Scott Leaney

An in situ estimation of anisotropic elastic moduli for a submarine shale

Direct arrival times and slownesses from wide-aperture walkaway vertical seismic profile data acquired in a layered anisotropic medium can be processed to give a direct estimate of the phase slowness surface associated with the medium at the depth of the receivers. This slowness surface can, in turn, be fit by an estimated transversely isotropic medium with a vertical symmetry axis (a TIV medium). While the method requires that the medium between the receivers and the surface be horizontally stratified, no further measurement or knowledge of that medium is required. When applied to data acquired in a compacting shale sequence (here termed the Petronas shale) encountered by a well in the South China Sea, the method yields an estimated TIV medium that fits the data extremely well over 180 o of propagation angles sampled by 201 source positions. The medium is strongly anisotropic. The anisotropy is significantly anelliptic and implies that the quasi-shear mode should be triplicated for off-axis propagation. Estimated density-normalized moduli (in units of km2/s 2) for the Petronas shale are All = 6.99 -+ 0.21, A33 = 5.53 _ 0.17, A55 = 0.91 _+ 0.05, and A13 - 2.64 _+ 0.26. Densities in the logged zone just below the survey lie in the range between 2200 and 2400 kg/m 3 with an average value close to 2300 kg/m3.

A Monte Carlo Method to Quantify Uncertainty in the Inversion of Zero-Offset VSP Data

Summary We describe in this paper a method that uses zero-offset Vertical Seismic Profile (VSP) data to infer the characteristics of a layered medium below the receivers. We quantify the non-uniqueness, or uncertainty, of the solution by adopting a Bayesian approach that defines a posterior distribution of layered media for the given VSP data and prior knowledge (for example, on likely values of acoustic impedance). This formulation also ensures that the sampled layered medium solutions are simple, i.e., do not contain layers that are not required by the data. We describe a variation of the commonly used Monte Carlo Metropolis-Hastings algorithm that efficiently obtains a sample from the posterior distribution of layered media. The Monte Carlo method is illustrated with an example where VSP data were acquired for pore pressure prediction. The method quantifies the uncertainty inherent to predicting the depth of overpressured zones from VSP data, and demonstrates how this uncertainty is reduced by additional travel time-depth ties acquired during drilling.

Offset-to-angle Transformations For PP And PS AVO Analysis

The quest for quantitative interpretations of seismic amplitudes requires that higher-order approximations supplant those considered as “industry standard”. A layered earth with VTI anisotropy requires a non-hyperbolic moveout equation to reproduce long-offset arrival times. If use is to be made of long-offset amplitude information, we must honor this theory in each step. We describe an offsetto-angle transformation that is applicable for both PP and PS reflections, based on the non-hyperbolic equation. The effects are illustrated on a model and on field data.