This in an excerpt from this book
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.
Geophysics is finding more and more usages in other fields like that of archaeology though the greatest hurdle in this is the relatively high cost of the equipment. In instrumentation, the automation of reading and data storage, which was only just becoming significant in the late 1980s, has proceeded apace. Virtually all the new instruments coming on to the market incorporate data loggers as well as the modern devices like that of automatic levelling which results in sophisticated performances saving a lot of time and easy to handle. The usage of laptop in these operations ensures two contrasting outcomes: On the one hand, the need for specialist skills in the field personnel actually operating the instruments has been reduced, and this is leading to a general decline in the quality of field notes. On the other hand, much more can now be done in the field by way of processing and data display, and even interpretation. Comments made in the first edition on the need to record information about the survey area as well as geophysical data have equal, and perhaps even more, force in these instances, but it is obviously usually neither practical nor appropriate to make individual notes relating to individual readings.
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