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In-depth Analysis

Shear velocity, AVO, and Anisotropy

Shear wave velocities;


From standard VSP surveys (P-source) we can in many cases observe strong converted downgoing shear waves on the horizontal components. By picking one of these events we get estimates for interval shear wave travel times and velocities. The S-velocities can be combined with P-velocities and the density log to estimate important reservoir characterization parameters such as Vp/Vs, bulk modulus, shear modulus and Poisson’s ratio.



Walkaway VSP survey used for AVO calibration


By placing the receiver array just above the reflector of interest, the variation of reflection strength observed in the true amplitude reflected P-Wave and S-Wave common receiver gathers is a function of the ray reflection angle.


Identification of reflector


The reflector is identified on the common receiver gathers, optionally with anisotropic, long-spread NMO corrections applied. The amplitude of the wavelet as a function of reflection angle and source offset is then calculated.


AVO/ AVA analysis


The AVO/AVA results can be compared to pre-stack surface seismic results and to synthetic AVO responses generated from wireline logs.


Anisotropy estimation (VTI)


Estimates of the anisotropy can be obtained from walkaway, offset VSP’s and for combinations of borehole seismic and surface seismic. Several approaches are offered;


Average anisotropy estimation based on analysis of Walkaway VSP data first time breaks.

Derive a 1D model for the vertical P- and S- velocities, then compute the direct P-travel times for a range of epsilon and delta values. For each combination of epsilon and delta compute the least square difference between the computed and observed travel-times.


Local anisotropy estimation based on analysis of Walkaway VSP data first time breaks.

Estimate the local anisotropy based on observation of the vertical as well as the horizontal slowness values. The method, which requires the wavefield to be recorded for a vertical and a horizontal component, is very suitable to estimate the anisotropy parameters at the depth of the receivers for data acquired with the Walkaway VSP geometry.


Local anisotropy estimation based on use of the rig source VSP first time breaks in a deviated well. Vertical velocities are derived from a Normal Incidence VSP. The isotropic model is achieved by minimizing the difference between the isotropic modeled and the observed first time breaks. Then the Rig Source VSP direct P-Wavefield is modeled using an initial anisotropic model. The anisotropy model is updated by minimizing the difference between the anisotropic modeled and observed first time breaks.


Other survey geometries can be used for estimation of anisotropy, including the combination of surface seismic data and rig source data to broaden the propagation angles covered by a rig source VSP.  Fixed Offset VSP can be used in the same way to estimate local anisotropy if the geology between the vertical well and the shot location is not too complex.


Anisotropy estimation (TTI);


A walkaway VSP with a downgoing wavefield recorded through dipping layers may show a tilted anisotropy axis. Symmetry around zero offsets for an isotropic model will no longer be the case. REX Geo Solution offer forward modelling to build theta models (angle of the tilted transverse isotropy). Not including a TTI model for dipping layers in a surface seismic migration will normally lead to a significant horizontal displacement of the target reflectors.

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