We expect that the Bakken Shale example investigated here represents
about the most anisotropic geological specimen likely to be encountered
in the laboratory.
If this is true then
the traveltimes measured in similar laboratory core-sample experiments
should also represent vertical phase velocities,
*assuming*
the critical ratio of core-sample height to transducer width is
no greater than it was in their experiment (i.e., ).
If the core-sample height to transducer width is larger it becomes
more likely that some indeterminate quantity representing neither vertical
phase nor vertical group velocity is being sampled; in case of doubt
equations (7) and (8) can be used
to estimate whether there might be a problem for the measurements
in a particular experimental configuration.

Our modeling indicates that as an additional safeguard
first breaks should be picked instead of first peaks whenever possible.
This can be important in cases like the *q*P measurement
in our example that are close to being detectably delayed
due to side-slipping of the wavefront.
In any case, note that it is highly unlikely a
core-sample experiment could accidentally measure vertical group velocities;
our numerical modeling suggests the core-sample height to transducer width
ratio needs to be on the order of 20 (preferably even larger)
to be reasonably sure of sampling vertical group velocities.

While it is comforting to know that core-sample traveltime measurements
sample phase velocity, this still does not mean we are assured of
finding accurate values for all the various anisotropy parameters we
might wish to know.
At least some sorts of errors are avoidable, and *should* be avoided.
If we want to measure Thomsen's anisotropy parameter we should clearly state whether we are using
equation (4) or (5);
the choice of equation makes a difference (unless the anisotropy is very weak).
We find equation (5) to be the more useful.
Unless we have reason to discount some of our measurements
we should try to reconcile and use all available information;
the more independent measurements we can use the more confidence
we can have in the results.
In particular, the SV measurement is often the
single most important piece of data for constraining *C _{13}*.

Some errors may be unavoidable, unfortunately. For this reason
it is important to have at least a rough idea of the
reliability of the measured traveltimes and to use those input errors
to determine what the corresponding output errors in the calculated
anisotropy parameters may be.
If the standard deviations in the measured traveltimes are too
great the values of *C _{13}* and derived from them may
be effectively meaningless.
For this reason when publishing elastic constants we should state
what measurements we used to calculate the tabulated constants
and what magnitude of error we expect in them.
Similarly, when interpreting elastic constants published
by someone else we must be wary if this information is not provided
(Seriff and Sriram, 1991).

12/18/1997