Geophysics

In the decade and a half since the preparation of the first
edition of this script there have been few fundamental changes
in the methods used in various geophysical surveys conducted
within the small scale range. Not only that, the past few years
has been witness to great changes in applications and
sophistication of instrumentation. Surface energy sources used in
VSP surveys are much the same as those used in surface
seismic surveys. They include dynamite, vibrators, air guns, and
mechanical impulse source. Buried dynamite charges are widely
used as the surface energy source for VSP because of their
effectiveness in producing seismic body waves. However,
maintaining a consistent wavelet shape when shooting several
tens of shots requires a great deal of care. Vibrators are
attractive for use in VSP work. A pilot sweep can be designed
and input to the ground that satisfies whatever resolution is
required in VSP recording. Sweep parameters such as number
of units, length of sweep, and number of sweeps can be
selected that provides the desired signal-to-noise ratio. Also,
cross-correlation of Vibroseis sweeps enhances signal-to-noise
ratio by discriminating against noise outside of the sweep
frequency range. Coherent noise with frequencies in the sweep
bandwidth may present a problem but these can usually be
solved in the data processing stage.
Mechanical impulse sources exist that can apply a vertical
impulsive force to generate seismic energy. However, these
sources should be tested for an area before being used. There
is a major difference in the shape, size, and construction of a
geophone used for surface recording and a borehole geophone
used to record a VSP survey, as shown in Fig. 4.50. A typical
land geophone is about 10 cm long, has a diameter of about 3
cm, and weighs around 200 gm.
By contrast, a downhole geophone is 3 m long, has a diameter
of 10 cm, and weighs 100 kg. The size of a downhole
geophone results from its being within a massive housing that
is designed to withstand the high pressures and temperatures
found in deep wells. Also within this housing is the mechanical
deployment system that anchors the geophone to the borehole
wall and electronic amplifier and telemetry circuits. The 24-bit
recording systems used for surface seismic surveys will record
down- hole geophone data and near-field monitor geophone
responses with more than adequate resolution to capture
high-resolution wavefronts. The near-field wavelet should be
recorded in all marine VSP surveys.
 
This is particularly true when using an energy source, such as
an untuned airgun that creates a long wavelet. Signature
deconvolution can be used to compress the source wavelet and
a recording of the near-field wavelet is needed for this purpose.
Rayleigh waves, or ground roll, propagate along the earth's
surface in all directions away from the energy source,
interfering with the signal from deep reflectors in land
exploration. These waves are undesirable signals and they
prevent optimum imaging of stratigraphic, and structural
conditions.
 
However, the VSP does not record Rayleigh or Love waves,
because they do not reach the depth of the geophone.
Random noises are caused, in some wells, by formation
irregularities, fluid movement behind the casing or in the well.
There will be no further discussion of theses noises but there
are other noises that deserve some discussion.