Noise tests

From SEG Wiki
Revision as of 18:09, 9 January 2020 by Ageary (talk | contribs) (moved image)
(diff) ← Older revision | Latest revision (diff) | Newer revision → (diff)
Jump to navigation Jump to search

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?


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.


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


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.

Continue reading

Previous section Next section
Response of a triangular array Selecting optimum field methods
Previous chapter Next chapter
Seismic equipment Data processing

Table of Contents (book)

Also in this chapter

External links

find literature about
Noise tests
SEG button search.png Datapages button.png GeoScienceWorld button.png OnePetro button.png Schlumberger button.png Google button.png AGI button.png