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In a fast formation, where shear
velocity is faster than the velocity
of the drilling fluid, the SDT
obtains direct measurements for
shear, compressional, and
Stoneley wave values. In a slow
formation, the SDT obtains realtime measurements of
compressional, Stoneley, and mud
wave velocities. Shear wave
values can then be derived from
these velocities. The multireceiver
sonic tool, with its linear array of
eight receivers, provides more
spatial samples of the propagating
wavefield for full waveform
analysis than the standard tworeceiver tools. This arrangement
allows measurements of wave
components propagating deeper
into the formation past the altered
zone.
The depth of investigation cannot
be easily quantified; it depends on the spacing of the detectors and on
the petrophysical characteristics of the rock such as rock type, porosity
(granular, vacuolar, fracture porosity), and alteration. For sourcedetector spacings of 3-5 ft, 8-10 ft, and 10-12 ft the depth of
investigation ranges from 2 in to 10 in (altered/invaded and undisturbed
formation, respectively), 5 in to 25 in, and 5 in to 30 in. The vertical
resolution is 2 ft (61 cm).
Porosity and "pseudodensity" log
The sonic transit time can be used to compute porosity by using the
appropriate transform and to estimate fracture porosity in carbonate
rocks. In addition, it can be used to compute a "pseudodensity" log
over sections where this log has not been recorded or the response was
not satisfactory.
Seismic impedance