Seismic inversion transforms seismic reflection data into quantitative rock-property descriptions of a reservoir, e.g., P-impedance, S-impedance, and density. Regardless of the inversion method used, the accuracy of and confidence in the inversion results rely highly on the quality of gathers or stacks obtained from seismic imaging.
Seismic data bandwidth is limited by signal-to-noise ratio (S/N), absorption, source wavelet, and shot and receiver ghosts. As a result, conventional seismic data lack low frequencies below 7 Hz. A typical deterministic seismic inversion workflow uses the low frequencies of existing well logs by extrapolating or interpolating along stratigraphic layers. The interpolation result is often biased on the well locations and quality of the well logs and can be affected by the interpolation method.
We propose a 3-stage method to minimize the dependency of seismic inversion on a well-log based initial model and improve confidence in the final result. The method includes 1) pre-migration deghosting to remove ghosts in the seismic data, subsequently extending seismic signal to lower frequencies; 2) high-resolution velocity model building with full waveform inversion (FWI) and fault-constrained tomography (FCT) to improve velocity resolution, extending the spectrum to higher frequencies; and 3) simultaneous seismic inversion using the FWI-derived model as the initial model to invert for P-impedance and Vp/Vs.
Presentation Date: Monday, September 25, 2017
Start Time: 3:55 PM
Presentation Type: ORAL
Full waveform inversion (FWI) usually gives a poor update of the sediment velocity when in the close vicinity of the top of salt (TOS) reflection. This phenomenon is a common practical challenge and is due to the strong velocity contrast between the sediment and salt—although its exact cause is not yet well understood. We investigated the relationship between FWI’s sediment velocity update and the role of salt insertion, namely the accuracy of the salt interface used in the FWI input velocity model. The results indicated that the initial model with salt inserted helps update the sediment model, and furthermore, using a more accurate TOS (i.e., the TOS interpretation used to insert the salt is closer to the true TOS) provides a better sediment velocity update. However, an accurate TOS is not available unless the sediment velocity, particularly directly above the TOS, has been correctly updated. Therefore, we propose a workflow to obtain a more accurate TOS for the FWI sediment velocity update using iterative FWI and salt interpretation. Using 2D synthetic data and 3D real data, we demonstrate that our workflow yields a better FWI update above the salt compared to using the sediment model as the initial model.
FWI aims to minimize the misfit of phase and amplitude between real shot gathers and synthetic shot gathers (Lailly, 1983; Tarantola, 1984; Sirgue and Pratt, 2004; Virieux and Operto, 2009). Updating the velocity in areas of high impedence and/or velocity contrast, such as the sedimentsalt boundary, is challenging for FWI. Two approaches are commonly used to address this overburden velocity update problem. Kapoor et al. (2012) used sediment models created from ray-tracing tomography as FWI input. Chen et al. (2014) used a salt model approach that ran an initial pass of FWI with the sediment model to update the sediment velocity before creating a salt model for the second pass of FWI to update the sediment velocity again. However, few studies have been performed to understand the impact of the TOS interpretation on the sediment velocity update using FWI. By using band limited data and conventional FWI, the TOS singularities are not automatically updated at each iteration.
The presence of shallow gas anomalies continues to be a challenge in seismic imaging in the Gulf of Mexico (GOM). Due to the strong absorption property and irregular shape of a gas cloud, the seismic image below it could suffer from frequency, amplitude, and phase distortion. Q tomography and pre-stack depth Q migration (Q-PSDM) are efficient tools for compensating these distortion effects caused by absorptive heterogeneities. However, deriving Q models is a challenging task, especially when the shape of gas anomalies are complex and the velocity inside or close to the gas cloud area is not correct. Standard ray-based tomography fails to capture the detailed rapid velocity variation for the areas with or beneath a high lateral or vertical contrast (e.g., gas pocket, shale, salt, and carbonate). Full waveform inversion (FWI) provides high-resolution velocity details but is still an expensive process with its success highly dependent on the quality of the initial model. We present a case study at East Breaks, GOM, that combines adaptive data-selection FWI and Q tomography to invert the velocity and absorption model around a gas cloud area to improve the seismic image.
Elebiju, Bunmi (BP America) | Ariston, Pierre-Olivier (BP America) | van Gestel, Jean-Paul (BP America) | Murphy, Rachel (BP America) | Chakraborty, Samarjit (BP America) | Jansen, Kjetil (BP America) | Rodenberger, Douglas (Shell America) | White, Roy C. (Shell America) | Chen, Yongping (CGG) | Hren, David (CGG) | Hu, Lingli (CGG) | Huang, Yan (CGG)
Using the Kepler and Ariel Fields as a case study, this paper discusses the processing challenges and solutions applied to a 4D co-processing of Wide Azimuth Towed Streamer (WATS) on Narrow Azimuth Towed Streamer (NATS) data. Unlike a dedicated 4D acquisition, WATS on NATS 4D has relatively low repeatability in terms of acquisition geometry and bandwidth differences. All these factors can negatively impact the extraction of a meaningful 4D signal. In this paper, we demonstrate how processing techniques can help to increase repeatability and enhance 4D signal. We focus on the following 4D processing procedures: 4D co-binning, data matching, and post-migration co-denoise. Due largely to these techniques, the final co-processed volumes show an optimized 4D seismic signal with a median Normalized Root Mean Square (NRMS, which measures the repeatability between base and monitor. Details refer to Kragh and Christie, 2002) of 0.10 along the water bottom and 0.28 above the reservoir.
Para-Maranhao Basin offshore Brazil is well-known for its shallow geological complexities. Rugose topography and complex V-shaped paleo-canyons directly below the water bottom create challenges for ray-based migration velocity analysis from prestack depth imaging. In particular, when using a global grid-based tomography, generating velocity updates at a sufficiently small scale to conform to these shallow structures can be difficult. Image distortions, characterized by non-geologic depth undulations and amplitude variations, are often observed below the shallow structures. These artifacts can affect the accuracy of reservoir interpretation. We propose a weighted, high-resolution workflow utilizing dip-constrained, non-linear slope tomography to resolve small scale velocity variations at shallow depths. As a result, image distortions below the near-surface geological complexities are attenuated, and the common image gather flatness is improved.