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.