Circumferential Velocity Analysis (CVA)

This is a technique which can be used to assess in situ stress orientations. Very simply, we measure the P-wave velocity across the diameter of a core at various azimuthal positions along the circumference. One of course measures the diameter simultaneously and corrects velocities for length changes. If the material is homogeneous and isotropic, one should record a constant unchanging velocity at each azimuthal position (see schematic below).


The actual bench top apparatus is shown in the figure below. This arrangement shows a shale sample from roughly 16,000 feet and a set of P-wave transducers.
In the schematic above, the measured velocity is independent of angle and the material is isotropic. Observations are made at increasing values of phi. If the rock has been subjected to unequal in situ stress and brought to the surface, the process of stress relief, re-equilibration, will not be symmetric. The process is facilitated through microcracking or microfracturing. The cracks will open so that the normal to the crack face will orient parallel to the maximum stress. Thus when one measures velocities on a stress relieved core, one does not expect a constant velocity. Velocities measured parallel to the crack faces will be faster than that measured perpendicular to the crack faces. The results for simple systems are sinusoidal to a 2*phi dependence.

Above are results on three samples from Red Oak field in Oklahoma.  Note the simple angular dependence and the difference in the magnitudes of the velocity changes. The amplitude of the sinusoids is directly proportional to the crack density. Calibration of crack density with stress relief magnitude is an ongoing project. The azimuthal position of the minima or maxima among the three samples is consistent as one should suspect if the prevailing stresses are governing this process.

Another example of such analysis is presented below as a simple azimuthal plot of the observed waveforms.

Here we can clearly see that the first energy arriving at the receiver transducer scribes an elliptical pattern and not a circle. The magnitude of the ellipticity is proportional to crack density and the major and minor axes indicate the prevailing in situ stress directions.

A more complicated pattern is detected if the material is anisotropic. An example of this is shown in the CVA analysis for a coal.

Now instead of having a simple 2*phi dependence, the pattern is complicated by the cleating system of the coal

CVA works well on old core. Orientation is needed in some form to register the laboratory observations back to field orientations. This orientation can be obtained at the time of core retrieval as oriented core or through post orientation of core using FMI or televiewer logs or as crudely as using sedimentary features to establish initial borehole orientations. It is most convenient in establishing directions and not magnitudes.

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