# Noise tests

Series Geophysical References Series Problems in Exploration Seismology and their Solutions Lloyd P. Geldart and Robert E. Sheriff 8 253 - 294 http://dx.doi.org/10.1190/1.9781560801733 ISBN 9781560801153 SEG Online Store

## 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 $\displaystyle A_{1} -A_{2}$ 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 $\displaystyle A_{2} -A_{1}$ loses its early cycles with distance because it is dispersive. Its apparent velocity is about 90 m/s. At $\displaystyle A_{2}$ the peak-to-peak period is about 70 ms or 15 Hz frequency, so its wavelength is about 6 m. The dominant frequency near $\displaystyle A_{1}$ 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 $\displaystyle B_{1} =B_{2}$

### 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.