We present a new approach to estimate frequency-based texture attributes from the local 2D Fourier spectrum. A set of new attributes is derived tailored to highlight near-horisontal layers, steep-dipping layers, and areas without any dominant orientation. Their suitability for discriminating between salt structures and layered structures with arbitrary dip is discussed, and the best attributes are used in an automatic segmentation algorithm that finds a smooth, continuous border between the salt structure and the surrounding areas. The results correspond very well with a manually interpreted contour.
We have developed an optimization method for automatic dyke delineation from observed magnetic and gravity gradient traverse data. A non-linear least squares algorithm is used to find model dyke parameters that best fit the computed gradient tensor data to the observed data. The eigen-system of the observed magnetic gradient tensor data is used to provide starting model dyke parameters for an iterative non-linear least squares solver. This greatly enhances the ability of the solver to find a plausible dyke model for matching observed and synthetic tensor gradients locally. The method works well on synthetic examples. Multiple surveys using a Full Tensor Magnetic Gradient (FTMG) signal instrument from IPHT, have been made in Southern Africa. A real case study with remanence, taken from the Platreef near Pretoria, shows that the gross observed gradient features can be recovered by our procedure, but the residuals in the gradient fit hint strongly at the need for more complex dyke models. There is more directly inferable structural geology in this tensor signal than can be found in a conventional TMI signal.
Shales are often regarded as inactive barriers in the reservoir simulation model and the surrounding rocks. Whilst this appears the correct approach for fluid flow modeling purposes, it is inaccurate for the pressure component of this process. In most clastic reservoirs experiencing pressure depletion, the sands naturally compact. This leads to the well documented extension of the non-reservoir rocks, but also the extension of the intra-reservoir shales. Less well known is that the shales have a finite, but small, permeability and pressure equilibration will occur with the reservoir sands. This diffusion process opposes the geomechanical effects. Numerical computation for a range of shale permeabilities suggests that intrareservoir shales of 1m to 10m thickness should be considered as active when quantitatively assessing the 4D seismic signature. Also it is observed that pressure depletion in the reservoir can ‘propagate’ distances of as much as 50m into the shale over/under burden during the production time scale. The integration of these coupled mechanisms into forward modelling of time lapse seismic shows vertical time shift profiles different from those proposed for geomechanics alone.
Yin, Zhiheng (China University of Petroleum) | Li, Xiangyang (China University of Petroleum) | Di, Bangrang (China University of Petroleum) | Wei, Jianxin (China University of Petroleum) | Zhang, Sihai (China University of Petroleum)
A physical modeling experiment has been conducted to study the effects of different offset-depth ratio on imaging and the capability for fracture detection. The model consists three horizontal layers, where the top and bottom layers are isotropy and the middle layer is HTI medium. There are a dome and a fault block in the fractured layer. Two modeling data are acquired with the same geometry parameters except for offset-depth ratio. Two sets of data are used by the similar analysis methods to detect the fracture from stacking velocity and AVO gradient. The results show that the larger offset-depth benefits the detection of fracture orientation.
In conventional marine CSEM methods, we need a survey vessel that tows a long cable to which both an EM transmitter and receivers are attached. Therefore, it is difficult to survey shallow sub-seafloor structure below the seafloor of complex topography around submarine massive sulphides (SMS) because of the risk of cable-tangling. In this research, we propose a new marine CSEM method to solve this problem using two autonomous underwater vehicles (AUV). Using this method, it is possible to keep a low height of diving AUVs from the seafloor, so we can carry out the exploration of SMS effectively. We discussed the possibility of new CSEM method employing the 2.5-D FEM program. From numerical results, it is possible to detect the rough existent area of SMS and the rough thickness of SMS.
