Freelance Gephysical Consultants Ltd

VIBROSEIS SWEEP ANALYSIS


SWEEP DESIGN ANALYSIS

The sweep design is quite important, whether is non-linear , linear, varisweep, custom sweep, etc. If the wrong sweep is selected, this issue it can damage the mechanicsof the vibrator. The image above shows thet a -6 dB/Oct log sweep can overdrive the Pump FLow and at the same time generate source generated noise and in real time, damage the pump of the machine.

Freelance Geophysical Consultants Ltd.can remotely fetch and analyzed vibroseis data files such as Sercel, Pelton or Force 3 VCU prodiving full comprehensive report about the performanceof the selected sweep and how could affect the servo mechanism if the sweeo has wrong parameters.

We can also design the correct sweep depending on your target depth and frequency needed to visualize the geological objective, see sample below:

We can see that the -3dB/Oct sweep stays in low and high frequencies throughout the lenght of the sweep evenly on high and low, then on the -6 dB/OCt the sweep goes out of the low frequenciesfaster and then remain on high frequencies for shallow targets and detection of methane hydrates. Then on the + 6dB/Oct the sweep enhances better the low frequencies anhigh frequencies. The LINEAR sweep is a total waste of time and MONEY and should not be considered for seismic acquisition, neither the seismic contractor or the oil company client. Then the + 3dB/Hz, the sweep totally ehnances the high frequencies and get ooutof the low as soo as possible. Finally the -3 dB/Hz sweep enhances the low frequencies for depper penetration and deeper geological target. We can design your custom sweep dependingon your geological target and if you have two targets, we can design that sweep also.

LINEAR SWEEP: With a linear sweep from 14-56 Hz, the time spent injecting energy is the same for the 14- 18 Hz and 52-56 Hz ranges. Since the higher-frequency components of reflections from depth are severely attenuated in the earth, you need to provide more signal input at the higher frequencies to achieve a satisfactory signal-to-background noise ratio. By increasing the sweep time, this objective can be accomplished with a linear sweep, but adding sweep time is expensive. With a nonlinear sweep, you can spend relatively more time at the higher frequencies and input more high-frequency energy without increasing total sweep time.

With linear sweeps, neither more sweep time nor more vibrators would help solve this dynamic range problem. But nonlinear sweeps can help by boosting the amplitude of the relatively weak, high-frequency components of reflections as compared to the relatively strong, low-frequency components of source-generated noise.

1) Determine the attainable upper-frequency limit. To do this record a test with a wideband sweep. Try several narrow-band sweeps (for example, if an upper frequency of 100 Hz is desirable, try sweeps of 70-80, 80-90,90-100, and l00- 110 Hz). Note ringy events that correlate with the objective reflections in time and in moveout. Where correlatable events are present, the frequencies used did not exceed the attainable upper-frequency limit.

2) Use an upsweep with either a dB/Hz boost (logarithmic sweep) or a dB/octave higher-frequency boos.

3) Choose the sweep. Select an upper-frequency limit from the narrow-band tests. Use a 2.5-3 octave sweep (this sets the lower-frequency limit). Choose a total high-frequency boost of, usually, about 10-l (20 dB), or up to about 20- 1 (26 dB) for maximum emphasis on high frequencies, or only about 6- 1(16 dB) when high-frequency recording is less critical. (These boosts have worked well for me in a wide range of applications using sweep lengths of at least 6 s to limit the rate of change of frequency at the lowest frequencies.) Test 2-3 different sweeps in a sweep and array test, and make a final choice.

4) Monitor the field data. In many cases, you will see at least periodic glimpses of reflections in the correlated data.

If these events disappear or broaden, repeat steps l-3 (broadening might indicate a loss of the higher frequencies).The choice of the upper-frequency limit is critical. If the earth varies markedly in the prospect area, you might need to test at more than one location. Choose a test site that is typical for the line or lines to be recorded.Be moderately optimistic in choosing the upper-frequency limit, but temper your optimism with the realization that you will spend much of your recording time sweeping frequencies near the upper limit. For example, with a 20-120 Hz sweep with 12 dB/octave high-frequency boost, 23 percent of the sweep time is at frequencies greater than 110 Hz and 42 percent at frequencies greater than 100 Hz. If these frequency components of reflections of interest are not present in the final stacked section, you have wasted much field time.

Conclusions. Nonlinear sweeps should be used routinely because they are cost-effective and they can improve resolution by improving both the signal-to-background noise ratio and the signal-to-source-generated noise ratio for the higher frequency components of reflections from depth.


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