Noise tests

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Problem 8.11a

The noise test shown in Figure 8.11a used 36 geophones spaced 10 m apart and six sources spaced 360 m apart. The event indicates ground roll. What are the velocites, dominant frequencies, and wavelengths of the noise trains? What length of a geophone group will attenuate them?

Background

A noise test (noise profile) uses single geophones that are closely spaced (as little as 1–3 m apart) and recorded individually. The profile is studied to identify the characteristics (especially apparent velocities) of noise wavetrains so that arrays can be designed to attenuate them.

A pulse is composed of many frequency components, each traveling with a phase velocity that at times varies with the frequency; in this case the pulse changes shape and travels with the group velocity (see problem 2.7c), an effect called dispersion.

Ground roll is discussed in problems 2.14 and 8.6.

Solution

Measurements on Figure 8.11a are very crude. The ground-roll wavetrain loses its early cycles with distance because it is dispersive. Its apparent velocity is about 90 m/s. At the peak-to-peak period is about 70 ms or 15 Hz frequency, so its wavelength is about 6 m. The dominant frequency near is about 5 Hz so the wavelength is about 18 m and a linear geophone array of an integral number of wavelengths would attenuate it. The wavetrain that arrives at long offsets at about 1.8 s has a velocity of about 170 m/s and frequency of about 10 Hz, or wavelength about 17 m. An array about 18 m long would attenuate both wavetrains.

Figure 8.11a.  Walkaway noise test.

Problem 8.11b

Explain the alignment

Solution

This event is probably a result of spatial aliasing (problem 9.25) although it might be a backscattered surface wave, scattered from a source not necessarily inline.

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