In the wave propagation simulation by finite difference time domain (FDTD), the perfectly matched layer (PML) is often applied to eliminate the reflection artifacts due to the truncation of the finite computational domain. In the acoustic Logging-While-Drilling (LWD) FDTD simulation, due to high impedance contrast between the drill collar and fluid in the borehole, the stability and efficiency of PML scheme is critical to simulate complicated wave modes accurately. In this paper, we compare four different PML implementations in FDTD in the acoustic LWD simulation, including splitting PML (SPML), Multi-axis PML (MPML), Non-splitting PML (NPML), and complex frequency-shifted PML (CFS-PML). The simulation indicates that NPML and CFS-PML can more efficiently absorb the guide wave reflection from the computational boundaries than SPML and MPML. For large simulation time, SPML, MPML and NPML are numerically instable. However, stability of MPML can be improved further to some extent. Among all, CFS-PML is the best choice for LWD modeling. The effects of CFS-PML parameters on the absorbing efficiency are investigated, including damping profile, frequency-shifted factor, scaling factor and PML thickness. For a typical LWD case, the best value for maximum of quadratic damping profile
Virieux, J. (ISTerre, Université Joseph Fourier – CNRS) | Operto, S. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Asnaashari, A. (ISTerre, Université Joseph Fourier – CNRS) | Brossier, R. (ISTerre, Université Joseph Fourier – CNRS) | Castellanos, C. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Etienne, V. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Gholami, Y. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Hu, G. (ISTerre, Université Joseph Fourier – CNRS) | Pageot, D. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Prieux, V. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Ribodetti, A. (Géoazur - Université Nice Sophia-Antipolis – CNRS) | Roques, A. (ISTerre, Université Joseph Fourier – CNRS)
Full waveform inversion is a promising imaging technology for high-resolution images of the subsurface, but suffers from intrinsic non-linearities that require adequate strategies for overcoming them. One crucial issue is related to the starting model from which one has to start when considering local optimization strategies. Depending on the complexity of the data often related to the frequency content of the waves, the initial model selection may range from a crude smooth model for low frequencies to already quite accurate models for high frequencies. We shall investigate relations between the data complexity either when recorded or after time transformation, the model discretisation as well as the optimization strategy in order to better quantify acquisition design and related data interpretation one should consider.
Ajo-Franklin, Jonathan (Lawrence Berkeley National Laboratory) | Daley, Thomas (Lawrence Berkeley National Laboratory) | Butler-Veytia, Belinda (URS Corporation) | Peterson, John (Lawrence Berkeley National Laboratory) | Wu, Yuxin (Lawrence Berkeley National Laboratory) | Kelly, Bob (ARS Technologies) | Hubbard, Susan (Lawrence Berkeley National Laboratory)
We present results from the first deployment of a fully automated multi-source seismic tomography system designed to monitor hydrofracture initiation and propagation in near-surface environments. We utilized this system to track an induced fracture utilized as part of an enhanced bioremediation in a tight clay-rich formation at a TCE contaminated site. Several hundred full tomographic datasets were acquired with a temporal resolution of 3–4 minutes; this fine sampling in time allowed us to successfully capture localized P-wave velocity reductions associated with fracture emplacement. In addition to velocity changes, alterations in amplitude were observed as well as diffracted events and secondary scattered arrivals. This unique dataset suggests that real-time active source imaging strategies has a potential role in mapping induced fractures both in the shallow subsurface and in deeper environments such as enhanced geothermal and shale gas deposits.
In this paper we discuss an efficient way to apply Gaussian Packet method to data representation and seismic imaging. Similar to Gaussian Beam method, wavefield radiating from a seismic source as a set of Gaussian Packets can represent synthetic seismic data, and recorded wavefield at the surface can be expressed and downward continued by a set of Gaussian Packets at surface as well. The evolution of Gaussian Packet is determined by parameter of central frequency, local time, local space and ray emergent angle, and the shape of Gaussian Packet is determined by its initial value. Each Gaussian Packet is directly related to the ray emergent angle, and propagates along a central ray. Therefore, representation of seismic data using Gaussian Packets provides the local time slope and location information at certain central frequency, while summation of Gaussian Packets'' evolutions constructs the corresponding propagated wavefield. These properties also make seismic imaging using Gaussian Packets easily be understood and implemented. The method becomes efficient because 1) to represent seismic data, Gaussian Packets with given initial value can be used and inner product can be applied to obtain the useful information of seismic data; 2) with proper initial value, only Gaussian Packets with few central frequencies are needed for representing propagated seismic wavefield. Numerical examples on impulse responses and a 4layer zero-offset data are calculated to demonstrate the valid of the method.
We propose to use the Gaussian beam method to calculate finite-frequency sensitivity kernels for transmitted wave and for prestack migration geometry. The Gaussian beam method has the advantage of high efficiency and does not have angle limitations. Thus it is suitable for generating sensitivity kernels for high-frequency and long-distance propagation and for wide-angle waves including the turning waves. The limitation of this method is that the velocity model should be relatively smoothed. We use numerical examples to demonstrate this technique. The resulted sensitivity kernels can be used for velocity tomography and migration velocity analysis.