Geophysics - E-book - ePub

In the decade and a half since the preparation of the firstedition of this script there have been few fundamental changesin the methods used in various geophysical surveys conductedwithin the small scale range. Not only that, the past few yearshas been witness to great changes in applications andsophistication of instrumentation. Surface energy sources used inVSP surveys are much the same as those used in surfaceseismic surveys.
They include dynamite, vibrators, air guns, andmechanical impulse source. Buried dynamite charges are widelyused as the surface energy source for VSP because of theireffectiveness in producing seismic body waves. However, maintaining a consistent wavelet shape when shooting severaltens of shots requires a great deal of care. Vibrators areattractive for use in VSP work. A pilot sweep can be designedand input to the ground that satisfies whatever resolution isrequired in VSP recording.
Sweep parameters such as numberof units, length of sweep, and number of sweeps can beselected that provides the desired signal-to-noise ratio. Also, cross-correlation of Vibroseis sweeps enhances signal-to-noiseratio by discriminating against noise outside of the sweepfrequency range. Coherent noise with frequencies in the sweepbandwidth may present a problem but these can usually besolved in the data processing stage.
Mechanical impulse sources exist that can apply a verticalimpulsive force to generate seismic energy. However, thesesources should be tested for an area before being used. Thereis a major difference in the shape, size, and construction of ageophone used for surface recording and a borehole geophoneused to record a VSP survey, as shown in Fig. 4.50. A typicalland geophone is about 10 cm long, has a diameter of about 3cm, and weighs around 200 gm.
By contrast, a downhole geophone is 3 m long, has a diameterof 10 cm, and weighs 100 kg. The size of a downholegeophone results from its being within a massive housing thatis designed to withstand the high pressures and temperaturesfound in deep wells. Also within this housing is the mechanicaldeployment system that anchors the geophone to the boreholewall and electronic amplifier and telemetry circuits.
The 24-bitrecording systems used for surface seismic surveys will recorddown- hole geophone data and near-field monitor geophoneresponses with more than adequate resolution to capturehigh-resolution wavefronts. The near-field wavelet should berecorded in all marine VSP surveys. This is particularly true when using an energy source, such asan untuned airgun that creates a long wavelet. Signaturedeconvolution can be used to compress the source wavelet anda recording of the near-field wavelet is needed for this purpose.
Rayleigh waves, or ground roll, propagate along the earth'ssurface in all directions away from the energy source, interfering with the signal from deep reflectors in landexploration. These waves are undesirable signals and theyprevent optimum imaging of stratigraphic, and structuralconditions. 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 formationirregularities, fluid movement behind the casing or in the well. There will be no further discussion of theses noises but thereare other noises that deserve some discussion.