Kirchhoff integral-based migration algorithm in which the traveltime computation comprises the major task. Among the existing traveltime computation algorithms such as ray tracing with interpolation, ray bending, and eikonal solvers, a direct or a brute force approach of traveltime computation is generally highly robust. We have modified a direct method of first-arrival P-wave traveltime computation in 2D media that accounts for TTI. The algorithm requires that the group velocity be computed at each gridpoint, using either an analytic solution or by an approximate Fourier series expansion. The P-wave traveltime contours computed for complex geologic models show the pronounced effects from TTI media. Our results, using laboratory P-wave data collected over a physical model of an anisotropic thrust sheet, reveal that a 2D Kirchhoff migration based on our traveltime algorithm (TTI model) images the structure beneath the thrust sheet very well. The vertical transversely isotropic (assuming a vertical axis of symmetry) or isotropic imaging introduces false anticlinal structures. We compare our results with those obtained by a recursive-extrapolation method and find that our approach images the underside of one of the thrust sheets better.

Traveltime calculation and prestack depth migration in tilted transversely isotropic media

[1] A seismic experiment composed of streamer and ocean bottom seismometer (OBS) surveys was conducted in the summer of 2002 at southern Hydrate Ridge, offshore Oregon, to map the gas hydrate distribution within the hydrate stability zone. Gas hydrate concentrations within the reservoir can be estimated with P wave velocity (Vp); however, we can further constrain gas hydrate concentrations using S wave velocity (Vs), and use Vs through its relationship to Vp (Vp/Vs) to reveal additional details such as gas hydrate form within the matrix (i.e., hydrate cements the grains, becomes part of the matrix frame or floats in pore space). Both Vp and Vs can be derived simultaneously by inverting multicomponent seismic data. In this study, we use OBS data to estimate seismic velocities where both gas hydrate and free gas are present in the shallow sediments. Once Vp and Vs are estimated, they are simultaneously matched with modeled velocities to estimate the gas hydrate concentration. We model Vp using an equation based on a modification of Wood's equation that incorporates an appropriate rock physics model and Vs using an empirical relation. The gas hydrate concentration is estimated to be up to 7% of the rock volume, or 12% of the pore space. However, Vp and Vs do not always fit the model simultaneously. Vp can vary substantially more than Vs. Thus we conclude that a model, in which higher concentrations of hydrate do not affect shear stiffness, is more appropriate. Results suggest gas hydrates form within the pore space of the sediments and become part of the rock framework in our survey area.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2007JB004993

Gas hydrate concentration and characteristics within Hydrate Ridge inferred from multicomponent seismic reflection data

[1] P-wave velocity increases in the presence of gas hydrates and decreases in the presence of free gas in the sediments, making it an excellent means to investigate gas hydrate systems. However, seismic velocity is typically derived from surface seismic data without consideration of seismic anisotropy. The presence of anisotropy in the hydrate bearing sediments adds an additional complexity in data analysis; however anisotropy can help reveal the distribution of hydrates. Here we report on the evidence of seismic anisotropy at Hydrate Ridge along the Cascadia convergent margin. We find that the south summit is anisotropic, while the basin side (east of south summit) is isotropic. Anisotropy is likely caused by the hydrate veins. We interpret the anisotropy parameters in terms of the distribution and fabric of gas hydrates.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2005GL023945

Seismic anisotropy at Hydrate Ridge

Analysis of Impact of Thermal and Permeability Heterogeneity on SAGD Performance Using a Semi-Analytical Approach

Gas hydrate is an ice-like substance that contains low molecular weight gases (mostly methane) in a lattice of water molecules (Sloan, 1998). Gas hydrates are widely present in permafrost and deep oceanic environments around the world. Hydrate Ridge, offshore Oregon, is one of the areas where hydrates are found in relatively high concentration. In marine environments, methane hydrates are usually stable at temperatures of 0–20° C, water depths greater than 500 m, and in sediments up to 300 m below the seafloor. Small amounts of free gas are often present below the gas-hydrate stability zone (GHSZ). The hydrate-to-free-gas contact gives a strong acoustic impedance contrast, which is evident on seismic sections as a bottom-simulating reflection (BSR). Offshore gas hydrate systems have received attention from the scientific community because of their potential to be:

Dhananjay Kumar

Papers

Traveltime calculation and prestack depth migration in tilted transversely isotropic media

Gas hydrate concentration and characteristics within Hydrate Ridge inferred from multicomponent seismic reflection data

Seismic anisotropy at Hydrate Ridge

Analysis of Impact of Thermal and Permeability Heterogeneity on SAGD Performance Using a Semi-Analytical Approach

Seismic characteristics of gas hydrates at Hydrate Ridge, offshore Oregon