Prediction of Elastic Properties Within CO2 Plume at Sleipner Field Using AVS Inversion Modified for Thin‐Layer Reflections Guided by Uncertainty Estimation. Issue 11 (3rd November 2020)

• Record Type: Journal Article
• Authors: Ghosh, Ranjana; Ojha, Maheswar
• Is Part Of: Journal of geophysical research. Volume 125:Issue 11(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-11-03
• Subjects: thin layer -- AVS/AVO -- VFSA -- elastic property -- uncertainty -- CO2 sequestration; Geomagnetism -- Periodicals; Geochemistry -- Periodicals; Geophysics -- Periodicals; Earth sciences -- Periodicals; 551.1
• DOI: 10.1029/2020JB019782
• Languages: English
• ISSNs: 2169-9313

Abstract

Abstract: Existing amplitude variation with offset (AVO) and slowness (AVS) theories fail to interpret observed amplitudes in terms of actual elastic properties of the media for a reflection event resulting from a stack of thin layers. We propose a method to replace the stack of thin layers with an equivalent medium of elastic properties using the Backus averaging theory. Numerical examples show considerable deviation in intercept calculated from the modified AVO and AVS theories from the conventional methods. AVS theory is preferred as it can be applied irrespective of impedance contrast in both precritical and postcritical reflections, and no spherical divergence correction is required. P and S wave velocities, density, and thickness of thin layers are determined using the inversion scheme of very fast simulated annealing (VFSA). Uncertainty in prediction is evaluated using an approximate marginal posterior probability density function and a parameter correlation matrix. We demonstrate our methodology on synthetic and real seismic data from the Sleipner field, which is a good example of enhanced seismic amplitudes due to interference among reflections from thin layers. We select 11 common depth point (CDP) gathers for amplitude analysis of four identified reflectors along a line of 3‐D seismic data acquired in 2008. Predicted mean models of CO2 ‐saturated sand layers at all locations show that the thickness, density, and P and S wave velocities vary from 5–7 m, 1.8–2.0 g/cm 3, and 1, 460–1, 490 and 630–650 m/s, respectively. Our analysis suggests that the model parameters are well constrained and independent except at a few locations. Key Points: We propose a modified AVO/AVS theory to account the amplitude alteration in the presence of a seismically thin layer. Our method also aids to resolve the tuning effect due to the presence of a thin layer The proposed method efficiently predicts the elastic properties and thickness of thin stratified layers using a VFSA inversion scheme