About 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.

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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.

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